Nick,

An interesting experiment.

Where is the 'sensor' for a Baro altimeter normally located, and does it have a profile that attemps to minimize extraeous input?
I understand that the active and static pitot tubes are located so as to try and minimize the unwanted.


Dave J.

Nick, yes it does! Not only that but while I am raising the lever to take off the RCDI indicates a rate of descent (an increase in pressure). I and many others have taught this as a check that the static vents are clear before going flying, especially instrument flying.

If I hover the helo OGE so thrust = weight and could magically introduce the ground at an instant say 5' below the wheels then the aircraft would climb if I did nothing with the controls. Why? because the ground has caused the pressure increase to occur which opposes IF, improves Cl, reduces Cd etc.etc.
BUT if I lower the lever to maintain height, the same thrust = weight is true but I have achieved it using less pitch angle, less AoA and less power.
You will say it is because the induced drag has reduced and I will agree but only because I believe that the increase of pressure under the wing has caused this to happen, whether through the action of vortices or whatever.

nick
 

you said > Of course pressure is part of the flow equation, but the issue I am countering here is the assertion that the ground somehow allows pressure to build up (a ground cushion).

so, ground effect = higher pressure under the disc for the same pitch aplied.

ground cussion, only in the hover, is a extra build up of pressure within the disc area simply because it cant get away. thrust goes down and then out, some would have to go in!

CRAB ; does the RCDI indicate a pressure rise when slowing to the hover?

power curves always start on a lower power at hover (cussion) then increase slightly (as you lose the cussion) before decreasing as translational lift comes in. all this is done in ground effect!
why wouldnt pressure build up in this area?

Crab,
You said "yes it does" about the altimeter, but gave no numbers. For an S-76, there is an altitude drop when lifting, then it returns to the proper altitude when stable in an IGE hover. What does yours read when in the hover?

Vortecy,
the issue is that pressure must return to the baseline (same as OGE) when the lever is reduced to hold the hover. While in the hover IGE, the pressure is the same below the helicopter as when in an OGE hover. The ground does not raise the pressure under the helicopter, the pitch lever does! The ground allows you to use less pitch.

Guys, we are now again going around and around. Try to think of this this way:
If the pressure is greater below the helicopter in a certain circumstance, how does it act on the helicopter? How would it be measured?

You must break out of the old words and physical models that your instructors taught you if you are to fully understand the concept of Ground Effect. The ground supresses the tip losses, and makes the blades/wing behave as if they were much longer, so they need less angle of attack. This means less tip loss, and less induced drag/power. Pressure does not build up between the aircraft and the ground, there is no "cushion"

If you cannot understand this, that is too bad, but that does not make pressure build up!

Nick, yes I believe it returns to the pre hover figure once the helo is in the hover as does the RCDI, which shows that as you are raising the lever you are increasing the pressure under the disc - surely you cannot argue with that.
Our sticking point is that you say once established in the hover, the pressure distribution is the same in an IGE hover as it is in an OGE hover - and I agree!!!
But I think that you are allowed a lower lever position and less power in the IGE hover because if you did not the helo would climb because the proximity of the ground had caused an increase of pressure under the disc.
The proximity of the ground produces 'excess lift' for want of a better term and you dump the 'excess lift by lowering the lever. Having lowered the lever you are no longer producing 'excess lift' only the 'exact lift' required to hover. In the steady state IGE hover I agree that there is not a cushion of high pressure air under the disc but in coming to the hover or lifting to the hover, the high pressure under the disc would make you climb if you did not reduce your IF by lowering the lever.
To sum up - Cause of GE = increase of pressure under the disc making the blades more efficient by appearing bigger to the air- Effect - lower lever position to hover and less power required.

Crab,

This is, like many pprune threads, a really good one, and an eye opener. We are then left with this paradox:

The pressure under the disk IGE is the same as the pressure under the disk OGE. This means that IGE does not have a "ground cushion" of pressure. There is NO ground cushion, no pressure increase. The change in lift is due to the earth creating, in effect, end plates that block the sidewash of the airfoils.

OED

BTW, the downward flexing of the altimeter during the pitch pull is due to the static ports getting bathed in the downwash of the rotor, not a general pressure increase. Once the flow becomes steady state, the altimeter is a good indicator of the pressure under the disk, and when it shows no significant difference from the field elevation, it tends to prove the non-pressure case. I did some number crunching, for a 15% drop in power to be caused by pressure alone, the pressure would have to change about -150 feet's worth of altitude.

Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all.

Skirting the issue ??
 

Nick sez,

"Note that real GEM's do create a pressure below the aircraft, and they have a skirt to allow the pressure to build. A manometer that reads the pressure under the skirt would show a significant pressure rise. Helos in ground effect are not the same beast at all."


The previously mentioned web page on Ground Effect sez,

" The principle of Wing-in-ground-effect or (W.I.G) has been known for a long time. Ground effect is a natural phenomenon, as an aerofoil approaches the ground its lifting ability increases and the drag reduces"
It never uses the word 'skirt'


Nick, you appear to be basing the cause of ground effect totally on tip vortices. This is contrary to what I have been able to discover.

Perhaps the tip vortices contribute to ground effect, by acting as a kind of skirt, and this helps retain the pressure.
~or~
Perhaps the increased pressure results in the blades operating in a slightly denser medium and they are therefor slightly more efficient.


:confused: :confused: :confused:
Dave J.
_____________________________

.... and furthermore

Having no (good) reputation to protect, I'll go out on a limb and make the following totally unheard of and totally unsupported claim. :eek: [list=1][*]Pressure is an increase in air density.[*]Higher air density results in greater rotor efficiency.[/list=1] Therefor the higher pressure caused by ground effect results in a greater thrust for a given collective pitch settling.

Tear that to pieces, ~ kindly. :(

Dave J.

Got my hat in front of me...getting ready to eat it...but I'm still not convinced, and I suspect 73,000 other helo drivers aren't either...yet;)

If downwash from the rotor hits the ground there has to be an opposite and equal reaction. .....pressure :confused:

Try rephrasing your explanation of it after all american is a foreign language and there is a lot lost in translation:D

Nick, I am nearly with you in that I agree that the pressure IGE below the disk is the same as OGE and that there is not a 'cushion (cussion for Vorticey)':D of high pressure air.

I still maintain that an increase in pressure caused by the vortices being modified by the presence of the ground is the mechanism by which the Ground Effect phenomenon is created. Without an initial increase in pressure there would be no way for the lift to increase and no reason to lower the lever to prevent a climb.

BTW just to muddy the waters again, I went on a training sortie last night, set the QFE just before take off which gave me +10' on the Bar Alt. As I lifted to the hover the RCDI showed a RoD and the Bar Alt reduced to - 30' AND stayed there once established in the 10' hover. This is with a reasonable altimeter with a vibrator (although why it needs one when it's mounted in a Sea King I don't know).

DJ don't forget temperature in your statement about prssure and density eg if temperature stays constant then rising pressure will increase the density.

crab,

Yes. It is assumed that the temperature is a constant.

The idea in my last post was an alternative to the concept that there is a higher pressure below the blades pushing upwards. What might be considered as the 'air cushion'.

Perhaps the ground increases the air pressure (density) in whole area in which the rotor is working. I.e. above, below and to the sides of the rotor. The rotor therefor performs as if it was operating at a much lower elevation then it actually is. In other words, better thrust for the same power.

Dave J.


Oops!! ~ Edited to apologize and correct above error.

The thrust of the rotor does not vary with altitude. It is the power of a naturally aspirated reciprocating engine that varies with altitude. :( :(

nick
 

you dont need to repeat youself in every post, things that were leant at heli school. of caurse the pressure never increases if you want to hold a hight above the ground, as the pilot keeps the pressure constant with the collective. did you even think about what i said on the power chart? it indicates how much pressure you could put under the helicopter at a particular speed (power available). it distinctly shows slightly less power at hover than a couple of knots. this is the cushion im talking about. your right, the pressure doesnt actualy rise, because the pilot reduces the collective to save it. the saved pressure now shows on the performance chart.
so for you nick, i wont call it ground cushion anymore. its now, THE CONSERVATION OF POTENTIAL GROUND PRESSURE!:D

DAVE
thrust REQUIRED is constant.
turbine engines also suffer at altitude, air is less dence. but on RPT jets the engine is designed to operate at a particular altitude so flying below or above this hight the engine is inefficient.

Pressure?

http://www.dynamicflight.com/aerodyn...ground_effect/

The rest of the site may halt some of the other crap theories posted here as well.

Mr Lappos gets my vote!

RE: Crap
 

John,

I don't think that any of the posters have disagreed with the information on your referenced web page.

But; ~

1. This web page does not disprove 'pressure'.
2. Bell has said that 'pressure' on the underside of the fuselage contributes to ground effect. This is a third reason, which is in addition to the two that were mentioned on the page.

__________________

Quote from your referenced web page; "First and most important is the reduction of the velocity of the induced airflow. Since the ground interrupts the airflow under the helicopter, the entire flow is altered. This reduces downward velocity of the induced flow.

The above is a superficial answer.

What enables the rotor to produce a lesser velocity, and yet, produce the same lift?
OK it's the ground, but, the air is the only connection between the helicopter and the ground.
What change has taken place to the air?


More camel dung added :D

OK guys, that's that. Increased pressure under the helicopter has nothing to do with ground effect. The pressure under an OGE helo is the same as that under an IGE helo.

If you want, you can believe otherwise, it is fine. That is why we run horse races, isn't it?

I'm not a believer of the pressure having an effect on ground effect theory, but I have a question to which I have no answer.

If the presence of the ground reduces the induced flow, then wouldn't the lower induced flow result in a greater pressure due to Bernoulli's principle?

Before everyone's blades start spinning, I'm not saying aerofoils produce lift because of Bernoullie (also not saying they don't)...I just don't want to get into that discussion.
_____________________

I agree that the engines don't have to work as hard IGE than OGE (actually seen this phenomenon myself ;) ). I agree that the rotors produce the same amount of lift in a stable hover IGE and OGE. From that I deduce that the extra work done OGE is due to drag...the only one that can change is blade induced drag. This is where it gets complicated. I seem to recall that blade induced drag is a function of angle of attack, density, geometry, and airspeed. The trick here is that density is still air density, ie not affected by dynamic pressures.

The only quantity in the blade induced drag equation that changes IGE vs OGE is the angle of attack (alpha). alpha is a result of induced flow, relative airflow, blade pitch and rotation of the blade. IGE vs OGE you change induced flow and blade pitch. Blade pitch is changed by the pilot (collective increase). Induced flow would increase slightly as a result of the collective increase, but would decrease overall due to the resistance to allowing the air to move through the ground. At the tips the vortex is changed, but that also causes a change in induced flow and a subsequent change in angle of attack.

Stopping here, the OGE vs IGE argument is explained simply without resorting to pressures.

However, pressure does play a role. It is the mechanism that changes the vortex and restricts the induced flow. If you really wanted to, I'm sure the equations could all be rewritten very complexly (that a word?) so that pressure is used and induced flow is ignored. That doesn't mean pressure causes anything, its accepted that the most simple explanation is the how that things work, other explanations are useful only in specific studies.
___________________

Someone pointed out that pressure doesn't increase below fixed wing aircraft flying very close to the ocean. That's correct, but in that case the "downwash" isn't moving straight down. Instead there is a wake behind the aircraft. That wake wants to flow, but the ocean won't allow it. The restriction in flow causes an increase in pressure (this is actually Bernoulli's theorem in a different light) in the wake. The equations of lift and drag still don't need that pressure change to explain ground effect, so I stand by those that insist pressure isn't a factor.
___________________

I know, I talked too much. Pressure is the mechanism that causes some of the changes that result in the rewards of ground effect. Ground effect (at the wing/blade) is fully explained without reference to the pressure of the air.

Food for thought: What about compressibility right at the blade? We know it happens...we've all seen contrails coming off the blades in the hover. Does that change IGE vs OGE? If so, it gives a bit of ammunition to the pressure explanation enthusiasts.
___________________

Back to original question. It's been said that the pressure beneath is the same IGE vs OGE yet the induced flow changes. Why doesn't the change in velocity of flow change the dynamic and hence the total air pressure?

PRESSURE
 

I still say that pressure changes are the ultimate way to describe aerodynamic phenomena. OK other factors are involved but the vast majority of airflow changes are as a result of pressure, it has to be so as this is the only way to apply force and change momentum of the air. Yes many different explanations can be used without mentioning pressure, but pressure is still there.
Is the pressure increased in ground effect? This can and has been answered intwo ways:
1. In a steady state hover the average pressure difference over the blades must be the same to give the same force (=weight). So you can say there is no pressure difference.
2. Going back to the original question of the effect on an autorotation...a helo arriving close to the ground will produce a pressure build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in pressure.
We can all argue until we are blue in the face. I for one believe pressure explains an awful lot. We can all use different tools to help us visualise how the air moves, computer programs that predict air flows use different methods: sinks, sources and vortices, purely mathematical models from imaginary worlds, and even finite element blocks that use physical properties at the boundaries with neighbouring blocks. Non of these are 100% accurate, only the real world is, but it shows how the same thing can be described in many ways.
If nothing else this thread has got many of us looking at different viewpoints, and hopefully, given us new tools to look at other situations.
PS I have been out of this for a while due work commitments, glad it was still going when I got back.

DeltaFree,

Your logic is circular, neat and precise, and unfortunately wrong.
You say "a helo arriving close to the ground will produce a pressure build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in pressure."

Why not say, with equal evidence and equal physical data, "a helo arriving close to the ground will produce a little red demon build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in little red demons."

There is no pressure buildup. That is the old pilot saying, like an old wives tale, told 10,000 times, so it must be correct. Were the pressure higher below the helicopter close to the ground, it would be measurable in many ways, such as your altimeter. No measurement shows higher pressure.

I am sad to see the thread last this long, and that the 40-zillionth post starts off where the first did. Oh well.

Nick,

You are against using the concept of 'pressure' in the discussion of ground effect. But, you have previously commented that the activity under discussion takes place at the speed of sound. The very definition of the word 'Sound' is "Oscillating pressure waves passing through a medium. I believe that 'pressure' is at the very heart of the discussion.

To me, the question is simply 'how does ground effect cause a change(s) in pressure'. The answer is obviously not as simple as the question.
_____________

As a wild guess, I suggest that the ground restricts the airflow. It causes a congestion of air molecules under the disk and it inhibits the replenishment of air molecules above the disk. Hence a pressure differential between the upper and the lower surfaces of the disk (and the fuselage).

PS Merry Xmas
_________________

Edited to elaborate on the possibility of reduced pressure above the disk.

When out of ground effect, the air that is drawn down through the disk is replaced by air from a very large sphere surrounding the helicopter. Some of this replenishment is originating from below the disk.

In ground effect, the ground will restrict the flow from the large area below the disk. This causes some of the replenishment to come from air that has previously gone down through the disk. This 'used' air that is about to be 'reclaimed' is heading upward at a certain velocity and with a certain inertia. It is now called upon to change its direction 180-degrees and head back down through the disk again. The thing that is 'calling it back down' is partial vacumn.

Dave,
Nice wild guess, but the pressure is the same in or out of ground effect. There is no "restriction" and no pressure rise. The flow is bent by the ground, which reduces the outflow, and makes the rotor behave as if it were larger. The efficiency rise of the rotor means it can make the lift needed with less power.

The issue that all the pressure fans miss is that there is more power efficiency, not more lift. This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.

However, since we have gone through 138 posts to circle back to this, and the last several posts all simply repeat the pressure assertions of the initial posts, we are truly stuck. If you cannot see that the answer to the ground effect efficiency improvement is not a pressure rise, it is simply a shame. Quel damage, as they say in Paris.

Have a Merry Christmas, guys!

The pressure is on.
 

I'm Starting ro think that Dave is really Lu in disguise!

Cheers, :eek: OffshoreIgor :eek:

Oh, Dave. You are Lu in disguise. What does that site you posted have to do with the question of how ground effect works?

I feel like those who debated with the Flat Earth Society back in the 1800's. The Flat Earthers were a group who argued that the earh is really flat. No matter what evidence was presented, they simply ignored it and went on with their assertions.

I guess Dave you are President of the Ground Pressure Society.

http://www.alaska.net/~clund/e_djubl...rthsociety.htm

Nick,

This thread was initially about 'Autorotation and Ground-Effect'. As part of this discussion, the 'effects' of the ground were considered. This thread then moved on to consider the 'causes' of ground effect.

There is obviously a distinction between 'Cause' and 'Effect'.

I politely suggest that;

*~ Your mentioning of; reduced angle of attack, reduced drag, and reduced power, are actually part of the 'Effect'. They are not part of the 'Cause'.

* ~ You deny that pressure is a 'Cause' of ground effect. I submitted the previous web page because it very clearly presents the relationship that exists between an airfoil and pressure, and by extension between, an airfoil, pressure and the ground.

____________________

There is probably no definitive answer, and as you say, this thread may have run its course, but, if you wish to continue, it will be appreciated if you attack the subject and not the person.


Yours truly;

Lu

oops Dave J. http://www.unicopter.com/drinking_smile.gif

im sick of this post too
 

NICK
you said > The issue that all the pressure fans miss is that there is more power efficiency, not more lift.(the power required reduces because the extra lift is extinguished by reducing collective.) This efficiency is brought about becuse the ground changes the direction of the flow, NOT its pressure.
why are'nt blades made to produce this changed direction of flow and have the extra efficiency all the time??

DeltaFree, spot on!


:confused:

Simulations show increased pressure
 

I found a couple of computer images that seem to support the idea that pressure does increase under the airfoil in ground effect quite dramatically.

http://was.kewlhair.com/hammer/gifs/grnd-effect.jpg
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.

http://was.kewlhair.com/hammer/gifs/grnd-effect.gif

Here's another simulation showing the air pressure gradients as a wing travels over uneven terrain. As the terrain rises the ground effect is increased and pressure builds as indicated by the color chart.

-Chiplight

Chiplight,
Take care with your interpretation, the two plots at the top do not show the same case, and do not prove the erroneous assumption that ground effect raises the pressure under the helicopter. Those plots (which are quite accurate) do not show the difference between IGE and OGE with the same angle of attack. Note that the deep orange lower plot has more lift, and higher angle of attack than the upper plot, that is why the pressure is higher. In other words, the upper plot shows the greater pressure with ground effect because the lift is greater, since the CHANGE IN CIRCULATION DUE TO THE GROUND contributes to the airfoil's lift. This would cause the helicopter to climb, so the collective would be reduced, equalizing the pressure.
In other words, the pressure under the airfoil, and under the helicopter, is the same in ground effect and out of ground effect, there is NO ground cushion of high pressure air.

The plots do show the circulation that the ground induces, and therefore the way the ground allows you to achieve the same lift with lower collective. Those plots show WHY IGE needs lower angle of attack to achieve the same lift, and that it is not pressure that allows hover IGE to be done at lower power.

Equivalent theories?
 

Nick,

it would be nice to have an animation showing the case where the pilot adjusts collective to hold lift constant; then the pressure increase may drop out of the equation so to speak.

Nevertheless, it is obvious that if nothing is done to lower collective, and we fly into ground effect that the 'reds" will light up the plot as above, leading one to ponder cause and effect.

I'm not any kind of expert, but could it be that there is some kind of equivalence between "pressure" based explanations and momentum based explanations of ground effect? I'm sure this has been mentioned in the thread before, but I've not read all 10 pages of it.(!)

-Chiplight

Chiplight,

The answer is obvious. The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE. There is no ground cushion. Read your altimeter in a hover if you have any doubt about this. There is no "ground cushion" of any kind.

Nick

Angle of Attack and Angle of Collective Pitch
 

Perhaps we are all envisioning the same thing, but in a different way.

Nick said; "The pressure below the helicopter IGE is the same as the pressure below the helicopter OGE."

Assuming that the helicopter is hovering in both of the above situations, I don't think that anyone disagrees with this statement. Nick has also said that the angle of attack will be the same in both situations and nobody probably disagrees with this, either. The thrust will also be the same.

What is different between hovering OGE and IGE is the angle of the collective pitch. In other words, the effect of the ground is that of allowing the helicopter to hover with a reduced collective pitch (plus a reduced rotor drag and a reduced power).

The question is; 'What is the cause of this effect'?

Pressure is a potential &/or partial answer.

Dave,
The cause of this is the fact that the ground suppresses the outflow, and allows the blade to deliver the same lift with less angle of collective. This is a bent flow discussion, not a pressure discussion. Note in the plot that Chiplight provides shows the equivilent circulation that the ground "provides" that helps explain the change in flow.

Cushion: From the French. A soft place to rest or to provide support.
 

This will raise some eyebrows or increase someone’s blood pressure. A while back I made a comment to Nick Lappos indicating that the field of engineering is in a constant state of change. I further stated that sometimes the new kid on the block would change the rules and redefine a problem in opposition to the facts as taught to the older guys. I don’t dispute what Nick is saying in his explanation. But, I also feel that based on what is taught to most helicopter pilots and this includes the older guys with no engineering training there is such a thing as a ground cushion and by implication the cushion exists due to an increase in the localized pressure field. It takes a long time to reshape a persons perception when it comes to learning a new theory while discarding the old theory. There is a possibility that new helicopter pilots will get Nicks explanation in their POF courses but until the last of the old breed dies off they will still believe in a “Ground Cushion” and an increase in pressure.

Even the FAA Helicopter book is in support of Nicks' theory.

:rolleyes:

BENT FLOW?
 

This is a bent flow?

Nick,
You refuse to accept that pressure has anything to do with ground effect, you have slagged me and my arguments several times without actually giving anything of substance to prove pressure advocates wrong, other than wonderful phrases that do not explain any physical behaviours. Now you tell us it is not pressure it is bent flow.......OK OK Please now tell me in words of one syllable, so we can all follow your your extraordinary logic, just how does the flow bend without pressure differentials?

PS I really thought my last post was quite reasonable. You say yourself as we approach the ground lift increases due to greater efficiency, and we are able to reduce collective to maintain the same lift. So now you tell me a little red demon produces this extra lift not an increase in pressure! I thought even your logic had already stated extra lift equals extra pressure, OK you said equal lift means equal pressure, surely that is the same thing.

PPS I am trying not to get personal but would accept a sound argument in preference to references to Sunday school attendance, how good a doctor my mum is, how great my first instructor was or even little red demons. Come on Nick real physics not my imaginary life please.