Helicopter Urban Myths

[Dave_Jackson]

Overt Auk says that Nick says;

""Explain how proximity to the ground affects aspect ratio.""

Then Ovet Auk says that he says;

"The correct answer is that extreme proximity reduces it. (You knock your wingtips off)."

Then The Shadow is about to says;

"Everyone is grammatically incorrect in the use of aspect ratio, because Wikipedia says;

""In linguistics, the grammatical aspect of a verb defines the temporal flow (or lack thereof) in the described event or state. For example, in English the difference between I swim and I am swimming is a difference of aspect.""

Then The Shadow throws in a couple of extra quotation marks to make sure that the above quotes have all been properly closed off, grammatically speaking.

"""

[waspy77]

The consistent frame of reference is that of the direction of the aerofoil through the air. Nick referred to it earlier as the free stream direction. This is generally used as it is the one real constant, whether on an aircraft or in the wind tunnel.

Other frames of reference can be used and in a fixed wing are usually closely mechanically coupled to the above. The fun thing about designing helicopters is that these many frames of reference often change dynamically with respect to each other.

Apply the equations concerned to the aerofoil, and then translate them anywhich way you want. Afterall it's the aerofoil that flies, everything else just hitches a ride.

[delta3]

Pressure Bubble Myth

Induced drag according to the simulator


R44-I, see figure for detailed boundary conditions


IGE (arbritarily choosen as 3ft):





OGE (30 ft):





Remark that the profile drag is (almost) the same, but induced drag is higher in the case of OGE.


So I guess my definitions must be similar to Nick's ones.


d3

PS remark the drag also becomes "induced lift", at small angles this is mostly neglectable


Added defintions according to my doc (is so complicated that even I need to read back!)

2. MAIN ROTOR DRAG

Rotor DM equation (rotor drag small angle approximation used in force calculations for non stalled profile)
In the precise calculations stall is incorporated. The precise and approximate calculations are performed at the end of each simulation to give an idea of approximation errors.

Drag = Profile Drag + Induced Drag
= Drag along IF projected by cos(alfaIF) + Lift by sin(alfaIF)
where cos(alfaIF) is almost= 1 and sin(alfaIF) = alfaIF

and alfaIF is the inflow angle of the relative airflow with respect to the HP plane, that is the plane perpendicular to the rotor axis. Remark that this plane is tilted forward by approximately 3 degrees in the case of the R44.

[Dave_Jackson]

. . . . OK No more jokes.


delta3,

You might find this information and the links of value.

Dave

[delta3]

Dave


Yes and .... no

I agree with your general definitions but I did not see a precise coordinate reference

The reference I choose is :

1. Project everything to the HP plane (= perpendicular to MR axis)

2. Reduce(=project) all flow-speed components a foil section sees to that reference plane (= rotational speed, inflow of air (aircraft motion+induction), flap and when fully dynamic rotor pitch and roll rates)

3. Calculate foil drag and lift with respect to that inflow angle

4. Induced drag is then defined by the angle of that vector makes with respect to the HP-plane

d3

[Graviman]

D3, you will have to explain those diagrams more clearly. The text is hard to read, and we are not familiar with your simulation. The polar plots slightly differ from one plot to the next, but i have no way of understandiing what this is telling me. Why is the plot slanted?

Agreed that in hover it is the induced drag which goes down. To me, the reason for this is the induced downwash velocity goes down. The blades will see a similar static pressure difference top to bottom (since heli weight has not changed), but the pitch will be less. So if you use axis plane as datum then the lift vector is less rearwards, so torque hence power is reduced.

It would help if you described how you accounted for the reduction in downwash velocity from ground effect. To me this is a complex CFD problem (using vortices or finite difference), and i am not sure how you can do such a complex calculation real time. Prouty would describe the way that the downwash wake contrction becomes reduced as the ground is brought nearer, so velocity goes down.

If you wish email me directly, since i am worried that this thread may be losing the pilot community for which it is intended.

[puntosaurus]

I think I've just realised the wisdom in Nick's and Waspy's comments about relating drag to the free stream.

I'm guessing the aerodynamicists definition of induced drag is quite narrow, ie. the cost of tip vortex production in the direction of the free stream. Therefore without tip vortices (ie with an unbounded wing) there can be no induced drag as far as an aerodynamicist is concerned.

I'm further guessing that Prouty, Wagtendonk et al and anyone else responsible for educating pilots may have appropriated the term induced drag to cover ANY gain in ROTOR drag as a result of lift production from the pilot's perspective.

If my guesses are correct the circle is squared, and everybody can be right according to their different definitions.

[delta3]

Pressure Bubble myth

Graviman,

For publication puposes resolution is reduced, I thought still readable, but I guess this may not be the case.

Your comments on boudary conditions are right : same weight, just a different hover height.

The graph shows :

Blue : profile drag
Green : induced drag
Color : total drag

all according to the previous definitions.

Why is the induced drag so slanted, up to a negative dip on the retreating blade, this is because the rotor is already flapping given the CoG of the load, see figure: cyclic is between 2.12 and 2.25 degrees backward. This can lead to a situation that induced drag gets propulsive when the blade is flapping down. The simulator allows me to look at all data, even point per point blade elements, but then again details get so volumenous, that this would be diffult to present clearly.


d3

[Graviman]

D3, thanks for the clarification. I think the slant is an example of where this can get confusing, depending on whether you reference axis plane control plane or tip-path plane. For the purpose of this discussion it is fair to comment that 1D momentum theory describes a uniform downwash relative to tip-path plane (which would be slanted in axis plane). Once the machine has flapped to equality, we are both agreed that this average velocity goes down with ground effect.

What i am still confused over, and central to the discussion, is how have you accounted for the way the ground plane affects the downwash velocity? Prouty shows how induced downwash Vige/Voge is a function of height, but this data appears to be either from wind tunnels or CFD simulations. I am guessing that you have used an empitical curve, possible taken from very accurate flight test data, to simulate ground effect in your model.

My only concern here is that we are trying to establish the cause of ground effect reducing Vige/Voge in a hovering helicopter. There are two competing ideas:
1. The complex 3D flow means that ground effect reduced downwash velocity through the rotor by reducing wake contraction.
2. The viscosity in the flow means that the ground resists the outwards flow, causing a buildup of stagnation pressure under the rotor.

Although your model is clearly very good, and probably quite accurate, it's reasonable to comment that we can not use it to bust (or otherwise) this myth. I get the impression you had reached a similar conclusion.

[delta3]

Pressure bubble myth

Right on Graviman (see PM)

whether this busts the myth or not, is a personal call.

I would go for plausible, because static pressure under the rotor disk does go up. I had by the way the possibilty to do my "static" flying test yesterday, all beith not very scientific
at 2/3 of fuel and 1 pob the alti lowered almost 20 ft just before lift off. (surprised me that as a pilot I didn't really notice this before, perhaps to buzy doing other things in lift off, thanks Shawn...).

The alti cannot be used as a reference any more when climbing and is not precise enough to use it as an instrument to validate lower static pressure OGE...

The increased static pressure also may not be used in an extrapolative way to extend to some spherical ball kind of picture, this is why I also would be willing to rest my case.


d3

[Graviman]

D3, we missed each other there. For reference i hope you don't mind if i put highlights of your PM on this post. I think this will increase interest in your model:

Originally Posted by Delta3

I think my model is best classified as : Blade element augmented with impuls theory. That is iterations are performed to balance impuls(=induction) and local aerodynamics on each full rev, according to Glauert.

Near ground I put an extra rule on (OGE) impuls theory consistent with Leishmans formula's (p187), which is backed up by experimental data:
Tige/Toge = 1 / ( 1 - (R/4z)²)

no CFD calculation of the wake is done.

To improve precision, I can add aeroelasticity and tips losses (this can already be activated), but compairing during check mode of the program those reveils differences that are of second/third order and as such of less "full heli dynamic" interest.

Interesting that the altimeter goes down 20'. Don't forget that the altimeter will only read correct when the flow past the static port is the same velocity as the free field. Although i would have thought the downwash velocity would increase the altimeter reading, not decrease it. On the other hand if you had sideslip, from say cross wind, the static port would pick up some dynamic pressure (no instrument is perfect).

I think we have to consider other effects here too. There will be a lag which causes the altimeter to low pass the pressure signal. This is caused not least by the capacity in the VSI.

The other assumption in the aerodynamic model is that the freefield flow is static. It is quite possible for an acoustic wave to build up under the rotor, since the rotor would literally act as a very large diameter loudspeaker as you changed collective. In fact it would not supprise me if a microphone nearby would pick up a subaudio pressure wave, every time collective is changed. This is obviously nibling away at the incompressibility assumption.

I think the best test would be for a 2nd pilot to determine how an altimeter set for Qfe would vary in a low hover into wind, but then if the downwash is not over the static port, does the test count? The more i think about it the more i realise that Mathew's comment about instrumentation accuracy is the limitation. There may just be no practical way to determine if the air just under the rotor compresses slightly in a stable hover...

[Shawn Coyle]

My observations on the pressure altimeter are that (in a no-wind situation) as the collective is raised from flat pitch, the altimeter starts to decrease in reading. This indicates an increase in pressure sensed by the static ports. The altimeter reading continues to decrease as the collective is raised, reaching it's lowest reading just as the helicopter lifts off.
The altimeter then starts to return towards the 'correct' reading and when the helicopter is out of ground effect, the altimeter is once more reading correctly (using known external references for height, as a radar altimeter is not accurate enough for comparison).
So, when OGE, the pressure altimeter is pretty accurate.
The difference in height/pressure at the 'just lifting off point' is minimal and makes no difference to any performance data. It's of academic interest, aside from modeling ground effect as has been so spectacularly done in earlier posts.

[delta3]

Pressure Bubble Myth

Shawn,

I fully agree what every word you state, for the intended use the altimeter resumes correct operation OGE and in forward flight because ..... the bubble got away (not the intent to put these words in your mounth, just picking up)

I also tend to believe in IGE it is a correct read out of the pressure increase.

1. read out in line with theory (could call this of course result biased reasoning...)
2. looks to me that given its position and function, it is capable to measure correctly under the scenario you described and which I litterally replicated.


d3

[southernweyr]

After all that has been said it seems to me that the pressure does increase under the rotor when IGE and this is what slows the induced flow. Would Bernoullis principle be at work here? Slower velocity would increase pressure, right? Of course, the pressure above the rotor probably increases as well. Please correct me if I am wrong.
I definately see the altimeter lower when IGE and my static ports are at the bottom of the helicopter on both sides just in front of the rotor mast.

Delta3, I read correctly and the idea of less performance over grass is a myth according to what you are saying. I do not claim otherwise. I have heard others claim that the tall grass slows the outward flow of air underneath the rotor system which allows the vortex rings to enlarge.

As to the rest of your post when you replied to me . . . I think you are saying that the R44 would dip at 8knts because of the distance of the rotor above the ground and the value of the induced flow velocity.

[delta3]

Pressure Bubble Myth

Southernweyr

Q1. Bernoulli : right on, at same trust levels, the pressure above MR proportionally increases with the pressure below when induction speed (and induced drag) decreases

Q2. I claimed the opposite, IF the grass slows down the outflow it may reduce induced speed, so I claimed a few feet higher hover when compaired to sleek concrete, but this is SPECULATIVE, as some pointed out I don't model this with CFD, so need to rely on experimental data. I observed this experimentally but not using a scientific set up.

Q3. Indeed, I claim that from 8-12 knts onwards indeed no dip will occur, starting at lower headwinds, a dip should occur

d3

[Graviman]

I'm stumped for words! So there is an increase in static pressure in ground effect. The next trick is to understand whether this effect is the same from ground level to rotor height. Good work, Delta3.

Matthew, i concede - but i'm at a loss to understand what causes this. I guess incompressibility is an assumption to make the maths simpler, but it shows that aerodynamics always finds ways to suprise you...

Shawn, what is your experience of torque or manifold pressure over long grass vs tarmac?

[Shawn Coyle]

I have only had the opportunity to look at this a couple of times but in all the cases I've tried moving from hovering over hard concrete to hovering over even short grass without moving the collective resulted in a definite decrease in height over the grass, and a return to the same height when back over the concrete.

Probably too much flight time at my disposal without definite things to do.... so I can pay attention to trivia....

[Dave_Jackson]

definite decrease in height over the grass

Speculation: Perhaps the reason for this is that grass has a lesser surface friction (smooth) than does concrete (rough).
_____________

A thought that just keeps bubbling up to the surface.

• Meteorological charts show pressure and each line represent a difference of 4 hectopascals (hPa)
• Tire pressure is measured in PSI.
• A tire pressure of 36 PSI = 2,500 hPa
• We tend to think of pressure in the context of the much higher tire pressure.
• A meteorological pressure difference of a few hectopascal can create a wind.
• This wind can remove the 'whats the thing' out from under a helicopter when it is hovering in ground effect over a fixed point.
• I would suggest that meteorically induced air pressure is blowing away the rotor induced air pressure.

Perhaps aerodynamists do not talk in terms of pressure because 99% of their computations relate to aerodynamic activities that have nothing to do with the ground.

However; pressure is pressure.

Dave

[Graviman]

Weather is actually a good example. It is definately free stream yet the centre of a hurricane is not moving and is at low pressure.

I just had a discussion with another chap who has fluid dynamics experience (dredging ships, so slightly larger than helicopters). The conclusion we reached is that directly below the rotor at ground level you would expect an increase in static pressure (due to change of direction of dynamic pressure), but further out you would expect a decrease as venturi effect took over. Certainly i would expect this on the ground, but i'm still amazed that the air itself actually compresses under the rotor.

Originally Posted by Shawn Coyle

Probably too much flight time at my disposal without definite things to do.... so I can pay attention to trivia....

Lucky so&so! C&C is an enjoyable read BTW.

[delta3]

Pressure bubble myth

Graviman

That is correct, according to Impuls theory / Bernoulli, the wake tunnel below the MR in OGE contracts and will progressively half surface down stream (so square root for diameter), whilst induction doubles from vi to 2vi with the corresponding static pressure drop starting from below the rotor

In IGE the geometry is different, very turbulent, so too complex, so the whole argument starts upstream from the rotor (once we agreed upon the effects on trust and vi-IGE) and continues down stream to just below the rotor up to that point where the wake stops OGE-like contracting because of the ground proximity. I did not make any prognosis on the speeds/pressures from there on.

This is why measurements should be reasonably close to the MR, but not too close to reduce the "loadspeaker" pulses (a few feet i would say) and increase setup safety.

d3