Autorotation and Ground-Effect
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There is ground effect!
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!!!
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!!!
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hummmm....
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 ?
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-so far my instructor is still alive-
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-
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.
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.
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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
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
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!
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!
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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.
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.
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!
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?
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!
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?
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.
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.
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.
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.
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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.
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.
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.
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.
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cushion creep
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)
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)
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Ground Effect
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.
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.
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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.
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.
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!!)
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!!)
Last edited by helmet fire; 9th Nov 2002 at 00:44.
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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?
Am I way off base or is this close?
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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.
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.
Crazy little thing called lift
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.
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.
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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.
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.
Last edited by Dave Jackson; 9th Nov 2002 at 23:43.
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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.
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.