Autorotation and Ground-Effect
OffshoreIgor 
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Oh, Dave. You are Lu in disguise. What does that site you posted have to do with the question of how ground effect works?
I feel like those who debated with the Flat Earth Society back in the 1800's. The Flat Earthers were a group who argued that the earh is really flat. No matter what evidence was presented, they simply ignored it and went on with their assertions.
I guess Dave you are President of the Ground Pressure Society.
http://www.alaska.net/~clund/e_djubl...rthsociety.htm
I feel like those who debated with the Flat Earth Society back in the 1800's. The Flat Earthers were a group who argued that the earh is really flat. No matter what evidence was presented, they simply ignored it and went on with their assertions.
I guess Dave you are President of the Ground Pressure Society.
http://www.alaska.net/~clund/e_djubl...rthsociety.htm
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Nick,
This thread was initially about 'Autorotation and Ground-Effect'. As part of this discussion, the 'effects' of the ground were considered. This thread then moved on to consider the 'causes' of ground effect.
There is obviously a distinction between 'Cause' and 'Effect'.
I politely suggest that;
*~ Your mentioning of; reduced angle of attack, reduced drag, and reduced power, are actually part of the 'Effect'. They are not part of the 'Cause'.
* ~ You deny that pressure is a 'Cause' of ground effect. I submitted the previous web page because it very clearly presents the relationship that exists between an airfoil and pressure, and by extension between, an airfoil, pressure and the ground.
____________________
There is probably no definitive answer, and as you say, this thread may have run its course, but, if you wish to continue, it will be appreciated if you attack the subject and not the person.
Yours truly;
Lu
oops Dave J.
This thread was initially about 'Autorotation and Ground-Effect'. As part of this discussion, the 'effects' of the ground were considered. This thread then moved on to consider the 'causes' of ground effect.
There is obviously a distinction between 'Cause' and 'Effect'.
I politely suggest that;
*~ Your mentioning of; reduced angle of attack, reduced drag, and reduced power, are actually part of the 'Effect'. They are not part of the 'Cause'.
* ~ You deny that pressure is a 'Cause' of ground effect. I submitted the previous web page because it very clearly presents the relationship that exists between an airfoil and pressure, and by extension between, an airfoil, pressure and the ground.
____________________
There is probably no definitive answer, and as you say, this thread may have run its course, but, if you wish to continue, it will be appreciated if you attack the subject and not the person.
Yours truly;
Lu
oops Dave J.
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im sick of this post too
NICK
you said > The issue that all the pressure fans miss is that there is more power efficiency, not more lift.(the power required reduces because the extra lift is extinguished by reducing collective.) This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.
why are'nt blades made to produce this changed direction of flow and have the extra efficiency all the time??
DeltaFree, spot on!
you said > The issue that all the pressure fans miss is that there is more power efficiency, not more lift.(the power required reduces because the extra lift is extinguished by reducing collective.) This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.
why are'nt blades made to produce this changed direction of flow and have the extra efficiency all the time??
DeltaFree, spot on!
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Simulations show increased pressure
I found a couple of computer images that seem to support the idea that pressure does increase under the airfoil in ground effect quite dramatically.
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.
Here's another simulation showing the air pressure gradients as a wing travels over uneven terrain. As the terrain rises the ground effect is increased and pressure builds as indicated by the color chart.
-Chiplight
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.
Here's another simulation showing the air pressure gradients as a wing travels over uneven terrain. As the terrain rises the ground effect is increased and pressure builds as indicated by the color chart.
-Chiplight
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Chiplight,
Take care with your interpretation, the two plots at the top do not show the same case, and do not prove the erroneous assumption that ground effect raises the pressure under the helicopter. Those plots (which are quite accurate) do not show the difference between IGE and OGE with the same angle of attack. Note that the deep orange lower plot has more lift, and higher angle of attack than the upper plot, that is why the pressure is higher. In other words, the upper plot shows the greater pressure with ground effect because the lift is greater, since the CHANGE IN CIRCULATION DUE TO THE GROUND contributes to the airfoil's lift. This would cause the helicopter to climb, so the collective would be reduced, equalizing the pressure.
In other words, the pressure under the airfoil, and under the helicopter, is the same in ground effect and out of ground effect, there is NO ground cushion of high pressure air.
The plots do show the circulation that the ground induces, and therefore the way the ground allows you to achieve the same lift with lower collective. Those plots show WHY IGE needs lower angle of attack to achieve the same lift, and that it is not pressure that allows hover IGE to be done at lower power.
Take care with your interpretation, the two plots at the top do not show the same case, and do not prove the erroneous assumption that ground effect raises the pressure under the helicopter. Those plots (which are quite accurate) do not show the difference between IGE and OGE with the same angle of attack. Note that the deep orange lower plot has more lift, and higher angle of attack than the upper plot, that is why the pressure is higher. In other words, the upper plot shows the greater pressure with ground effect because the lift is greater, since the CHANGE IN CIRCULATION DUE TO THE GROUND contributes to the airfoil's lift. This would cause the helicopter to climb, so the collective would be reduced, equalizing the pressure.
In other words, the pressure under the airfoil, and under the helicopter, is the same in ground effect and out of ground effect, there is NO ground cushion of high pressure air.
The plots do show the circulation that the ground induces, and therefore the way the ground allows you to achieve the same lift with lower collective. Those plots show WHY IGE needs lower angle of attack to achieve the same lift, and that it is not pressure that allows hover IGE to be done at lower power.
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Equivalent theories?
Nick,
it would be nice to have an animation showing the case where the pilot adjusts collective to hold lift constant; then the pressure increase may drop out of the equation so to speak.
Nevertheless, it is obvious that if nothing is done to lower collective, and we fly into ground effect that the 'reds" will light up the plot as above, leading one to ponder cause and effect.
I'm not any kind of expert, but could it be that there is some kind of equivalence between "pressure" based explanations and momentum based explanations of ground effect? I'm sure this has been mentioned in the thread before, but I've not read all 10 pages of it.(!)
-Chiplight
it would be nice to have an animation showing the case where the pilot adjusts collective to hold lift constant; then the pressure increase may drop out of the equation so to speak.
Nevertheless, it is obvious that if nothing is done to lower collective, and we fly into ground effect that the 'reds" will light up the plot as above, leading one to ponder cause and effect.
I'm not any kind of expert, but could it be that there is some kind of equivalence between "pressure" based explanations and momentum based explanations of ground effect? I'm sure this has been mentioned in the thread before, but I've not read all 10 pages of it.(!)
-Chiplight
Last edited by Chiplight; 3rd Jan 2003 at 03:00.
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Chiplight,
The answer is obvious. The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE. There is no ground cushion. Read your altimeter in a hover if you have any doubt about this. There is no "ground cushion" of any kind.
Nick
The answer is obvious. The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE. There is no ground cushion. Read your altimeter in a hover if you have any doubt about this. There is no "ground cushion" of any kind.
Nick
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Angle of Attack and Angle of Collective Pitch
Perhaps we are all envisioning the same thing, but in a different way.
Nick said; "The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE."
Assuming that the helicopter is hovering in both of the above situations, I don't think that anyone disagrees with this statement. Nick has also said that the angle of attack will be the same in both situations and nobody probably disagrees with this, either. The thrust will also be the same.
What is different between hovering OGE and IGE is the angle of the collective pitch. In other words, the effect of the ground is that of allowing the helicopter to hover with a reduced collective pitch (plus a reduced rotor drag and a reduced power).
The question is; 'What is the cause of this effect'?
Pressure is a potential &/or partial answer.
Nick said; "The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE."
Assuming that the helicopter is hovering in both of the above situations, I don't think that anyone disagrees with this statement. Nick has also said that the angle of attack will be the same in both situations and nobody probably disagrees with this, either. The thrust will also be the same.
What is different between hovering OGE and IGE is the angle of the collective pitch. In other words, the effect of the ground is that of allowing the helicopter to hover with a reduced collective pitch (plus a reduced rotor drag and a reduced power).
The question is; 'What is the cause of this effect'?
Pressure is a potential &/or partial answer.
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Dave,
The cause of this is the fact that the ground suppresses the outflow, and allows the blade to deliver the same lift with less angle of collective. This is a bent flow discussion, not a pressure discussion. Note in the plot that Chiplight provides shows the equivilent circulation that the ground "provides" that helps explain the change in flow.
The cause of this is the fact that the ground suppresses the outflow, and allows the blade to deliver the same lift with less angle of collective. This is a bent flow discussion, not a pressure discussion. Note in the plot that Chiplight provides shows the equivilent circulation that the ground "provides" that helps explain the change in flow.
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Cushion: From the French. A soft place to rest or to provide support.
This will raise some eyebrows or increase someone’s blood pressure. A while back I made a comment to Nick Lappos indicating that the field of engineering is in a constant state of change. I further stated that sometimes the new kid on the block would change the rules and redefine a problem in opposition to the facts as taught to the older guys. I don’t dispute what Nick is saying in his explanation. But, I also feel that based on what is taught to most helicopter pilots and this includes the older guys with no engineering training there is such a thing as a ground cushion and by implication the cushion exists due to an increase in the localized pressure field. It takes a long time to reshape a persons perception when it comes to learning a new theory while discarding the old theory. There is a possibility that new helicopter pilots will get Nicks explanation in their POF courses but until the last of the old breed dies off they will still believe in a “Ground Cushion” and an increase in pressure.
Even the FAA Helicopter book is in support of Nicks' theory.
Even the FAA Helicopter book is in support of Nicks' theory.
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BENT FLOW?
This is a bent flow?
Nick,
You refuse to accept that pressure has anything to do with ground effect, you have slagged me and my arguments several times without actually giving anything of substance to prove pressure advocates wrong, other than wonderful phrases that do not explain any physical behaviours. Now you tell us it is not pressure it is bent flow.......OK OK Please now tell me in words of one syllable, so we can all follow your your extraordinary logic, just how does the flow bend without pressure differentials?
PS I really thought my last post was quite reasonable. You say yourself as we approach the ground lift increases due to greater efficiency, and we are able to reduce collective to maintain the same lift. So now you tell me a little red demon produces this extra lift not an increase in pressure! I thought even your logic had already stated extra lift equals extra pressure, OK you said equal lift means equal pressure, surely that is the same thing.
PPS I am trying not to get personal but would accept a sound argument in preference to references to Sunday school attendance, how good a doctor my mum is, how great my first instructor was or even little red demons. Come on Nick real physics not my imaginary life please.
Nick,
You refuse to accept that pressure has anything to do with ground effect, you have slagged me and my arguments several times without actually giving anything of substance to prove pressure advocates wrong, other than wonderful phrases that do not explain any physical behaviours. Now you tell us it is not pressure it is bent flow.......OK OK Please now tell me in words of one syllable, so we can all follow your your extraordinary logic, just how does the flow bend without pressure differentials?
PS I really thought my last post was quite reasonable. You say yourself as we approach the ground lift increases due to greater efficiency, and we are able to reduce collective to maintain the same lift. So now you tell me a little red demon produces this extra lift not an increase in pressure! I thought even your logic had already stated extra lift equals extra pressure, OK you said equal lift means equal pressure, surely that is the same thing.
PPS I am trying not to get personal but would accept a sound argument in preference to references to Sunday school attendance, how good a doctor my mum is, how great my first instructor was or even little red demons. Come on Nick real physics not my imaginary life please.
