Helicopter Urban Myths

[Shawn Coyle]

From my observations - in a Bell 206, at a medium weight, the difference in pressure from rotors stopped to just lifting off the ground (which is where the pressure difference is most noticeable) is about 50' on the altimeter.
Isn't that easier to use as a measure - it's really insignificant as far as performance goes.
An interesting effect, but of little practical value.

And for whoever it was that copied the stuff from my book - I don't mind. The plugs are appreciated!

As far as the 'bubble' stuff goes - in a perfect no wind day, there may be a slight tendency to push the helicopter when the thrust vector has been tilted, but I would defy anyone to measure it.

I seem to remember there used to be a requirement to measure static stability in the hover - turned out to be impossible to do.

[delta3]

The bubble myth

Shawn

That look OK to me. The figures I published for the R44 show 1.09mb or 33 feet.
Be interested in imperical proof. The static tube should be reasonally close to the MR, and averaging the 5-7 Hz range correctly or be fast enough to register all pulses to allow for averaging out.


Nick, ho-ho...

1. I did say the induced power REDUCES because of ground effect : the induction SPEED is reduced because of the blockage, so is the induced power, and because of Bernoulli's fault the pressure increases below the rotor.
I suggest you take more time to read my stuff before you jump on it.

2. You say the induced drag is reduced because the angle of attack is reduced. FULLY correct, don't seem to have stated anything against that, see the posted vector diagrams in the stated scenarios that just illustrate that:
OGE:

IGE 3ft:

IGE 0ft:


You know all these equations are inter-related and there are many interdependent variables. This reduces to a chicken and egg situation. Why is the induction flow reduced, why is the angle of attack reduced, what is the impact on the static pressure...
And just to help understand the pictures : in the given attitude the effective aerodynamical angles of attack (solid lower mesh) are nicely symmetric around the rotor axis.
The mechanical angles (transparent upper mesh) are not symmetrical because the rotor is "flapping" to adjust its angle with respect to the hub-plane, needed for the given load/attitude of the heli. The mechanical angles are also greater because of induced flow.

3. I will not comment on the desktop universe. I had fun and I am proud that during many months I put 5500 lines of Math together, including detailed foil models of MIT applied to the R44 blades, fins and hull and including very complex 3D rigid body dynamics.
The results of the model gave me many sleepness nights, but all proved to be correct once I got over some apparently wrong predispositions.
I know math doesn't sell well and do not expect any changes soon. It's a relieve to me that history showed that some mathematical theorems took 100 years to be accepted. So I firmly and patiently stand by the figures I published.

4. I got the time despite some wintery weather out here to do some testing this afternoon in the following conditions
R44-II, 1 pob 2/3 fuel, 5-10 knts of surface wind.
(a) Hover Up wind : 19 map, steady hover at 11 feet over concrete (measured by radalt), keeping collective and moving over to long grass gives 14 feet.
(b) Starting from 11 feet up wind with gentle cyclic and fixed collective: take off without sinking: In line with prediction that once above 5 knts the translational lift, and in your words -Nick- reduced induced drag, takes over from the loss of ground effect
(c) Doing same down wind : needed to abort (increase collective) because of risk of touching down.
Cheers,
d3

[NickLappos]

delta 3,

OK, your work is truly great, and I can see the reduction on angle of attack and power. It is truly a desktop universe, and I mean that in a good way.

The problem is that without careful explanation, one could actually think you show how the "pressure bubble" under the rotor is what ground effect is, and that would be untrue.

Ground effect is when the ground plane flattens out the flow, causing the rotor to behave as if the blades were much longer. As a result, the tip losses for the blades are sharply reduced, causing a sharp reduction in the induced power requirements.

The reason why ground effect does not show very well at 100 knots is that induced power is very low there, but ground effect is still present at 100 knots.

For an airplane, the case is easier to picture. As the wing comes down into ground effect, the tip wash reduces sharply, and the wing behaves as if it were infinitely long. This requires less angle of attack (which had caused a large drag rise prior to entering the ground effect). The induced drag wastes power, so ground effect shows itself as a reduction in angle of attack and a reduction in the power required.

Ground effect is not caused by a pressure rise, nor is the advantage to the aircraft due to any pressure that pushes the aircraft up (or any other such pressure bubble nonsense.)

[delta3]

Pressure bubble myth

Nick,

Thx, the discussion made me think of the electrical resistor equations : is it the voltage that makes the current go, or does the current create a voltage differential...
People have personal preferences or (thought) habits to take one route or the other. For some scenario's one route may seem more appropriate : voltage creates current looks "more intuitive" when hooking a resistor to a battery, but current creates voltage looks more appropriate when hooking up the resistor to a coil. Some people may even talk about electrons jumping orbit and take great fun at using this route (could be freaky on this forum)


In the pressure bubble case, my preferred route is looking at the induced speeds, I only calculated pressure because of your myth (so blame Bernoulli and yourself...)


Concerning the plank / helo :

- difference between a line and a circle : locally on segments not a big difference, globally totally different topolygical (read mathematical) objects. So at some point this will show up somewhere.

- ground effect : to have the effect of a plank wing close to the ground (say cord width to height) I personally think we are more talking about a grass machine or an dangerously low flying inverted heli or a RC-model. If it so low as a landing plane, I even think some people would be surprised of the aerodynamical results (vortexes, vibrations etc???) But yes you are right the effects are there in both cases, but at some point it matters wether they are significant or not with respect to the other phenomena. Just as an example : in the three hover conditions above (OGE/IGE3/IGE0) rotor trust is slightly different because of the effects of down wash on the frame...

d3


added : some russian mathematicians take the coordinate references at the rotor blade and make the world go round. This is even difficult for me to grasp.

[Dave_Jackson]

Matthew says;

"Pressure Enhancer"
Careful, Nick. DaveJ will consider that idea as public domain and you won't be able to patent it.

Dave says;

"****! Someone already has patented it.

US Patent 5,197,446 ~ Vapor Pressure Enhancer and Method"

____________________


Nick says;

In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.

Me says;

The reason for this is very well understood by the Mongolian physicists. This physiological phenomenon is also apparent in the reducing diameters of a streamtubes. Simply explained; - the vortices compress the air. Then the gravitational force draws these denser masses of air toward each other. This is also very simply explained by three other forces.

_____________________


Urban Myth says;

12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees

Lu mental telepathes;

"The guys just don't get it. Everything is now very clear."

_____________________


Is this an Urban Myth, a Rural Myth or a Non Myth?

19) Gross Weight’s Effect on Autorotation

[delta3]

Dave


"This is also very simply explained by three other forces"

I always believed that quarks are responsible for VRS. I am quite relieved now that at last some physicists proved that... I leave it to you to convince the rest...
Personal experience: if somebody emphazises his point by using "very simply", look very sceptically at the claim...


19) http://www.aviationtoday.com/rw/issu...ts/engineering

No yoke but my simulator not only confirms the claim, it also predicts better autorotation with weight. Perhaps the next though test for the simulator. So I'll second this as a myth.


d3

[southernweyr]

Thank you all for your post, I am learning a lot.

Delta3 it is interesting that you mentioned that the R44 may not be affected by the Low Altitude Vortex because of its high rotor blades. The helicopter that I fly most definitely is affected by the Low Altitude Vortex because to perform a normal take off without getting the big dip is not possible unless certain wind conditions exists. The distance from the rotor blades to the ground is usually about 10 ft with a 2 ft skid height.

It is also interesting that you got more altitude at a given power setting over the grass than over the concrete. (If I read it correctly.) Was there a building closer to the concrete than the grass? Or is the idea that you have less performance over grass just another myth?

So, if I understand right, ground effect causes the air at the tip to flow more horizontally which increases AoA and also reduces induced flow. Both of these things help reduce the vortex rings generated at the tip.

[delta3]

southernweyr

I refer to studies done by Sheridan&Weisner 1977, also refered to by Leishman p189 explaining difference in accelerating forward flight between a OGE hover versus IGE hover. Their measurements shows that in OGE you just ride the power curve, whereas from IGE the trust will first decrease, requiring more power to compensate up to 10 knts, from which point it starts resembeling the OGE curves. To quantify: many studies show a 10% gain at Z/R of 0.75 (relative height of rotor above ground), and a max power needed at 8-12 ktns.

As stated before "one way to model" that is to look at average induced velocities. My simulator uses the Cheeseman&Bennetts formula for calculating induced velocities that is consistent with the above observations.

It is however my believe that higher loading (higher induced speeds) and closeness to the ground will make the effect more pronounced : not only will it create more ground effect -which I think nobody will contest- but also will delay the effect to greater speeds. As a result I would put it for a R44 at the 8 knt range. Caution: it requires quite precise measurements because we are getting below my 1% precision target, so I personally am not equipped to back this up with precise measurements other then it is in the 5-10 knt range.

With respect to the effect of grass you claim the opposite of what I observe. My explanation is that the grass creates more ground effect because it slows down the downwash more than concrete does. I also remember that some pilots quoted that the bubble the downwash creates in water also produces the same effect (don't remember the thread anymore)


d3

[Graviman]

D3,

That is a nice piece of work on the simulation. Is it using something which would run on a generic PC, or did you write it in (say) delphi? is it realtime, or do you have to set up the conditions?

I've been doing some homework to make sure i understand this properly. What really bothered me is is that since altimeter static port is in an airflow then, like a carburettor, shouldn't the static pressure go down (hence altitude up). My understanding now is that the freestream stagnation pressure remains the sum of atmospheric pressure P0 and reference frame dynamic presure 1/2RhoV^2. So for an altimeter static port pressure only varies from atmospheric if the flow past the port varies from TAS (near to a wing for example).

For your simulation this means that the stagnation (or total) pressure will be the same as atmospheric reference frame anywhere in the system. In the flow field the velocity, hence dynamic pressure, will vary. This means that you would expect static pressure to drop where the velocity increased. I think what your model shows is that in ground effect the downwash velocity slows down (as expected), and so the local static pressure goes up. However the stagnation pressure does not alter.

Basically the ground effect bubble has burst - or myth busted, Matthew...

[delta3]

Pressure bubble

Graviman


The logic (which is in fact used by the impuls theory) is as follows:

1. Start upstream at infinity : we have Pinf = Patm (inf cannot of course be that far that we have atmospheric pressure drop conditions, so a few hundred ft or so will do)

2. Just before the rotor we have vi and a static pressure drop according to the Bernoulli formula you quote.

3. The rotor system now interferes with the wake tunnel in that sense that it gives an average pressure impuls equal to T/A

Completing the impuls theory which calulates the vi results is (same numbering as before):

1. Pstat = Pinf = Patm

2. Pstat = Pinf -1/4 T/A

3. Pstat = Pinf -1/4 T/A + T/A = Pinf + 3/4 T/A

Simulator : written in Mat Lab, running at approx 600 frames/second, enough to have full blade dynamics. Calculates vi, pressure in derived via Bernoulli. Runs real-time on a PC (see the AVI-films I published on low-G roll over


d3

[Graviman]

Post a link, i'd like to see this work. Maybe i should buy MatLab...

I'm bothered about your number 3 condition - this just doesn't make sense. In a carburettor there is just no way to make the static pressure go above atmospheric (otherwise why design an expensive mechanical turbo charger).
The only way you can go above atmospheric, if this is the Pinf condition, is to push dynamic pressure way up (which leads to the interesting condition of negative static pressure - that's how low vacuum capture pumps work). For example where extremely high speed downwash impinges the ground could be where measured pressure went up, but not in the free stream.

I've done a few simulations in my time, and can vouch for the fact that sometimes you get the right result with an incorrect assumption. Clearly you have spent a great deal of effort getting this to work (and well deserved congratulations are in order), but the physics just doesn't make sense to me.


Mathew, for your static pressure bomb i would try to keep the bomb at the centre of the rotor downwash. For several different lengths get a measure of static pressure for hover at different heights above the ground. For completeness if the bomb also had a pitot tube you could determine stagnation pressure. If you feel that flight safety isn't compromised maybe get some records of pitot (stagnation) and static pressures at various alternate positions in the downwash (ie hand held). The same could be done in transition. Clearly i don't need to comment to record ambient pressure of the day.

My bet (one international standard engineer's pint) is that if you work out stagnation pressure (dynamic + static) for that flight condition, then you always get the same pitot pressure result, and static pressure is never above ambient.

[delta3]

Pressure bubble

Graviman


I'll pm you later tonight or tomorrow on the sim.

As a scientist, I fully agree with your point : (nice) theory, but needs to be backed up with hard data = measurement. I am not aware of publications in that field, and as a pilot, I realise that my altimeter does not really prove the theory, although Shawn did mension some readings in this direction, perhaps an idea to do some "static" flying with the robby.
I wonder though that people doing model tests should somehow have instrumented that.

So I guess the eyes are on Matthew, looking forward to some measurements.

d3

[Matthew Parsons]

This has left the urban myths world and ventured into the area where engineers and scientists think about the same things in different ways.

Try to forget about how to measure pressure, because that introduces so many other effects that it becomes easy to lose sight of the basic principles.

Think more about Newton's first law. Take one molecule of air moving swiftly toward the surface of the earth. It encounters ground. More molecules are charging up behind it. If you want to ease calculation and make the air incompressible, you have to increase pressure (and temperature will increase as a result). If you decide to be a realist and make the air compressible, you either increase pressure or temperature or both, however at some point you'll find you have to increase pressure. You're probably thinking that you don't need to increase pressure in either case because there is somewhere for the molecule to go, hence the pressure "spills" out. However, that molecule is going to keep going downward unless "compelled to change direction by a force impressed upon it". So we need a force that is pointed upwards (to reduce the vertical component) and outwards (to accelerate away) from the center of where the downwash strikes the surface. That force comes from the interaction with the molecules in the higher pressure area (its 3D so if you want to call it a bubble, go ahead). You can call it stagnation pressure if you like, but that is still a real pressure.

Now for the measurement. If I get a chance to do this (bjc, you in on this discussion?), then the biggest issue will be in measurement error. The differences in pressure we'll be looking for will be quite small, we will require much more precision in height than normal, and normal local variations in pressure could be greater than some of the numbers we try to measure. Or, we could discover a giant bubble of pressure (not likely).

Now for the urban myth. I haven't changed my stance. I do believe there is a pressure gradient formed underneath the hovering helicopter. It is an essential part of determining the flow of air in the vicinity of the helicopter. Also, when you move the helicopter forward, there is a sinking feeling. When you are trying to keep the helicopter still, it feels statically unstable, as though trying to balance a baseball on top of a basketball. In fact, it feels like you're hovering on a bubble. Two different facts, but I think this is where the urban myth holds to an extent. Just because the pressure "bubble" is there, doesn't mean that is why the helicopter feels like it does.

Of course, next time I describe how to hover, I still might refer to this sensation (and will caveat that it may not be scientifically accurate).

Matthew.

[delta3]

Pressure bubble

Two remarks

1. The fact that I claim that the static pressure under that rotor is higher in IGE than in OGE does NOT mean I am advocating an extrapolation of the metaphore to the point of stating that the heli sits on a (spherical) pressure bubble. May be a better metaphore could be : put a propeller on the top part of a tube that you put vertically on the ground. On the bottom side of the tube you have some air escape holes, that you can make smaller (IGE) or bigger (OGE)

2. Instability comes from a combination of all effects, among which the reaction of rotor efficiency with respect to height and cyclic angles as inputs to a comprehensive dynamic model.

Personally I never used pressure as any "driver" in the model nor in my "personal" thinking, I fully stand behind Nick's reasoning of induced flows, less induced drag etc as "a thinking" model".

But as I stated before impuls theory implies the above claim....


Two "ex aburdo" reasonings (without the need of going to the molecular level, which -as you state- is of course also possible, but only will make the model and convincing others more difficult).

1. If pressure does not increase how do axial pumps work, in particular all the turbines many of the PPruners use every day...

2. If impuls theory is wrong many schools will have to rewrite their books...

d3

[MightyGem]

why don't you tell me how a chunk of air changes its direction by 90 degrees when it hits an open surface squarely (ie downwash hitting ground).

Because it's got nowhere else to go...???

[deeper]

Oh Nick,
What have you done,

Urban myths have now become Global myths,

Some of these posts show an astonishing lack of general knowledge and add to the overall confusion of what is "simple aerodynamics", now we have low altitude vortex, what next.

Some of these posts make me feel that i have been in imminent danger for years of flying and was totally unaware of it.

[Backward Blade]

Buddy if you don't think that you are in "imminent danger" anytime flying a helicopter then maybe your in the wrong buisiness...All the above is a discussion which I'm sure all of us can use so that we mitigate the risk of that bustup that's always over the horizon for every flight we do

BWB

On with the thread

[just the driver]

Theory of flight

Couple of points guys:

Isn't all of the above just a theory ? - It was called theory of flight when I was taught it anyway.

Correct me if I'm wrong, but wasn't this thread meant to be a light-hearted fun thing. Some people have either A) Too much time on their hands or B) No life! You should get out more.

[Graviman]

Despite the fact that we seem to be at odds, actually this counts as a light hearted discussion! My motive here is not to disagree with Matthew and D3, but to get to the true nature of what is happening. I have a great deal of respect for both.

The point about axial flow compressors is a good one, the point about what causes the air to change direction is also a good one. Basically the answer, for me, is in the fact that a helicopter is operating in a free stream while a turbine stage isn't. The rotor also tries to generate the increase in static pressure, like the turbine stage, but the pressure leaks out. This is what causes the tip vortices, and is also why turbines have many overlapping blades. In a fenestron that tip vortex becomes recirculation around the tail fin - so again i would expect no static pressure diffence for the same flow velocity. For change of flow directtion, the ground is subject to the total stagnation pressure. The ground will see a pressure greater than atmospheric (after all it is supporting the weight of a flying machine), so a pitot tube would also register this increase in total pressure. The static pressure flowing with the airmass would not increase, and for any flow velocity should decrease. This stuff gives me headaches too...

Regarding the hover ball, well whatever imagery works for you. To me the dynamic instability is just driven by a finite response time and the risk of pilot overcontrolling. Tail last fixed wings have the same problem with longitudinal response - that's what often causes the landing bounce.

[Matthew Parsons]

Graviman, would there be any pressure on the front of a kite? If not, why is it harder to hold the string when there is wind?