Autorotation and Ground-Effect

[Dave Jackson]

During autorotation to touchdown, does ground-effect reduce the final descent rate by any significant amount?

Thanks
Dave J.

[GLSNightPilot]

Not that I've noticed, but it's hard to tell for sure. There are lots of things happening at the same time - you're flaring, pulling pitch, & leveling, all at pretty much the same time. The only way I can see to tell for sure is just autorotate into the ground without changing anything, & seeing if you slow up during the last 20' or so. I'll let someone else try that.

Remember, the airflow is up through the rotor in autorotation, so how can the ground below it affect it? To tell the truth, I've never really given this much thought, so maybe I ought to. :confused:

[DeltaFree]

Ground effect exists at all heights it just becomes negligible very quickly as you climb. Logically all wing borne ac fly by exerting a downward force on the air around them, this is balanced by a pressure in turn being exerted on the ground below, so the proximity of the ground below will have an effect on your "wings".
During a flare at the bottom of the auto there is not a steady state of pressure and your deceleration is brought about by accelerating air downward. At first the ground is unaware of this but it doesn't take long to realise and exert a presure to resist the downward movement of air which in turn pushes you back. So I guess I am saying there will be ground effect but not as noticeable as in a stable state constant low level flight/hover. To be honest as previously mentioned, amongst all the other effects it will be barely noticeable.

[SASless]

Now if the ground was as slow to react as I am due to my advanced age and alcohol ravaged brain.....could the ground effect become operable well after I touchdown.....and the followup question must be to see if the ground up north amongst all dem yankee's reacts any quicker than southern soil? Could we get NASA to study this concept....and maybe come up with another isogonic line similar to variation on our maps...to depict areas of differing rates of ground reaction to rotorwash.....thus allowing us to be more aware of just where we might anticpate slow vs quick reacting ground? How would we factor that into our CAT A performance graphs.....now that is a very interesting question! Could it be our power assurance numbers are not taking into effect slow reacting ground?;)

[DeltaFree]

Believe me the ground will react before you hit it but more so when you do! Actually as you may remember from ground school sometime in the dim and distant, different surface do give different response to low flying helos. Long grass vs hard concrete etc. Sorry perhaps more previous explanation was a little too simple I could give more in depth physics and aerodynamics if you wish, but my fingers cannot take the keyboard bashing.

[Barannfin]

This is just an idea here, but I would think that at the very last stage of an auto, when your pulling pitch, the ground effect would be present. Because in this part of the auto you are actually pushing air down.
Although you would like to be on the grnd as soon as possible as the Nr is getting quite low by this time. Just my 2cents.

[Flight Safety]

Agree with Barannfin, there MUST be ground effect near touchdown.

The Nr energy is being used to power the last part of the touchdown, thus the rotor is producing downwash. Downwash and lift are affected by close proximity to the ground, or else the HIGE and HOGE specs would be meaningless.

I don't see that it should make any difference if either the stored energy in the rotor or an engine is providing the power for the downwash and lift, when the collective is pulled up right before touchdown.

I think though that prior to that last collective pitch up, ground effect would not be much of a factor, since air is flowing up and through the rotor to maintain RPM, thus no downwash is being created.

[Bertie Thruster]

In my experience the effect of the ground in autorotation is to bring the rate of descent to zero.

[Nick Lappos]

Bertie has it right, the ground is always the winner!

There is ground effect at low AGL in auto, cruise and hover. It is not a major effect (a 20% reduction in power) but it contributes. The effect increases the rotor thrust (reduces the power needed, and power is rate of descent in autos) as you get near the ground. If you are looking for the effect while judging cyclic flair and collective pull, you will not find it, as all things are changing so rapidly you won't sort it out, but it is there.

[John Farley]

I don’t know anything about the existence or magnitude of the ground effect in question but I may still be able to help.

Once upon a time a ground effect device fitted to a Harrier increased the VTO weight by 1500 lb. Its effect tapered rapidly with height and had vanished by about 10ft wheel clearance.

When doing a normal VL the same device produced a negligible reduction in descent rate. The question is why?

Mr Newton helps us to appreciate what sort of acceleration or deceleration we get from applying a force to a mass. Any ground effect deceleration of the chopper during an autorotation to land (even if the deceleration was say a huge 1 g) would only actually reduce the touchdown rate if it was applied for a length of time. And time is likely to be very short doing a VL through ground effect.

Bottom line is any positive ground effect needs time, and quite a lot of it, to reduce a sink rate. And time is very short in the manoeuvre under consideration.

[Up & Away]

Surely 'ground effect' can only be measured from the moment the aircraft is 'levelled' and theoretically height maintained??

;)

[Nick Lappos]

The problem with the way we picture ground effect lies in the way it has been taught. It is not a cushion of high pressure air below the aircraft, it is not only effective at a steady hover, and it always affects lift-producing surfaces, rotors and wings, when they come close to a surface. Ground effect is essentially a reduction in the induced drag of the airfoil, because the ground helps reduce the angle of attack needed to produce the lift.

If ground effect cannot help an autorotation, why does it show itself when we approach the runway in an airplane during a glide? If ground effect does not help at speed, why did WWII bombers limp home half a wingspan above the Channel, using one engine's power to hold themselves aloft?

The small effect it has on power and thrust (about 20%) is not apparent during the auto flair,but it is there. Physics might be confusing, but it likes to be consistent.

[GLSNightPilot]

Nick, I have no problem with the fixed-wing ground effect you describe, but I still don't see how that applies to helicopter autorotation. In full autorotation, I've always believed that lift isn't being produced, the air is just moving upward through the rotors, keeping them turning. Now, on the bottom, when we start pulling pitch & producing lift to arrest the descent rate, it seems to me that ground effect does come into play, & is most likely a real effect, but as you said, a lot is happening there in a short time, so measurement would be difficult at best. Certainly your rate of descent will be stopped, either above, upon, or in the ground. The first is far preferable. ;)

[Dave Jackson]

To all who have responded; except SASless; A thank you. :)
To all who may possibly respond; A possible thank you. :eek:
To all who have not responded; #@*##%**##@. :mad:
To SASless;
Cannot decipher your post. Please advise which translator to use. [Mumbo Jumbo] to [English] [Pig Latin] to [English] [Gobble de Gook] to [English] [Esperanto] to [English] [Lower Slobovian] to [English] [Other] to [Another Other];)

Dave J.

[DeltaFree]

For GLS.... In auto the rotors are producing lift, hence the CONSTANT rate of descent, if there was no lift you would accelerate at 9.81m/s all the way to the ground, well OK fuselage drag would slow you down but auto is not freefall. Even a freefalling parachutist would have ground effect as the air in front of him is compressed as he reaches those last few millimetres, unfortunately it is a case of too little too late for him/her As indeed, in auto, it is kind of irrelevant but present nonetheless.

[PPRUNE FAN#1]

Yes, let's not confuse "ground-effect" and "ground-cushion."

I think a lot of helo pilots assume that an autorotating rotor produces no lift. But it should be clear that the rotor must be producing some lift or, as DeltaFree pointed out, the heliocopter would plummet to earth in freefall.

IF a rotor in auto produced no lift, how would we account for being able to arrest both our rate of descent and forward speed in the (ahem, Nicky?) flare with the pitch stick still firmly on the bottom stop?

Even at flat pitch, a rotor moving forward through the air still has the properties of an airfoil. Increase its angle of attack and you increase the amount of lift produced. Therefore, airplane-like ground-effect will apply, but only very close to the ground (probably less than the upper limit we usually associate with g/e in powered hovering flight).

As NL notates, the conditions are changing too rapidly in the flare/pitch-pull to quantify any "benefit" from ground-effect. At the bottom of a fairly steep auto, ground effect will be minimal, and not felt until the pitch-pull.

But come in fast and round-out low. Like an airplane, the helicopter would take advantage of the same type of ground-effect. Unlike an airplane which touches down at a fairly high lift-producing speed, the helicopter will quickly lose ground-effect as the speed bleeds back to zero. THEN, as we level and pull, ground-cushion comes into play.

[heedm]

Yes, the autorotating rotor is producing lift, that's why the cyclic is still effective. Other falling objects stop accelerating due to terminal velocity, but if that were the case for helicopters the cyclic wouldn't work and increasing collective wouldn't stabilize at a lower Nr, it would stop the rotor.

There is ground effect at the bottom of the autorotation. As Nick pointed out, the ground effect causes a reduction in induced drag. Hence, with the ground effect the same amount of cushion will slow the rotor less, availing more energy to stop the helicopter's descent.

[DeltaFree]

Excellent final point about rotor slowing less rapidly. Ground effect will not reduce as the machine slows in fast and low flare, it is purely a product of the lift being produced close to the ground and so will relate to rotor speed and incidence, so long as the machine is flying level at a constant height above ground the ground effect will remain constant but the ground cushion will vary.

[PPRUNE FAN#1]

DeltaFree:
Ground effect will not reduce as the machine slows in fast and low flare, it is purely a product of the lift being produced close to the ground and so will relate to rotor speed and incidence, so long as the machine is flying level at a constant height above ground the ground effect will remain constant but the ground cushion will vary.

Aha, caught me in a technicality! Excellent! Yes, I probably should have worded it thusly: the product of ground effect will diminish as airspeed reduces. That is, until the forward speed is at zero and the rotor must alternatively start producing lift by increasing the collective pitch. Umm...right?

Buqqery! Just when you think you've helicopters pretty much figured out someone will ask a question without a quick and easy answer. Lucky I've got you lot for new perspectives.

[sierra-papa]

I think we are getting pretty close to the facts on this interesting thread, but there must be an INCREASING ground effect at the end of the level off-phase. This is because we are increasing pitch at that point - which puts us aerodynamically closer to a normal IGE hover (air is moving from the top to bottom again). I am talking only about the very last phase of an touch down auto. In big helicopters that phase can actually last quite a bit.
sp

[DeltaFree]

It seems we are all singing off similar hymn sheets.
Well, we all agree that there is ground effect, that it is barely noticeable during the busy seconds before impact, and that autorotation is a time of lift, don't we?
I think the full understanding of ground effect is clearly one for the aerodynamicists. I find the easiest way to look at it is that the ground acts a little like a mirror, whatever you throw at it it throws back. If you blow air at it, it acts like someone is blowing air back at you, so the closer you get the bigger the effect. This of course doesn't seem to explain how it would work in autos. However, if you create higher pressures above it (as lift does) it does not allow the air to escape, expand and reduce that pressure so produces a relative increase in pressure underneath you, of course improving your lift.
Oh ****** I said this was the easiest way! Well I could have talked about the lift producing vorticity within a wing, the trailing horseshoes and the reflections in a solid surface!!!

[etnb]

Let see (forgive me for any grammar or spelling mistakes)

I am still a student so allow me to share with you what I have been learning:
Autorotation -
1) lower collective - to the bottom
Blades almost flat - angle of attack is efficient only at the (+-) center area of the blade.
rate of descent is decreased only because the relative airflow come from under the blade.
2) maintain forward speed (while maintaining Rrpm within limits)
3) slow down, start to level
Blades are still flat, and still ROD under control, because of our airflow that comes from below due our continues descent.
4) flare and level
The pitch is still flat, we are still on a descent path, which is slowed down only because of converting speed to power.
5) raise collective
Only now that the pitch has been change to positive angle, and our fall as been stopped and converted to forward "glide"
We might be subjected to GE, but I think we are still gliding forward relative fast to be under any GE.

Am I right ?

------------------------------------------
-so far my instructor is still alive-

[PPRUNE FAN#1]

etnb, you're just a little confused.

Think of the spinning rotor disk as a wing. Because it is. In forward flight it has the same characteristics as an aeroplane wing. Pull back on the stick to increase the angle of attack of the disk and you will get a corresponding increase in lift/drag. This occurs whether you're in powered flight OR autorotation, makes no difference. Just so long as you have some positive airspeed. And if our "wing" is producing lift, it must be producing a corresponding downforce.

Now, let's say we enter an auto. As we plummet to earth at 60 knots IAS or whatever, we'll need to start a flare at some point. As the ship passes through thirty feet or so (depending on the rotor diameter of which one we're in), believe it or not we'll start to feel the effects of our ground cushion. Will it be noticeable? That is, would you be able to tell any difference between an autorotative flare done close to the ground versus one done up at altitude? Personally, I've never compared them and the results would be very hard to quantify in any case. But it would be there. It *must* be there.

Once we're slowed to zero and the ship starts to settle, the airflow through the rotor has changed from horizontal to vertical, and the rotor is no longer acting like an airplane wing. We begin to pull up on the collective. With the now-downward flow of air from the rotor, the ship again gets to take advantage of ground-effect one more time.

[mjstrother]

A number of respodents to this subject do not seem to fully understand or appreciate the physics associated with the airflow reversal in the rotor in the last moments just prior to touchdown/run-on.

My own view is that ground effect must play some part during the final pitch pull, though I agree with respondents who have stated how difficult it is to quantify just how much (or how little).

One respondent mentioned the flare. The airflow is not reversed in the flare whilst still autorotating, and the physics in this portion of the maneovre should be studied separately.

Comments to all respondents made with respect and courtesy - does anyone disagree with me?

Mike Strother, Leeds

[[email protected]]

PPF1, the flare in autorotation does the same thing at altitude as it does during an engine off landing - it increases rotor thrust by changing the angle of attack - the increase in Nr, the reduction of forward speed and RoD are all a result of the flare effect.
Anyone who thinks they can detect ground effect during an EOL is deluding themselves - the airflow may finally make it from the top to the bottom of the disc in the latter stages of the cushioning part of the manoeuvre instead of the bottom to top movement that autorotation relies on, but lets not pretend it will stop us hitting the ground hard if we wait for ground effect to save us.

PS try a hover engine off and see what good ground effect is then!

[helix47]

I cannot believe the amount of perfectly good flying electrons spent (wasted?) on this subject. I can believe even less that I am contributing to it?? Must be the effects of Millers beer. won"t drink that again.
I think crab @SAA is getting close to the answer this subject deserves. Perhaps one of the other theoreticianscould go one step further.
How about building a 30 ft high helipad. This helipad should be only just larger than skid size in order to reduce the ground effect to an absolute minimum possible. Do a hovering auto on this helipad and then do one on the ground proper. Let us know if there is any difference.
I will have another beer thanks.

[helmet fire]

I really miss Lu during these discussions....I can just read his response about helicopters rolling off the back of ships in the '50s, etc, etc, etc, etc, etc.......;)

I am finding this thread fascinating, with some good educational points for me. Firstly, I guess it is fair to answer Dave's original question of

"During autorotation to touchdown, does ground-effect reduce the final descent rate by any significant amount?"

with a widely agreed "NO, not significantly".


There are some points raised that I would like to hear more about, in particular, the ground effect Nick has likened to a low flying aeroplane. Having conducted a few hours low level (<10ft AGL) I have not noticed any reduction in power required to fly level Vs flight at say 200ft AGL (above translational speeds of course). But I will test this theory at my earliest. On the other hand, I have seen the effect demonstrated in a fixed wing, and seen the telltale swirls over the water, even when going more than 350kts. Over about 40 kts do not recall seeing the same on the water behind a helo, but I will certainly pay more attention in the future.

Does the ground effect we are talking about here have to do with the cord, span and camber of the wing? I.E. the helo blade is so much smaller than the fixed wing "blade", thus it produces an entirely different pressure wave below and behind the wing. Also, the very next blade is slicing through the vorteicey's created by the preceeding blades and mixing the pressure areas up (plus fusleage and tail rotor disturbances), unlike a fixed wing that has no interference and can build a more constant effect. In otherwords, does the helo in it's forward flight have too much mixing of vorticey's to allow a fixed wing like ground effect to develop?

Perhaps the concept of time, as many of you have stated above applies here. The helicopter in the IGE hover has had enough time for ground effect to develop due to the consistants such as downwash dissipation over close proximity ground, height, wind, and pitch. For example, go to an IGE hover and note the TQ. Now do a quickstop to the same spot with a termination to the same heigh, but only pull the same TQ as previously noted. Ouch! :eek: But, if your helo could take it like the UH60 can (due to it's undercarriage design) you will eventually rise back up to the previous height. This demonstrates that the ground effect is always there, but it takes a finite time to develop into a power reduction that helps you. Thus in the auto, I reckon you would never notice it, and I would be suprised if it was even measurable. Besides, if I am going to take my mind of sex it will be to avoid heavy ground contact, not to try and notice any ground effect!!

Waddya reckon?

[PPRUNE FAN#1]

Helmut Fahr,

Interesting post! I can only offer one observation and one already-suggested suggestion.

1. In the 1980's there was on U.S. telly a show called "Airwolf." During the closing credits there was a shot of the 222 cruising low over the water, height and speed undetermined. The camera ship was above and just forward, shooting down and back. A very clear wake was visible on the water for quite some distance, more prominent on one side (advancing, if memory serves) than the other.

2. I would like to see an elevated platform, like an offshore drilling rig with a helideck made of grating (or otherwise porous) materiel well high up off the water. Hovering autos could then be performed and the results quantified versus doing them to solid ground. Should be interesting.

[helmet fire]

Good point PPF#1, I recall that shot. Whilst it certainly looked slower than 40kts, I am not convinced that the trail was not simply downwash on the water rather than the pressure wave you can see under an aeroplane wing. Or maybe that is simply downwash of the voticies as well?

Lastly, PPF#1, au contraire to your earlier post, I reckon etnb is not as confused as you think. I can only find fault with his concept of a "forward glide". On the other hand, your quote that the disc of a helicopter acts like an aeroplane wing is frought with misconceptions. I cannot imagine a scenrio in which it behaves like the wing of an aeroplane. You say that:

"In forward flight it has the same characteristics as an aeroplane wing. Pull back on the stick to increase the angle of attack of the disk and you will get a corresponding increase in lift/drag. "

Pull back on which stick? Cyclic will have the effect of increasing pitch on the advancing blade, and decreasing it on the retreating blade. Collective will increase the pitch of all blades, but that doesnt necessarily cause the nose to pitch up. So how is this like a fixed wing?

Perhaps your understanding of the above also led you to say that:
"At the bottom of a fairly steep auto, ground effect will be minimal, and not felt until the pitch-pull.
But come in fast and round-out low. Like an airplane, the helicopter would take advantage of the same type of ground-effect."

It doesn't matter how low and fast you round out (isn't that the idea of an auto anyway??) I dont reckon you would get any ground effect, certainly no significant effect, and definately nothing approaching an aeroplane. Your descent rate and forward speed energy is being used mostly to turn the rotor blades, and secondly to produce relatively little lift, just enough lift to stop you free falling. During the flare and cushion, you are diverting the energy that was turning the balde stored as blade inertia, into lift to arrest your rate of descent, for the first time in the auto you are generating significant lift. No where during this process is there anything "aeroplane like" going on, not even a glide.

[Nick Lappos]

Some commments:

Ground cushion and ground effect refer to the same thing, cushion is simply pilot lingo, and as usual is aerodynamically incorrect but linguistically interesting. It misleads, because we then picture a bubble of pressure, and all that wrong stuff. I have even had pilots say that the bubble must be under the aircraft, or there is no "ground cushion". Ouch!

The effect of the ground as we get closer is to reduce the angle needed by the airfoil. This reduces the induced power (the power that overcomes the drag due to the high angle of attack). In a hover, induced power is high, so the ground effect is more noticible. At Vne, the induced power is small, so we don't see as much ground effect, but it is there, the same percentage of induced power is reduced, but induced is a much smaller percentage of the total power, so the benefit of ground effect is small. At a hover, ground effect is worth a reduction in total power of about 20% for a 1 inch hover. At high cruise speed, it might be worth only 1 or 2%, because induced power is so small a part of the total power we are using.

There is no finite time for ground effect to "build up". It is not a cushion, and it does not involve perssure rising under the machine or any such thing. As the airfoil gets closer to the ground, the airflow is changed by the presence of the ground. The effect is "felt" at the speed of sound, so from 10 feet, it takes about 1/100 of a second for the full ground effect to form (10/1060).

On a still morning, cruise at 100 feet above a runway, carefully note the collective position and engine temp/manifold pressure needed to hold say 70 knots. Then slide down to 10 feet, and see if there is a difference after you level off. If flown carefully, you will see an appreciable reduction in collective, and in power.

[PPRUNE FAN#1]

Helmet - I think Nick just posted the definitive statements about ground-effect.

But to elaborate a bit on a different subject, you say:
"Pull back on which stick? Cyclic will have the effect of increasing pitch on the advancing blade, and decreasing it on the retreating blade. Collective will increase the pitch of all blades, but that doesnt necessarily cause the nose to pitch up. So how is this like a fixed wing?"

Look, a lot of pilots are confused about how rotors produce their lift, and I don't claim to be an expert. I'm certainly no Ray Prouty, but I do read him from time to time and try to understand his oft-confusing words, diagrams and formulae.

Pulling back on the cyclic does increase the pitch of the advancing blade, yes. And that tilts the rotor disk to a more nose-up position, yes. But increasing the pitch of the advancing blade does not produce a dramatic increase in total lift of the rotor (and remember, any lift that is generated is actually being produced on one side of the aircraft). Without a significant increase in total lift, the ship would merely change its attitude and not its rate of descent. But we know that we can reduce our rate of descent to zero if we like, even with the collective at flat pitch. Go fast enough in an auto and you could probably do a loop (my theory - don't quote me). How is this possible? What causes this tremendous increase in lift?

If we had a big round flat piece of plywood mounted above us, it would still have the properties of a fixed-wing in that if we increased its angle-of-attack with respect to the relative wind, it would produce lift (and downwash). The spinning rotor has solidity and thus does the same thing.

I just saw a movie of a wind tunnel test. In horizontal flight, smoke blowing toward a rotor went up and over the leading edge of the disk, just as it would when encountering an airplane's wing. Once it past the mast, the smoke was accellerated downward through the aft portion of the disk. The airflow (smokeflow?) looked very similar to the way we visualise a wing working in profile.

Like I said, I'm no expert, and maybe I'm completely off-base here. But I do know that even in autorotation, a rotor in forward flight works very much like an airplane wing, even if it's not exactly like one in the mechanics of it. As such, it will produce a certain amount of downwash. From there, we refer to Nick Lappos' post just above.

[etnb]

One exercise that my instructor thought me, and I was tested on for my PPL, was cushion creep take off.
I am pretty sure that for you, the experience pilots, it comes by now naturally, as for me, I still have to look for the downwash to see where transition lift comes to it effect.
How is it connected to all the discussion?
Well cushion creep take off (at least that what I have been learning) is where we start hover slowly forward into the wind, very slow, if the surface is like short grass, we can still see the downwash effect on the grass for a good few feet, and as we go further on we get to a point where we "meet the downwash", or where the downwash no longer blowing the grass, because we have reach the point of flying in equal speed to wind (I think), and at that point we will get a climb (just where we are loosing the cushion), well we have to options either to increase speed slowly and climb slowly according to the best profile, or to increase the speed fast and stay close to the ground and climb at about 35-40 kts (in H300).

All my text was just to point out on a way to see the ground effect - by looking for the downwash ahead off you, if you see it, you have a cousion (into wind cond).

I think

(apologizing for my english)

[Dave Jackson]

Nick,

Thanks for the interesting and informative comments in your last posting. All that you say makes sense, but the third paragraph raises a debatable point.

Some consider Ground Effect as a 'cushion of denser air' located under the lifting surface(s). This 'cushion' results in additional lift, caused by an effective increase in the angle of attack.

Your third paragraph suggests that this additional lift is the result of the sonic wave bouncing back off the ground at the speed of sound. This action is the basis of tuned exhausts, which are used in racing motorcycles and some recreational helicopters. A tuned exhaust causes the sonic wave from the opening exhaust port to be bounced back to this port, at the speed of sound, to increase the compression in the cylinder. It has the advantage of increasing the engine's torque, but this higher torque is limited to the very narrow range of rpm for which the exhaust pipe was tuned. Outside this narrow range, the torque is now lower than it would be with a conventional exhaust.

Using this analogy, an interesting concern, or question, is raised. Airplane's wings have large chords. This allows sufficient time for the 'shock wave' to bounce back to the wing and produce additional lift. Helicopters have a very low solidity ratio. The mathematics appears to suggest that roughly 90% of the 'shock waves' from the blades will rebound up between the blades. This, of course, would result in negligible additional lift.

Just a thought for consideration.

Dave J.

[DeltaFree]

Just a few thoughts.
Helicopters at speed at low level do leave a wake on the surface. Wessex over Irish Loughs definitely do. Yes it is more messy than that of a fixed wing machine due to all the mixing vorticeys, but it is there.
The analogy of a rotor disc to a solid disc is of course not 100% correct as air passes through a rotor disc! But in many respects the overall result is not too dissimilar, hence it is a useful visualising idea. Pulling the rotor disc back with cyclic is similar to pulling an aeroplane's stick back in that lift and drag increase resulting in a climb and deceleration.
Lift produced by a rotor during auto is the same as in the hover! That is equal the helo's weight. One of Newton's laws about a body remaining at rest or constant linear motion unless a force is applied to it. ie a steady state auto all forces balance out- lift=weight, same as in the hover.
Ground effect comes from the fact that air will not pass through the surface. When any heavier than air machine flies it tries to throw air downwards, the ground stops this and produces a higher pressure below the aircraft. The closer you come to the ground the more noticeable this effect becomes. In helos this "back pressure" reduces the induced flow and hence brings the blades lift vector nearer the vertical reducing drag. Induced drag is significant. In fixed wing bigger aspect ratios reduce induced drag, because this effect is significant most aeroplanes have relative high aspect ratios. If you could hover with your blades virtually touching the ground your power required would be similar to that needed to keep your blades spinning at zero pitch on the ground, as there is no lift induced drag. Unfortunately helicopters have their blades on top so you will not be able to prove this for yourself, but the physics is right.
Despite all this, back to original question the effect during an auto......negligible.

[helmet fire]

Thanks for all the info guys.

Nick, Your induced flow explanation of the effect of ground effect is great - thanks.

I have not yet fully understood the speed of sound bit yet though - although Dave J's third paragraph seems to pick up on the chord/camber question I asked before. Is this part of fluid dynamics, or gas properties? What I gather is that you are saying that any wing producing lift will cause a reaction with the ground that will manifest itself with the speed of sound. Why then is the downwash seen on the ground behind a forward flying aircraft? Or is the direction of reaction due to the relative airflow over the wing?


and for PPF#1: you say:
"any lift that is generated is actually being produced on one side of the aircraft"

I disagree. This would cause the aircraft to roll. Again, you cannot think of the disc as a big wing. This flows into your comments that:
"But we know that we can reduce our rate of descent to zero if we like, even with the collective at flat pitch..... How is this possible? What causes this tremendous increase in lift?"

You are not reducing your rate of descent to zero in an auto unless you are consuming your potential energy. You have not caused any such tremendous increase in lift - you have simply traded inertial energy for the increase in drag that you have incurred by pulling pitch and asking for more lift.

edited due to confusing myself - (again!!)

[heedm]

Nick, I think with so many poor resolution gauges around that reading the difference may be difficult. I think it may be easier to note what the airplane does when everything is set the same other than height AGL. Fly level at an airspeed towards a runway. Lower collective very slightly to initiate a slow descent while maintaining the airspeed. You may find yourself levelling in ground effect.

Am I way off base or is this close?

[Nick Lappos]

heedm,
You are right, noting everything the same except height is the best way (also scientific, since you try to control every variable but the one under study!) I do believe the T5/TGT gage on a turbine is very sensitive, with 3.5 degrees C of T5 equal to 1% power for most engines. Manifold pressure is pretty ratty on a recip, but if the gage allows you to read fractions of an ince (at least by interpolation) try that too.

Dave J, regrading the speed of sound, that enters the discussion because by definition, Mach 1 is the speed at which a signal propogates thru a medium. Sound is only a pressure difference, so it is directly a measure of how fast flow conditions pass thru air. In fact, Mach shock waves are really just the traffic jam as the pressure wave from the disturbence from the airfoil ends up being unable to go upstream because the wing is going faster than the pressure signal can.

Imagine that we have a wind tunnel, and we could see the effect upstream of having an airfoil in the tunnel. If we could make the airfoil change angle very quickly, we would see the effect exactly as fast as the speed of sound could carry the change to any new point in the tunnel. Similarly, the ground effect is propogated as quickly.

[PPRUNE FAN#1]

Helmet:

I did say that "...any lift that is generated is actually being produced on one side of the aircraft."

You say:
I disagree. This would cause the aircraft to roll.

What I implied or should have made clearer is that this amount of "lift" is insignificant. If it *were* significant the aircraft would definitely roll to the retreating side as lift does not adhere to the principle of gyroscopic precession. But it is not significant. The lift generated is only enough to cause the blade to flap up, which we know happens more or less ninety degrees later, causing a nose-up tilt of the disk.

You say:
Again, you cannot think of the disc as a big wing. This flows into your comments that: (quoting me, now)
"But we know that we can reduce our rate of descent to zero if we like, even with the collective at flat pitch..... How is this possible? What causes this tremendous increase in lift?"

You go on:
You are not reducing your rate of descent to zero in an auto unless you are consuming your potential energy. You have not caused any such tremendous increase in lift - you have simply traded inertial energy for the increase in drag that you have incurred by pulling pitch and asking for more lift.

Sorry to report, but you are wrong, old sport. Done many autos? I think you have one or two misperceptions about how helos fly.

First, consider a helicopter flying along in level cruise. If you pull aft cyclic without touching the collective pitch, the aircraft will climb. Why and how does it do this? Well, for *any* aircraft to climb, it must be producing more lift than gravity is pulling its weight.

Now, in an auto you do not have to "pull pitch" (are you referring to collective pitch?) to stop your rate of descent. Ever had a student flare too early and have the ship balloon? (Ever see a gyrocopter come in and land safely with the engine off? They don't even *have* collective pitch!)

I can assure you that if you are in an autorotative descent with sufficient airspeed and you haul back mightily on the cyclic, the aircraft will come to zero airspeed/zero rate of descent (or even begin a climb) with the rotor rpm still firmly up at 100% or higher. How is this possible? You may have traded something for something, but it is not rate-of-descent for rotor rpm. So again I ask: where does this extra lift come from? I doesn't appear magically.

Think about it aerodynamically. The *only* way to accomplish this level-off is if we have a surplus of lift. It must be thus. How does a fixed-wing glider convert its rate of descent into a climb? Right, it uses its stored-up energy by turning airspeed into lift - just like a rotorcraft. The principles are exactly the same.

I am comfortable in thinking of the rotor as a "wing" for it certainly behaves like one.

[Dave Jackson]

Nick,

All your comments are totally reasonable, except for "The [ground] effect is 'felt' at the speed of sound". I find two problems with this.

1/ By definition, sonic waves can be heard by the human ear. The rotor blades make relatively little noise, particularly when compared to an engine's un-muffled exhaust port. It therefor seems that the sonic forces propagated from the rotor blades will be quite insignificant.

2/ Any sonic forces that are produced by the blades will be done at all heights AGL. The ground will not be involved in the production of these sonic waves. All that the ground can do is reflect the waves back up towards the rotor disk. In addition, the ground will diffuse these waves. Also, since the blade area is only a small percentage of the disk area, very little 'sonic lifting force' will be received by the blades.

I suspect that a good analogy to hovering in ground effect would be to consider a helicopter that is OGE and maintaining its vertical height while experiencing an updraft. In both the above situations, the collective is lower than it would be for hovering in still air OGE.

The implication is that in ground effect less air is going down through the disk, just as in an updraft.

PPRUNE FAN#1

What you say make sense ~ I think.

Forgetting the helicopter's and the gyrocopter's rotational inertia for a moment. Lets say that a plane, a gyrocopter and a helicopter have forward linear inertia. While these craft have forward velocity, they are all producing vertical lift. When the (cyclic) stick is pulled further back for flare the crafts' overall pitch angles are increased and lift is maintained while forward velocity is reduced. In all three cases, at some reduced forward velocity, their airfoils will finally stall out.

I think that planes, gyrocopters and helicopters will all have to have some forward velocity at touchdown. The only exception to this is the helicopter which can also use up it rotational velocity by use of the collective, for a touchdown without any forward velocity.


Now to bend over and assume the Lu position :(

Dave J.

[Nick Lappos]

Dave,

I'm afraid the sonic thing now has a life of its own, and this is not correct!


The change in the air flow field around an object is felt by that field at 1060 feet per second, which is the speed at which pressure is propagated in that air. Any change to the filed effects flow at other places in the field but the change ripples through the field at 1060 feet per second. Forget sound, forget noise. If you make a change in a flow field, that change announces itself at that speed, 1060 feet per second.

[Thomas coupling]

There is some drivel discussed here...

Fact:

Ground effect will always be ready and present. As the object (a helo in this instance) nears the ground, it will compress the air ahead of it, albeit for a very short period. If the air column below the approaching object is compressed, then it will offer resistance to the vertical velocity of said object. However - change the profile/condition of the surface beneath the disc and the ground effect is considerably affected. Sloping ground, waves, grass, concrete all have an impact on ground effect.

Because the helo is in auto, that 'resistance' of air beneath the disc will translate aerodynamically to an increase in autorotational airflow (induced flow from BELOW the blades) thus increasing Angle of Attack, thus reducing rate of descent. Thus slowing helo down. This is a tangible but short lived effect.
In auto there is no downflow of air through the disc. [To be specific - most of the cross section of the blade generates lift, the remainder generates various levels of stall as recirculating air is dragged down through the disc from above].

Flare effect ALWAYS reduces rate of descent (ROD) it is the prime cause of reducing the ROD !!!! You are offering more induced flow from below the rotor disc, therefore the Aof A increases and the ROD decreases. You don't need collective to land from an auto, it is an aid to decelerating, but believe me a very well rehearsed engine off does not require collective pitch, so the only thing that prevents you from burying the helo is the flare.

A NORMAL auto to land however, culminates in collective application to convert all that stored (potential) energy in the blades, into kinetic energy providing significant increases in overal lift in the disc albeit decaying lift as the blades slow to a halt. This is the piece de resistance for any auto if you are to walk away from it. The flare reduces the vertical and horizontal airspeed to a manageable level, the coll pitch soaks up any rogue descent. Most 'autos' should be taught/practiced to as near as damit a zero zero touchdown.

Don't get your 'auto' mixed up with your 'EOL' Apart from the fact that the pedals work in the opposite manner(!), the aerodynamics of the bottom of an 'auto' vary significantly compared to those of an EOL.

Downwash which is what you see when you fly low over a lake, is not ground cushion!


Throughout an 'auto' the revs are there for you, With an EOL
its a one way trip.

[[email protected]]

TC you are right, there has been much drivel in this thread, a lot by people who have only a passing acquaintance with Engine Off Landings - the suggestion that you can complete an EOL (that doesn’t trash the aircraft) without using collective is my opinion ridiculous for the following reasons:

In autorotation the blades are kept rotating by the middle section of the blades – the root has too high an AoA and is therefore stalled, the tip section has a low AoA and whilst both produce lift, they also produce high levels of drag which put the total reaction facing rearwards (effectively slowing down the blades).

The flare moves the driving section of the blade outwards as the AoA is increased – the net result is an increase in rotor thrust and rotor RPM.
The flare therefore reduces RoD and forward speed but only while the AoA is being increased – once the flare is stopped the RoD will build up again because it is the air moving up through the blades that provides the driving force.

So in theory you could flare the aircraft from say 100’ all the way to the ground and you would (if you were gentle and progressive with the flare) continue to reduce forward speed and RoD to touchdown. BUT what attitude will the aircraft be at this stage – rather enormously nose-up I would suggest (not a great way to land anything except the space shuttle). Therefore in order to survive the landing you must level the skids/wheels/fuselage attitude – OH DEAR you are now reducing the AoA (negative flare effect if you like) so the rotor thrust and Nr reduce rapidly (try bunting in autorotation and see what happens to the Nr) just as you are accelerating the fuselage about it’s C of G towards the ground.

This is the point where you need collective to use up the remaining energy in the rotors to generate a short-lived, once-only burst of lift (and all the drag that goes with it and slows down the rotors) to cushion the touchdown.

I have stretched an EOL many times after the flare by selecting a level attitude and gradually milking the Nr as the aircraft sinks towards the ground – is this helped by ground effect? I don’t know but anything that reduces the drag on the blades that are trying to slow down, must be a good thing.

[Nick Lappos]

crab,
I have done touchdown autos in a light S-58, and not pulled collective pitch, but that is most unusual. The tail wheel helps, of course! If the disk loading is very low, and the helo configuration allows touchdown at a nose high attitude, a zero collective landing is possible, that's how airplanes land!

Unfortunately, a recent poster still refers to a pressure build-up as the ground effect mechanism. That is simply NOT correct, it is misleading. Run back to my previous post and note that ground effect is a help due to reduction in angle of attack, and induced drag, pressure has nothing to do with it.

[DeltaFree]

Now I always thought "induced flow" moved in the opposite direction to lift as it is induced by the lift. It seems TC you have invented induced flow from below the rotor! At a critical time like an auto this would be of great benefit to the helo flying world, could you tell us all how?

[PPRUNE FAN#1]

Got to agree with Nick. With any wind at all, a lightly-loaded S-58T can do a decent touchdown auto with no pitch-pull and very little ground roll. Wouldn't like to try it in a Blackhawk. Then again, it helps to remember that the S-58 was originally designed with only one engine (i.e. good autorotative qualities) and for shipboard duty (with 12 or 13 inches of stroke in the oleos and those huge balloon tyres). Ah, the memories...

Alternatively, some aircraft like the UH-1 can do very nice no-flare autos as well. Would be nice to have a ship with both choices available.

One puzzling thing in TC's post:
"Don't get your 'auto' mixed up with your 'EOL' Apart from the fact that the pedals work in the opposite manner(!), the aerodynamics of the bottom of an 'auto' vary significantly compared to those of an EOL."

Huh? In most aircraft I've flown, I've noticed that the pedals continue to work the same way whether the engine is on or off, and the aerodynamics at the bottom are the same, unless one is performing a power-recovery, of course. Perhaps some clarification is in order for the more dense of us?

[[email protected]]

Nick, I knew someone would point out the exception that proves my point - so very few helicopters are capable of a zero collective EOL, and those that can need specific situations (light weight, lots of wind) that the assertion that it can be done is almost fatuous.
Back to ground effect - your explanation is all well and good but HOW does the prescence of the ground reduce the induced drag on the aerofoil? does it reduce the upwash ahead of the wing or the downwash behind it, or both? or neither?
If a minute pressure change can be transmitted at the speed of sound along way ahead of the wing then the same must happen below it - and with the ground in the way the air can only compress itself against the ground and then recoil back up towards the wing again - sounds like an increase of pressure below the disc to me!

[Dave Jackson]

"Unfortunately, a recent poster still refers to a pressure build-up as the ground effect mechanism. That is simply NOT correct, it is misleading. Run back to my previous post and note that ground effect is a help due to reduction in angle of attack, and induced drag, pressure has nothing to do with it.

Nick,

I am not sure who the poster(s) you refer to is, but I, for one, have difficulty accepting that "pressure has nothing to do with [ground effect]".

Firstly, air is a compressible fluid. In addition, Actuator Disk Theory entails "sustaining a pressure differential between the upper and lower surfaces of the rotor.

You have previously mentioned that "Ground effect is essentially a reduction in the induced drag of the airfoil, because the ground helps reduce the angle of attack needed to produce the lift.". Few people, if any, will disagree with this statement. Unfortunately, it does not answer how the ground helps reduce the angle of attack. It cannot be by sonic wave since this wave consists of a compression plus an expansion of the air. The expansion offsets the compression and gives a net force of zero.

I suspect that ground effect is a cushion of compressed air, which is built up at a speed close to the speed of sound.

Dave J.

[Thomas coupling]

I thank NL for confirming that EOL's can be done without collective.
I still believe there is a pressure build up under the disc adjacent to the ground. My vertical speed indicator tells me as it shows a decrease the moment one applies collective when lifting into the hover. All that downwash, though it quickly cascades away from the underside of the disc, does 'back up' and produce a pressure build up - maybe i'm misunderstanding what you are saying?

Delta3: W.R.T. induced flow from beneath: If you draw your vector diagram for a steady state auto/EOl you will see that the little green arrow comes from beneath and the line drawn from this produces your AofA. There is no green arrow pointing down from above the blade chord, in an auto?

Pprune:

At the bottom of an auto (powered flight), when you raise the collective to recover (either to a landing or to the hover), you apply more torque (coll) and therefore require counter thrust from the tail rotor to prevent the helo from rotating in the OPPOSITE direction to the rotor disc. One applies the POWER pedal.

At the bottom of an EOL, as you apply collective to burn up remaining potential energy, the rotor speed decays and one applies OPPOSITE PEDAL to prevent the helo from rotating in the SAME direction as the rotors (due to friction).

Looking back, I meant to describe that pedal requirement was reversed - ;)

[imabell]

where did induced drag come from?

it's induced flow, and the angle of attack is increased or reduced with increases or reductions in the induced flow.

basic aerodynamics. nothing of any consequence has happened to light to medium helicopter aerodynamics since the introduction of the 47 in '46.

are you blokes trying to reinvent the wheel, the knowledge of some posters is very sadly lacking for (i presume) professional pilots.

zero collective touchdown autos are not impossible and you only need a few knots on the nose and a bit of timing and lots of practice. :eek: :eek:

[Thomas coupling]

Who said the induced drag bit????
Can't find it....

[[email protected]]

imabell, when you induce the airflow downwards by passing it over an aerofoil with an AoA, you produce lift - you also produce drag (acts at 90 degrees to the lift vector in the diagram and gives you the total reaction).
This drag is induced drag.
Not the same as the profile drag caused by skin friction between the wing and the air.
DJ - I want to know how the ground effect increases lift and decreases drag without altering the pressure below the wing. The only other way I can think of altering Cl and Cd is by changing the aerofoil shape.

[vorticey]

you said> ground effect is not a cushion, and it does not involve perssure rising under the machine or any such thing.

the ground is resisting the airflow from the rotor, the same as clean air in foward flight, reducing induced flow. if you're pushing air down and it cant go anywhere it has to increase in presure, doesnt it?

ground cussion > in the hover air from the disc is forced downward but is stopped by the ground (ground effect) so then has to move horizontally away, inward and outward. the air that goes inward stops when it meets the air directly opposite coming in. so the air in the centre has reduced velosity theorfore increasing preasure in all directions i would think.

to prove it > creeping takeoff,- as foward movement is achieved, adjusting collective for the offset lift vector, an additional collective pull will be required to hold the helicopter up after the cussion is lost behind you and just before translation.

you dont give a reason for the reduced induced airflow. pressure is the reason!

DAVE
shockwaves might have somthing to do with tuned length exaust pipes and inlet runners but i think it has more to do with the volume that is moved and the inertia of that volume.
as for helicopter blades....na! the shock waves would miss the blades when i adjust rpm.

[Nick Lappos]

For the Ground Effect is Pressure advocates:

Take care that the physical analogies you construct to help you understand the aerodynamics don't overpower the real factors. What about the behavior of any aircraft makes you think that the machine pushes on anything, exerts any pressure, to fly? This whole construct is not even true in the simplest little experiment. Look at a heavy bomber moving across the water at 100 knots, its wing within 1/3 of a span of the water, using much less power due to ground effect. Its wake is hundreds of feet behind it, yet you say it rides on a bubble of pressure. Wrong! It is going 115 miles per hour, yet you say ground effect disappears at forward speed. Wrong!

If you are going to conduct thought experiments to help understand the physics of flight, at least use real data!

[Thomas coupling]

You've confused me now Nick:

For my own benefit:

1. Are you saying ground effect and ground cushion are 2 seperate things?

2. When a helo is hovering within a factor of its rotor span to the surface, are you saying that the air beneath the rotor while it is rushing downward and outward...does NOT exert a pressure on the surface?

Thanks

[Dave Jackson]

crab,

" DJ - I want to know how the ground effect increases lift and decreases drag without altering the pressure below the wing."

I don't think it does. I suspect that hover in ground effect and that hover in an updraft are similar, and they both differ from hover OGE because their collectives are lower. In other words, the lift and the drag of the first two are less while all three are maintaining a constant elevation.
______________________

vorticey,

I agree with most of your comments and concur with the rest. :)
_____________________

Nick,

" Look at a heavy bomber moving across the water at 100 knots, its wing within 1/3 of a span of the water, using much less power due to ground effect. Its wake is hundreds of feet behind it."

Perhaps this distant wake is from the more powerful and 'horizontal' propwash and not from the wings.
____________________

A pictorial thought, supporting the 'bubble of air' ;

Envision a tube that has diaphragms covering the openings at both ends. If one diaphragm is flicked, then the other diaphragm will experience roughly the same displacement, but with a small delay cause by the time it takes the wave to travel down the tube, at the speed of sound.

Now envision the tube where one diaphragm is replaced by a solid plug and the other diaphragm is replaced by a plunger. If the plunger is pushed in an inch and held in, a wave will be produced. This wave will travel at the speed of sound down the tube. When it hits the plug at the other end, it will bounce back. It will continue bouncing back and forth between the plug and the plunger. Segments of the wave will be traveling in both directions. These interacting segments will quickly neutralize each other, and after a very short period of time there will be an increased but consistent pressure through the whole tube.

[vorticey]

you said > Look at a heavy bomber moving across the water at 100 knots, its wing within 1/3 of a span of the water, using much less power due to ground effect. Its wake is hundreds of feet behind it, yet you say it rides on a bubble of pressure. Wrong! It is going 115 miles per hour, yet you say ground effect disappears at forward speed.

I didnt say ground effect dissapears at forward speed, i said the ground cussion does.
The bomber does not hover with a ring of downward velosity around it. theorfore no ground cussion (bubble). also it doesnt ride on a bubble of air, it is riding on air that is resisting being diverted downward by the wing.

my question to you is : why do you think you need less power to hover in nill wind than to take off after the initial power to overcome the offset thrust is used?

[[email protected]]

Nick, if the pressure change at the stagnation point on the leading edge is transmitted forward at the speed of sound, it must be transmitted equally in all directions at the same speed.

Just because the bomber is flying at 100 kts doesn't mean that the increase of pressure the wing causes has to be felt behind it - the downwash does but that air has already been affected by the pressure change and passed over the wing.

My point therefore is that a pressure increase must be felt ahead and below the wing/bomber which creates a carpet of higher pressure air which cannot escape because the ground is there.

As for the wing pressing on the air - well unless Isaac Newton has cocked it all up an equal and opposite reaction exists between the wing and the air - each exerting a force on the other.

[DeltaFree]

You can draw what you like in a vector diagram, the flow coming from below a blade in auto is rate of descent flow, your little green arrow should ( if drawn true to physics and the real world) always go downwards as long as you are producing positive lift. It is INDUCED downwards in opposition to the lift force.:D

[Dave Jackson]

Many times over the past 30 or 40 years, a video clip has been shown on TV documentaries. It shows an atomic explosion and the effect of this explosion on a wooden building and a number of tall evergreen trees.

If I recall it correctly, there is a sequence of events coming from the explosion. First, there is the visual effect of light (and probably radiation). Then a shock wave could actually be seen as it past by the building and the trees. Final, there was a force of air that blew the building and the trees over.

Can anyone recall this film clip and what, if any, time delay there was between the shock wave and the force of the explosion?

Dave J

[Nick Lappos]

Yep, you're right guys, the real thing about ground effect is pressure. The Earth pushes the aircraft up harder when you get close to it. It isn't induced drag.

Uh Huh.

[Dave Jackson]

Nick,

This thread was initially about the effect of the ground on autorotation. In an attempt to answer this, the question of what is Ground Effect arose. It appears to be a valid one. Even Leishman, University of Maryland, says "The [ground] effect has long been recognized but the aerodynamics are still not fully understood."
______________________

Here is a description from the Internet, which represents your position, I think.

Ground effect

Another common phenomenon that is misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by 50% just before it touches down. There is a great deal of confusion about ground effect. Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.

To understand ground effect it is necessary to have an understanding of upwash. For the pressures involved in low speed flight, air is considered to be non-compressible. When the air is accelerated over the top of the wing and down, it must be replaced. So some air must shift around the wing (below and forward, and then up) to compensate, similar to the flow of water around a canoe paddle when rowing. This is the cause of upwash.

As stated earlier, upwash is accelerating air in the wrong direction for lift. Thus a greater amount of downwash is necessary to compensate for the upwash as well as to provide the necessary lift. Thus more work is done and more power required. Near the ground the upwash is reduced because the ground inhibits the circulation of the air under the wing. So less downwash is necessary to provide the lift. The angle of attack is reduced and so is the induced power, making the wing more efficient.
_____________________

Here is another statement by Bell, when describing their proposed Quad configuration.

.... with a ground effect cushion that provides a significant up-load on the airframe.

_____________________
Here's another.

The ground reduces the vortices coming off the rotor blade tips, reducing induced drag.

_____________________

:confused: & :confused: & :confused: & :confused: & :confused:

Dave J.

[etnb]

If you hover over grass (tall one), you can see the effect of the downwash spread for some distance around the heli (in no wind condition).

I was taught that this is the ground effect - or cushion.

As long as it is there, in front of the heli (in forward flight - slow one) we are in ground effect.

I don’t remember seeing that on the ground while I was doing Auto's (as a student, it took me a while to get my eyes up front).

Have you seen it ?

[vorticey]

im dissapointed you gave up without explaning what makes the induced flow reduce although you expained the effect of it on the aircraft verry well. i thought you might say that the velocity increases theorfore pressure decreases so there's no pressure under the wing.

you said
> The effect of the ground as we get closer is to reduce the angle needed by the airfoil.(BUT HOW?) This reduces the induced power (the power that overcomes the drag due to the high angle of attack).

> The change in the air flow field around an object is felt by that field at 1060 feet per second, which is the speed at which pressure is propagated in that air.

> What about the behavior of any aircraft makes you think that the machine pushes on anything, exerts any pressure, to fly? This whole construct is not even true in the simplest little experiment.

im not picking your posts to pieces, im just asking you to explain to me why you think the induced flow is reduced?

[RobboRider]

Thought I’d wade in with Wagtendonk’s two cents worth. (From the W.J.Wagtendonk’s book Principles of Helicopter Flight, which I used as my bible when I was doing my exams.)

W.J.W.’s answer mirrors Nick’s (or vice versa)

He specifically says the bubble of high pressure is an oversimplification but if it helps you remember the practicalities then “so be it”.

Induced flow is the rate of bulk movement of air downwards through the disc.
When there is nothing below the disc (eg. HOGE) the downwash can dissipate into the surrounding air below the disc and induced flow remains constant.
When there is a flat surface (eg the ground) below the disc the air flow is impeded and it can’t dissipate as rapidly. The flow through the disc is thus slowed because it can only flow in as fast as the air below can dissipate.

Wagtendonk doesn’t approach whether there really IS a higher pressure under the disc or not. I wonder has anybody actually got under the blades during HIGE and actually measured the pressure? Might be an interesting point.


Wagtendonk then gives a series of those airfoil diagrams with arrows going everywhere which translates decreased induced flow into 1. Increased Angle of Attack
2. Same Drag
3. Greater Rotor thrust

(I wish I knew how to draw and post the diagrams but I can’t)

If you wanted to hover (IGE) you must maintain a constant but now lower Angle of Attack (and thus the same Lift). You are able to reduce the pitch on the blades, decrease the drag and suddenly be able to hover with less effort from the engine.

[RobboRider]

I have posted scans of Wagtendonk's section on Ground effect on my web site. Have a read at:

http://helipics.homestead.com/hige.html

[Nick Lappos]

This is an interesting thread for three very intriguing reasons:

1) The things we were taught (and that are propagated in the literature) are often simplifications and rules of thumb. They are not strictly correct, but serve as memory aids and physical analogs to help us understand complex situations.
Gyroscopic precession
Centrifugal Force
Ground Cushion
LTE

2) When we try to bend the simplifications to fit other situations, they often break down, and actually hurt our understanding.

3) many Pprune posts are attempts to straighten out theses broken rules of thumb

Thanks Roborider for helping out in this lively debate.
I posted my last wise-guy post to actually try and make someone research the question, and I am very glad it worked!
This thread is an example of how Pprune is great. Thanks, guys.

Nick

[[email protected]]

Robborider, I'm afraid your wonderfully named Wagtendonk has given the standard explanation of ground effect that every student QHI can draw in his sleep. His disclaimer that the real explanation for ground effect is far more complex, just indicates that he doesn't know either.

No-one doubts that the AoA in an IGE hover is reduced by the effect of the ground slowing down the downwash - what is missing is a credible expanation for how it happens without being a function of a pressure increase .

In some mysterious way, despite the concept of divergent ducts giving a reduction in speed and an increase in pressure, we must believe that simply the presence of the ground slows down the downwash and reduces induced flow.

At least the explanation that Dave Jackson posted shows how the circulation theory of lift can give an explanation without using pressure or magic - unlike some of the posts here.

[Dave Jackson]

RobboRider;

Thanks for the scans. It looks like Wagtendonk may have 'blown it' from the start. His first sketch shows an expansion of the streamtube instead of a contraction.
___________________________

Crab,

What you are saying makes sense. If we assume that the ground IS the cause of Ground Effect, then we must question HOW the ground does this. The only 'connection' that I can see between the craft and the ground is the air. Air can transmit; heat, vibration and pressure. All of these entail an increased activity amongst the molecules of the air, within the area that is available to them. Heat creates pressure, vibration creates an oscillating, self-canceling pressure, and pressure is pressure.

In ground effect, at a given thrust, I suspect that the induce velocity and the induced drag are less because the angle of attack is less, not that the angle of attack is less because the induced drag is less. I think that a reduced angle of attack result in less "air must shift around the wing (below and forward, and then up).

IF pressure differentiation is the reason, then the "canoe paddle" analogy may be up the creek without a paddle.


Dave J

[Thomas coupling]

Good grief DJ, I've actually read through your post without falling asleep for once:D :D

Funny how - meteorologically speaking, air that descends from the stratosphere and then diverges when hitting the earth's surface, is known as a High pressure system....

What is the difference then:confused:

Anything that restricts the flow of any gas or fluid must translate into back pressure. That's the bread and butter of it all. Other phenomena that exist in and around that basic concept go towards making aerodynamics the complex theory it is;)

I'm with crab and DJ on this one..........

[ShyTorque]

A few thoughts.

Lift = a force acting at 90 degrees to the relative airflow. By definition, of course there is lift in autorotation.

If it wasn't POSSIBLE to complete an EOL without using collective every autogyro/gyrocopter flight would be it's first and last. I've done a collective-free EOL in a Whirlwind 10 but you DO need a good undercarriage (the groundcrew afterwards checked it and confirmed the Whirlwind u/c is a good one).

The key to how well a rotor system works is related to the amount of induced (downwards) flow. More induced flow = lower angle of attack = lower rotor efficiency.

Flare effect changes the direction of the relative airflow and reduces the induced flow, resulting in a reduced demand for power whilst increasing the angle of attack. It is, of course, a transitory effect that is taken advantage of during the penultimate stage of an EOL. The last stage is normally the application of collective to gain effective rotor thrust at the expense of Nr.

As far as the original question goes, YES there probably is some ground effect at the bottom of an EOL but as the aircraft is in ground effect suddenly and briefly, the effect is unnoticeable or hidden.

[helmet fire]

PPF#1,
Sorry it has taken me three pages to reply to your post - those pesky bush fires are burning again!!

You, "sport" (as you like to say) are a crack up.:D :D :D

You totally (and condescendingly I might add) disagreed with my statement that :

You are not reducing your rate of descent to zero in an auto unless you are consuming your potential energy. You have not caused any such tremendous increase in lift - you have simply traded inertial energy for the increase in drag that you have incurred by pulling pitch and asking for more lift.

BUT then to proove me wrong you end your explanation with:

"How does a fixed-wing glider convert its rate of descent into a climb? Right, it uses its stored-up energy by turning airspeed into lift - just like a rotorcraft. The principles are exactly the same".

Good one. Do you know what potential energy is? Is this potentially condescending?

You go ahead and imagine that a rotor disc behaves as a big wing - just spare any of your junior pilots/students from the same affliction.

You also say:
"Think about it aerodynamically. The *only* way to accomplish this level-off is if we have a surplus of lift. It must be thus."

Alright - I have thought about this aerodynamically,and..No. There is no surplus of lift in this situation. Refer to my post to you re "creating lift". There is only potential energy. Potentially.

but all this is potentially off thread......:D

[PPRUNE FAN#1]

Oh dear. It seems I have offended the sensitive Helmut. If so, I apologise, sir! I apologise if your understanding of aerodynamics is so rudimentary that you can't keep up, and I'll try to be less condescending to your sort in the future.

Let's try this. In a glide, if you wish to decrease your rate of descent, your lift vector *must* become bigger than the combination of your drag and weight vectors, hmm? That is, unless you've cleverly figured out how to magically decrease either your drag or weight. This is not rocket science, old chap. But if you disagree, please explain to the class how a rate of descent can be slowed, or a rate of climb begun *without* excess, surplus.....or MORE lift than your wing was already producing.

[[email protected]]

PPF#1, which drag vector are you talking about - the parasite drag from the airframe and rotor system or the induced drag produced by the wing/blade as it displaces the air? If you are going to belittle other peoples aerodynamic knowledge you really need to be more precise with your own.
If your rotor thrust (the vertical component of the Total Reaction) is greater than the aircraft weight then the RoD will reduce if you are descending or a climb will commence if you are not.
A reduction in induced flow not only gives a higher AoA (therefore more Cl and Cd) but also moves the TR vector towards the axis of rotation of the rotor giving an increase in rotor thrust and a decrease in rotor drag.

ShyTq whilst I have to admit is is possible to do a cyclic only EOL in favourable conditions in a helicopter with low disc loading - modern helicopters (and I discount the Whirlwind here) have much higher disc loadings and far less benign autorotative characteristics. No doubt Nick will have flown a test aircraft which has a high disc loading and does collective- free EOLS but he is expert at shooting holes in my arguments.

[PPRUNE FAN#1]

Crab:
If your rotor thrust (the vertical component of the Total Reaction) is greater than the aircraft weight then the RoD will reduce if you are descending or a climb will commence if you are not.

Precisely! And that is my sole point. An autorotating rotor still has a vertical thrust component. "Vertical component of the Total Reaction" = LIFT.

You chaps are overcomplicating this bit. Nevermind your vector diagrams and formulae. You said it, Crab! In very simple terms, to slow down a descent or to start a climb, the upward force (lift) must exceed the downward force (gravity). Hence, in a steady-state descent, to slow the RoD you need to grab some more lift from somewhere.

Many illustrations of helicopters in forward flight depict a downward flow of air through both the front and rear parts of the rotor. If you've flown for any length of time you probably intuitively know that this is incorrect. In wind tunnel footage that I have seen, smoke blowing at a translated rotor goes right up and over the front part of the disk and does not come down through it (transverse flow, anyone?) until just about the point that it passes the mast. As long as the rotor is through ETL this will be the case, including during autorotation.

The misconception that an autorotating rotor "sees" a predominantly vertical column of air throughout the entire disk is just that, a misconception. Anyone is free to disagree. But some of you lot need to read more. Start with Ray Prouty. I do not make a habit of arguing with him.

[vorticey]

you said > Many illustrations of helicopters in forward flight depict a downward flow of air through both the front and rear parts of the rotor. If you've flown for any length of time you probably intuitively know that this is incorrect. In wind tunnel footage that I have seen, smoke blowing at a translated rotor goes right up and over the front part of the disk and does not come down through it (transverse flow, anyone?)

this must be just in translation, inflow roll (transverse flow effect) happens because the front most part of the disc is cutting clean air, not paddeling in a vortex which would cause the smoke to go over the front of the disk.
http://www.dynamicflight.com/aerodynamics/translational_lift/
http://www.dynamicflight.com/aerodynamics/transverse_flow_eff/

you said > Pulling back on the cyclic does increase the pitch of the advancing blade, yes. And that tilts the rotor disk to a more nose-up position, yes. But increasing the pitch of the advancing blade does not produce a dramatic increase in total lift of the rotor (and remember, any lift that is generated is actually being produced on one side of the aircraft).

pulling back on the cyclic does produce lift! because some of the total rotor thrust that was being used for overcoming parasidic drag (forward flight) now is producing lift aswell. aslo pulling back radicaly reduces the induced flow (instead of the air coming in from the top, it now wants to come in from underneith) so angle of attack is incraesed. all this lift on the blades causes them to cone, aswell as autorotative force increasing, so the rrpm will increase. i would say airspeed is traded for total rotor thrust and rrpm.

[Thomas coupling]

1 nil to vorticey, I'd say...
You do talk bollocks sometimes PPFan#1:D

[DeltaFree]

I really cannot see what is so difficult about this increased pressure business.
In a hover OGE air flows freely downward slowly dissipating as it goes, ultimately spreading out and going back up to replace the air which is still being drawn down thro the rotor. This takes the form of a very large doughnut vortex. We all know about the strong bit at the rotor tip but in a prolonged steady state this vortex has a reducing effect out to some considerable distance.
Now closer to the ground the downward flow is stopped at the surface. In order to stop motion a force must be present, or in this case a pressure increase. This pressure resists the downward flow of air so reducing the induced flow. For a given angle of attack/lift, this means lower pitch, more vertical lift vector and reduced rotor drag.
It is true to say the pressure under the rotor does not add lift, so NL is right in saying that. But the increased pressure is the mechanism by which the induced flow is reduced so it does reduce the power required.
Any lift produced by a heavier than air machine is ultimately supported by a higher pressure on the ground surface. Remember the old school days question....
A van weighing 1 ton has 1 ton of budgies sat on perches in the back. It weighs 2 ton. All the budgies takeoff and fly around inside the van, how much does the van weigh now?
Yes 2 ton! The birds are now supporting their weight with air which must in turn be supported by what is underneath.
The difference between OGE and IGE is only how localised the increased surface pressure is. OGE the increased pressure is spread over a large area and hence has little feedback, IGE on the otherhand the effect is very localised and hence has a larger return effect.
I am sure this is really quite a simple process and hope it seems so to you all. If I am missing something let me know.

[helmet fire]

:D :D :D

Sorry to you too PooFan#1, I failed to grasp your appreciation of basic physics. Rather than disregard vector diagrams and formulae, perhaps they might help you in your obvious confusion about lift. No one has argued against lift having to exceed wieght to climb. Not in any of the posts. Nobody. Nada. Nyet.

So perhaps you might avoid continually restating the blindingly obvious to give the inference that you have adressed the questions raised? Or do you restate the simple to avoid addressing the complex?

Your wind tunnel footage is irrelevant to the discussion, and your perceptions/assumptions about it are wrong. Lu, are you there?

PS: To climb, lift has to exceed wieght. :D

[Nick Lappos]

DeltaFree,
The whole pressure increase buisiness is interesting, it "seems" right, and it is not correct. This "mystery" was studied about 3/4 of a century ago, here is one reference that discusses the ability of ground proximity to reduce induced power. Note that in 1928 there seemed to be no need to use the word "pressure" in the NACA study that measured and explained ground effect:

http://naca.larc.nasa.gov/reports/1928/naca-report-265/

[[email protected]]

Nick, yet again you have eluded the question - we all know that ground effect exists, the link you provided is to a report that shows reduction in rpm and increase in Cl for an aircraft in ground effect.

It does not say how it happens, only that it does.

If it is not pressure related then what is it - all the reitieration that it is not pressure related is not verified by any other evidence to the contrary. You can tell someone the world is round (alright an oblate spheroid) as much as you like but without proof they are unlikely to believe you.

If you do not know then say so instead of treating us like errant schoolchildren who don't understand their teacher (because he hasn't explained something properly).

[ATPMBA]

I recently purchased a copy Ray Prouty’s book Helicopter, Performace, Stability, and Control. It was a good buy as it was used, a new copy is over $100. He has a section on ground effect and some good diagrams that address the issue but not the main question of the topic.

Angle of Attack (AoA) is the same for both IGE, and OGE. A specific angle of attack will produce a constant amount of lift (other things being equal). The amount of lift produced is the same for IGE and OGE hovers.

The induced flow through the rotor system slows down when close to the ground, the ground hinders the air from escaping. There is now a decrease of the induced velocity flowing down through the rotor. The blade pitch angle decreases and that causes a more vertical lift component and the vector for induced drag lessens (less power needed). AoA is the same for both IGE and OGE even with a decreased pitch angle, this is because the induced velocity is less.

[DeltaFree]

NL, that report does not mention pressure. But the theory is based on trailing vortices, and their mirror image in the ground. So what they do is effectively push air upward to meet the downward flow from the downwash to cancel it out at the surface leaving zero airflow through the surface. Yes again no pressure need be mentioned to achieve this result, but again in order to slow down an airflow a pressure gradient must be introduced. Newton's Laws. So in comparison to flight at altitude there will be an increased pressure under an aircraft at v.low heights.

[Dave Jackson]

To simplify the activity, the following is my impression of what will take place during a vertical (no forward velocity) autorotation to the ground.

There is no change in the collective or cyclic stick, therefor the pitch on all blades, at all azimuths, and at all heights above ground, will not change. Near the ground the rotor disk's mean angle of attack will increase because of the pressure (attempted upwash) below the blades. This will increase the area of and the thrust from the driven (outer) region of the disk.

This increased angle of attack will reduce the descent rate, partially because of an additional pressure created under the fuselage, and partially because of an increase in the induced thrust. The induced drag will also increase. This is because the higher pressure (attempted upwash) will be attempting to pushing more air from under the blade upward, and some of it will move forward and up around the front of the blades.

The rotor's rpm may not slow because the additional thrust will increase the coning angle and the Coriolis effect.
_____________

The above only relates to the rotation energy in the disk. Should the helicopter also have forward velocity, then it will also have linear energy. This will result in additional effects, similar to those experienced by a plane's wings or a gyrocopter's rotor disk. These additional effects must then be combined with those from above.


Just a reasoned guess.

Dave J.

[Nick Lappos]

DeltaFree,
I once asked a Physics professor why it was that light behaved a certain way, and tried to couch my question in physically analogeous ways, such as you try with the pressure thing. He commented to me that attempting to draw such analogies was fine as a memory trick, but virtually always fell down when you tried to extend the analogy in any way. He admonished me to stop trying to make such parallels, as they block true understanding.

The old "size of a pea" thing is an example. The Sunday suppliment science reporter can explain anything, and in a few words completely ball up any knowledgable person with analogies like "the energy in a piece of uranuim the size of a pea...."

The EXPLANATION is that the ground effects the airflow around the wing to make it wash outward less, and this reduces the angle of attack of the wing/blade so that more lift is produced at a lesser angle of attack. The effect is to magically make the wing behave as if it were longer, with higher aspect ratio and more efficiency. The lesser angle means less insuced drag, more efficient lift. That EXPLANATION has appeared in about 45 or so posts on this thread. The rotor blade is a wing, and it follows the same rules as it gets close to the ground, of course.

You, DeltaFree then take that EXPLANATION and screw it up with a physical analog that makes you comfortable, you turn it into a pressure argument, and then you feel secure and comfy. That is nice, it is keen, it makes you all warm and fuzzy, and it is wrong, but you should keep it up because the idea of angles of air motion is obviously very uncomfortable for you. In this process of screwing it up, you then insist that your fuzzy little pet story is correct. Please be assured that it is not. Please be assured that the instructor who first said "ground cushion" to you was wrong, in his delightful way. He gave you a simple way to understand that when the ground is near, the helo flies gooder, but he was wrong. Now, in observance of his infallability, you must make all explanations of ground effect fit what you were first told. You were probably the very best student in Sunday school, where faith was the guiding requirement toward understanding. This is not sunday school.

As the last attempt, here is yet again another, separate, more full, complete, different example of a non-pressure explanation of ground effect:

"Ground effects may be explained by the interaction of the aircraft wingtip vortices with the ground. This interaction reduces the strength of these vortices. The weakened wingtip vortices reduce the wing downwash which increases the lift and decreases the induced drag, or drag caused by lift." - Dynamic Ground Effects Flight Test of an F-15 Aircraft, NASA Technical Memorandum 4604 at:
http://www.dfrc.nasa.gov/DTRS/1994/PDF/H-1999.pdf

Forgive me getting crotchity in this post, it must be the pressure I feel with this whole thing ;-)

[Thomas coupling]

Nick: hang in there, we're not having a go at you personally, please believe this. may I also suggest that when someone of your calibre with your knowledge, tries to explain simple theories (in your mind) it may prove difficult for you to understand why mere mortals like us are having problems getting it:)
ALL of the UK military, and I would argue all civilian trained helo pilots, are taught that air under the disc, when close to the ground, is compressed thus exerting a back pressure down the line. Its effect on AofA/Induced Flow etc is another issue.

How, then are we to be asked to accept that this concept is incorrect?

Question: When a blade/wing/rotor, moves through the air, do the molecules of air in front of that leading edge (albeit for a millisecond) get compressed because they cannot move out of the way fast enough? Does this become a serious problem when that object moving through the air, reaches close to Mach 1?
Is compressibility not the same as pressure build up, and does it not have an effect on the air flow over/around that leading edge?


Please be gentle with me, as I think I represent 90%+ of all helo pilots...

[Nick Lappos]

Thomas (and all the brethren who watch as he throws his hat through the door!) I do know that you are not getting personal, and I do see the frustration when I come along and say something so outlandish.

It is absolutely true that the state of a fluid flow must contain information on its density, pressure, viscosity and velocity, so we can be sure pressure is an important piece of information, but the lift that the disk sees as the aircraft enters ground effect is NOT a product of a cushion of air under the machine that gives it an extra lift! That old piece of bull called "ground cushion" is an English language statement that does not describe the pressure under the aircraft.

Those great pilots who taught us used this term, the same way we say that your Mom says that you "catch a cold" when you feel chilly after a virus attacks your respiratory system. Your Mother tells you to wear a sweater so you don't catch a cold, a completely wrong medical advice, (the chill you feel is the symptom of the disease you already have, not the cause of the further symptoms you are now guaranteed to get!) Your instructor told you about Ground Cushions, and he was a great instructor, your Mom is a great Mom. Your instructor was not an aero guy, and your Mom was not a Doctor (of course, I now speak metaphorically, your Mom could be a Doctor, and so on, but what the heck....)

I think of the movie "Sand Pebbles" where Steve McQueen tells the Chinese helper about the great steam God in the boiler who gets hot and angry when the boiler is fired. Where does this all end!!

[Dave Jackson]

Nick

Re NACA Report;

In no way can I see how this 1928 report arrives at its conclusion that "The induced drag of an airplane is reduced upon approaching the ground ...'.

Only the [propeller RPM] and the [forward velocity] were used as inputs to this report. These two variables, plus a homemade 'fudge factor', the , are the constructs of the lift/drag graph. I would suggest that the only conclusion, which can be drawn from this report, is that the ground improves the lift/drag ratio.

[Angle of attack] also improves the lift/drag ratio, but the report does not even consider the [angle of attack]. I believe that a good argument can be put forth that states 'The angle of attack is increased in ground effect. The drag is increased in ground effect, also; but not to the extent that the lift/drag ratio is increased.'


Re NASA Technical Memorandum 4604:

[i]"Ground effects generally caused an increase in the lift, drag and nose down pitching moment coefficients". This appears to support the above argument.

The reduction of tip vortices obviously improves lift and the ground may well reduce these vortices. This is may be especially beneficial for aircraft with highly swept wings, where the lift is "near the wingtips of a swept wing".

Vortices reduction is probably only one component of ground effect. Perhaps at 150-170 knots the F-15 is out in front of other ground effects.


Dave J.

[Nick Lappos]

Dave,

I don't know what your point was in the last post. Having reported what the papers said, you seem to think they support your position that ground effect is a pressure thing. Huh?
Nick

[ShyTorque]

Thomas C,

You aren't correct in saying that ALL UK military pilots are taught that air under the disc becomes compressed. At least it wasn't so in my time. I was taught that the ground cushion air behaved differently to "free" air, ie. air higher than 2/3rds rotor span. I don't ever recall it being referred to as compressed air, but more as a static or constrained dome of air beneath the aircraft which altered the induced flow through the rotor disc.

Perhaps you mean "military" in the army sense?

[Thomas coupling]

DeltaFree: I love the analogy with the birds in a container, but I don't think it's right:)
Correct me if I'm wrong, but when you use the word 'weight' then you are considering the definition of weight which includes its correlation with resistance. I.e. weight only acts on a surface, it is gravity based. If there is no reaction (surface) then there is no 'weight'.
For instance, two men standing on a weighing scales, one on top of the other! When the top man levitates away from the guy underneath, the scales will reflect a lesser weight!!
The same happens with your birdies, in that when they take off and fly inside the container which is on a weighing scale, the scales will indicate a reduced figure:)

When a helo takes off away from the ground, it weighs nothing:eek:

What doesn't change is the MASS. This remains the same.
So the mass of the container and birds in your analogy is 2 tonnes, and will always be 2 tonnes. The weight of the combined lot is 1 tonne (birds in flight).
That was one reason why all up weight (AUW) for aircraft was changed to all up mass (AUM) all those years ago, so that it represented more closely what was going on....

What do you think?

[Nick Lappos]

The birds are lifted by the air that they accelerate downward. This air transfers the momentum to the container, so the weight of the container never changes, (it presses on a scale with the same weight) regardless if the birds are airborne or walking around inside the container.

[DeltaFree]

TC the birds in a container do not become weightless- their weight is supported by the air, agreed weight is a much misused term and I probably did use it incorrectly here. But as agreed by NL the container would still weigh the same and yes it's mass would still be the same. All heavier than air machines do have weight and support this weight with lift. Balloons etc when floating do not have weight but do have mass. Make sense?

NL I am prepared to bet a hefty sum that the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away. Get your fluid dynamic calculations, finite element / source,sink vorticey programs working on that and tell us all the results.

I have just read that last NASA report and correct me if I am mistaken, (I am sure you will) but they use radar/ optical height measurements since the aircraft static pressures are affected by ground effect! But hang on ground effect does not affect pressure, or does it NL?

[Dave Jackson]

TC

Don't give in yet. Go back and edit you post so it says 'open' container (like a birdcage with the s--t pan removed). ;)


Nick,

"I don't know what your point was in the last post. Having reported what the papers said, you seem to think they support your position that ground effect is a pressure thing."

There was no point. It's just the fun of arguing. :eek:

Seriously, what I was saying is that neither paper disproves pressure as a cause of ground effect.

1/ "Note that in 1928 there seemed to be no need to use the word "pressure" in the NACA study that measured and explained ground effect:" This study did not measure ground effect, nor did it explain ground effect.

2/ The F-15 makes mention of tip vortices as a cause of ground effect. For the reason previously stated, I feel that tip vortices will play a much smaller role in a helicopter then an F-15.

Bell has referred to pressure under the fuselage as being one contributor to ground effect. Few would argue with this.

Heck, I learned to fly in an airplane with low wings. During dual instruction, we got the craft proposing to the extent that we bent the blade tips. On another occasion at Vancouver International, the tower had 'all traffic hold' while I attempted three times, on the same pass, to put the little sucker on the ground. The first time a friend took my plane up by himself, he did three circuits before he got it down. He said that if he could not get it down on the fourth try, he was going to open the canopy a step out. On these attempted landings, there was definitely a lot of pressure. There had to have been, since pressure causes heat and we were sure sweating. :(


DeltaFree

"... the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away."

The operation of a pitot tube supports your statement.


Dave J.

[[email protected]]

The circulation theory of lift can best be applied here I think – this assumes that the wing is at the centre of a vortex spinning in a clockwise direction when viewed from the tip of the wing towards the root with the leading edge on the left. The circulation around this vortex creates a force at 90 degrees to the free stream flow in the direction of rotation ie upwards. This is because the flow above the vortex speeds up and that below it slows down ( at the point where the trailing edge flow around the vortex is moving forwards and meets the free stream flow going the other way) – a stagnation point of increased (or less reduced pressure). The force is a lifting force and the relationship between the pressures above and below the vortex is apparently called the Kutta-Zhukovsky relationship.
Back to ground effect and the area of relatively high pressure beneath the wing is interfered with by the ground, not only increasing the area of stagnation but changing the way the flow goes up and over the leading edge – instead of coming rear to front around the bottom of the vortex it now can only come from ahead which would alter the inflow angle and the AoA, Cl and Cd which we have agreed occurs in ground effect.
What do you reckon Nick?

PS Weight is a force = mass x acceleration (due to gravity)


PPS I wouldn't trust a Physics professor to explain light, does it behave as a particle or as a wave or both? I believe the answer is both but this doesn't fit in with their theories to explain how it does it - waving streams of particles anyone?

[DeltaFree]

I totally accept the line vortex theory of lift, this vortex "contained" in the wing cannot just end, and does leave the wing as the trailing vortex we all know and love from a wing tip (well as a sheet vortex which has increasing strength to the tip, which all bundles up as one just behind the wing). Ground effect can be explained by the mirror image of this vortex system in the ground, as I think I have mentioned earlier.

The effect of the mirror image trailing vortices is to push the real trailing vortices further outwards, this has the effect of increasing the aspect ratio of the wing, reducing downwash, and induced drag hence producing the efficiency improvement we are all talking about. Also by effectively increasing the aspect ratio we also reduce the strength of the vortex, as previously mentioned.(Lift/unit span is proportional to vorticity and airspeed, so longer span same lift and airspeed give less vorticity)

There you go all explained without even mentioning pressure!

BUT air will continue in its state of rest or constant linear motion unless a force is applied to it. Pressure is the force that makes air deviate around an aerofoil, if we are to change this flow field we must also change the pressure distribution. So despite vortices being able to explain ground effect, there must be pressure changes if there is to be a different flow pattern. So I say the pressure directly under a wing is increased by ground effect. I am not saying that this pressure on the underside of the wing makes more lift, but it does resist downwash hence reducing induced flow, reducing induced drag, and for our helo allowing a lower pitch angle for given alpha, bringing the lift vector closer to the vertical and reducing the power requirement.

I will produce diagrams if it helps.

Light photons behave according to probability, which is apparently a great explanation! To who?

PS I did not go to Sunday school for very long so again Nick you are mistaken.

[ShyTorque]

Perhaps the earth DOES repel ugly helicopters..........:D

[GLSNightPilot]

I think that's incorrect. Opposites attract, so the earth must repel beautiful helicopters, & it sucks the ugly ones down into smoking holes. :eek:

[Nostone]

I am not a rotor head, sorry, but I have been reading this thread with interest and there seems to be aerodynamic misconceptions creeping in.
There will be an increase in ground effect during an autoration landing over a powered landing due to the fact that with the former you presumably have a higher rate of descent until just prior to touchdown and therefore require more lift to both support the weight of the helicopter and reduce its vertical speed therefore increasing the pressure below each rotor blade. The proximity of the ground affects the ability of that pressure to escape.
A rotor blade is just a wing and behaves in the same way. It generates lift by a combination of (positive) angle of attack and relative airspeed so in powered flight and autorotation the relative airflow must be coming from below the cord line producing the usual decrease in pressure over the top surface, increase in pressure on the lower surface and trailing edge downwash. In powered flight the rotor blade (wing) is moved forward by a force applied to the root in just the same way as a powered aircraft with a fuselage mounted engine. In autorotation the rotor blade (wing) is allowed to glide in the same way as a gliders wing and keeps moving for exactly the same reason that a glider keeps moving.

[[email protected]]

Sorry Nostone - you used the word 'pressure' -therefore half the audience aren't listening anymore :D

PS Nostone - a rotor blade in autorotation keeps moving because a section of the blade is not only producing lift but a total reaction that is forward of the rotational axis (ie a driving force without which the blades would slow down and stop).

[Nostone]

Hi Crab,
Ref. your PS. Yes in just the same way as a glider keeps forward speed, as it climbs in a thermal for example. I just think it is easier to visulise than lots of whirling maths.

[vorticey]

why is there a vortex around the wing? maybe trailing edgers should be round not to inhibit this vortex? dont think so. the air on top and under the trailing edge are leaving it at the same speed although slightly closer to the wing tip than on entry.


DeltaFree
would'nt the ground help vortice's develop, making outward flow, which is easily ingested back into the top of the disk? a building near by increases the ingestion further making the flow go up.

by 'sheet vortex', do you mean outward flowing air?



PREASURE doesnt the asi measure velocity with a diaphragm in the wind (not directly), that is deflected with pressure?
lets say this asi is made from a tube facing into the airflow and a diaphragm at the end. i guess the air speed, by looking at the deflection of the diaphragm (it doesnt matter). im saying the pressure in the tube is going to be the same as in the diaphragm at any airspeed, air speed in the tube would be 0. at the entrance of the tube there will also be a cone of presure sticking out from the middle where air builds up and flows to the sides aswell. take the tube away, now the pressure cone will now be on the front of the diaphragm.

relation to this thread (incase you were wondering), if the diaphragm was under the helicopter it would measure the downward velocity with preasure in relation to the ground.
now at 30 000 feet the same diaphram is held under the helicopter with surrounding helium balloons to overcome gravity only. the diaphragm measures pressure but some is exerted to the ouside air which cant suport it so the diaphragm measures less pressure.
theorfore the downward velocity increases pressure near the ground which reduces induced flow. and yes nick, i think that the cone of pressure i mentioned above would be under the centre of the helicopter simply cause it cant escape. where the air can escape there's less pressure (around the edges).
:confused:

[Tail Bloater]

This is an interesting question and from my limited knowledge of this aspect of heli v ground. For my part there is NO ground effect because unless the landing profile is truely vertical for some reasonable period of time No ground effect will be produced. As most practice EOLs will start from a suitable hight and speed the resultant landing will be performed with some run-on forward speed utilising 'Translational Lift' and leaving the ground effect behind, but catching up. Unfortunately by the time the GE has caught up the RRPM is unable to support the helicopter.

What do you think about the Translational Lift idea. I know it exhists and I think it's there for me to use.

Any thoughts?

For RobboRider....

You can see the pressure that a rotating rotor produces.

Sit in your heli. Adjust the Altimeter to Zero. Start up and once at Flying RPM look at the altimeter. Note reading. Lift off into a very low hover (skids just clear of the ground) look at the altimeter again. In each case you will see that according to the altimeter you have descended (ie pressure increase)

And for others. And my three cents worth. Ignor the Grouind Effect theory and think 'Translational Lift' as the benefactor helping to cushion the landing

DeltaFree. I actually agree that when an aerofoil is in close proximitry to the surface a pressure increase will affect the AoA giving a benefit of less power required. There will be a Translational Lift element reducing as forward speed reduces and Ground Effect will be increasing as the heli slows. During this phase of an EOL the RRPM will be reducing fairly quickly and my logic forces me to consider that 'yes ' there will be a GE but in reality of no damm use 'cause what little there was to start with is dissipated in milliseconds and there isn't going to be any real benefit.

A wonderful subject which has increased global warming which that in it's self must reduce lift by the factor of 'n' due to the reduction in density.

Just one last point.
During my Military Helicopter Course and thereafter we were taught and practiced a Constant attitude EOL. Timing was all important and the application of too much collective lever at the point of landing would certainly produce an effect of blowing grass and an unconfortable feeling of running out of collective lever for the final touchdown. But we never did. Can I put it down to Ground Effect? In my mind I shall.

[[email protected]]

Vorticey, I didn't make up the circulation theory of lift, someone light years cleverer than me did that. It is one of 3 well accepted theories to explain the production of lift - the other 2 are: momentum theory and dimensional analysis.
It is a bit like the physics professor explaining the behaviour of light - sometimes it is easier to believe it is a wave and at othertimes a stream of discrete particles - which one is correct I couldn't say.
The ground effect phenomenon is easily demonstrated but, as we have seen on this thread, difficult to explain clearly and concisely.

The vortex theory is extended by the realisation that you cannot have a vortex with open ends and the resulting Horseshoe Vortex is the end game of the circulation theory. It can be demonstrated by placing a flat piece of wood in a bath and tilting it to produce an AoA. Move the wood forwards and the vortex is formed at the trailing edge first indicating a circulation of water around the chord of the wood.

The effect of the building you are talking about is recirculation which is nothing to do with ground effect.

As to your ASI, the pitot pressure (the air rushing down the pitot tube) is made up of static pressure ie the measurable pressure of the air free from the influcence of the pitot; and dynamic pressure - caused by ramming the air down the tube. Inside the ASI you compare pure static pressure with dynamic pressure, across a diaphragm in your case, and since the static elements cancel each other you are left with a way to measure dynamic pressure by the deflection of the diaphragm.

I don't think anyone doubts that the pressure under the disc is greater than above it - the helo wouldn't fly otherwise, it is a question of how the ground influences it that is the nub here.
In a free air hover the air is being pushed down against air which has not been through the disc and will offer resistance to the moving air (equal and opposite reaction) - in a low hover the air pushes against the ground which also resists it (clearly more effectively) so the rotor has to do less work to achieve the same end. Is it a function of increased pressure though? Discuss....

Which is how this thread has become so long

[DeltaFree]

Nostone, you are right in both what you say and the thoughts behind it. Agood point in stressing about the increase in lift due to stopping the rate of descent, which of course produces greater ground effect, though I think we all agree that with rotors high mounted, as they are, as opposed to low wings on an ac, the effect would be barely noticeable, especially in the mayhem that is usually going on then.
As Crab said, the pressure argument is really what has kept this topic going, and I note it has all gone quiet on the "pressure has nothing to do with ground effect" campaign. As you seem to appreciate pressure is the be all and end all of aerodynamics, OK viscosity, and density play there parts too. It is after all pressure differences that give us the lift we all seek to attain.

Vorticey, the tip vortex is if you like the result of the pressure differential between top and bottom of a wing, the average difference must be the same to produce the same lift. What the ground does is even out this pressure difference moving more of the lift to the tips, so effectively increasing aspect ratio which in turn slightly reduces the vortices.

Tail bloater, ground effect is present with any type of lifting wing, and of course for a very slow eol there will be no translational lift.

[Dave Jackson]

Oh! Oh! Talking about pressure;

While considering the meaning of life and other important things, such as how to get the lint out of ones navel, a thought arose. If thrust outside of ground effect is less than thrust in ground effect then thrust in the vacuum of space must be even less.

A quick search for the answer uncovered the following statement. ~ " a space shuttle main engine can produce 1.67 million N of force during liftoff and 2.1 million N of thrust in a vacuum. " :(

Oh sh:t. Back to the navel.

[Irlandés]

Hey Crab,
according to Quantum Electrodynamics (QED - one of the few unassailable areas of physics and also one of its proudest creations), the particle nature of light is the one that takes first (and only?) prize. At least that's according to Richard P. Feynman who practically wrote the book on it. And who am I to argue with a nobel winning physicist the stature of Einstein! OK sorry, back to helicopters! :D

Irlandés

[[email protected]]

Irlandes, so how does he explain that light is part of the electro magnetic spectrum in which every other type of energy travels in waves with another (magnetic) wave at 90 degrees to it? Maybe I should just read his book.....hmmmmm maybe not, my brain gets fried just trying to decide what to have for breakfast without trying to cogitate over the meaning of the universe!

DJ - surely the thrust IGE or OGE is the same - ie it balances the weight of the aircraft in order for it to hover. The difference is how much work the rotor has to do to produce that thrust.
As for the space shuttle in a vacuum - I will refer my learned friend to the end of the previous paragraph

[Irlandés]

Irlandes, so how does he explain that light is part of the electro magnetic spectrum in which every other type of energy travels in waves with another (magnetic) wave at 90 degrees to it?

Crab, that's easy. It's all gyroscopic precession!!! Ha ha :D

Seriously though, I'll let you read the book. That lets me off the hook.
"QED, The Strange Theory of Light and Matter" - Richard P. Feynman (Penguin)
I'll stop now before I get into trouble for digressing on this thread! ;)

Irlandés

[vorticey]

i looked at your wood in the water theory, and i only seen a rolling of water laging behind and across the trailing edge in the wake turbulence. if i stalled the wood the rolling water comes up and rides on the back of it. theres two normal tip vortices. although i do know a soccerball curls the way it spins and if you throw a wooden ruler away from you horazontaly, spinning it backwards it flies up, but dont see wing in a vortex theory being a good one.

you said > this assumes that the wing is at the centre of a vortex spinning in a clockwise direction when viewed from the tip of the wing towards the root with the leading edge on the left.
this is the part i dont believe. smoke over a wing doesnot go around it.

and > The effect of the building you are talking about is recirculation which is nothing to do with ground effect.
i was thinking recirculation would be bigger in ground effect (at the same thrust as oge) and more-so a hill or buildings. recirculation is just a vorticey.

the presure is increased because the helicopter disk is just a compresser pump mechanism. if air cant get away, it compresses. if it is let go it will flow. anything restricting flow compresses the air. also reduced rotational flow due to ground friction (same reason wind vears or backs near the ground) increasing the inflow air speed reducing power required.
i rest my case.
gotta go, bit of wet season thunder comin'on.
:D

[[email protected]]

Vorticey, I told you I'm not the author of the vortex/circulation theory but what you describe is what the theory suggests ie that a vortex is formed at the root, tip and the trailing edge - giving a horseshoe shape. When you pull the wing through the water/air it leaves the trailing edge vortex behind it, parallel to the trailing edge itself. When you stall the wing and break the circulation the vortex isn't left behind any more and it catches up with the wing.
As for the rest, you said 'pressure' as well so you will probably be ex-pprunicated and banished to the wilderness, even though I agree that most of what we have been talking about on this thread is pressure related.........Oh no, now I've said it again!

[DeltaFree]

How can you separate light from the rest of the magnetic spectrum? It is all the same, just that our eyes detect visible light. So it is all particulate, little quanta. Its weird wave-like behaviour is due to probability waves as Scroedinger's cat will tell you, or not depending on his neither alive nor dead state just prior to opening his box.

[heedm]

Light: An example of the problems with the particle theory is that when you fire photons at a diffraction grating, instead of passing straight through or getting blocked, they fan out and form an interference pattern...not explained if they're particles. An example of the problems with the wave theory is that you can get wavelike interference patterns when you allow only one photon to pass through a diffraction grating at a time. This works only if you don't try to figure out exactly where the photon passes through. Really weird. What works in the end is a 'wave function' that really just indicates energy and likely locations to find a particle (if you were to look for a particle-like quality). Won't get into that much more.

Space Shuttle: Produces thrust by firing particles out the end. With atmosphere behind it, there is resistance. The pressure in the combustion chamber must exceed one atmosphere just to start firing stuff out...it then increases to some number much bigger than one atm.

[Dave Jackson]

heedm

"Space Shuttle: Produces thrust by firing particles out the end. With atmosphere behind it, there is resistance. The pressure in the combustion chamber must exceed one atmosphere just to start firing stuff out...it then increases to some number much bigger than one atm.

Ah! But why is there a greater force in space, where there's no "atmosphere behind it"? :confused:

Dave J.:

[Irlandés]

Light: Heedm, see that book by Feynman that I recommended to Crab. The answer to your question is there. Actually I'll add to the list another book by the same author which also deals with it: Six Easy Pieces (also Penguin). Both books are highly readable and aimed (more or less) at the layman (although maybe not helicopter pilots! ;) ).

Space Shuttle: By an interesting coincidence, Feynman was one of the principal members of the team that investigated the Challenger disaster.

[heedm]

Dave: No atmosphere behind means no resistance to the expulsion of stuff. This means they come out moving faster. Total change in momentum of the stuff is greater so the produced thrust is greater.

Irlandes: I didn't actually ask a question. I was a physicist prior to flying helicopters. I have read those books and regularly recommend "Six Easy Pieces" to non-physics types. Also out is "Six Not-so-Easy Pieces" haven't read it yet. The explanation I gave for light is accurate but lacks a lot of explanation. If you want more detail, pm or email me. Not really stuff for this forum.

[Dave Jackson]

`
An interesting page on Ground Effect, with picture, graph and computer simulations. (http://home.mira.net/~radacorp/ground_effect.html)
`

[[email protected]]

From DJs link:

Quote 'You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.

A WIG craft, like a Hovercraft, rides on a cushion of air. The difference is that a Hovercraft generates a static air cushion (by blowing air into a bag) whereas a WIG craft has a free ride on a cushion that is created by its own forward movement. There are also lots of other advantages of WIG craft over Hovercraft, not the least being operational altitude.' Unquote

Oh dear, now they have said it's pressure too!!!! However their explanation makes perfect sense to me.

[Nick Lappos]

I guess its time to jump back in, please look at the pressure plot on DJ's link, where the pressure field is plotted. Note that the ground effect plot (the lower one) has higher pressure (darker red color).

Wow, higher pressure for ground effect! But also note that both airfoils are at the same angle of attack. If the lower airfoil were reduced in angle, to compensate for the increased lift due to ground effect, it would have the same pressure distribution.

In fact, please think about it carefully. If the pressure below is greater, the lift would be greater, since the blade area and rpm are a constant. If the lift is greater (due to more pressure) the aircraft would climb.

In other words, the pressure must be the same in all cases where the lift is the same. I.E. the ground effect case has the same pressure below it as the non-ground effect case.

[[email protected]]

Nick, if the pressure distribution changes when the lower aerofoil is reduced in angle then you have proved the pressure theory of GE.

The increased pressure below the wing produces an increase in CL and therefore lift which allows you to reduce the pitch angle to stop it climbing (exactly what you do in an IGE hover). Not only is the Cd reduced by the increased pressure making the wing think it is bigger than it is and therefore more efficient but the further reduction in Cd caused by reducing the AoA means less power needed to drive the wing through the air (just like in an IGE hover).

It is a little like chicken and egg, once you have compensated for the extra lift in GE by reducing the pitch angle, THEN the pressure distribution will be the same and you will have produced the same lift (ie just enough to keep the wing level) but with less AoA and consequently less drag.

[Nick Lappos]

Crab,
We are getting somewhere. If the pressure below the wing/rotor in the IGE case is the same as the pressure beneath the wing/rotor in the OGE case, tell me again how the pressure against the ground has an effect?
Of course pressure is part of the flow equation, but the issue I am countering here is the assertion that the ground somehow allows pressure to build up (a ground cushion).

In fact the pressure must be equal in both cases, else the aircraft would rise in the IGE case.

Proof:
Set Baro altimeter to zero while flat pitch, lift to hover, read altimeter. If pressure is higher below rotor, altimeter would show altitude drop (higher pressure). Does it?

[Dave Jackson]

Nick,

An interesting experiment.

Where is the 'sensor' for a Baro altimeter normally located, and does it have a profile that attemps to minimize extraeous input?
I understand that the active and static pitot tubes are located so as to try and minimize the unwanted.


Dave J.

[[email protected]]

Nick, yes it does! Not only that but while I am raising the lever to take off the RCDI indicates a rate of descent (an increase in pressure). I and many others have taught this as a check that the static vents are clear before going flying, especially instrument flying.

If I hover the helo OGE so thrust = weight and could magically introduce the ground at an instant say 5' below the wheels then the aircraft would climb if I did nothing with the controls. Why? because the ground has caused the pressure increase to occur which opposes IF, improves Cl, reduces Cd etc.etc.
BUT if I lower the lever to maintain height, the same thrust = weight is true but I have achieved it using less pitch angle, less AoA and less power.
You will say it is because the induced drag has reduced and I will agree but only because I believe that the increase of pressure under the wing has caused this to happen, whether through the action of vortices or whatever.

[vorticey]

you said > Of course pressure is part of the flow equation, but the issue I am countering here is the assertion that the ground somehow allows pressure to build up (a ground cushion).

so, ground effect = higher pressure under the disc for the same pitch aplied.

ground cussion, only in the hover, is a extra build up of pressure within the disc area simply because it cant get away. thrust goes down and then out, some would have to go in!

CRAB ; does the RCDI indicate a pressure rise when slowing to the hover?

power curves always start on a lower power at hover (cussion) then increase slightly (as you lose the cussion) before decreasing as translational lift comes in. all this is done in ground effect!
why wouldnt pressure build up in this area?

[Nick Lappos]

Crab,
You said "yes it does" about the altimeter, but gave no numbers. For an S-76, there is an altitude drop when lifting, then it returns to the proper altitude when stable in an IGE hover. What does yours read when in the hover?

Vortecy,
the issue is that pressure must return to the baseline (same as OGE) when the lever is reduced to hold the hover. While in the hover IGE, the pressure is the same below the helicopter as when in an OGE hover. The ground does not raise the pressure under the helicopter, the pitch lever does! The ground allows you to use less pitch.

Guys, we are now again going around and around. Try to think of this this way:
If the pressure is greater below the helicopter in a certain circumstance, how does it act on the helicopter? How would it be measured?

You must break out of the old words and physical models that your instructors taught you if you are to fully understand the concept of Ground Effect. The ground supresses the tip losses, and makes the blades/wing behave as if they were much longer, so they need less angle of attack. This means less tip loss, and less induced drag/power. Pressure does not build up between the aircraft and the ground, there is no "cushion"

If you cannot understand this, that is too bad, but that does not make pressure build up!

[[email protected]]

Nick, yes I believe it returns to the pre hover figure once the helo is in the hover as does the RCDI, which shows that as you are raising the lever you are increasing the pressure under the disc - surely you cannot argue with that.
Our sticking point is that you say once established in the hover, the pressure distribution is the same in an IGE hover as it is in an OGE hover - and I agree!!!
But I think that you are allowed a lower lever position and less power in the IGE hover because if you did not the helo would climb because the proximity of the ground had caused an increase of pressure under the disc.
The proximity of the ground produces 'excess lift' for want of a better term and you dump the 'excess lift by lowering the lever. Having lowered the lever you are no longer producing 'excess lift' only the 'exact lift' required to hover. In the steady state IGE hover I agree that there is not a cushion of high pressure air under the disc but in coming to the hover or lifting to the hover, the high pressure under the disc would make you climb if you did not reduce your IF by lowering the lever.
To sum up - Cause of GE = increase of pressure under the disc making the blades more efficient by appearing bigger to the air- Effect - lower lever position to hover and less power required.

[Nick Lappos]

Crab,

This is, like many pprune threads, a really good one, and an eye opener. We are then left with this paradox:

The pressure under the disk IGE is the same as the pressure under the disk OGE. This means that IGE does not have a "ground cushion" of pressure. There is NO ground cushion, no pressure increase. The change in lift is due to the earth creating, in effect, end plates that block the sidewash of the airfoils.

OED

BTW, the downward flexing of the altimeter during the pitch pull is due to the static ports getting bathed in the downwash of the rotor, not a general pressure increase. Once the flow becomes steady state, the altimeter is a good indicator of the pressure under the disk, and when it shows no significant difference from the field elevation, it tends to prove the non-pressure case. I did some number crunching, for a 15% drop in power to be caused by pressure alone, the pressure would have to change about -150 feet's worth of altitude.

Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all.

[Dave Jackson]

Nick sez,

"Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all."


The previously mentioned web page on Ground Effect (http://home.mira.net/~radacorp/ground_effect.html) sez,

" The principle of Wing-in-ground-effect or (W.I.G) has been known for a long time. Ground effect is a natural phenomenon, as an aerofoil approaches the ground its lifting ability increases and the drag reduces"
It never uses the word 'skirt'


Nick, you appear to be basing the cause of ground effect totally on tip vortices. This is contrary to what I have been able to discover.

Perhaps the tip vortices contribute to ground effect, by acting as a kind of skirt, and this helps retain the pressure.
~or~
Perhaps the increased pressure results in the blades operating in a slightly denser medium and they are therefor slightly more efficient.


:confused: :confused: :confused:
Dave J.
_____________________________

.... and furthermore

Having no (good) reputation to protect, I'll go out on a limb and make the following totally unheard of and totally unsupported claim. :eek:
[list=1]
Pressure is an increase in air density.
Higher air density results in greater rotor efficiency.
[/list=1] Therefor the higher pressure caused by ground effect results in a greater thrust for a given collective pitch settling.

Tear that to pieces, ~ kindly. :(

Dave J.

[Thomas coupling]

Got my hat in front of me...getting ready to eat it...but I'm still not convinced, and I suspect 73,000 other helo drivers aren't either...yet;)

If downwash from the rotor hits the ground there has to be an opposite and equal reaction. .....pressure :confused:

Try rephrasing your explanation of it after all american is a foreign language and there is a lot lost in translation:D

[[email protected]]

Nick, I am nearly with you in that I agree that the pressure IGE below the disk is the same as OGE and that there is not a 'cushion (cussion for Vorticey)':D of high pressure air.

I still maintain that an increase in pressure caused by the vortices being modified by the presence of the ground is the mechanism by which the Ground Effect phenomenon is created. Without an initial increase in pressure there would be no way for the lift to increase and no reason to lower the lever to prevent a climb.

BTW just to muddy the waters again, I went on a training sortie last night, set the QFE just before take off which gave me +10' on the Bar Alt. As I lifted to the hover the RCDI showed a RoD and the Bar Alt reduced to - 30' AND stayed there once established in the 10' hover. This is with a reasonable altimeter with a vibrator (although why it needs one when it's mounted in a Sea King I don't know).

DJ don't forget temperature in your statement about prssure and density eg if temperature stays constant then rising pressure will increase the density.

[Dave Jackson]

crab,

Yes. It is assumed that the temperature is a constant.

The idea in my last post was an alternative to the concept that there is a higher pressure below the blades pushing upwards. What might be considered as the 'air cushion'.

Perhaps the ground increases the air pressure (density) in whole area in which the rotor is working. I.e. above, below and to the sides of the rotor. The rotor therefor performs as if it was operating at a much lower elevation then it actually is. In other words, better thrust for the same power.

Dave J.


Oops!! ~ Edited to apologize and correct above error.

The thrust of the rotor does not vary with altitude. It is the power of a naturally aspirated reciprocating engine that varies with altitude. :( :(

[vorticey]

you dont need to repeat youself in every post, things that were leant at heli school. of caurse the pressure never increases if you want to hold a hight above the ground, as the pilot keeps the pressure constant with the collective. did you even think about what i said on the power chart? it indicates how much pressure you could put under the helicopter at a particular speed (power available). it distinctly shows slightly less power at hover than a couple of knots. this is the cushion im talking about. your right, the pressure doesnt actualy rise, because the pilot reduces the collective to save it. the saved pressure now shows on the performance chart.
so for you nick, i wont call it ground cushion anymore. its now, THE CONSERVATION OF POTENTIAL GROUND PRESSURE!:D

DAVE
thrust REQUIRED is constant.
turbine engines also suffer at altitude, air is less dence. but on RPT jets the engine is designed to operate at a particular altitude so flying below or above this hight the engine is inefficient.

[John Bicker]

Pressure?

http://www.dynamicflight.com/aerodynamics/ground_effect/

The rest of the site may halt some of the other crap theories posted here as well.

Mr Lappos gets my vote!

[Dave Jackson]

John,

I don't think that any of the posters have disagreed with the information on your referenced web page.

But; ~ This web page does not disprove 'pressure'.
Bell has said that 'pressure' on the underside of the fuselage contributes to ground effect. This is a third reason, which is in addition to the two that were mentioned on the page.


__________________

Quote from your referenced web page; "First and most important is the reduction of the velocity of the induced airflow. Since the ground interrupts the airflow under the helicopter, the entire flow is altered. This reduces downward velocity of the induced flow.

The above is a superficial answer.

What enables the rotor to produce a lesser velocity, and yet, produce the same lift?
OK it's the ground, but, the air is the only connection between the helicopter and the ground.
What change has taken place to the air?


More camel dung added :D

[Nick Lappos]

OK guys, that's that. Increased pressure under the helicopter has nothing to do with ground effect. The pressure under an OGE helo is the same as that under an IGE helo.

If you want, you can believe otherwise, it is fine. That is why we run horse races, isn't it?

[heedm]

I'm not a believer of the pressure having an effect on ground effect theory, but I have a question to which I have no answer.

If the presence of the ground reduces the induced flow, then wouldn't the lower induced flow result in a greater pressure due to Bernoulli's principle?

Before everyone's blades start spinning, I'm not saying aerofoils produce lift because of Bernoullie (also not saying they don't)...I just don't want to get into that discussion.
_____________________

I agree that the engines don't have to work as hard IGE than OGE (actually seen this phenomenon myself ;) ). I agree that the rotors produce the same amount of lift in a stable hover IGE and OGE. From that I deduce that the extra work done OGE is due to drag...the only one that can change is blade induced drag. This is where it gets complicated. I seem to recall that blade induced drag is a function of angle of attack, density, geometry, and airspeed. The trick here is that density is still air density, ie not affected by dynamic pressures.

The only quantity in the blade induced drag equation that changes IGE vs OGE is the angle of attack (alpha). alpha is a result of induced flow, relative airflow, blade pitch and rotation of the blade. IGE vs OGE you change induced flow and blade pitch. Blade pitch is changed by the pilot (collective increase). Induced flow would increase slightly as a result of the collective increase, but would decrease overall due to the resistance to allowing the air to move through the ground. At the tips the vortex is changed, but that also causes a change in induced flow and a subsequent change in angle of attack.

Stopping here, the OGE vs IGE argument is explained simply without resorting to pressures.

However, pressure does play a role. It is the mechanism that changes the vortex and restricts the induced flow. If you really wanted to, I'm sure the equations could all be rewritten very complexly (that a word?) so that pressure is used and induced flow is ignored. That doesn't mean pressure causes anything, its accepted that the most simple explanation is the how that things work, other explanations are useful only in specific studies.
___________________

Someone pointed out that pressure doesn't increase below fixed wing aircraft flying very close to the ocean. That's correct, but in that case the "downwash" isn't moving straight down. Instead there is a wake behind the aircraft. That wake wants to flow, but the ocean won't allow it. The restriction in flow causes an increase in pressure (this is actually Bernoulli's theorem in a different light) in the wake. The equations of lift and drag still don't need that pressure change to explain ground effect, so I stand by those that insist pressure isn't a factor.
___________________

I know, I talked too much. Pressure is the mechanism that causes some of the changes that result in the rewards of ground effect. Ground effect (at the wing/blade) is fully explained without reference to the pressure of the air.

Food for thought: What about compressibility right at the blade? We know it happens...we've all seen contrails coming off the blades in the hover. Does that change IGE vs OGE? If so, it gives a bit of ammunition to the pressure explanation enthusiasts.
___________________

Back to original question. It's been said that the pressure beneath is the same IGE vs OGE yet the induced flow changes. Why doesn't the change in velocity of flow change the dynamic and hence the total air pressure?

[DeltaFree]

I still say that pressure changes are the ultimate way to describe aerodynamic phenomena. OK other factors are involved but the vast majority of airflow changes are as a result of pressure, it has to be so as this is the only way to apply force and change momentum of the air. Yes many different explanations can be used without mentioning pressure, but pressure is still there.
Is the pressure increased in ground effect? This can and has been answered intwo ways:
1. In a steady state hover the average pressure difference over the blades must be the same to give the same force (=weight). So you can say there is no pressure difference.
2. Going back to the original question of the effect on an autorotation...a helo arriving close to the ground will produce a pressure build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in pressure.
We can all argue until we are blue in the face. I for one believe pressure explains an awful lot. We can all use different tools to help us visualise how the air moves, computer programs that predict air flows use different methods: sinks, sources and vortices, purely mathematical models from imaginary worlds, and even finite element blocks that use physical properties at the boundaries with neighbouring blocks. Non of these are 100% accurate, only the real world is, but it shows how the same thing can be described in many ways.
If nothing else this thread has got many of us looking at different viewpoints, and hopefully, given us new tools to look at other situations.
PS I have been out of this for a while due work commitments, glad it was still going when I got back.

[Nick Lappos]

DeltaFree,

Your logic is circular, neat and precise, and unfortunately wrong.
You say "a helo arriving close to the ground will produce a pressure build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in pressure."

Why not say, with equal evidence and equal physical data, "a helo arriving close to the ground will produce a little red demon build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in little red demons."

There is no pressure buildup. That is the old pilot saying, like an old wives tale, told 10,000 times, so it must be correct. Were the pressure higher below the helicopter close to the ground, it would be measurable in many ways, such as your altimeter. No measurement shows higher pressure.

I am sad to see the thread last this long, and that the 40-zillionth post starts off where the first did. Oh well.

[Dave Jackson]

Nick,

You are against using the concept of 'pressure' in the discussion of ground effect. But, you have previously commented that the activity under discussion takes place at the speed of sound. The very definition of the word 'Sound' is "Oscillating pressure waves passing through a medium. I believe that 'pressure' is at the very heart of the discussion.

To me, the question is simply 'how does ground effect cause a change(s) in pressure'. The answer is obviously not as simple as the question.
_____________

As a wild guess, I suggest that the ground restricts the airflow. It causes a congestion of air molecules under the disk and it inhibits the replenishment of air molecules above the disk. Hence a pressure differential between the upper and the lower surfaces of the disk (and the fuselage).

PS Merry Xmas
_________________

Edited to elaborate on the possibility of reduced pressure above the disk.

When out of ground effect, the air that is drawn down through the disk is replaced by air from a very large sphere surrounding the helicopter. Some of this replenishment is originating from below the disk.

In ground effect, the ground will restrict the flow from the large area below the disk. This causes some of the replenishment to come from air that has previously gone down through the disk. This 'used' air that is about to be 'reclaimed' is heading upward at a certain velocity and with a certain inertia. It is now called upon to change its direction 180-degrees and head back down through the disk again. The thing that is 'calling it back down' is partial vacumn.

[Nick Lappos]

Dave,
Nice wild guess, but the pressure is the same in or out of ground effect. There is no "restriction" and no pressure rise. The flow is bent by the ground, which reduces the outflow, and makes the rotor behave as if it were larger. The efficiency rise of the rotor means it can make the lift needed with less power.

The issue that all the pressure fans miss is that there is more power efficiency, not more lift. This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.

However, since we have gone through 138 posts to circle back to this, and the last several posts all simply repeat the pressure assertions of the initial posts, we are truly stuck. If you cannot see that the answer to the ground effect efficiency improvement is not a pressure rise, it is simply a shame. Quel damage, as they say in Paris.

Have a Merry Christmas, guys!

[Dave Jackson]

Flight ~ and Daniel Bernoulli (http://www.rod.beavon.clara.net/flight.htm)

[offshoreigor]

I'm Starting ro think that Dave is really Lu in disguise!

Cheers, :eek: OffshoreIgor :eek:

[Nick Lappos]

Oh, Dave. You are Lu in disguise. What does that site you posted have to do with the question of how ground effect works?

I feel like those who debated with the Flat Earth Society back in the 1800's. The Flat Earthers were a group who argued that the earh is really flat. No matter what evidence was presented, they simply ignored it and went on with their assertions.

I guess Dave you are President of the Ground Pressure Society.

http://www.alaska.net/~clund/e_djublonskopf/Flatearthsociety.htm

[Dave Jackson]

Nick,

This thread was initially about 'Autorotation and Ground-Effect'. As part of this discussion, the 'effects' of the ground were considered. This thread then moved on to consider the 'causes' of ground effect.

There is obviously a distinction between 'Cause' and 'Effect'.

I politely suggest that;

*~ Your mentioning of; reduced angle of attack, reduced drag, and reduced power, are actually part of the 'Effect'. They are not part of the 'Cause'.

* ~ You deny that pressure is a 'Cause' of ground effect. I submitted the previous web page because it very clearly presents the relationship that exists between an airfoil and pressure, and by extension between, an airfoil, pressure and the ground.

____________________

There is probably no definitive answer, and as you say, this thread may have run its course, but, if you wish to continue, it will be appreciated if you attack the subject and not the person.


Yours truly;

Lu

oops Dave J. http://www.unicopter.com/drinking_smile.gif

[vorticey]

NICK
you said > The issue that all the pressure fans miss is that there is more power efficiency, not more lift.(the power required reduces because the extra lift is extinguished by reducing collective.) This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.
why are'nt blades made to produce this changed direction of flow and have the extra efficiency all the time??

DeltaFree, spot on!


:confused:

[Chiplight]

I found a couple of computer images that seem to support the idea that pressure does increase under the airfoil in ground effect quite dramatically.

http://was.kewlhair.com/hammer/gifs/grnd-effect.jpg
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.

http://was.kewlhair.com/hammer/gifs/grnd-effect.gif

Here's another simulation showing the air pressure gradients as a wing travels over uneven terrain. As the terrain rises the ground effect is increased and pressure builds as indicated by the color chart.

-Chiplight

[Nick Lappos]

Chiplight,
Take care with your interpretation, the two plots at the top do not show the same case, and do not prove the erroneous assumption that ground effect raises the pressure under the helicopter. Those plots (which are quite accurate) do not show the difference between IGE and OGE with the same angle of attack. Note that the deep orange lower plot has more lift, and higher angle of attack than the upper plot, that is why the pressure is higher. In other words, the upper plot shows the greater pressure with ground effect because the lift is greater, since the CHANGE IN CIRCULATION DUE TO THE GROUND contributes to the airfoil's lift. This would cause the helicopter to climb, so the collective would be reduced, equalizing the pressure.
In other words, the pressure under the airfoil, and under the helicopter, is the same in ground effect and out of ground effect, there is NO ground cushion of high pressure air.

The plots do show the circulation that the ground induces, and therefore the way the ground allows you to achieve the same lift with lower collective. Those plots show WHY IGE needs lower angle of attack to achieve the same lift, and that it is not pressure that allows hover IGE to be done at lower power.

[Chiplight]

Nick,

it would be nice to have an animation showing the case where the pilot adjusts collective to hold lift constant; then the pressure increase may drop out of the equation so to speak.

Nevertheless, it is obvious that if nothing is done to lower collective, and we fly into ground effect that the 'reds" will light up the plot as above, leading one to ponder cause and effect.

I'm not any kind of expert, but could it be that there is some kind of equivalence between "pressure" based explanations and momentum based explanations of ground effect? I'm sure this has been mentioned in the thread before, but I've not read all 10 pages of it.(!)

-Chiplight

[Nick Lappos]

Chiplight,

The answer is obvious. The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE. There is no ground cushion. Read your altimeter in a hover if you have any doubt about this. There is no "ground cushion" of any kind.

Nick

[Dave Jackson]

Perhaps we are all envisioning the same thing, but in a different way.

Nick said; "The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE."

Assuming that the helicopter is hovering in both of the above situations, I don't think that anyone disagrees with this statement. Nick has also said that the angle of attack will be the same in both situations and nobody probably disagrees with this, either. The thrust will also be the same.

What is different between hovering OGE and IGE is the angle of the collective pitch. In other words, the effect of the ground is that of allowing the helicopter to hover with a reduced collective pitch (plus a reduced rotor drag and a reduced power).

The question is; 'What is the cause of this effect'?

Pressure is a potential &/or partial answer.

[Nick Lappos]

Dave,
The cause of this is the fact that the ground suppresses the outflow, and allows the blade to deliver the same lift with less angle of collective. This is a bent flow discussion, not a pressure discussion. Note in the plot that Chiplight provides shows the equivilent circulation that the ground "provides" that helps explain the change in flow.

[Lu Zuckerman]

This will raise some eyebrows or increase someone’s blood pressure. A while back I made a comment to Nick Lappos indicating that the field of engineering is in a constant state of change. I further stated that sometimes the new kid on the block would change the rules and redefine a problem in opposition to the facts as taught to the older guys. I don’t dispute what Nick is saying in his explanation. But, I also feel that based on what is taught to most helicopter pilots and this includes the older guys with no engineering training there is such a thing as a ground cushion and by implication the cushion exists due to an increase in the localized pressure field. It takes a long time to reshape a persons perception when it comes to learning a new theory while discarding the old theory. There is a possibility that new helicopter pilots will get Nicks explanation in their POF courses but until the last of the old breed dies off they will still believe in a “Ground Cushion” and an increase in pressure.

Even the FAA Helicopter book is in support of Nicks' theory.

:rolleyes:

[DeltaFree]

This is a bent flow?

Nick,
You refuse to accept that pressure has anything to do with ground effect, you have slagged me and my arguments several times without actually giving anything of substance to prove pressure advocates wrong, other than wonderful phrases that do not explain any physical behaviours. Now you tell us it is not pressure it is bent flow.......OK OK Please now tell me in words of one syllable, so we can all follow your your extraordinary logic, just how does the flow bend without pressure differentials?

PS I really thought my last post was quite reasonable. You say yourself as we approach the ground lift increases due to greater efficiency, and we are able to reduce collective to maintain the same lift. So now you tell me a little red demon produces this extra lift not an increase in pressure! I thought even your logic had already stated extra lift equals extra pressure, OK you said equal lift means equal pressure, surely that is the same thing.

PPS I am trying not to get personal but would accept a sound argument in preference to references to Sunday school attendance, how good a doctor my mum is, how great my first instructor was or even little red demons. Come on Nick real physics not my imaginary life please.