Ground effect/Urban myth
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Crab, stick your hand out the car window when you're going 30mph and again at 60 mph. the speed increases, but you'll notice the pressure increases, too. The speed of the downwash being blocked by the ground or someone's lungs is what slows down the downwash and is also the source of the dynamic pressure increase.
There is a clear measurable pressure increase under a helicopter. Don't make the myth worse by disputing it. The accepted explanation ignores that pressure, because the pressure is an intermediate consideration. It's what changes speed/direction of the flow.
The problem is, to use that dynamic pressure increase to explain increased efficiency of the disk, you have to change everything else we're told about how the disk makes lift. Keep to the flow angles and speeds and everything is simplified and understandable.
Matthew.
There is a clear measurable pressure increase under a helicopter. Don't make the myth worse by disputing it. The accepted explanation ignores that pressure, because the pressure is an intermediate consideration. It's what changes speed/direction of the flow.
The problem is, to use that dynamic pressure increase to explain increased efficiency of the disk, you have to change everything else we're told about how the disk makes lift. Keep to the flow angles and speeds and everything is simplified and understandable.
Matthew.
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I love this block, we can go round and round it forever!
Some points:
1) I never said that ground effect was a myth, I said that the "pressure bubble" was a myth! Ground effect is very real, and amounts to a typical 15% power savings for most helicopters (where the rotor cannot get closer to the ground that about .3 radius.
2) Those who love the pressure bubble theory are welcome to it, believing myths is not against the law! However, ask yourself why ground effect disappears when you hover over long grass - and don't tell me that grass absorbs pressure! The grass slows the outflow, which is the mechanism that changes the blade angle of attack and reduces the hover power.
3) The pressure bubble theory cannot explain why ground effect only reduces the induced drag of the blades, if that theory is correct, more velocity makes more pressure, and thus saves more power. If the air velocity gets banged against the ground, and pressure builds, then a higher velocity should make even more pressure, and more ground effect, right? No, wrong! Maximum ground effect is about +15% power, regardless of how fast the downwash velocity is. In fact, ground effect is no different for highly loaded rotors, with faster downwash, than it is for low disk loading rotors with gentle downwash.
4) If pressure bubbles push the aircraft up, then high speed airplanes, with the wake hitting the ground hundreds of meters behind the aircraft, should see no ground effect. But they do! That is because the effect is on the wing, where the angle of the flow around the wing is changed by the presence of the ground.
5) Ground effect makes the wings or blades act as if they are longer, and this cuts the tip losses that make the induced drag. They have nothing to do with the pressure under the blade or wing, they have everything to do with the reduction in outflow, and the reduction in the tip vortex pattern due to that outflow.
Some points:
1) I never said that ground effect was a myth, I said that the "pressure bubble" was a myth! Ground effect is very real, and amounts to a typical 15% power savings for most helicopters (where the rotor cannot get closer to the ground that about .3 radius.
2) Those who love the pressure bubble theory are welcome to it, believing myths is not against the law! However, ask yourself why ground effect disappears when you hover over long grass - and don't tell me that grass absorbs pressure! The grass slows the outflow, which is the mechanism that changes the blade angle of attack and reduces the hover power.
3) The pressure bubble theory cannot explain why ground effect only reduces the induced drag of the blades, if that theory is correct, more velocity makes more pressure, and thus saves more power. If the air velocity gets banged against the ground, and pressure builds, then a higher velocity should make even more pressure, and more ground effect, right? No, wrong! Maximum ground effect is about +15% power, regardless of how fast the downwash velocity is. In fact, ground effect is no different for highly loaded rotors, with faster downwash, than it is for low disk loading rotors with gentle downwash.
4) If pressure bubbles push the aircraft up, then high speed airplanes, with the wake hitting the ground hundreds of meters behind the aircraft, should see no ground effect. But they do! That is because the effect is on the wing, where the angle of the flow around the wing is changed by the presence of the ground.
5) Ground effect makes the wings or blades act as if they are longer, and this cuts the tip losses that make the induced drag. They have nothing to do with the pressure under the blade or wing, they have everything to do with the reduction in outflow, and the reduction in the tip vortex pattern due to that outflow.
I was just thinking it's nice to see that rotary-wing people have their theological debate too. In the fixed wing world it is of course about what generates lift.
I'm all for a bit of theory myself (and in fact this thread has prompted me to buy myself a copy of "Helicopter Aerodynamics") but at the end of the day why does it actually matter? Ground effect is real; increased pressure under the rotor is also real, though it may only be incidentally part of the explanation for ground effect.
Incidentally it would be good if the theologians could agree on whether long grass increases or decreases ground effect - I've seen both in this thread.
n5296s
I'm all for a bit of theory myself (and in fact this thread has prompted me to buy myself a copy of "Helicopter Aerodynamics") but at the end of the day why does it actually matter? Ground effect is real; increased pressure under the rotor is also real, though it may only be incidentally part of the explanation for ground effect.
Incidentally it would be good if the theologians could agree on whether long grass increases or decreases ground effect - I've seen both in this thread.
n5296s
Matthew - I didn't say there was no pressure increase, just not enough to cause a problem with breathing and, since the action of breathing requires lower pressure inside the lungs than outside, higher pressure would help not hinder (eg pressure breathing for FJ pilots).
As with the hand out the window example there must be a slight increase in pressure at the point of contact (a stagnation point like on an aerofoil leading edge) and I agree that the slight increase in pressure is what causes the deflection of airflow in all the cases we are considering here. The pressure rise cannot be great since, as you have said, the air is effectively incompressible and it is not contained (as in a pitot tube for example) so any rise in pressure tries to dissipate (just like highs filling lows in the atmosphere).
N5296s - it is my belief that the long grass scenario leads to an increase in the air recirculated at the tips of the blades, reducing the effectiveness of the blades due to increased tip losses and reducing the power benefits of ground effect
As with the hand out the window example there must be a slight increase in pressure at the point of contact (a stagnation point like on an aerofoil leading edge) and I agree that the slight increase in pressure is what causes the deflection of airflow in all the cases we are considering here. The pressure rise cannot be great since, as you have said, the air is effectively incompressible and it is not contained (as in a pitot tube for example) so any rise in pressure tries to dissipate (just like highs filling lows in the atmosphere).
N5296s - it is my belief that the long grass scenario leads to an increase in the air recirculated at the tips of the blades, reducing the effectiveness of the blades due to increased tip losses and reducing the power benefits of ground effect
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Aerodynamics 101 refresher.
Just to make sure we are all talking the same language. I've done some reading up... 
Flowfield: The aerodynamic system being considered.
Streamline: The path an air particle follows.
Total pressure: This is the pressure that would be measured if a streamline was brought to a halt. For this reason it is often called the stagnation pressure. This is measured by a pitot tube
Static pressure: This is the pressure measured if moving along with the air. This is measured by a static port.
Dynamic pressure: This is the pressure contribution from velocity^2. This is measured by a pitot-static tube.
Total Pressure = Static Pressure + Dynamic Pressure
Concepts important to helicopters:
The total pressure in a flow field does not need to be a constant, but the total pressure along a streamline must be a constant. A pitot-static tube only works if it is in the same streamline or if the total pressure is more or less constant.
For a helicopter rotor this means that the total pressure can vary under the disk. On average it must be higher than free stream total pressure or the helicopter would not produce lift. The rotor effectively pumps up the total pressure. The thrust produced by a rotor disk is equal to total pressure increase times area.
The presence of ground alters the flow field. This does not change the total pressure under the helicopter rotor, since the thrust remains constant. The ground effectively adds a reflection of the downwash to the flow. This means the rotor is effectively seeing an upwash, which means it works less hard to generate the same total pressure increase. The 3D flow is a little more complex, with downwash spread out near the ground, but the basics still hold.
So Nick and Matthew are both quite right: directly under the rotor there is no pressure bubble, but the induced velocity can be lower for the same total pressure. Near to the ground there will be a static pressure increase, but this comes as no suprise since we are converting dynamic to static pressure to keep the total pressure along each streamline constant.
Hope this helps, but i'll clarify if it doesn't.
Flowfield: The aerodynamic system being considered.
Streamline: The path an air particle follows.
Total pressure: This is the pressure that would be measured if a streamline was brought to a halt. For this reason it is often called the stagnation pressure. This is measured by a pitot tube
Static pressure: This is the pressure measured if moving along with the air. This is measured by a static port.
Dynamic pressure: This is the pressure contribution from velocity^2. This is measured by a pitot-static tube.
Total Pressure = Static Pressure + Dynamic Pressure
Concepts important to helicopters:
The total pressure in a flow field does not need to be a constant, but the total pressure along a streamline must be a constant. A pitot-static tube only works if it is in the same streamline or if the total pressure is more or less constant.
For a helicopter rotor this means that the total pressure can vary under the disk. On average it must be higher than free stream total pressure or the helicopter would not produce lift. The rotor effectively pumps up the total pressure. The thrust produced by a rotor disk is equal to total pressure increase times area.
The presence of ground alters the flow field. This does not change the total pressure under the helicopter rotor, since the thrust remains constant. The ground effectively adds a reflection of the downwash to the flow. This means the rotor is effectively seeing an upwash, which means it works less hard to generate the same total pressure increase. The 3D flow is a little more complex, with downwash spread out near the ground, but the basics still hold.
So Nick and Matthew are both quite right: directly under the rotor there is no pressure bubble, but the induced velocity can be lower for the same total pressure. Near to the ground there will be a static pressure increase, but this comes as no suprise since we are converting dynamic to static pressure to keep the total pressure along each streamline constant.
Hope this helps, but i'll clarify if it doesn't.
Last edited by Graviman; 22nd May 2009 at 17:55. Reason: General tidy up - i know some pilots appreciate this stuff.
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Graviman:
I think what Matthew is trying to get across is that the pressure increase is negligible and shouldn't be bothered with to explain the increase in lift.
How about we try a theoretic example: if the pressure increase has any effect, it must be measurable... Let's say a 10,000 lbs helicopters lift is increased by 15% in ground effect; doesn't that translate to that the increase in pressure must be able to support 1,500 lbs?
So basically we should be able to prove that the increase in air pressure is insignificant for the purpose of the ground effect argument, by computing the required increase in pressure to support that weight?
Or am I comparing apples and oranges here?
I think what Matthew is trying to get across is that the pressure increase is negligible and shouldn't be bothered with to explain the increase in lift.
How about we try a theoretic example: if the pressure increase has any effect, it must be measurable... Let's say a 10,000 lbs helicopters lift is increased by 15% in ground effect; doesn't that translate to that the increase in pressure must be able to support 1,500 lbs?
So basically we should be able to prove that the increase in air pressure is insignificant for the purpose of the ground effect argument, by computing the required increase in pressure to support that weight?
Or am I comparing apples and oranges here?
Last edited by Phil77; 20th May 2009 at 21:34.
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Phil,
This is similar to the discussion i had with Dave J. the other day...
The misconception here is the belief that a helicopter in flight is a natural state to be in. The engine(s) struggles uphill all the way to keep a helicopter airborne. Anything that reduces the power required helps win the stuggle.
When you are in ground effect there is no total pressure increase for the same MAUW. But the engine works less hard to generate the same total pressure. This means that you have some power in reserve for your extra 1'500lbs.
Ground effect alters the flow field but not the total pressure under the rotor. In the same way that a gentle descent (above VRS) also reduces the engine power for a given MAUW. Imagine a mirror image helicopter below the ground that is blowing its downdraught upwards to help keep you airborne. Near the ground you are effectively in a mild descent, so the collective goes down a little. But the weight of your heli hasn't changed therefore total pressure change across the rotor can't have changed.
The pressure only goes up (or altimeter down) because suddenly all of this downdraught is being redirected. So for the same total pressure, dynamic pressure goes down and static pressure goes up. Again this makes sense because with a mild descent (collective goes down a little) the induced velocity goes down. Same total pressure, less dynamic pressure gives you more static pressure.
The concepts here can be a little hard to grasp, but do work...
This is similar to the discussion i had with Dave J. the other day...
The misconception here is the belief that a helicopter in flight is a natural state to be in. The engine(s) struggles uphill all the way to keep a helicopter airborne. Anything that reduces the power required helps win the stuggle.
When you are in ground effect there is no total pressure increase for the same MAUW. But the engine works less hard to generate the same total pressure. This means that you have some power in reserve for your extra 1'500lbs.
Ground effect alters the flow field but not the total pressure under the rotor. In the same way that a gentle descent (above VRS) also reduces the engine power for a given MAUW. Imagine a mirror image helicopter below the ground that is blowing its downdraught upwards to help keep you airborne. Near the ground you are effectively in a mild descent, so the collective goes down a little. But the weight of your heli hasn't changed therefore total pressure change across the rotor can't have changed.
The pressure only goes up (or altimeter down) because suddenly all of this downdraught is being redirected. So for the same total pressure, dynamic pressure goes down and static pressure goes up. Again this makes sense because with a mild descent (collective goes down a little) the induced velocity goes down. Same total pressure, less dynamic pressure gives you more static pressure.
The concepts here can be a little hard to grasp, but do work...
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There is intellectual property and there are practical uses of intellectual property. There are laws of physics and there are practical applications of the laws of physics. You do not need to have a detailed understanding of the underlying physics to understand how to apply them in appropriate circumstances.
So as a student or an operator I would be more concerned about the ramifications of ground effect versus the technical debate of how it happens.
Pilots do not typically have Phd's in physics. (although I do know of one) Teaching a pilot the physics as opposed to teaching the ramifications seems silly. Even if we all understood the physics, how should we apply the physics? Trial and error? Not a good idea.
What a student should know is why ground effect matters and how it is applied to power requirements in all situations, environments and weather conditions. That would be the appropriate use of instruction. The US requirements is a practical test and I would much rather have a DPE ask questions about the applications of power requirements and ground effect as opposed to the physics of ground effect. I want the pilot to know when he is going to run out of power and break my machine or hurt someone. Understanding detailed physics can be helpful, but is in no way necessary.
I spouted off the physics to my DPE and he could not care less. He cared if I knew how to apply it in practical situations.
So as a student or an operator I would be more concerned about the ramifications of ground effect versus the technical debate of how it happens.
Pilots do not typically have Phd's in physics. (although I do know of one) Teaching a pilot the physics as opposed to teaching the ramifications seems silly. Even if we all understood the physics, how should we apply the physics? Trial and error? Not a good idea.
What a student should know is why ground effect matters and how it is applied to power requirements in all situations, environments and weather conditions. That would be the appropriate use of instruction. The US requirements is a practical test and I would much rather have a DPE ask questions about the applications of power requirements and ground effect as opposed to the physics of ground effect. I want the pilot to know when he is going to run out of power and break my machine or hurt someone. Understanding detailed physics can be helpful, but is in no way necessary.
I spouted off the physics to my DPE and he could not care less. He cared if I knew how to apply it in practical situations.
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This is similar to the discussion i had with Dave J. the other day...
Thanks for the explanation! It does make sense... although that means that my idea of a simplified DISPROVE of the pressure bubble doesn't work then
I just figured it would be nice to be able to say: "Hey, the increase/redistribution in pressure is not enough to support even half the additional 1,500lbs or 15% less torgue!"
Diethelm: agreed with your sentiment that it is not important for the initial student to know what precisely happens in aerodynamics; its probably waaay over his head.
But let me explain to you why it is important for me to know more:
I am a person who needs a practical application for all mathematical/physical problems to understand the process. I sucked in school because nobody could/wanted to give me practical, interesting example why I needed to know all that BS with tangents ending nowhere, hypotenuses and adjacent legs and so forth.
Ever since I started flying I realized how fascinating and useful all that information can be, to understand and explain the complex interrelationships - even for practical applications: vortex ring state, settling with power, front/back side of the power curve, autorotation, high altitude flying (in Jets) etc.
There is more to it when you learn to fly than to accept that it is what it is, no explanation necessary - but I do understand that that is not your intention either and I agree, first and foremost you need to know the practical application.
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Phil,
Fair enough. You are absolutely correct that we should not simply use blind trust for if we did, leaches would still be the cure for most that ails us.
But a practical example......
A business man and an engineer are standing in the hallway 15 feet from a rather attractive nude female. A math professor explains that each individual can proceed 1/2 the remaining distance until they are satisfied.
The engineer turns and walks away yet the business man remains. The teacher asks the engineer why he is leaving. He explains that given the rules, he will never reach the beautiful young damsel and as such he can by the laws of math never be satisfied.
Then the professor looks at the business man with great disdain and asks why do you stay? The engineer is correct!
The business man looks the teacher in the eye and states.....
"In theory I may never get there but for practical purposes I will get close enough"
Fair enough. You are absolutely correct that we should not simply use blind trust for if we did, leaches would still be the cure for most that ails us.
But a practical example......
A business man and an engineer are standing in the hallway 15 feet from a rather attractive nude female. A math professor explains that each individual can proceed 1/2 the remaining distance until they are satisfied.
The engineer turns and walks away yet the business man remains. The teacher asks the engineer why he is leaving. He explains that given the rules, he will never reach the beautiful young damsel and as such he can by the laws of math never be satisfied.
Then the professor looks at the business man with great disdain and asks why do you stay? The engineer is correct!
The business man looks the teacher in the eye and states.....
"In theory I may never get there but for practical purposes I will get close enough"
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diethelm,
That is one of my favourite jokes, but the version i tell is the physicist and the engineer!
I am trying to explain ground effect to those that do want to understand a little better. I'm only asking for three concepts to be grasped:
total pressure
dynamic pressure
static pressure
Why bother with an altimeter when you can guestimate the height? Why bother mentioning exact quantities of fuel remaining? Why worry about whether NR is at 97%? A little understanding can go a long way.
By me taking the time to give an exact explanation, it will save this debate going round and round for the next ten years. It may improve split second judgement when figuring if grass will work as well as well as water for ground effect - in grass the reflected helicopter is a little blury.
Does that get close enough for all practical purposes?
That is one of my favourite jokes, but the version i tell is the physicist and the engineer!
I am trying to explain ground effect to those that do want to understand a little better. I'm only asking for three concepts to be grasped:
total pressure
dynamic pressure
static pressure
Why bother with an altimeter when you can guestimate the height? Why bother mentioning exact quantities of fuel remaining? Why worry about whether NR is at 97%? A little understanding can go a long way.
By me taking the time to give an exact explanation, it will save this debate going round and round for the next ten years. It may improve split second judgement when figuring if grass will work as well as well as water for ground effect - in grass the reflected helicopter is a little blury.
Does that get close enough for all practical purposes?
Last edited by Graviman; 22nd May 2009 at 17:59.
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If we get back to the start of this thread, the poor chap is going to become an instructor. I was one a long time ago, and a bit reactionary. Many things that we teach are very complicated and it serves no purpose to blind the poor student with science. I remember being asked at an Instrument rating ground check to tell this pumped up self important IRI at what revolutions did the Mk 6 artifical horizon gyro rotate at. I was much junior to him and replied that I did not know, did not want to know, that it was irrelevant, but that I did know how to recognise when it had failed and what to do about it. Hauled in front of the CO I explained my case and was exonerated. The IRI never spoke to me again!! Ground effect can be easily demonstrated in a practical fashion and simply explained, if, like much of helicopter aerodynamics we just accept that it happens and don't dig toooooo deeply.KISS is a good watchword when instructing. Leave the deep theory to the instructors crewroom and teach the student to fly safely and enjoy the thrill of flight in a machine clearly not designed with that in mind. My father in law was a test pilot in the Ciervia auto gyro (C30?) and he had little idea of the theory but was an ace pilot.Only hit a tree once.
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I'm fairly certain that no one here is suggesting over explaining this to the student. There are two simple explanations to offer, and the discussion is which one is the better to use. We get into the physics to support our position, but never intended to offer all of this detail to the ab-initio students.
Don't talk about pressure bubbles, physics, or gyroscopic precession. Just mention that when you get close to the ground, you require less power. If the student asks why, just say that the flow of the air is changed. That should be enough.
Don't talk about pressure bubbles, physics, or gyroscopic precession. Just mention that when you get close to the ground, you require less power. If the student asks why, just say that the flow of the air is changed. That should be enough.
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Leave the deep theory to the instructors crewroom...
...wait! it's already called Professional Pilots Rumor Network!
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PHIL 77
And why not! Problem is instructors crewrooms can be very boring places! Pprune is named as you describe - but is clearly populated by a huge number without the BIG P ps: that is NOT aimed at anyone here
My point was that the originator of this thread was asking about myths. I was trying to suggest that he should not worry too much about the theory when he was teaching. I think Matthew Parsons put it very nicely.
And why not! Problem is instructors crewrooms can be very boring places! Pprune is named as you describe - but is clearly populated by a huge number without the BIG P
ps: that is NOT aimed at anyone hereMy point was that the originator of this thread was asking about myths. I was trying to suggest that he should not worry too much about the theory when he was teaching. I think Matthew Parsons put it very nicely.
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Originally Posted by Matthew Parsons
Don't talk about pressure bubbles, physics, or gyroscopic precession. Just mention that when you get close to the ground, you require less power. If the student asks why, just say that the flow of the air is changed. That should be enough.
A simple explanation should be accurate. That means the guy explaining it should know just a little more about the subject. If more of an explanation is saught then post flight find a white board, draw a stick-helicopter, draw the ground plane then draw a dotted helicopter reflection. Just say that the reflection generates an updraught that makes the helicopter think it is in a gentle descent. If you want to really impress sketch the downdraught OGE, then show it widened and deflected outwards by the ground. Then explain that the airflow slows down through the rotor so the collective goes down.
You don't need in depth understanding of fluid dynamics for that explanation...
So this thread now contains two explanations for ground effect...
1. The ground in effect slows down the column of air passing through the rotor, so the rotor "thinks" it is in a gentle updraft and is therefore working less hard to provide the necessary lift. [That one makes perfect sense to me]
2. Earlier, and from someone who definitely appeared to believe they were right... the air moving horizontally away from the rotor changes the vector of the air's movement through the rotor, so that in effect there is a higher component of spanwise flow. Somehow (???) this increases the lift of the rotor. [This one makes no sense to me, but then I'm a computer scientist, not an aerodynamicist. Why would the rotor blade care about how much spanwise flow there is? In a different context, the whole point of swept wings is that they effectively convert some of the aerofoil's forward velocity into spanwise flow, precisely because it DOESN'T affect the aerofoil's operation...???]
n5296s/n9888s/confused of Mountain View
1. The ground in effect slows down the column of air passing through the rotor, so the rotor "thinks" it is in a gentle updraft and is therefore working less hard to provide the necessary lift. [That one makes perfect sense to me]
2. Earlier, and from someone who definitely appeared to believe they were right... the air moving horizontally away from the rotor changes the vector of the air's movement through the rotor, so that in effect there is a higher component of spanwise flow. Somehow (???) this increases the lift of the rotor. [This one makes no sense to me, but then I'm a computer scientist, not an aerodynamicist. Why would the rotor blade care about how much spanwise flow there is? In a different context, the whole point of swept wings is that they effectively convert some of the aerofoil's forward velocity into spanwise flow, precisely because it DOESN'T affect the aerofoil's operation...???]
n5296s/n9888s/confused of Mountain View
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Nick
i've tried a few times to read and decipher your post but its my head that keeps going round, not the theory. I don't think that pressure builds up in long grass but power is certainly absorbed, thus requiring more power.
Of course the longer the grass the further away one will be from something solid and the argument defuses somewhat.
Also when one vertically descends into a hole in the trees the close proximity of waving leaf and branch absorbs loads of power more than descending through open airspace (say up to 5%) and must be considered when calculating load out. what has that to do with outflow?
HOGE also gives a bubble under yon disc the abscence of which is easily seen when one goes to shift away from it, as one does when moving towards translation.
apart from that there is considerable pressure in excess under the stationary disc with any pitch pulled, well according to my ears and altimeter there is.
when doing surveys for spot height checking I always allow time for the whole deal to settle down at flat pitch before reading the altimeter.
regards tet
i've tried a few times to read and decipher your post but its my head that keeps going round, not the theory. I don't think that pressure builds up in long grass but power is certainly absorbed, thus requiring more power.
Of course the longer the grass the further away one will be from something solid and the argument defuses somewhat.
Also when one vertically descends into a hole in the trees the close proximity of waving leaf and branch absorbs loads of power more than descending through open airspace (say up to 5%) and must be considered when calculating load out. what has that to do with outflow?
HOGE also gives a bubble under yon disc the abscence of which is easily seen when one goes to shift away from it, as one does when moving towards translation.
apart from that there is considerable pressure in excess under the stationary disc with any pitch pulled, well according to my ears and altimeter there is.
when doing surveys for spot height checking I always allow time for the whole deal to settle down at flat pitch before reading the altimeter.
regards tet