Autorotation and Ground-Effect
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
crab
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).
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).
Join Date: Apr 2002
Location: UK
Posts: 45
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Last edited by Tail Bloater; 28th Nov 2002 at 13:34.
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
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
Join Date: Oct 2002
Location: scotland
Posts: 79
Likes: 0
Received 0 Likes
on
0 Posts
More of the Same......some high number
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.
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.
Last edited by DeltaFree; 26th Nov 2002 at 19:37.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2002
Location: Dun Laoghaire
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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!
Irlandés
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!
Irlandés
Last edited by Irlandés; 27th Nov 2002 at 09:17.
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
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
Join Date: Jun 2002
Location: Dun Laoghaire
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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?
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
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
crab
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.
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.
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!
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!
Join Date: Oct 2002
Location: scotland
Posts: 79
Likes: 0
Received 0 Likes
on
0 Posts
Light
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.
Join Date: Jan 2001
Location: AB, Canada
Posts: 420
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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"?
Dave J.:
"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"?
Dave J.:
Join Date: Jun 2002
Location: Dun Laoghaire
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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.
).Space Shuttle: By an interesting coincidence, Feynman was one of the principal members of the team that investigated the Challenger disaster.
Join Date: Jan 2001
Location: AB, Canada
Posts: 420
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Guest
Posts: n/a
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.
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.
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.
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.
Guest
Posts: n/a
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?
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?