Indeed you have. Previous page courtesy of Letsby Avenue.

Bugger!

I hate it when that happens!

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

ground effect
 

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.

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

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

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:

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:

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:

Here's the link:

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;)

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.

Nicks comments can be found found here:
http://www.pprune.org/rotorheads/300...ml#post3739638

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).

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/...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

Flying Bull: please read what Nick Lappos wrote (and the FAA supports FWIW): http://www.pprune.org/rotorheads/300...ml#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/Cha...s/image4JK.jpg
Taken from the FAAtest Publication: Ground Effect

(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.

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.

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.

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.

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!