Ground effect/Urban myth
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Ground effect/Urban myth
I'm starting an instructor course on Tuesday so I've been diligently going through the urban myths to ensure that I'm not guilty of propagating them.
Can anyone point me to the thread that contains the full polemic about ground effect not being to do with pressure ? I've got Nick's urban myth thread which touches on the subject and refers to another thread, but I can't find the full monty.
Can anyone help with a reference ? If possible, I'd rather not rehash the debate here, at least until I've read the last one.
Can anyone point me to the thread that contains the full polemic about ground effect not being to do with pressure ? I've got Nick's urban myth thread which touches on the subject and refers to another thread, but I can't find the full monty.
Can anyone help with a reference ? If possible, I'd rather not rehash the debate here, at least until I've read the last one.
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Ground effect
puntosaurus
Gordon Leismans book gives a good account on ground effect, including the effect of forward speed.
For instance: T at height Z / T OGE = 1 / ( 1 - (R/2z)^2)
The way I modelled this in my R44 simulator is by using a similar formula to calculate the induced speed by the rotor. The ground effect blocks the downwash, so the induced speed gets reduced. I augmented the classic Glauert formula the calculates the Vi, with a similar term as above and get the same trust calculations as Leishman suggests.
The reduction in induced speed increases the rotor performance, and as a consequence the trust.
A similar reasoning can be applied on the effect of forward speed on the rotor induction speed. This leads to a behaviour where at a given (low altitude) the power first increases as the blockage disappears because the downwash escapes backwards (not blocked anymore), and this happens before translation lift starts taking over (= Glauert), that reduces induction speed which in turn increases trust.
Hope that helps
Delta3
Gordon Leismans book gives a good account on ground effect, including the effect of forward speed.
For instance: T at height Z / T OGE = 1 / ( 1 - (R/2z)^2)
The way I modelled this in my R44 simulator is by using a similar formula to calculate the induced speed by the rotor. The ground effect blocks the downwash, so the induced speed gets reduced. I augmented the classic Glauert formula the calculates the Vi, with a similar term as above and get the same trust calculations as Leishman suggests.
The reduction in induced speed increases the rotor performance, and as a consequence the trust.
A similar reasoning can be applied on the effect of forward speed on the rotor induction speed. This leads to a behaviour where at a given (low altitude) the power first increases as the blockage disappears because the downwash escapes backwards (not blocked anymore), and this happens before translation lift starts taking over (= Glauert), that reduces induction speed which in turn increases trust.
Hope that helps
Delta3
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Thanks guys. I'm pretty comfortable with the idea that the downwash is blocked, and that as a result the relative airflow changes to more horizontal, and that the TR vector therefore moves more vertical etc. etc. From a teaching perspective that ought to pretty much cover it.
But suppose your student has done a bit more reading or listening in the flying club bar and comes up with the 'urban myth' of a ground cushion. My problem with simply dismissing it as an urban myth is that it seems to have a grain of truth in it.
My issue concerns the mechanism whereby the blocked downwash alters the airflow at the rotor ? Surely the dynamic pressure of the inflow hitting the ground is the mechanism. Now maybe to talk about as a 'ground cushion' is sloppy terminology, but surely it IS a pressure effect.
One of the counterarguments that is put foward against this 'myth' is the ground effect of a fixed wing which since it is travelling along cannot have a ground cushion. Well maybe not in the static sense, but you can easily envisage a dynamic ground cushion forming, a bow wave if you like, which is again a pressure effect causing the relative airflow changes.
I stress I'm not trying to make a strong theoretical point here, simply to try and separate the mythical from the factual. Maybe you can dismiss the ground cushion as sloppy terminology, but not dismiss pressure effects from being involved with Ground Effect ?
But suppose your student has done a bit more reading or listening in the flying club bar and comes up with the 'urban myth' of a ground cushion. My problem with simply dismissing it as an urban myth is that it seems to have a grain of truth in it.
My issue concerns the mechanism whereby the blocked downwash alters the airflow at the rotor ? Surely the dynamic pressure of the inflow hitting the ground is the mechanism. Now maybe to talk about as a 'ground cushion' is sloppy terminology, but surely it IS a pressure effect.
One of the counterarguments that is put foward against this 'myth' is the ground effect of a fixed wing which since it is travelling along cannot have a ground cushion. Well maybe not in the static sense, but you can easily envisage a dynamic ground cushion forming, a bow wave if you like, which is again a pressure effect causing the relative airflow changes.
I stress I'm not trying to make a strong theoretical point here, simply to try and separate the mythical from the factual. Maybe you can dismiss the ground cushion as sloppy terminology, but not dismiss pressure effects from being involved with Ground Effect ?
Well mate, of course it's a pressure effect; air doesn't go anywhere unless there's differential pressure to make it do so, so what's the big deal about describing a change in airflow brought about by the proximity of some kind of deflecting medium (eg. the ground) as something that affects the pressure on certain air molecules in the area? As they say in the classics, Crikey!!
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Well I'm pretty much with you, but to see the vehemence with which the pressure connection has been skewered in the past, I just wanted to be sure that I wasn't missing something.
It is possible that the 'myth' part of this is where someone gets the idea that it is the 'cushion' itself which is solely responsible for keeping the helicopter in the air in the hover, and in that case I can see why you would want to put them right.
It is possible that the 'myth' part of this is where someone gets the idea that it is the 'cushion' itself which is solely responsible for keeping the helicopter in the air in the hover, and in that case I can see why you would want to put them right.
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cor blimey - the whole point of explaining this stuff is so that studes will remember it and carry the knowledge with them for the rest of thier flying career - Can I suggest that the formula above introduces a soporific quality to the proceedings! What on earth is wrong with describing it as a build up of pressure under the disc and in the hover the helicopter sits on it (thus reducing power etc, etc) wether this description is right or wrong it is easy to understand and helps explain a lot of other things later on - Keep it simple.
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How about:
The ground blockage reduces the air leaking out from the region below the rotor disk. In order to provide the disk pressure required to stay airborne, the rotors thus need to supply less air into this region. Power required therefore goes down.
How about the other myth about grass producing less ground effect than tarmac?
Mart
The ground blockage reduces the air leaking out from the region below the rotor disk. In order to provide the disk pressure required to stay airborne, the rotors thus need to supply less air into this region. Power required therefore goes down.
How about the other myth about grass producing less ground effect than tarmac?
Mart
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Graviman - Omigod. Is that a myth ? I thought that was true, at least for long grass.
Letsby Avenue (Great name ! Almost as good as Paul McKeksdown) - In fairness to Delta3 I think the formulae were for my benefit rather than the students. I don't mind simplifying things but the idea of a pressure bubble 'supporting' the helicopter seems a bridge too far to me. It's not supporting the helicopter - it's making the rotor more efficient.
For the PPL they have to know about induced drag, how autorotation works etc. so there's no avoiding some basic exposure to vector diagrams, and no great additional complexity to show them how those diagrams change when in ground effect.
Letsby Avenue (Great name ! Almost as good as Paul McKeksdown) - In fairness to Delta3 I think the formulae were for my benefit rather than the students. I don't mind simplifying things but the idea of a pressure bubble 'supporting' the helicopter seems a bridge too far to me. It's not supporting the helicopter - it's making the rotor more efficient.
For the PPL they have to know about induced drag, how autorotation works etc. so there's no avoiding some basic exposure to vector diagrams, and no great additional complexity to show them how those diagrams change when in ground effect.
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Letsby Avenue,
Instead of lying to them about pressure below the rotor, why don't we tell them there is a magnet in the transmission that repels the earth, and is worth 15% power when you get close? It fits, everyone knows that grass isn't magnetic, so ground effect is weaker over grass!
How about we tell them that there is a big invisible elephant below the aircraft, and we have to squash it wnem we land, and elephant flesh is particularly tough, so it takes 15% of the weight of the aircraft to squash it!
We could lie about everything, to make it easier to learn! The max rotor rpm should be observed, because the rotor remembers each rotation, and it gets pissed if we eat its life too quickly!
Why don't we ppruners invent better ways to explain things, instead of that silly, hard to understand truth!
Instead of lying to them about pressure below the rotor, why don't we tell them there is a magnet in the transmission that repels the earth, and is worth 15% power when you get close? It fits, everyone knows that grass isn't magnetic, so ground effect is weaker over grass!
How about we tell them that there is a big invisible elephant below the aircraft, and we have to squash it wnem we land, and elephant flesh is particularly tough, so it takes 15% of the weight of the aircraft to squash it!
We could lie about everything, to make it easier to learn! The max rotor rpm should be observed, because the rotor remembers each rotation, and it gets pissed if we eat its life too quickly!
Why don't we ppruners invent better ways to explain things, instead of that silly, hard to understand truth!
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The build up of pressure under a helicopter in the hover is observable on a light wind day - the altimeter goes down as ground effect occurs, but only to the tune of about 30 ft (1mb). However the ground reducing the induced flow is the major effect. The increased pressure diverts the flow outwards (it has to go somewhere).
The benefits of low hover are a balance between the good news of the GE and the bad news of enhanced recirculation at the rotor tips. Hence, long rotor blades benefit most and wide fuselages reduce the leakage of the downwash up the centre of the rotor disc. This is why some helicopters have a demonstrably better "ground cushion" than others. The R22 is rather poor in this respect
(short blades and egg shaped fuselage).
Long grass slows the outflow and allows it to be picked up by the rotor tip vortices, which reduces the rotor thrust.
The benefits of low hover are a balance between the good news of the GE and the bad news of enhanced recirculation at the rotor tips. Hence, long rotor blades benefit most and wide fuselages reduce the leakage of the downwash up the centre of the rotor disc. This is why some helicopters have a demonstrably better "ground cushion" than others. The R22 is rather poor in this respect
(short blades and egg shaped fuselage).
Long grass slows the outflow and allows it to be picked up by the rotor tip vortices, which reduces the rotor thrust.
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HaHaHa. Now that is funny.
Rotorfossil - that's the first time I've heard that as an explanation of the long grass effect (not a myth then ?) & it sounds plausible. Previous explanations I've heard centre around the work done by the downwash on the medium (water or grass) resulting in lower pressure and less reduction in inflow - also plausible. Maybe a combination of both ?
I've also read elsewhere about the altimeter in ground effect, I've never noticed it but will check next time I fly.
Rotorfossil - that's the first time I've heard that as an explanation of the long grass effect (not a myth then ?) & it sounds plausible. Previous explanations I've heard centre around the work done by the downwash on the medium (water or grass) resulting in lower pressure and less reduction in inflow - also plausible. Maybe a combination of both ?
I've also read elsewhere about the altimeter in ground effect, I've never noticed it but will check next time I fly.
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rotorfossil, this old saw will not die, and it is fun every time! The old, pop-corn texts used to train today's instructors were written by pilots who did not understand the issues. An expert can find a dozen substantively wrong statements in the typical "aerodynamic" discussions in training manuals.
Your observation is a continuation of the urban myth. Actually the altimeter jumps because the elephant is screaming and the grass waves because the invisible elephant is struggling below the aircraft.
Seriously, the altimeter is affected in both directions on many helicopters, it goes up first during the lift-off! The effect is mostly due to the screwy hover flow past the static ports, which are not designed for hover readings. Try two things:
1) calculate the amount of equivilent lift due to 30 feet of altitude's worth of pressure, to see if it is the 15% that IGE gains you
2) Look at the altimeter in an OGE hover compared to a fly-by at that altitude to see if the "pressure" is the reason for the movement IGE
3) If there were truly 30 feet worth of pressure under an R-22, and the aircraft were "packing the air down" with that much force, the force exerted on the helo would be nearly enough to shut off the engine and just sit there on the ball of air!
Your observation is a continuation of the urban myth. Actually the altimeter jumps because the elephant is screaming and the grass waves because the invisible elephant is struggling below the aircraft.
Seriously, the altimeter is affected in both directions on many helicopters, it goes up first during the lift-off! The effect is mostly due to the screwy hover flow past the static ports, which are not designed for hover readings. Try two things:
1) calculate the amount of equivilent lift due to 30 feet of altitude's worth of pressure, to see if it is the 15% that IGE gains you
2) Look at the altimeter in an OGE hover compared to a fly-by at that altitude to see if the "pressure" is the reason for the movement IGE
3) If there were truly 30 feet worth of pressure under an R-22, and the aircraft were "packing the air down" with that much force, the force exerted on the helo would be nearly enough to shut off the engine and just sit there on the ball of air!
Last edited by NickLappos; 12th Jun 2005 at 19:18.
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Ground effect
Hi Guys,
I have seen over and over how difficult it is to explain behaviour of a Helo (to pilots if you allow me...).
It is maybe plausible to say, looking at the airflow (the whole tube of air that is acted upon by the rotor), and derive that because of the blockage by the ground, precisily at the ground level the static air pressure probably will be higher than the static pressure measured in the same tube of air without the ground.
But even trying to explain it with pressure, I think one should make a difference between static and dynamic pressures, and also look at the pressure just above the rotor with and without ground effect. I am shure this could be developped into a model, but the pressure under the rotor would only be part of the story (pressure above the rotor could also be higher).
My simulator tries to model the helo based on the fundamental dynamics and aerodynamical rules. In that model the pressure at the ground does not come into play, in the same way as the pressure measured at the runway does not directly come into play when studying the influence of the ground on a wing of a airplane.
What the rotor sees and acts upon is a moving air mass.
It is the speed of that air mass that counts. The ground just changes that speed (making abstractions of more detailed vortex effects) : it reduces the speed, making the rotor more efficient in exactly the same way as forward speed makes the rotor more efficient.
Delta3
I have seen over and over how difficult it is to explain behaviour of a Helo (to pilots if you allow me...).
It is maybe plausible to say, looking at the airflow (the whole tube of air that is acted upon by the rotor), and derive that because of the blockage by the ground, precisily at the ground level the static air pressure probably will be higher than the static pressure measured in the same tube of air without the ground.
But even trying to explain it with pressure, I think one should make a difference between static and dynamic pressures, and also look at the pressure just above the rotor with and without ground effect. I am shure this could be developped into a model, but the pressure under the rotor would only be part of the story (pressure above the rotor could also be higher).
My simulator tries to model the helo based on the fundamental dynamics and aerodynamical rules. In that model the pressure at the ground does not come into play, in the same way as the pressure measured at the runway does not directly come into play when studying the influence of the ground on a wing of a airplane.
What the rotor sees and acts upon is a moving air mass.
It is the speed of that air mass that counts. The ground just changes that speed (making abstractions of more detailed vortex effects) : it reduces the speed, making the rotor more efficient in exactly the same way as forward speed makes the rotor more efficient.
Delta3
Last edited by delta3; 12th Jun 2005 at 22:22.
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Delta3 said, "It is the speed of that air mass that counts."
That's only part of the story. If the pressure doesn't count, then that whole Mt Everest thing is nothing to brag about.
Nick, I agree that putting that hover bubble argument to rest is way overdue. I also agree that the truth is the way to go. I think it's important to remember that throughout even the most pure theory, assumptions have been made. While talking about reducing induced velocity is satisfactory to explain ground effect, that does not mean that pressure gradients can be ignored.
In the IGE case, the earth is effectively an infinite flat plate in the path of the induced velocity (downwash). The earth changes the direction of the down wash the same way air flows across/around flat plates. There will be a stagnation point. A static pressure difference can be measured. How big? This is where we realize why we bin the pressure argument from the start.
Matthew.
PS, will Gulfstream be in Paris? Time for a beer?
That's only part of the story. If the pressure doesn't count, then that whole Mt Everest thing is nothing to brag about.
Nick, I agree that putting that hover bubble argument to rest is way overdue. I also agree that the truth is the way to go. I think it's important to remember that throughout even the most pure theory, assumptions have been made. While talking about reducing induced velocity is satisfactory to explain ground effect, that does not mean that pressure gradients can be ignored.
In the IGE case, the earth is effectively an infinite flat plate in the path of the induced velocity (downwash). The earth changes the direction of the down wash the same way air flows across/around flat plates. There will be a stagnation point. A static pressure difference can be measured. How big? This is where we realize why we bin the pressure argument from the start.
Matthew.
PS, will Gulfstream be in Paris? Time for a beer?
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Ground effect
Matthew,
You are making my point...
Of course the overall pressure counts and this only because it changes the overall density. I am afraid that all these 'loose' comparisons/ metaphores are the core of the problem...
Good night...
You are making my point...
Of course the overall pressure counts and this only because it changes the overall density. I am afraid that all these 'loose' comparisons/ metaphores are the core of the problem...
Good night...
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Thanks again guys, I think most students are going to be happy with the increase in rotor efficiency as a result of ground reduced inflow. That won't cause any urban myths to be propagated, and at the same time ensures they have a base of knowledge and tools to find out more if they want to.
Matt - you've left it dangling a bit, what a tease - are you implying that a quantitative analysis of the situation would lead to better insight ?
Matt - you've left it dangling a bit, what a tease - are you implying that a quantitative analysis of the situation would lead to better insight ?
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Have I missed something here? What is this urban myth?
Surely:
The induced flow hits the ground and spreads out in all directions essentially forming a divergant duct (reduced velocity = increased pressure) therfore increasing relative pressure below the disk.
An example of this can be witnessed on many helicopters (dependant on the position of the static vents I think)when you pull in power to take off and the VSI indicates a rate of descent (relative pressure increasing)
Surely:
The induced flow hits the ground and spreads out in all directions essentially forming a divergant duct (reduced velocity = increased pressure) therfore increasing relative pressure below the disk.
An example of this can be witnessed on many helicopters (dependant on the position of the static vents I think)when you pull in power to take off and the VSI indicates a rate of descent (relative pressure increasing)