Helicopter Lift Theory - Equal Transit, Skipping Stone & 1/2 a Venturi - all wrong...

[JB77UK]

OK so here is a frustrating situation for a new Helicopter CFI working on how to teach lift theory:

The 2012 FAA Helicopter Flying Handbook explains the creation of lift as a combination of half a venturi (the bottom half is the airfoil) and air pushing up on the bottom of the airfoil (like a water skier).

The 2012 FAA Helicopter Instructor's Handbook explains lift as the upper surface having a longer distance for the air molecules to travel than the lower surface and so causing the airflow above to speed up to make sure the molecules meet at the trailing edge.

Two different theories in books released within a month or two of each other!!! To make things even more frustrating, both explanations are completely wrong according to NASA:

Incorrect Lift Theory
Incorrect Lift Theory
Incorrect Lift Theory

So apart from a moan at the FAA, the question is what do you teach a new helicopter student:

1. What the Helicopter Instructor's Handbook says you should teach - 'Equal Transit Time' theory
2. What the Helicopter Flying Handbook teaches - a combination of 'Skipping Stone' & '1/2 a Venturi' theories
3. What NASA (who put a man on the moon using a pocket calculator) says - ' Turning of a fluid'

I WANT to teach option 3, but will they suffer in written tests and at the hands of DPEs because of it?

Note to the FAA - the NASA theory has been established for a long time - even longer than the amount of time ADFs have NOT been used in the average cockpit... ;-)

[paco]

I know someone who was trying to revise the Rotorcraft book but was bombed out because they just didn't want the changes. Did they finally get the tail rotor drift diagram right? I had to point out that the arrows were going the wrong way.

We have the same problem in Europe - according to EASA, the only reason aerofoils produce lift is because of the upper surface reduction in pressure, in the first 25% of chord.

So tell me - based on that, where do helicopters get downwash from? They should read Stick and Rudder!

Like you, I find it frustrating to have to say "this is what they want in the exams, but this is what really happens!" Which is what we do, otherwise they will not pass the exams in the first place (with the lies), or the tech interviews after that (the truth).

The truth is that all those theories work in combination. None of them work entirely alone.

[SilsoeSid]

Whenever I see the geese migrate south at this time of year, flying in close formation the following birds using less energy than the leader due to the airflow disruption, I wonder how this transfers to rotor blades.

The V formation greatly boosts the efficiency and range of flying birds, particularly over long migratory routes. All the birds except the first fly in the upwash from the wingtip vortices of the bird ahead. The upwash assists each bird in supporting its own weight in flight, in the same way a glider can climb or maintain height indefinitely in rising air. In a V formation of 25 members, each bird can achieve a reduction of induced drag by up to 65% and as a result increase their range by 71%.[1] The birds flying at the tips and at the front are rotated in a timely cyclical fashion to spread flight fatigue equally among the flock members.

V formation - Wikipedia, the free encyclopedia

[jymil]

@JB77UK: thx for sharing the links. I haven't really spend much time thinking about it, but the half venturi never added up for me. It's not like the atmosphere is a gigantic other half of a venturi. Any why should there be some magic force requiring the molecules to meet again at the end of the flow ? In reality, they don't. The NASA explanation does absolutely make sense for me. It's basically Newton's second law of motion combined with the physical properties of a fluid.

Here's the whole "turning of a fluid" explanation:
Lift from Flow Turning

[RVDT]

Try this -

[Thomas coupling]

Voila:

How wings really work - Research - University of Cambridge

The end.................

[RVDT]

TC,

Thats hilarious - the University of Cambridge doesn't understand it either!

Amazing. Maybe Holger has too much "tunnel vision" - pun intended.

[army_av8r]

I use a good mix of "all" the reasons why helicopters fly. but if someone doesn't get it, i just have them put there hand out a car window. start at zero pitch, and slowly increase. they can then feel the upward force on their palm. i also use symmetric airfoils to help explain why the upper/lower stuff isnt the only thing at play.

[lelebebbel]

All models are wrong - some are useful.
George E. P. Box

Teach what will get them through the exams. Mention that "this explanation works for practical purposes but is not fully accurate, and here are some links to NASA with more information if you are really interested in the subject. Now lets move on in the syllabus. What is next, ah yes, LTE.."

[paco]

Don't you mean overpitching?

Phil

[Thomas coupling]

RVDT: would you care to point out where they got it wrong at Cambridge Uni?

I'm sure their all ears listening to another aerodynamics research scientist

For those of us who have been teaching it for decades, I for one would be most grateful for further pointers to the holy grail.

[jymil]

The cambridge website says "it’s the curvature that creates lift". NASA says the deflection of air creates lift. Therefore, a flat wooden board also produces lift at a positive angle of attack. The shape does have an influence on how much the air is deflected, but it is not the source of lift. The coefficient of lift from the lift formula is a combination of both the angle of attack and the effects of the geometry of the airfoil, which may have either a positive, negative, or no effect on the deflection.

[[email protected]]

Well, whilst it is clear from the Cambridge video that the air doesn't meet again at the same point after the trailing edge of the wing, it is also clear that almost no air is deflected downwards - the streamlines only a short distance below the wing are completely undisturbed!

How does the Newtonian 'deflects air downwards therefore produces lift upwards' school of thought explain that?

The 'flat plate' effect at least is understandable since it relates exactly to what happens when you stick your hand out of a car window - the air pushes against your hand and forces it upwards.

I have yet to listen to the whole of the video RVDT has posted but the first 10 minutes are not that enthralling - he tries to rubbish benoulli lift theorem by saying that an aircraft in a 2G turn doesn't increase its speed so it cannot increase the acceleration over the top of the wing - I don't think he has heard of angle of attack.

[puntosaurus]

I should imagine the Newton 'school of thoughters' would say F=MA, and they would be wondering about why you were concerned about the displacement of air rather than the acceleration applied to it.

I think they would also consider that they'd moved on from the 'school of thought' phase.

[jymil]

it is also clear that almost no air is deflected downwards

If you compare the height of each line on the left with the right side, you'll see that air is indeed deflected downwards. And we all know this phenomenon from reality as downwash.

[Shawn Coyle]

I finally realized that it's a bit of Bernoulli and a bit of Newton. Mostly Newton, but the curvature of the upper surface does help to deflect the mass of air down.
You might also look at 'See How it Flies', a Fixed wing book freely available online. Great physics of flight stuff there.

[Ascend Charlie]

It's a combination of straight flow and rotational flow - the rotation goes from under the wing, around the leading edge, over the top surface, and down off the trailing edge - see the beautiful attachment point on those smoke lines, with the initial part of the airflow over the wing coming from underneath, not in front.

Try it yourself - blow up a ballon, and try to push it forward.

Now try it again with some backspin, and watch how it climbs and moves forward, having deflected the air down and back. Add frontspin and the balloon dives at the ground big time.

Sticking your arm out the window is a simple example of deflecting flow, without the rotation- the air is deflected down, and your arm goes up and back, the total reaction. This can be split into lift, and drag.

The rotation comes from the aerofoil shape, which makes the straight Newtonial deflection into a rather prosaic rotational flow with streamlines, inflow, downwash and angels singing. Mr Coanda joins in the chorus.

[canterbury crusader]

Warning

Clicking on this link will compel you to part with some of your cash.

Then you can make up your own minds on the deflection from the top surface. Worth every cent (check the other versions before purchasing).

https://itunes.apple.com/au/app/wind...1296?mt=8&ls=1

[[email protected]]

Any downwash caused by the curvature of the wing is preceded by an upwash. Anyone done the maths to see how these might cancel each other out? The streamlines show no net downward movement of the air, so where is the lift for a 747?

Any acceleration imparted by the curvature of the wing would (if you ignore benoulli and think Newton) impart a downward force onto the wing itself as the centre of curvature (somewhere below the wing) is where that force must be trying to push towards (like centripetal force).

The air on the underside of the wing pushes the wing up.

[RVDT]

the streamlines only a short distance below the wing are completely undisturbed!

Well it is in a tunnel so where is it going to go?

A WING IS NOT A HALF VENTURI
You might find it amusing to note that ground school courses
often introduce the venturi as an example of how wings fly.
The presentation includes moving one of the walls of the
venturi so far away as to not influence the other wall. What is
left is a wall with a hump.The instructors
tell the students that because of Bernoulli’s principle this “half
venturi” has lift. But you now know that this is wrong.The
wall blocks the downwash, so there can be no lift. After
leaving the hump, the air is traveling at the same speed and in
the same direction as before the hump. As we know, if there is
no net change made to the airflow there cannot be lift. So,
what do you do when you see this on the FAA written exam?
Well, if you want to pass, you will have to give them the
answer they want to hear, even though it is wrong!

Forget Bernoulli’s theorem
WOLFGANG LANGEWIESCHE, STICK AND RUDDER, 1944

In brief, the air bends around the wing because of the Coanda effect.

When the air bends around the surface of the wing it tries to separate from the airflow above it. But since it has a strong reluctance to form voids, the attempt to separate lowers the pressure and bends the adjacent streamlines above. The lowering of the pressure propagates out at the speed of sound, causing a great deal of air to bend around the wing.This is the source of the lowered pressure above the wing and the production of the downwash behind the wing.

This reduction in pressure causes acceleration of the air via the Bernoulli effect

The acceleration of air over the top of a wing is the result of the lowered pressure and not the cause of the lowered pressure.

As is popularly quoted.

For me it goes -

Coanda -> Newton -> And some interesting little Bernoulli effects along the way.

The lift of a wing is proportional to the amount of air diverted per
time, times the vertical velocity of that air.

The Popular Description of Lift
Most of us have been taught what we will call the “popular description
of lift,” which fixates on the shape of a wing. The key point of
the popular description of lift is that the air accelerates over the top
of the wing. Because of the Bernoulli effect, which relates the speed
of the air to the static pressure, a reduced static pressure is produced
above the wing, creating lift. The missing piece in the description is
an understanding of the cause of the acceleration of the air over the
top of the wing. A clever person contributed this piece with the introduction
of the “principle of equal transit times,” which states that the
air that separates at the leading edge of the wing must rejoin at the trailing edge.
Since the wing has a hump on the top, the air
going over the top travels farther. Thus it must go faster to
rejoin at the trailing edge. The description is complete.
This is a tidy explanation and it is easy to understand. But
one way to judge an explanation is to see how general it is. Here one
starts to encounter some troubles. If this description gives us a true
understanding of lift, how do airplanes fly inverted? How do
symmetric wings (the same shape on the top and the bottom) fly?
How does a wing flying at a constant speed adjust for changes in load,
such as in a steep turn or as fuel is consumed? One is given more
questions than answers by the popular description of lift.One might also ask
if the numbers calculated by the popular description really work.
Let us look at an example. Take a Cessna 172,
which is a popular, high-winged, four-seat airplane. The wings must
lift 2300 lb (1045 kg) at its maximum flying weight. The path length
for the air over the top of the wing is only about 1.5 percent greater
than the length under the wing. Using the popular description of lift,
the wing would develop only about 2 percent of the needed lift at 65
mi/h (104 km/h), which is “slow flight” for this airplane. In fact, the
calculations say that the minimum speed for this wing to develop
sufficient lift is over 400 mi/h (640 km/h). If one works the problem
the other way and asks what the difference in path length would have
to be for the popular description to account for lift in slow flight, the
answer would be 50 percent. The thickness of the wing would be
almost the same as the chord length.

Understanding Flight, Second Edition
By David Anderson, Scott Eberhardt

David Anderson is a pilot and a lifelong flight enthusiast. He has degrees from the University of Washington, Seattle, and a Ph.D. in Physics from Columbia University. He has had a thirty-year career in High Energy Physics at Los Alamos National Laboratory, C.E.R.N in Geneva Switzerland and at the Fermi National Accelerator Laboratory.

Scott Eberhardt - Aerodynamics Engineer - Boeing (current) Professor University of Washington July 1986 – April 2006 (19 years 10 months)
Engineer NASA Ames Research Center - September 1994 – June 1996 (1 year 10 months)

Scott Eberhardt's Education - Stanford University Ph.D., Aeronautics and Astronautics 1982 – 1985 - Massachusetts Institute of Technology
M.S., Aeronautics and Astronautics - 1980 – 1981 - Massachusetts Institute of Technology - B.S., Aeronautics and Astronautics - 1976 – 1980

Available on Google Books or

Amazon

Amazon

in a range of digital or hard media.

Me? I'm sticking with the physicist and the Boeing dude!