Ground effect/Urban myth

[puntosaurus]

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.

[NickLappos]

Some hits:

http://www.pprune.com/forums/showthread.php?s=&threadid=100881&highlight=ground+effect

http://www.pprune.com/forums/showthread.php?s=&threadid=128379&highlight=ground+effect

[delta3]

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

[puntosaurus]

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 ?

[Arm out the window]

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!!

[puntosaurus]

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.

[Letsby Avenue]

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.

[Graviman]

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

[puntosaurus]

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.

[NickLappos]

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!

[rotorfossil]

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.

[puntosaurus]

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.

[ThomasTheTankEngine]

Rottorfosil

Would that be a fried egg or boiled egg

[NickLappos]

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!

[JerryG]

S**t, I never realised we were squashing poor elephants!!??
It's running landings for me from now on.

[delta3]

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

[Matthew Parsons]

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. :ooh:


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?

[delta3]

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...

[puntosaurus]

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 ?

[Chicken Leg]

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)

[puntosaurus]

Indeed you have. Previous page courtesy of Letsby Avenue.

[Chicken Leg]

Bugger!

I hate it when that happens!

[Letsby Avenue]

Don’t worry Chicken Leg - apparently the basic theory you have worked to for the last 20 odd years’ is complete baloney and it’s now something like - T at height Z / T OGE = 1 / ( 1 - (R/2z)^2) minus white elephants.:ok:

I'll be learning all about fenestron stall next:p

[rotorfossil]

Nick
I wasn't suggesting that the increased pressure under the disc was responsible for ground effect. Quite the reverse. It just serves to direct the downwash outwards (mostly). The effect is purely due to the reduction of induced flow because the ground is in the way, to put it simply.
What is observable is the variation of perceived "ground effect" between different types under the same conditions. I repeat the observation that blade length and fuselage cross section seem to be factors.
Has anybody tried strakes along the outer edges of the fuselage to contain the leakage of flow up the centre of the rotor? I seem to remember that the strakes/gunpods on the Harrier were found to benefit hover performance.
It is a little discussed topic but the recirculation of hot exhaust gas can't be benificial to low hover performance on turbine helicopters either.

[Banjo]

Damn, glad you know about the elephants also Nick...thought I was the only one that could see them.

Now maybe the other pilots won't be so scared to fly with me...I'm not mad after all.:D

[Milt]

Let's see how the MYTHBUSTERS handle this one on their TV program. Their series is a hoot!!!

[bast0n]

Is it true that the bubble of air that the aircraft sits on has a certain surface tension? If so the release of Fairy Liquid into this bubble would increase surface tension and so enhance the beneficial effects. Thinking this through a bit more, I suspect that this is not done as it would make the elephants eyes sting and an angry elephant could induce a lot of inflow roll.........:ok:

[nigelh]

Quite frankly my dear fellow WHO gives a S**T ? Does it make you a better pilot ? If so then all those old boys in Nam were obviouslt not very good pilots because they believed this myth :eek: or maybe you should just get out a little more .....:rolleyes:

[Hughes500]

Er who said planes have no ground effect ? While not quite a plane who has seen the programme on the Russian ( aircraft) that would skim across the Black sea. They from memory were about 150 tons and relied on the ground cushion from the wings being in close ( less than 20 ft ) proximity to the sea.
Think they were capable of 250 kts with a fraction of the fuel burn of an equivelent aircraft.:uhoh:

[oldbeefer]

Here's the link:

NpGacOmSeHc

[Hughes500]

Nick

Now you have seen the video of the Caspian sea monster so is ground efect a myth ?? Actually just played it back and there is a Kraken holding it up;)

[[email protected]]

Ground effect is not a myth but the notion that there is a bubble of high pressure air underneath the aircraft (helicopter or ekranoplane) is a myth - that is what Nick tried to get across to people.

[Phil77]

Nicks comments can be found found here:
http://www.pprune.org/rotorheads/300955-helicopter-urban-myths-6.html#post3739638

[ReverseFlight]

The way I was taught is that the "ground cushion" is a myth. The "ground effect" is the correct term for describing the phenomenon in both fixed wing and helicopters. There is no "pressure area" under a chopper at the hover, as it is the reduction in induced flow which reduces the power requirement. FWs glide further when in ground effect because there is likewise a reduction in induced drag due to the reduced production of tip vortices (which tries to flow from underneath to above the wing via the wing tips).

[Flying Bull]

Hi all,

one thing to learn about the ground effect and sit in the helo and seeing, how it works - and how you loose it over a slope
another thing to be underneath a helicopter and seeing, what happens.
Did some water rescue training last year and - instead of just doing it from the pilot seat or from the skid, also went into the water to get rescued.
http://www.polizeifliegerstaffel.de/images/Wasserret_Archivbild.jpg
Being in the water, you can actually see, that the water gets pushed down in a circle roughly around the bladetips.
If the helo doesn´t close and keeps the "survivor" in that area, he might panic cause you can hardly breathe - independent of the direction you face.
On that ring there is a remakable pressure difference!
Closer inside the ring you have a relativ calm area (around rotorcenter)
I guess, the air being pushed down isn´t capable of getting away fast enough to make space for the air coming from the rotor - so you have more air there and therefore a higher pressure.
Greetings Flying Bull

[Phil77]

Flying Bull: please read what Nick Lappos wrote (and the FAA supports FWIW): http://www.pprune.org/rotorheads/300955-helicopter-urban-myths-6.html#post3739638.

IT IS NOT A PRESSURE BUBBLE!

You surely experience strong winds from the downdraft and it might create a higher air pressure but that is NOT the reason for the increase in lift available. What happens is, that when the air (downdraft) hits the surface it will be deflected and changes the inflow angle (it becomes more shallow) and therefore resulting in decreased induced drag (= increased angle of attack)!

http://www.faatest.com/books/FLT/Chapter17/Ground%20Effect_files/image4JK.jpg
Taken from the FAAtest Publication: Ground Effect (http://www.faatest.com/books/FLT/Chapter17/GroundEffect.htm)

(yes we are talking helicopters, but after all a helo's rotorblade is just a fast spinning wing)

Certainly the reduction in wing tip vortices plays a role too - the deflected downdraft pushes the vortices further outboard and therefore increases the area of clean, undisturbed air the blade can work with.

[Matthew Parsons]

For almost all of helicopter aerodynamic theory, we start with the assumption that air is incompressible. If we keep that (fairly valid) assumption, then there is no way we can force more pieces of air into the same volume of space, keeping the temperature constant. That's just another way of saying we can't increase STATIC pressure.

In physics there are many ways of describing the cause of an observed effect. Some are less intuitive but more accurate, others are more intuitive and less accurate, and some are just a different way of explaining things. This is especially true in aviation, where the complete physics solution is just far to complex to provide any amount of insight.

Throughout incompressible aerodynamics, we try to explain things using airflow velocities because it is both intuitive and accurate. We can explain ground effect using airflow velocities. It is consistent with the rest of the explanations, is accurate, and is somewhat intuitive.

I don't think it is necessarily wrong to try to help a student understand the handling consequences of ground effect by describing it as a cushion of air, but the instructor needs to make sure that the student isn't mislead by that analogy. Unfortunately, just as in the gyroscopic precession analogy, students are getting mislead. They are taking the convenient explanations to heart, and passing them on as utter truths, which ultimately leads to redundant posts on pprune. Because of that, I think sticking to the 'book answer' that describes ground effect with respect to flow is the best way to describe ground effect.


One small caveat, and one that I'm certain the aerodynamicists and perhaps even the exalted posters should agree to, is that pressure is the only mechanism through which air can change the flow of air. The flow description of ground effect demands that pressure exists and is measurable beneath the hovering helicopter. It is the same pressure that causes a depression in the surface of the water, the same pressure that can make a small boat sink, the same pressure that can be felt by people beneath the helicopter. However, it is a DYNAMIC pressure.

Summary: Describe ground effect using flow. Don't get mislead by the symptoms of pressure underneath the helicopter.

[Phil77]

Matthew, could you do me a favor and elaborate a bit more from a physicists standpoint how we cannot compress static air but, according to Bernoulli, create a pressure differential by increasing the velocity above the wing? So non-static air we can compress, but not the static air... but hold on, the downdraft is moving!? Or am I missing something? (I am not arguing the correctness of your theory btw.)
...and what about weather? How come we have low-pressure and high-pressure areas if we cannot force more air-molecules into a volume of air - is it because it is flowing? But the flow was initiated by the pressure differential in the first place, right? :confused:
To a certain extent I find that very interesting, but as you mentioned earlier, there will be a point where for the sake of scientific accuracy the understandability will suffer ;) - there must be a middle way for the interested aviator with his "in-depth-half-knowledge" (you'll loose me when you start using formulas for example) to understand your theory.

As far as the redundancy of PPruNe goes, I think its inevitable. We hardly can expect everybody to dig out posts from 3 years+ ago and read everything of the wealth of knowledge stored here. I for one am still working on reading through the whole "S-76 Ask Nick Lappos" thread (I made it to page 43):ok:! But we can point somebody in the right direction and expect him to read at least the newest posts on a certain thread.

[Matthew Parsons]

Phil,

The pressure differential above and below the wings is a dynamic pressure.

The air is in fact compressible, a quality that defines it as a gas rather than a liquid, but the amount of compression that we see in helicopter aerodynamics is for the most part negligible. It can be added to the equations, but it makes such a small difference to the answer, and yet a large difference to the complexity.

The exception is tip effects, but rather than complicate life by doing everything with compressible aerodynamics, in flight test we just keep on the lookout for tip effects, and then if discovered handle them empirically.

As far as the redundancy, agreed. If everyone was never mislead we'd have little theory to talk about on this forum.

EDIT: I was wrong. The pressure differential across the wing is a static pressure. In order for Bernoulli's equations to work with incompressible air you have to make different assumptions.

Matthew.

[[email protected]]

Flying Bull - it is not high pressure under the aircraft that makes it difficult to breathe, it is the high speed of the downwash carrying water spray that makes it so uncomfortable (try breathing with your head under the shower and you will get a similar feeling).

The 'dishing' of the water surface is something that the ground cushion advocates like to use to 'prove' that it is increased over the water because of the divergent duct it seems to provide and thus, by their reckoning, reduces velocity and increases pressure under the disc - it isn't so but they like it!

[Matthew Parsons]

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.

[NickLappos]

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.

[n5296s]

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

[[email protected]]

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

[Graviman]

Just to make sure we are all talking the same language. I've done some reading up... :ok:

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. :8

[Phil77]

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?

[Graviman]

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... :ok:

[diethelm]

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.

[Phil77]

This is similar to the discussion i had with Dave J. the other day...

Sorry about that, when I read you guys discussions you sometimes loose me after a paragraph or two... that's why I tend to ignore the more philosophical posts between you and Dave - I promise to be better in the future :ok:

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 :ooh:

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.

[diethelm]

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"

[Graviman]

diethelm,

That is one of my favourite jokes, but the version i tell is the physicist and the engineer! :ok:

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? :)

[bast0n]

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.:)

[Matthew Parsons]

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.

[Phil77]

Leave the deep theory to the instructors crewroom...

Then I herewith declare PPruNe my virtual crewroom! :ok:

...wait! it's already called Professional Pilots Rumor Network! :cool:

[bast0n]

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.

[Graviman]

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.

Spot on Matthew! :ok:

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... :8

[n5296s]

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

[topendtorque]

It takes a lotta energy to wave a mob of grass around,
you only gotta look at them hula dancers.

[bast0n]

Svenestron

Big PEEES!

Either or and maybe both!:)

[topendtorque]

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

[Doinmabest]

Well, I never. I joined this string when it had grown to page 4. I even started reading from page 1. But then I skipped to page 4 and to discover we are still on the same topic (although there were a few attempts to divert the discussion

Many of the points made about the pressure under the disc reducing the induced airflow, as seen on a vector diagram of the airflows and forces acting on a rotor blade were agreeable to me.

However, when I did my Instructor rating a few years ago, a wise old instructor once taught me:

I.F.O.I.T.A.

Does this ring any bells with anyone?

Just
DOINMABEST

[Shawn Coyle]

Going right back to basics - Since the helicopter 'weighs' the same, it takes the same amount of lift force to hold the helicopter at 1 foot above the ground as it does at 100 feet above the ground at the same atmospheric conditions. It also takes the same amount of lift to hold the helicopter at 10 feet above the ground whether you're at sea level or 10,000' above sea level.
The power required to overcome the drag of the blades and produce the lift is the thing that changes.

[[email protected]]

TeT - if you fly a cushion creep style transition in ground effect, you will notice that the maximum power required is just before the onset of translational lift - this is because the rotors are battling their way through the tip vortices and are suffering loss of effectiveness at the tips.

The same thing happens when you transition from an OGE hover - you don't fall of a bubble of air you just need more power to accelerate the aircraft through the translational stage.

Grass and confined areas create recirculation which increases tip losses and requires more power to compensate for.

[Graviman]

Shawn, building on your well laid foundation:

Since the lift is the same, at 1' and 100' , the Relative Angle of Attack that each blade section requires is the same. Since the induced velocity to produce this lift is lower near the ground ("mirror image" helicopter is generating a gentle descent flowfield) this means that the pitch goes down to maintain the same Rel AOA. So the same lift force (and hence total pressure) is generated from a lower collective position. Thus a lower induced torque, hence power, is required. :ok:

[helisphere]

Graviman, maybe you could give a more detailed description of static, dynamic and total pressure and how they change with ground effect and without...

[Ascend Charlie]

Helisphere, do a search on "Helicopter Urban myths" - this whole thread has been beaten to death on that one.

[Graviman]

Helisphere. i don't get the time to check rotorheads so often these days..

The total pressure under the disk is what is holding you up - this is referred to in textbooks as disk actuator theory. Total pressure is the sum of static and dynamic pressure, so the rotor generates either an increase in downwash velocity (hence wake contraction) or an increase in static pressure (since the actual streamlines are revectored as the aerofoil passes). Since the weight of the heli stays the same so does the total pressure. The rotor is accelerating air down to generate the lift, which just either side of the rotor produces a step in static pressure.

In ground effect you are changing the 3D picture around heli. So for the same static pressure there can be a greater static pressure step and a smaller dynamic pressure step. In other words the air needs be accelerated less. This comes about because the ground "blockage" has the effect of superimposing a "mirror image" of heli below the ground. So heli is operating in climbing air.

The root of my arguement comes from vortex panel theory, which is still used as a pleliminary prediction for aircraft performance. The subject can, and does, still boggle my mind. ;)

[helisphere]

Ok, I think the confusing part is static pressure and total pressure. I've read some different things and it doesn't seem real black and white.

If we look at Bernoulli's p + q = p0

And p0 is total pressure and doesn't change, p is static pressure and q is dynamic pressure.

So IGE: q is smaller and p is bigger

OGE: q gets bigger and p gets smaller but p0 remains the same for both.

So then what is the static source measuring? I assume it's measuring p, but if this is the case then OGE/IGE will show up in the altimeter. But if the static source is measuring p0 this makes no sense because it couldn't read as high as a pitot tube vertically placed in the rotorwash.

Do you see my confusion? :ugh:

:confused:

[SimFlightTest]

What gets considered as static pressure or total pressure depends on the context.

For an aircraft flying through the atmosphere, then static pressure is the same as the ambient pressure, and total pressure is the sum of this static pressure plus the dynamic pressure resulting from the aircraft accelerating the surrounding air to the same velocity as the aircraft.

For an aircraft in a wind tunnel (open return) with the air traveling past it, then total pressure is the pressure of the ambient air in the room containing the wind tunnel and the static pressure is a pressure below this ambient pressure value based on how fast the wind tunnel is moving the air.

The situation gets complicated beneath a rotor because the rotor adds energy to the flow, hence increasing the total pressure.

[Graviman]

Helisphere, what you have to remember is that total pressure (static + dynamic) only stays constant along a streamline. So in an ideal world the pitot-static ports both operate on the same streamline.

In a rotor the inflow "streamlines" gets muddled up as each blade passes, so if you are only looking at the larger picture then it appears that there is a step increase in total pressure across the rotor "disk". Since engineers (like myself) are trying to produce a useable model then this is all nicely wrapped up in the name "disk actuator theory". Both above and below the rotor, along each streamline, Bernoulli applies as normal...