Autorotation and Ground-Effect
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Nick,
This thread was initially about the effect of the ground on autorotation. In an attempt to answer this, the question of what is Ground Effect arose. It appears to be a valid one. Even Leishman, University of Maryland, says "The [ground] effect has long been recognized but the aerodynamics are still not fully understood."
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Here is a description from the Internet, which represents your position, I think.
Ground effect
Another common phenomenon that is misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by 50% just before it touches down. There is a great deal of confusion about ground effect. Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.
To understand ground effect it is necessary to have an understanding of upwash. For the pressures involved in low speed flight, air is considered to be non-compressible. When the air is accelerated over the top of the wing and down, it must be replaced. So some air must shift around the wing (below and forward, and then up) to compensate, similar to the flow of water around a canoe paddle when rowing. This is the cause of upwash.
As stated earlier, upwash is accelerating air in the wrong direction for lift. Thus a greater amount of downwash is necessary to compensate for the upwash as well as to provide the necessary lift. Thus more work is done and more power required. Near the ground the upwash is reduced because the ground inhibits the circulation of the air under the wing. So less downwash is necessary to provide the lift. The angle of attack is reduced and so is the induced power, making the wing more efficient.
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Here is another statement by Bell, when describing their proposed Quad configuration.
.... with a ground effect cushion that provides a significant up-load on the airframe.
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Here's another.
The ground reduces the vortices coming off the rotor blade tips, reducing induced drag.
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& & & &
Dave J.
This thread was initially about the effect of the ground on autorotation. In an attempt to answer this, the question of what is Ground Effect arose. It appears to be a valid one. Even Leishman, University of Maryland, says "The [ground] effect has long been recognized but the aerodynamics are still not fully understood."
______________________
Here is a description from the Internet, which represents your position, I think.
Ground effect
Another common phenomenon that is misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by 50% just before it touches down. There is a great deal of confusion about ground effect. Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.
To understand ground effect it is necessary to have an understanding of upwash. For the pressures involved in low speed flight, air is considered to be non-compressible. When the air is accelerated over the top of the wing and down, it must be replaced. So some air must shift around the wing (below and forward, and then up) to compensate, similar to the flow of water around a canoe paddle when rowing. This is the cause of upwash.
As stated earlier, upwash is accelerating air in the wrong direction for lift. Thus a greater amount of downwash is necessary to compensate for the upwash as well as to provide the necessary lift. Thus more work is done and more power required. Near the ground the upwash is reduced because the ground inhibits the circulation of the air under the wing. So less downwash is necessary to provide the lift. The angle of attack is reduced and so is the induced power, making the wing more efficient.
_____________________
Here is another statement by Bell, when describing their proposed Quad configuration.
.... with a ground effect cushion that provides a significant up-load on the airframe.
_____________________
Here's another.
The ground reduces the vortices coming off the rotor blade tips, reducing induced drag.
_____________________
& & & & Dave J.
Last edited by Dave Jackson; 15th Nov 2002 at 06:03.
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That what I mean .....
If you hover over grass (tall one), you can see the effect of the downwash spread for some distance around the heli (in no wind condition).
I was taught that this is the ground effect - or cushion.
As long as it is there, in front of the heli (in forward flight - slow one) we are in ground effect.
I don’t remember seeing that on the ground while I was doing Auto's (as a student, it took me a while to get my eyes up front).
Have you seen it ?
I was taught that this is the ground effect - or cushion.
As long as it is there, in front of the heli (in forward flight - slow one) we are in ground effect.
I don’t remember seeing that on the ground while I was doing Auto's (as a student, it took me a while to get my eyes up front).
Have you seen it ?
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nick
im dissapointed you gave up without explaning what makes the induced flow reduce although you expained the effect of it on the aircraft verry well. i thought you might say that the velocity increases theorfore pressure decreases so there's no pressure under the wing.
you said
> The effect of the ground as we get closer is to reduce the angle needed by the airfoil.(BUT HOW?) This reduces the induced power (the power that overcomes the drag due to the high angle of attack).
> 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.
> What about the behavior of any aircraft makes you think that the machine pushes on anything, exerts any pressure, to fly? This whole construct is not even true in the simplest little experiment.
im not picking your posts to pieces, im just asking you to explain to me why you think the induced flow is reduced?
you said
> The effect of the ground as we get closer is to reduce the angle needed by the airfoil.(BUT HOW?) This reduces the induced power (the power that overcomes the drag due to the high angle of attack).
> 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.
> What about the behavior of any aircraft makes you think that the machine pushes on anything, exerts any pressure, to fly? This whole construct is not even true in the simplest little experiment.
im not picking your posts to pieces, im just asking you to explain to me why you think the induced flow is reduced?
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Thought I’d wade in with Wagtendonk’s two cents worth. (From the W.J.Wagtendonk’s book Principles of Helicopter Flight, which I used as my bible when I was doing my exams.)
W.J.W.’s answer mirrors Nick’s (or vice versa)
He specifically says the bubble of high pressure is an oversimplification but if it helps you remember the practicalities then “so be it”.
Induced flow is the rate of bulk movement of air downwards through the disc.
When there is nothing below the disc (eg. HOGE) the downwash can dissipate into the surrounding air below the disc and induced flow remains constant.
When there is a flat surface (eg the ground) below the disc the air flow is impeded and it can’t dissipate as rapidly. The flow through the disc is thus slowed because it can only flow in as fast as the air below can dissipate.
Wagtendonk doesn’t approach whether there really IS a higher pressure under the disc or not. I wonder has anybody actually got under the blades during HIGE and actually measured the pressure? Might be an interesting point.
Wagtendonk then gives a series of those airfoil diagrams with arrows going everywhere which translates decreased induced flow into 1. Increased Angle of Attack
2. Same Drag
3. Greater Rotor thrust
(I wish I knew how to draw and post the diagrams but I can’t)
If you wanted to hover (IGE) you must maintain a constant but now lower Angle of Attack (and thus the same Lift). You are able to reduce the pitch on the blades, decrease the drag and suddenly be able to hover with less effort from the engine.
W.J.W.’s answer mirrors Nick’s (or vice versa)
He specifically says the bubble of high pressure is an oversimplification but if it helps you remember the practicalities then “so be it”.
Induced flow is the rate of bulk movement of air downwards through the disc.
When there is nothing below the disc (eg. HOGE) the downwash can dissipate into the surrounding air below the disc and induced flow remains constant.
When there is a flat surface (eg the ground) below the disc the air flow is impeded and it can’t dissipate as rapidly. The flow through the disc is thus slowed because it can only flow in as fast as the air below can dissipate.
Wagtendonk doesn’t approach whether there really IS a higher pressure under the disc or not. I wonder has anybody actually got under the blades during HIGE and actually measured the pressure? Might be an interesting point.
Wagtendonk then gives a series of those airfoil diagrams with arrows going everywhere which translates decreased induced flow into 1. Increased Angle of Attack
2. Same Drag
3. Greater Rotor thrust
(I wish I knew how to draw and post the diagrams but I can’t)
If you wanted to hover (IGE) you must maintain a constant but now lower Angle of Attack (and thus the same Lift). You are able to reduce the pitch on the blades, decrease the drag and suddenly be able to hover with less effort from the engine.
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I have posted scans of Wagtendonk's section on Ground effect on my web site. Have a read at:
http://helipics.homestead.com/hige.html
http://helipics.homestead.com/hige.html
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This is an interesting thread for three very intriguing reasons:
1) The things we were taught (and that are propagated in the literature) are often simplifications and rules of thumb. They are not strictly correct, but serve as memory aids and physical analogs to help us understand complex situations.
Gyroscopic precession
Centrifugal Force
Ground Cushion
LTE
2) When we try to bend the simplifications to fit other situations, they often break down, and actually hurt our understanding.
3) many Pprune posts are attempts to straighten out theses broken rules of thumb
Thanks Roborider for helping out in this lively debate.
I posted my last wise-guy post to actually try and make someone research the question, and I am very glad it worked!
This thread is an example of how Pprune is great. Thanks, guys.
Nick
1) The things we were taught (and that are propagated in the literature) are often simplifications and rules of thumb. They are not strictly correct, but serve as memory aids and physical analogs to help us understand complex situations.
Gyroscopic precession
Centrifugal Force
Ground Cushion
LTE
2) When we try to bend the simplifications to fit other situations, they often break down, and actually hurt our understanding.
3) many Pprune posts are attempts to straighten out theses broken rules of thumb
Thanks Roborider for helping out in this lively debate.
I posted my last wise-guy post to actually try and make someone research the question, and I am very glad it worked!
This thread is an example of how Pprune is great. Thanks, guys.
Nick
Robborider, I'm afraid your wonderfully named Wagtendonk has given the standard explanation of ground effect that every student QHI can draw in his sleep. His disclaimer that the real explanation for ground effect is far more complex, just indicates that he doesn't know either.
No-one doubts that the AoA in an IGE hover is reduced by the effect of the ground slowing down the downwash - what is missing is a credible expanation for how it happens without being a function of a pressure increase .
In some mysterious way, despite the concept of divergent ducts giving a reduction in speed and an increase in pressure, we must believe that simply the presence of the ground slows down the downwash and reduces induced flow.
At least the explanation that Dave Jackson posted shows how the circulation theory of lift can give an explanation without using pressure or magic - unlike some of the posts here.
No-one doubts that the AoA in an IGE hover is reduced by the effect of the ground slowing down the downwash - what is missing is a credible expanation for how it happens without being a function of a pressure increase .
In some mysterious way, despite the concept of divergent ducts giving a reduction in speed and an increase in pressure, we must believe that simply the presence of the ground slows down the downwash and reduces induced flow.
At least the explanation that Dave Jackson posted shows how the circulation theory of lift can give an explanation without using pressure or magic - unlike some of the posts here.
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RobboRider;
Thanks for the scans. It looks like Wagtendonk may have 'blown it' from the start. His first sketch shows an expansion of the streamtube instead of a contraction.
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Crab,
What you are saying makes sense. If we assume that the ground IS the cause of Ground Effect, then we must question HOW the ground does this. The only 'connection' that I can see between the craft and the ground is the air. Air can transmit; heat, vibration and pressure. All of these entail an increased activity amongst the molecules of the air, within the area that is available to them. Heat creates pressure, vibration creates an oscillating, self-canceling pressure, and pressure is pressure.
In ground effect, at a given thrust, I suspect that the induce velocity and the induced drag are less because the angle of attack is less, not that the angle of attack is less because the induced drag is less. I think that a reduced angle of attack result in less "air must shift around the wing (below and forward, and then up).
IF pressure differentiation is the reason, then the "canoe paddle" analogy may be up the creek without a paddle.
Dave J
Thanks for the scans. It looks like Wagtendonk may have 'blown it' from the start. His first sketch shows an expansion of the streamtube instead of a contraction.
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Crab,
What you are saying makes sense. If we assume that the ground IS the cause of Ground Effect, then we must question HOW the ground does this. The only 'connection' that I can see between the craft and the ground is the air. Air can transmit; heat, vibration and pressure. All of these entail an increased activity amongst the molecules of the air, within the area that is available to them. Heat creates pressure, vibration creates an oscillating, self-canceling pressure, and pressure is pressure.
In ground effect, at a given thrust, I suspect that the induce velocity and the induced drag are less because the angle of attack is less, not that the angle of attack is less because the induced drag is less. I think that a reduced angle of attack result in less "air must shift around the wing (below and forward, and then up).
IF pressure differentiation is the reason, then the "canoe paddle" analogy may be up the creek without a paddle.
Dave J
Good grief DJ, I've actually read through your post without falling asleep for once
Funny how - meteorologically speaking, air that descends from the stratosphere and then diverges when hitting the earth's surface, is known as a High pressure system....
What is the difference then
Anything that restricts the flow of any gas or fluid must translate into back pressure. That's the bread and butter of it all. Other phenomena that exist in and around that basic concept go towards making aerodynamics the complex theory it is
I'm with crab and DJ on this one..........
Funny how - meteorologically speaking, air that descends from the stratosphere and then diverges when hitting the earth's surface, is known as a High pressure system....
What is the difference then
Anything that restricts the flow of any gas or fluid must translate into back pressure. That's the bread and butter of it all. Other phenomena that exist in and around that basic concept go towards making aerodynamics the complex theory it is
I'm with crab and DJ on this one..........
Avoid imitations
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A few thoughts.
Lift = a force acting at 90 degrees to the relative airflow. By definition, of course there is lift in autorotation.
If it wasn't POSSIBLE to complete an EOL without using collective every autogyro/gyrocopter flight would be it's first and last. I've done a collective-free EOL in a Whirlwind 10 but you DO need a good undercarriage (the groundcrew afterwards checked it and confirmed the Whirlwind u/c is a good one).
The key to how well a rotor system works is related to the amount of induced (downwards) flow. More induced flow = lower angle of attack = lower rotor efficiency.
Flare effect changes the direction of the relative airflow and reduces the induced flow, resulting in a reduced demand for power whilst increasing the angle of attack. It is, of course, a transitory effect that is taken advantage of during the penultimate stage of an EOL. The last stage is normally the application of collective to gain effective rotor thrust at the expense of Nr.
As far as the original question goes, YES there probably is some ground effect at the bottom of an EOL but as the aircraft is in ground effect suddenly and briefly, the effect is unnoticeable or hidden.
Lift = a force acting at 90 degrees to the relative airflow. By definition, of course there is lift in autorotation.
If it wasn't POSSIBLE to complete an EOL without using collective every autogyro/gyrocopter flight would be it's first and last. I've done a collective-free EOL in a Whirlwind 10 but you DO need a good undercarriage (the groundcrew afterwards checked it and confirmed the Whirlwind u/c is a good one).
The key to how well a rotor system works is related to the amount of induced (downwards) flow. More induced flow = lower angle of attack = lower rotor efficiency.
Flare effect changes the direction of the relative airflow and reduces the induced flow, resulting in a reduced demand for power whilst increasing the angle of attack. It is, of course, a transitory effect that is taken advantage of during the penultimate stage of an EOL. The last stage is normally the application of collective to gain effective rotor thrust at the expense of Nr.
As far as the original question goes, YES there probably is some ground effect at the bottom of an EOL but as the aircraft is in ground effect suddenly and briefly, the effect is unnoticeable or hidden.
PPF#1,
Sorry it has taken me three pages to reply to your post - those pesky bush fires are burning again!!
You, "sport" (as you like to say) are a crack up.
You totally (and condescendingly I might add) disagreed with my statement that :
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.
BUT then to proove me wrong you end your explanation with:
"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".
Good one. Do you know what potential energy is? Is this potentially condescending?
You go ahead and imagine that a rotor disc behaves as a big wing - just spare any of your junior pilots/students from the same affliction.
You also say:
"Think about it aerodynamically. The *only* way to accomplish this level-off is if we have a surplus of lift. It must be thus."
Alright - I have thought about this aerodynamically,and..No. There is no surplus of lift in this situation. Refer to my post to you re "creating lift". There is only potential energy. Potentially.
but all this is potentially off thread......
Sorry it has taken me three pages to reply to your post - those pesky bush fires are burning again!!
You, "sport" (as you like to say) are a crack up.
You totally (and condescendingly I might add) disagreed with my statement that :
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.
BUT then to proove me wrong you end your explanation with:
"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".
Good one. Do you know what potential energy is? Is this potentially condescending?
You go ahead and imagine that a rotor disc behaves as a big wing - just spare any of your junior pilots/students from the same affliction.
You also say:
"Think about it aerodynamically. The *only* way to accomplish this level-off is if we have a surplus of lift. It must be thus."
Alright - I have thought about this aerodynamically,and..No. There is no surplus of lift in this situation. Refer to my post to you re "creating lift". There is only potential energy. Potentially.
but all this is potentially off thread......
Oh dear. It seems I have offended the sensitive Helmut. If so, I apologise, sir! I apologise if your understanding of aerodynamics is so rudimentary that you can't keep up, and I'll try to be less condescending to your sort in the future.
Let's try this. In a glide, if you wish to decrease your rate of descent, your lift vector *must* become bigger than the combination of your drag and weight vectors, hmm? That is, unless you've cleverly figured out how to magically decrease either your drag or weight. This is not rocket science, old chap. But if you disagree, please explain to the class how a rate of descent can be slowed, or a rate of climb begun *without* excess, surplus.....or MORE lift than your wing was already producing.
Let's try this. In a glide, if you wish to decrease your rate of descent, your lift vector *must* become bigger than the combination of your drag and weight vectors, hmm? That is, unless you've cleverly figured out how to magically decrease either your drag or weight. This is not rocket science, old chap. But if you disagree, please explain to the class how a rate of descent can be slowed, or a rate of climb begun *without* excess, surplus.....or MORE lift than your wing was already producing.
PPF#1, which drag vector are you talking about - the parasite drag from the airframe and rotor system or the induced drag produced by the wing/blade as it displaces the air? If you are going to belittle other peoples aerodynamic knowledge you really need to be more precise with your own.
If your rotor thrust (the vertical component of the Total Reaction) is greater than the aircraft weight then the RoD will reduce if you are descending or a climb will commence if you are not.
A reduction in induced flow not only gives a higher AoA (therefore more Cl and Cd) but also moves the TR vector towards the axis of rotation of the rotor giving an increase in rotor thrust and a decrease in rotor drag.
ShyTq whilst I have to admit is is possible to do a cyclic only EOL in favourable conditions in a helicopter with low disc loading - modern helicopters (and I discount the Whirlwind here) have much higher disc loadings and far less benign autorotative characteristics. No doubt Nick will have flown a test aircraft which has a high disc loading and does collective- free EOLS but he is expert at shooting holes in my arguments.
If your rotor thrust (the vertical component of the Total Reaction) is greater than the aircraft weight then the RoD will reduce if you are descending or a climb will commence if you are not.
A reduction in induced flow not only gives a higher AoA (therefore more Cl and Cd) but also moves the TR vector towards the axis of rotation of the rotor giving an increase in rotor thrust and a decrease in rotor drag.
ShyTq whilst I have to admit is is possible to do a cyclic only EOL in favourable conditions in a helicopter with low disc loading - modern helicopters (and I discount the Whirlwind here) have much higher disc loadings and far less benign autorotative characteristics. No doubt Nick will have flown a test aircraft which has a high disc loading and does collective- free EOLS but he is expert at shooting holes in my arguments.
Crab:
Precisely! And that is my sole point. An autorotating rotor still has a vertical thrust component. "Vertical component of the Total Reaction" = LIFT.
You chaps are overcomplicating this bit. Nevermind your vector diagrams and formulae. You said it, Crab! In very simple terms, to slow down a descent or to start a climb, the upward force (lift) must exceed the downward force (gravity). Hence, in a steady-state descent, to slow the RoD you need to grab some more lift from somewhere.
Many illustrations of helicopters in forward flight depict a downward flow of air through both the front and rear parts of the rotor. If you've flown for any length of time you probably intuitively know that this is incorrect. In wind tunnel footage that I have seen, smoke blowing at a translated rotor goes right up and over the front part of the disk and does not come down through it (transverse flow, anyone?) until just about the point that it passes the mast. As long as the rotor is through ETL this will be the case, including during autorotation.
The misconception that an autorotating rotor "sees" a predominantly vertical column of air throughout the entire disk is just that, a misconception. Anyone is free to disagree. But some of you lot need to read more. Start with Ray Prouty. I do not make a habit of arguing with him.
If your rotor thrust (the vertical component of the Total Reaction) is greater than the aircraft weight then the RoD will reduce if you are descending or a climb will commence if you are not.
You chaps are overcomplicating this bit. Nevermind your vector diagrams and formulae. You said it, Crab! In very simple terms, to slow down a descent or to start a climb, the upward force (lift) must exceed the downward force (gravity). Hence, in a steady-state descent, to slow the RoD you need to grab some more lift from somewhere.
Many illustrations of helicopters in forward flight depict a downward flow of air through both the front and rear parts of the rotor. If you've flown for any length of time you probably intuitively know that this is incorrect. In wind tunnel footage that I have seen, smoke blowing at a translated rotor goes right up and over the front part of the disk and does not come down through it (transverse flow, anyone?) until just about the point that it passes the mast. As long as the rotor is through ETL this will be the case, including during autorotation.
The misconception that an autorotating rotor "sees" a predominantly vertical column of air throughout the entire disk is just that, a misconception. Anyone is free to disagree. But some of you lot need to read more. Start with Ray Prouty. I do not make a habit of arguing with him.
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PPRUNE FAN#1
you said > Many illustrations of helicopters in forward flight depict a downward flow of air through both the front and rear parts of the rotor. If you've flown for any length of time you probably intuitively know that this is incorrect. In wind tunnel footage that I have seen, smoke blowing at a translated rotor goes right up and over the front part of the disk and does not come down through it (transverse flow, anyone?)
this must be just in translation, inflow roll (transverse flow effect) happens because the front most part of the disc is cutting clean air, not paddeling in a vortex which would cause the smoke to go over the front of the disk.
http://www.dynamicflight.com/aerodyn...lational_lift/
http://www.dynamicflight.com/aerodyn...erse_flow_eff/
you said > 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).
pulling back on the cyclic does produce lift! because some of the total rotor thrust that was being used for overcoming parasidic drag (forward flight) now is producing lift aswell. aslo pulling back radicaly reduces the induced flow (instead of the air coming in from the top, it now wants to come in from underneith) so angle of attack is incraesed. all this lift on the blades causes them to cone, aswell as autorotative force increasing, so the rrpm will increase. i would say airspeed is traded for total rotor thrust and rrpm.
this must be just in translation, inflow roll (transverse flow effect) happens because the front most part of the disc is cutting clean air, not paddeling in a vortex which would cause the smoke to go over the front of the disk.
http://www.dynamicflight.com/aerodyn...lational_lift/
http://www.dynamicflight.com/aerodyn...erse_flow_eff/
you said > 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).
pulling back on the cyclic does produce lift! because some of the total rotor thrust that was being used for overcoming parasidic drag (forward flight) now is producing lift aswell. aslo pulling back radicaly reduces the induced flow (instead of the air coming in from the top, it now wants to come in from underneith) so angle of attack is incraesed. all this lift on the blades causes them to cone, aswell as autorotative force increasing, so the rrpm will increase. i would say airspeed is traded for total rotor thrust and rrpm.
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More of the Same
I really cannot see what is so difficult about this increased pressure business.
In a hover OGE air flows freely downward slowly dissipating as it goes, ultimately spreading out and going back up to replace the air which is still being drawn down thro the rotor. This takes the form of a very large doughnut vortex. We all know about the strong bit at the rotor tip but in a prolonged steady state this vortex has a reducing effect out to some considerable distance.
Now closer to the ground the downward flow is stopped at the surface. In order to stop motion a force must be present, or in this case a pressure increase. This pressure resists the downward flow of air so reducing the induced flow. For a given angle of attack/lift, this means lower pitch, more vertical lift vector and reduced rotor drag.
It is true to say the pressure under the rotor does not add lift, so NL is right in saying that. But the increased pressure is the mechanism by which the induced flow is reduced so it does reduce the power required.
Any lift produced by a heavier than air machine is ultimately supported by a higher pressure on the ground surface. Remember the old school days question....
A van weighing 1 ton has 1 ton of budgies sat on perches in the back. It weighs 2 ton. All the budgies takeoff and fly around inside the van, how much does the van weigh now?
Yes 2 ton! The birds are now supporting their weight with air which must in turn be supported by what is underneath.
The difference between OGE and IGE is only how localised the increased surface pressure is. OGE the increased pressure is spread over a large area and hence has little feedback, IGE on the otherhand the effect is very localised and hence has a larger return effect.
I am sure this is really quite a simple process and hope it seems so to you all. If I am missing something let me know.
In a hover OGE air flows freely downward slowly dissipating as it goes, ultimately spreading out and going back up to replace the air which is still being drawn down thro the rotor. This takes the form of a very large doughnut vortex. We all know about the strong bit at the rotor tip but in a prolonged steady state this vortex has a reducing effect out to some considerable distance.
Now closer to the ground the downward flow is stopped at the surface. In order to stop motion a force must be present, or in this case a pressure increase. This pressure resists the downward flow of air so reducing the induced flow. For a given angle of attack/lift, this means lower pitch, more vertical lift vector and reduced rotor drag.
It is true to say the pressure under the rotor does not add lift, so NL is right in saying that. But the increased pressure is the mechanism by which the induced flow is reduced so it does reduce the power required.
Any lift produced by a heavier than air machine is ultimately supported by a higher pressure on the ground surface. Remember the old school days question....
A van weighing 1 ton has 1 ton of budgies sat on perches in the back. It weighs 2 ton. All the budgies takeoff and fly around inside the van, how much does the van weigh now?
Yes 2 ton! The birds are now supporting their weight with air which must in turn be supported by what is underneath.
The difference between OGE and IGE is only how localised the increased surface pressure is. OGE the increased pressure is spread over a large area and hence has little feedback, IGE on the otherhand the effect is very localised and hence has a larger return effect.
I am sure this is really quite a simple process and hope it seems so to you all. If I am missing something let me know.
Sorry to you too PooFan#1, I failed to grasp your appreciation of basic physics. Rather than disregard vector diagrams and formulae, perhaps they might help you in your obvious confusion about lift. No one has argued against lift having to exceed wieght to climb. Not in any of the posts. Nobody. Nada. Nyet.
So perhaps you might avoid continually restating the blindingly obvious to give the inference that you have adressed the questions raised? Or do you restate the simple to avoid addressing the complex?
Your wind tunnel footage is irrelevant to the discussion, and your perceptions/assumptions about it are wrong. Lu, are you there?
PS: To climb, lift has to exceed wieght.
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DeltaFree,
The whole pressure increase buisiness is interesting, it "seems" right, and it is not correct. This "mystery" was studied about 3/4 of a century ago, here is one reference that discusses the ability of ground proximity to reduce induced power. 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:
http://naca.larc.nasa.gov/reports/1928/naca-report-265/
The whole pressure increase buisiness is interesting, it "seems" right, and it is not correct. This "mystery" was studied about 3/4 of a century ago, here is one reference that discusses the ability of ground proximity to reduce induced power. 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:
http://naca.larc.nasa.gov/reports/1928/naca-report-265/
Nick, yet again you have eluded the question - we all know that ground effect exists, the link you provided is to a report that shows reduction in rpm and increase in Cl for an aircraft in ground effect.
It does not say how it happens, only that it does.
If it is not pressure related then what is it - all the reitieration that it is not pressure related is not verified by any other evidence to the contrary. You can tell someone the world is round (alright an oblate spheroid) as much as you like but without proof they are unlikely to believe you.
If you do not know then say so instead of treating us like errant schoolchildren who don't understand their teacher (because he hasn't explained something properly).
It does not say how it happens, only that it does.
If it is not pressure related then what is it - all the reitieration that it is not pressure related is not verified by any other evidence to the contrary. You can tell someone the world is round (alright an oblate spheroid) as much as you like but without proof they are unlikely to believe you.
If you do not know then say so instead of treating us like errant schoolchildren who don't understand their teacher (because he hasn't explained something properly).