Autorotation and Ground-Effect
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I recently purchased a copy Ray Prouty’s book Helicopter, Performace, Stability, and Control. It was a good buy as it was used, a new copy is over $100. He has a section on ground effect and some good diagrams that address the issue but not the main question of the topic.
Angle of Attack (AoA) is the same for both IGE, and OGE. A specific angle of attack will produce a constant amount of lift (other things being equal). The amount of lift produced is the same for IGE and OGE hovers.
The induced flow through the rotor system slows down when close to the ground, the ground hinders the air from escaping. There is now a decrease of the induced velocity flowing down through the rotor. The blade pitch angle decreases and that causes a more vertical lift component and the vector for induced drag lessens (less power needed). AoA is the same for both IGE and OGE even with a decreased pitch angle, this is because the induced velocity is less.
Angle of Attack (AoA) is the same for both IGE, and OGE. A specific angle of attack will produce a constant amount of lift (other things being equal). The amount of lift produced is the same for IGE and OGE hovers.
The induced flow through the rotor system slows down when close to the ground, the ground hinders the air from escaping. There is now a decrease of the induced velocity flowing down through the rotor. The blade pitch angle decreases and that causes a more vertical lift component and the vector for induced drag lessens (less power needed). AoA is the same for both IGE and OGE even with a decreased pitch angle, this is because the induced velocity is less.
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NL, that report does not mention pressure. But the theory is based on trailing vortices, and their mirror image in the ground. So what they do is effectively push air upward to meet the downward flow from the downwash to cancel it out at the surface leaving zero airflow through the surface. Yes again no pressure need be mentioned to achieve this result, but again in order to slow down an airflow a pressure gradient must be introduced. Newton's Laws. So in comparison to flight at altitude there will be an increased pressure under an aircraft at v.low heights.
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To simplify the activity, the following is my impression of what will take place during a vertical (no forward velocity) autorotation to the ground.
There is no change in the collective or cyclic stick, therefor the pitch on all blades, at all azimuths, and at all heights above ground, will not change. Near the ground the rotor disk's mean angle of attack will increase because of the pressure (attempted upwash) below the blades. This will increase the area of and the thrust from the driven (outer) region of the disk.
This increased angle of attack will reduce the descent rate, partially because of an additional pressure created under the fuselage, and partially because of an increase in the induced thrust. The induced drag will also increase. This is because the higher pressure (attempted upwash) will be attempting to pushing more air from under the blade upward, and some of it will move forward and up around the front of the blades.
The rotor's rpm may not slow because the additional thrust will increase the coning angle and the Coriolis effect.
_____________
The above only relates to the rotation energy in the disk. Should the helicopter also have forward velocity, then it will also have linear energy. This will result in additional effects, similar to those experienced by a plane's wings or a gyrocopter's rotor disk. These additional effects must then be combined with those from above.
Just a reasoned guess.
Dave J.
There is no change in the collective or cyclic stick, therefor the pitch on all blades, at all azimuths, and at all heights above ground, will not change. Near the ground the rotor disk's mean angle of attack will increase because of the pressure (attempted upwash) below the blades. This will increase the area of and the thrust from the driven (outer) region of the disk.
This increased angle of attack will reduce the descent rate, partially because of an additional pressure created under the fuselage, and partially because of an increase in the induced thrust. The induced drag will also increase. This is because the higher pressure (attempted upwash) will be attempting to pushing more air from under the blade upward, and some of it will move forward and up around the front of the blades.
The rotor's rpm may not slow because the additional thrust will increase the coning angle and the Coriolis effect.
_____________
The above only relates to the rotation energy in the disk. Should the helicopter also have forward velocity, then it will also have linear energy. This will result in additional effects, similar to those experienced by a plane's wings or a gyrocopter's rotor disk. These additional effects must then be combined with those from above.
Just a reasoned guess.
Dave J.
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DeltaFree,
I once asked a Physics professor why it was that light behaved a certain way, and tried to couch my question in physically analogeous ways, such as you try with the pressure thing. He commented to me that attempting to draw such analogies was fine as a memory trick, but virtually always fell down when you tried to extend the analogy in any way. He admonished me to stop trying to make such parallels, as they block true understanding.
The old "size of a pea" thing is an example. The Sunday suppliment science reporter can explain anything, and in a few words completely ball up any knowledgable person with analogies like "the energy in a piece of uranuim the size of a pea...."
The EXPLANATION is that the ground effects the airflow around the wing to make it wash outward less, and this reduces the angle of attack of the wing/blade so that more lift is produced at a lesser angle of attack. The effect is to magically make the wing behave as if it were longer, with higher aspect ratio and more efficiency. The lesser angle means less insuced drag, more efficient lift. That EXPLANATION has appeared in about 45 or so posts on this thread. The rotor blade is a wing, and it follows the same rules as it gets close to the ground, of course.
You, DeltaFree then take that EXPLANATION and screw it up with a physical analog that makes you comfortable, you turn it into a pressure argument, and then you feel secure and comfy. That is nice, it is keen, it makes you all warm and fuzzy, and it is wrong, but you should keep it up because the idea of angles of air motion is obviously very uncomfortable for you. In this process of screwing it up, you then insist that your fuzzy little pet story is correct. Please be assured that it is not. Please be assured that the instructor who first said "ground cushion" to you was wrong, in his delightful way. He gave you a simple way to understand that when the ground is near, the helo flies gooder, but he was wrong. Now, in observance of his infallability, you must make all explanations of ground effect fit what you were first told. You were probably the very best student in Sunday school, where faith was the guiding requirement toward understanding. This is not sunday school.
As the last attempt, here is yet again another, separate, more full, complete, different example of a non-pressure explanation of ground effect:
"Ground effects may be explained by the interaction of the aircraft wingtip vortices with the ground. This interaction reduces the strength of these vortices. The weakened wingtip vortices reduce the wing downwash which increases the lift and decreases the induced drag, or drag caused by lift." - Dynamic Ground Effects Flight Test of an F-15 Aircraft, NASA Technical Memorandum 4604 at:
http://www.dfrc.nasa.gov/DTRS/1994/PDF/H-1999.pdf
Forgive me getting crotchity in this post, it must be the pressure I feel with this whole thing ;-)
I once asked a Physics professor why it was that light behaved a certain way, and tried to couch my question in physically analogeous ways, such as you try with the pressure thing. He commented to me that attempting to draw such analogies was fine as a memory trick, but virtually always fell down when you tried to extend the analogy in any way. He admonished me to stop trying to make such parallels, as they block true understanding.
The old "size of a pea" thing is an example. The Sunday suppliment science reporter can explain anything, and in a few words completely ball up any knowledgable person with analogies like "the energy in a piece of uranuim the size of a pea...."
The EXPLANATION is that the ground effects the airflow around the wing to make it wash outward less, and this reduces the angle of attack of the wing/blade so that more lift is produced at a lesser angle of attack. The effect is to magically make the wing behave as if it were longer, with higher aspect ratio and more efficiency. The lesser angle means less insuced drag, more efficient lift. That EXPLANATION has appeared in about 45 or so posts on this thread. The rotor blade is a wing, and it follows the same rules as it gets close to the ground, of course.
You, DeltaFree then take that EXPLANATION and screw it up with a physical analog that makes you comfortable, you turn it into a pressure argument, and then you feel secure and comfy. That is nice, it is keen, it makes you all warm and fuzzy, and it is wrong, but you should keep it up because the idea of angles of air motion is obviously very uncomfortable for you. In this process of screwing it up, you then insist that your fuzzy little pet story is correct. Please be assured that it is not. Please be assured that the instructor who first said "ground cushion" to you was wrong, in his delightful way. He gave you a simple way to understand that when the ground is near, the helo flies gooder, but he was wrong. Now, in observance of his infallability, you must make all explanations of ground effect fit what you were first told. You were probably the very best student in Sunday school, where faith was the guiding requirement toward understanding. This is not sunday school.
As the last attempt, here is yet again another, separate, more full, complete, different example of a non-pressure explanation of ground effect:
"Ground effects may be explained by the interaction of the aircraft wingtip vortices with the ground. This interaction reduces the strength of these vortices. The weakened wingtip vortices reduce the wing downwash which increases the lift and decreases the induced drag, or drag caused by lift." - Dynamic Ground Effects Flight Test of an F-15 Aircraft, NASA Technical Memorandum 4604 at:
http://www.dfrc.nasa.gov/DTRS/1994/PDF/H-1999.pdf
Forgive me getting crotchity in this post, it must be the pressure I feel with this whole thing ;-)
Nick: hang in there, we're not having a go at you personally, please believe this. may I also suggest that when someone of your calibre with your knowledge, tries to explain simple theories (in your mind) it may prove difficult for you to understand why mere mortals like us are having problems getting it
ALL of the UK military, and I would argue all civilian trained helo pilots, are taught that air under the disc, when close to the ground, is compressed thus exerting a back pressure down the line. Its effect on AofA/Induced Flow etc is another issue.
How, then are we to be asked to accept that this concept is incorrect?
Question: When a blade/wing/rotor, moves through the air, do the molecules of air in front of that leading edge (albeit for a millisecond) get compressed because they cannot move out of the way fast enough? Does this become a serious problem when that object moving through the air, reaches close to Mach 1?
Is compressibility not the same as pressure build up, and does it not have an effect on the air flow over/around that leading edge?
Please be gentle with me, as I think I represent 90%+ of all helo pilots...
ALL of the UK military, and I would argue all civilian trained helo pilots, are taught that air under the disc, when close to the ground, is compressed thus exerting a back pressure down the line. Its effect on AofA/Induced Flow etc is another issue.
How, then are we to be asked to accept that this concept is incorrect?
Question: When a blade/wing/rotor, moves through the air, do the molecules of air in front of that leading edge (albeit for a millisecond) get compressed because they cannot move out of the way fast enough? Does this become a serious problem when that object moving through the air, reaches close to Mach 1?
Is compressibility not the same as pressure build up, and does it not have an effect on the air flow over/around that leading edge?
Please be gentle with me, as I think I represent 90%+ of all helo pilots...
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Thomas (and all the brethren who watch as he throws his hat through the door!) I do know that you are not getting personal, and I do see the frustration when I come along and say something so outlandish.
It is absolutely true that the state of a fluid flow must contain information on its density, pressure, viscosity and velocity, so we can be sure pressure is an important piece of information, but the lift that the disk sees as the aircraft enters ground effect is NOT a product of a cushion of air under the machine that gives it an extra lift! That old piece of bull called "ground cushion" is an English language statement that does not describe the pressure under the aircraft.
Those great pilots who taught us used this term, the same way we say that your Mom says that you "catch a cold" when you feel chilly after a virus attacks your respiratory system. Your Mother tells you to wear a sweater so you don't catch a cold, a completely wrong medical advice, (the chill you feel is the symptom of the disease you already have, not the cause of the further symptoms you are now guaranteed to get!) Your instructor told you about Ground Cushions, and he was a great instructor, your Mom is a great Mom. Your instructor was not an aero guy, and your Mom was not a Doctor (of course, I now speak metaphorically, your Mom could be a Doctor, and so on, but what the heck....)
I think of the movie "Sand Pebbles" where Steve McQueen tells the Chinese helper about the great steam God in the boiler who gets hot and angry when the boiler is fired. Where does this all end!!
It is absolutely true that the state of a fluid flow must contain information on its density, pressure, viscosity and velocity, so we can be sure pressure is an important piece of information, but the lift that the disk sees as the aircraft enters ground effect is NOT a product of a cushion of air under the machine that gives it an extra lift! That old piece of bull called "ground cushion" is an English language statement that does not describe the pressure under the aircraft.
Those great pilots who taught us used this term, the same way we say that your Mom says that you "catch a cold" when you feel chilly after a virus attacks your respiratory system. Your Mother tells you to wear a sweater so you don't catch a cold, a completely wrong medical advice, (the chill you feel is the symptom of the disease you already have, not the cause of the further symptoms you are now guaranteed to get!) Your instructor told you about Ground Cushions, and he was a great instructor, your Mom is a great Mom. Your instructor was not an aero guy, and your Mom was not a Doctor (of course, I now speak metaphorically, your Mom could be a Doctor, and so on, but what the heck....)
I think of the movie "Sand Pebbles" where Steve McQueen tells the Chinese helper about the great steam God in the boiler who gets hot and angry when the boiler is fired. Where does this all end!!
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Nick
Re NACA Report;
In no way can I see how this 1928 report arrives at its conclusion that "The induced drag of an airplane is reduced upon approaching the ground ...'.
Only the [propeller RPM] and the [forward velocity] were used as inputs to this report. These two variables, plus a homemade 'fudge factor', the [influence coefficient], are the constructs of the lift/drag graph. I would suggest that the only conclusion, which can be drawn from this report, is that the ground improves the lift/drag ratio.
[Angle of attack] also improves the lift/drag ratio, but the report does not even consider the [angle of attack]. I believe that a good argument can be put forth that states 'The angle of attack is increased in ground effect. The drag is increased in ground effect, also; but not to the extent that the lift/drag ratio is increased.'
Re NASA Technical Memorandum 4604:
"Ground effects generally caused an increase in the lift, drag and nose down pitching moment coefficients". This appears to support the above argument.
The reduction of tip vortices obviously improves lift and the ground may well reduce these vortices. This is may be especially beneficial for aircraft with highly swept wings, where the lift is "near the wingtips of a swept wing".
Vortices reduction is probably only one component of ground effect. Perhaps at 150-170 knots the F-15 is out in front of other ground effects.
Dave J.
Re NACA Report;
In no way can I see how this 1928 report arrives at its conclusion that "The induced drag of an airplane is reduced upon approaching the ground ...'.
Only the [propeller RPM] and the [forward velocity] were used as inputs to this report. These two variables, plus a homemade 'fudge factor', the [influence coefficient], are the constructs of the lift/drag graph. I would suggest that the only conclusion, which can be drawn from this report, is that the ground improves the lift/drag ratio.
[Angle of attack] also improves the lift/drag ratio, but the report does not even consider the [angle of attack]. I believe that a good argument can be put forth that states 'The angle of attack is increased in ground effect. The drag is increased in ground effect, also; but not to the extent that the lift/drag ratio is increased.'
Re NASA Technical Memorandum 4604:
"Ground effects generally caused an increase in the lift, drag and nose down pitching moment coefficients". This appears to support the above argument.
The reduction of tip vortices obviously improves lift and the ground may well reduce these vortices. This is may be especially beneficial for aircraft with highly swept wings, where the lift is "near the wingtips of a swept wing".
Vortices reduction is probably only one component of ground effect. Perhaps at 150-170 knots the F-15 is out in front of other ground effects.
Dave J.
Last edited by Dave Jackson; 21st Nov 2002 at 05:24.
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Thomas C,
You aren't correct in saying that ALL UK military pilots are taught that air under the disc becomes compressed. At least it wasn't so in my time. I was taught that the ground cushion air behaved differently to "free" air, ie. air higher than 2/3rds rotor span. I don't ever recall it being referred to as compressed air, but more as a static or constrained dome of air beneath the aircraft which altered the induced flow through the rotor disc.
Perhaps you mean "military" in the army sense?
You aren't correct in saying that ALL UK military pilots are taught that air under the disc becomes compressed. At least it wasn't so in my time. I was taught that the ground cushion air behaved differently to "free" air, ie. air higher than 2/3rds rotor span. I don't ever recall it being referred to as compressed air, but more as a static or constrained dome of air beneath the aircraft which altered the induced flow through the rotor disc.
Perhaps you mean "military" in the army sense?
DeltaFree: I love the analogy with the birds in a container, but I don't think it's right
Correct me if I'm wrong, but when you use the word 'weight' then you are considering the definition of weight which includes its correlation with resistance. I.e. weight only acts on a surface, it is gravity based. If there is no reaction (surface) then there is no 'weight'.
For instance, two men standing on a weighing scales, one on top of the other! When the top man levitates away from the guy underneath, the scales will reflect a lesser weight!!
The same happens with your birdies, in that when they take off and fly inside the container which is on a weighing scale, the scales will indicate a reduced figure
When a helo takes off away from the ground, it weighs nothing
What doesn't change is the MASS. This remains the same.
So the mass of the container and birds in your analogy is 2 tonnes, and will always be 2 tonnes. The weight of the combined lot is 1 tonne (birds in flight).
That was one reason why all up weight (AUW) for aircraft was changed to all up mass (AUM) all those years ago, so that it represented more closely what was going on....
What do you think?
Correct me if I'm wrong, but when you use the word 'weight' then you are considering the definition of weight which includes its correlation with resistance. I.e. weight only acts on a surface, it is gravity based. If there is no reaction (surface) then there is no 'weight'.
For instance, two men standing on a weighing scales, one on top of the other! When the top man levitates away from the guy underneath, the scales will reflect a lesser weight!!
The same happens with your birdies, in that when they take off and fly inside the container which is on a weighing scale, the scales will indicate a reduced figure
When a helo takes off away from the ground, it weighs nothing
What doesn't change is the MASS. This remains the same.
So the mass of the container and birds in your analogy is 2 tonnes, and will always be 2 tonnes. The weight of the combined lot is 1 tonne (birds in flight).
That was one reason why all up weight (AUW) for aircraft was changed to all up mass (AUM) all those years ago, so that it represented more closely what was going on....
What do you think?
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The birds are lifted by the air that they accelerate downward. This air transfers the momentum to the container, so the weight of the container never changes, (it presses on a scale with the same weight) regardless if the birds are airborne or walking around inside the container.
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TC the birds in a container do not become weightless- their weight is supported by the air, agreed weight is a much misused term and I probably did use it incorrectly here. But as agreed by NL the container would still weigh the same and yes it's mass would still be the same. All heavier than air machines do have weight and support this weight with lift. Balloons etc when floating do not have weight but do have mass. Make sense?
NL I am prepared to bet a hefty sum that the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away. Get your fluid dynamic calculations, finite element / source,sink vorticey programs working on that and tell us all the results.
I have just read that last NASA report and correct me if I am mistaken, (I am sure you will) but they use radar/ optical height measurements since the aircraft static pressures are affected by ground effect! But hang on ground effect does not affect pressure, or does it NL?
NL I am prepared to bet a hefty sum that the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away. Get your fluid dynamic calculations, finite element / source,sink vorticey programs working on that and tell us all the results.
I have just read that last NASA report and correct me if I am mistaken, (I am sure you will) but they use radar/ optical height measurements since the aircraft static pressures are affected by ground effect! But hang on ground effect does not affect pressure, or does it NL?
Last edited by DeltaFree; 21st Nov 2002 at 20:20.
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TC
Don't give in yet. Go back and edit you post so it says 'open' container (like a birdcage with the s--t pan removed).
Nick,
"I don't know what your point was in the last post. Having reported what the papers said, you seem to think they support your position that ground effect is a pressure thing."
There was no point. It's just the fun of arguing.
Seriously, what I was saying is that neither paper disproves pressure as a cause of ground effect.
1/ "Note that in 1928 there seemed to be no need to use the word "pressure" in the NACA study that measured and explained ground effect:" This study did not measure ground effect, nor did it explain ground effect.
2/ The F-15 makes mention of tip vortices as a cause of ground effect. For the reason previously stated, I feel that tip vortices will play a much smaller role in a helicopter then an F-15.
Bell has referred to pressure under the fuselage as being one contributor to ground effect. Few would argue with this.
Heck, I learned to fly in an airplane with low wings. During dual instruction, we got the craft proposing to the extent that we bent the blade tips. On another occasion at Vancouver International, the tower had 'all traffic hold' while I attempted three times, on the same pass, to put the little sucker on the ground. The first time a friend took my plane up by himself, he did three circuits before he got it down. He said that if he could not get it down on the fourth try, he was going to open the canopy a step out. On these attempted landings, there was definitely a lot of pressure. There had to have been, since pressure causes heat and we were sure sweating.
DeltaFree
"... the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away."
The operation of a pitot tube supports your statement.
Dave J.
Don't give in yet. Go back and edit you post so it says 'open' container (like a birdcage with the s--t pan removed).
Nick,
"I don't know what your point was in the last post. Having reported what the papers said, you seem to think they support your position that ground effect is a pressure thing."
There was no point. It's just the fun of arguing.
Seriously, what I was saying is that neither paper disproves pressure as a cause of ground effect.
1/ "Note that in 1928 there seemed to be no need to use the word "pressure" in the NACA study that measured and explained ground effect:" This study did not measure ground effect, nor did it explain ground effect.
2/ The F-15 makes mention of tip vortices as a cause of ground effect. For the reason previously stated, I feel that tip vortices will play a much smaller role in a helicopter then an F-15.
Bell has referred to pressure under the fuselage as being one contributor to ground effect. Few would argue with this.
Heck, I learned to fly in an airplane with low wings. During dual instruction, we got the craft proposing to the extent that we bent the blade tips. On another occasion at Vancouver International, the tower had 'all traffic hold' while I attempted three times, on the same pass, to put the little sucker on the ground. The first time a friend took my plane up by himself, he did three circuits before he got it down. He said that if he could not get it down on the fourth try, he was going to open the canopy a step out. On these attempted landings, there was definitely a lot of pressure. There had to have been, since pressure causes heat and we were sure sweating.
DeltaFree
"... the pressure 10 ft under an autorotating/flying helicopter, will be comparatively higher for a machine 11ft off the ground, than for an identical helo in an identical situation except for the presence of the solid surface 11 ft away."
The operation of a pitot tube supports your statement.
Dave J.
The circulation theory of lift can best be applied here I think – this assumes that the wing is at the centre of a vortex spinning in a clockwise direction when viewed from the tip of the wing towards the root with the leading edge on the left. The circulation around this vortex creates a force at 90 degrees to the free stream flow in the direction of rotation ie upwards. This is because the flow above the vortex speeds up and that below it slows down ( at the point where the trailing edge flow around the vortex is moving forwards and meets the free stream flow going the other way) – a stagnation point of increased (or less reduced pressure). The force is a lifting force and the relationship between the pressures above and below the vortex is apparently called the Kutta-Zhukovsky relationship.
Back to ground effect and the area of relatively high pressure beneath the wing is interfered with by the ground, not only increasing the area of stagnation but changing the way the flow goes up and over the leading edge – instead of coming rear to front around the bottom of the vortex it now can only come from ahead which would alter the inflow angle and the AoA, Cl and Cd which we have agreed occurs in ground effect.
What do you reckon Nick?
PS Weight is a force = mass x acceleration (due to gravity)
PPS I wouldn't trust a Physics professor to explain light, does it behave as a particle or as a wave or both? I believe the answer is both but this doesn't fit in with their theories to explain how it does it - waving streams of particles anyone?
Back to ground effect and the area of relatively high pressure beneath the wing is interfered with by the ground, not only increasing the area of stagnation but changing the way the flow goes up and over the leading edge – instead of coming rear to front around the bottom of the vortex it now can only come from ahead which would alter the inflow angle and the AoA, Cl and Cd which we have agreed occurs in ground effect.
What do you reckon Nick?
PS Weight is a force = mass x acceleration (due to gravity)
PPS I wouldn't trust a Physics professor to explain light, does it behave as a particle or as a wave or both? I believe the answer is both but this doesn't fit in with their theories to explain how it does it - waving streams of particles anyone?
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I totally accept the line vortex theory of lift, this vortex "contained" in the wing cannot just end, and does leave the wing as the trailing vortex we all know and love from a wing tip (well as a sheet vortex which has increasing strength to the tip, which all bundles up as one just behind the wing). Ground effect can be explained by the mirror image of this vortex system in the ground, as I think I have mentioned earlier.
The effect of the mirror image trailing vortices is to push the real trailing vortices further outwards, this has the effect of increasing the aspect ratio of the wing, reducing downwash, and induced drag hence producing the efficiency improvement we are all talking about. Also by effectively increasing the aspect ratio we also reduce the strength of the vortex, as previously mentioned.(Lift/unit span is proportional to vorticity and airspeed, so longer span same lift and airspeed give less vorticity)
There you go all explained without even mentioning pressure!
BUT air will continue in its state of rest or constant linear motion unless a force is applied to it. Pressure is the force that makes air deviate around an aerofoil, if we are to change this flow field we must also change the pressure distribution. So despite vortices being able to explain ground effect, there must be pressure changes if there is to be a different flow pattern. So I say the pressure directly under a wing is increased by ground effect. I am not saying that this pressure on the underside of the wing makes more lift, but it does resist downwash hence reducing induced flow, reducing induced drag, and for our helo allowing a lower pitch angle for given alpha, bringing the lift vector closer to the vertical and reducing the power requirement.
I will produce diagrams if it helps.
Light photons behave according to probability, which is apparently a great explanation! To who?
PS I did not go to Sunday school for very long so again Nick you are mistaken.
The effect of the mirror image trailing vortices is to push the real trailing vortices further outwards, this has the effect of increasing the aspect ratio of the wing, reducing downwash, and induced drag hence producing the efficiency improvement we are all talking about. Also by effectively increasing the aspect ratio we also reduce the strength of the vortex, as previously mentioned.(Lift/unit span is proportional to vorticity and airspeed, so longer span same lift and airspeed give less vorticity)
There you go all explained without even mentioning pressure!
BUT air will continue in its state of rest or constant linear motion unless a force is applied to it. Pressure is the force that makes air deviate around an aerofoil, if we are to change this flow field we must also change the pressure distribution. So despite vortices being able to explain ground effect, there must be pressure changes if there is to be a different flow pattern. So I say the pressure directly under a wing is increased by ground effect. I am not saying that this pressure on the underside of the wing makes more lift, but it does resist downwash hence reducing induced flow, reducing induced drag, and for our helo allowing a lower pitch angle for given alpha, bringing the lift vector closer to the vertical and reducing the power requirement.
I will produce diagrams if it helps.
Light photons behave according to probability, which is apparently a great explanation! To who?
PS I did not go to Sunday school for very long so again Nick you are mistaken.
Last edited by DeltaFree; 22nd Nov 2002 at 19:46.
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Perhaps the earth DOES repel ugly helicopters..........
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Autorotation and Ground-Effect
I am not a rotor head, sorry, but I have been reading this thread with interest and there seems to be aerodynamic misconceptions creeping in.
There will be an increase in ground effect during an autoration landing over a powered landing due to the fact that with the former you presumably have a higher rate of descent until just prior to touchdown and therefore require more lift to both support the weight of the helicopter and reduce its vertical speed therefore increasing the pressure below each rotor blade. The proximity of the ground affects the ability of that pressure to escape.
A rotor blade is just a wing and behaves in the same way. It generates lift by a combination of (positive) angle of attack and relative airspeed so in powered flight and autorotation the relative airflow must be coming from below the cord line producing the usual decrease in pressure over the top surface, increase in pressure on the lower surface and trailing edge downwash. In powered flight the rotor blade (wing) is moved forward by a force applied to the root in just the same way as a powered aircraft with a fuselage mounted engine. In autorotation the rotor blade (wing) is allowed to glide in the same way as a gliders wing and keeps moving for exactly the same reason that a glider keeps moving.
There will be an increase in ground effect during an autoration landing over a powered landing due to the fact that with the former you presumably have a higher rate of descent until just prior to touchdown and therefore require more lift to both support the weight of the helicopter and reduce its vertical speed therefore increasing the pressure below each rotor blade. The proximity of the ground affects the ability of that pressure to escape.
A rotor blade is just a wing and behaves in the same way. It generates lift by a combination of (positive) angle of attack and relative airspeed so in powered flight and autorotation the relative airflow must be coming from below the cord line producing the usual decrease in pressure over the top surface, increase in pressure on the lower surface and trailing edge downwash. In powered flight the rotor blade (wing) is moved forward by a force applied to the root in just the same way as a powered aircraft with a fuselage mounted engine. In autorotation the rotor blade (wing) is allowed to glide in the same way as a gliders wing and keeps moving for exactly the same reason that a glider keeps moving.
Last edited by Nostone; 24th Nov 2002 at 20:56.
Sorry Nostone - you used the word 'pressure' -therefore half the audience aren't listening anymore
PS Nostone - a rotor blade in autorotation keeps moving because a section of the blade is not only producing lift but a total reaction that is forward of the rotational axis (ie a driving force without which the blades would slow down and stop).
PS Nostone - a rotor blade in autorotation keeps moving because a section of the blade is not only producing lift but a total reaction that is forward of the rotational axis (ie a driving force without which the blades would slow down and stop).
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Autorotation and Ground-Effect
Hi Crab,
Ref. your PS. Yes in just the same way as a glider keeps forward speed, as it climbs in a thermal for example. I just think it is easier to visulise than lots of whirling maths.
Ref. your PS. Yes in just the same way as a glider keeps forward speed, as it climbs in a thermal for example. I just think it is easier to visulise than lots of whirling maths.