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.

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.

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.

Induced Flow
 

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?

To flare or not to flare, dammit!
 

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?

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!

"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.

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 - ;)

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:

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

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.

nick
 

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.

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!

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

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.

nick
 

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?

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.

Re Induced Flow fo TC
 

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

Speed of sound vs. Speed of force.
 

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

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.