Autorotation and Ground-Effect
 

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

Thanks
Dave J.

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:

Ground effect and auto.
 

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.

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?;)

Slow responses
 

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.

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.

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.

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

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.

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.

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

;)

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.

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. ;)

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.

1. [Mumbo Jumbo] to [English]
2. [Pig Latin] to [English]
3. [Gobble de Gook] to [English]
4. [Esperanto] to [English]
5. [Lower Slobovian] to [English]
6. [Other] to [Another Other];)

Dave J.

Not producing lift!!!!!!
 

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.

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.

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.

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.

DeltaFree:


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.

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