The Bernoulli Effect in Blade design

[Vfrpilotpb]

Good Afternoon Rotorheads,

This is a question that is aimed at you chaps who are able to undo the technical jargon and reply to us mere mortals in earth type language,

I have read recently that a Professor in America has stated that the (forgive my spellings) Bernoulli effect and theory is wrong, and that if you replaced this with a wing or blade section that was of parallel or straight sided section (without all the convex and concave surface that we have now) it would work just as well, for the action of flight and or lift occurs because of the angle of attack of a blade or wing incidence allied to the speed/power applied to the A/C when pushed through the air.
Now this guy could be right , if he is would that mean that wings and blade profiles would change and be cheaper and stronger, or is the current metheod of design and way of manufacture correct, therefor making Mr Bernoulli spot on after all.
This story was in the Sunday Times about 2/3 weeks ago, and begs quite a few extra questions about "Old theories" mainly connected with the physics of engineering and transmission of bodies either solids or fluids.
Whilst obviously all current designs embody the tear drop type sections in wings and blades, what do you people out in Pprune land think about this will things alter or stay the same?
:confused:

[Nick Lappos]

VFRpilotpb,
This is an ancient and noteworthy conundrum, basically boiled down to Bernoulli or Newton, as the angle of attack guys are basically saying that the wing pushes air down, and waterskis on the momentum transfer that takes place, a reflection of Newton's third law.

In fact, both are right, and are the same thing, simply expressed in different Physical science terms. The pressure differential stuff got to the gate first, so everybody sketches it and shows the pressure relationships as pushing the wing up. Those sketches never show the inherent effect on the air behind the wing, which is thrown downward. That the air is propelled downward means that the wing is pushed up by the law of conservation of momentum.

It is a disservice for a professor to try to peel these two intrinsically identical explanations apart, and it adds little clarity to the issue of understanding lift. I admit that when I teach performance issues to pilots, I tell them to ignore Bernoulli and simply use momentum, as it is a straightforward look at the effect on the air, measurable and practical.

[Vfrpilotpb]

Hi Nick,

Whilst I realise my question posed is like
"How long is a piece of string" and must have infinate explanations and diagrams along with calculations to explain it to its absolute end, thank you for giving the short answer that I must admit I understand what you write. Is the other effect the " Coander" or somthing like that, however it is things like this that set the old grey matter racing, whether for cover or open ground I'm not sure but it is deeply thought provoking.
Thank you Nick :)

[heedm]

Well done, Nick. This is similiar to the argument I was trying to make in another post. The same physics is buried in different explanations, but sometimes the alternate explanations are more clear.

I always wondered why the more obscure explanation (Bernoulli) was favoured over the intuitive explanation (Newton).

Matthew.

[Nick Lappos]

Heedm said:
I always wondered why the more obscure explanation (Bernoulli) was favoured over the intuitive explanation (Newton).

Nick sez:
Good observation! I have found that when people fall back on complex explanations, it is because they really don't understand the subject! I imagine the pay of a Sorcerer in the Middle Ages was higher if he could explain things so well that nobody could understand but the sorcerer.

[Dave Jackson]

The point has been expressed that certain activities can be explained by two or more methods.

For the brash fun of it, I will suggest that this is not always so. Newtonian physics and quantum mechanic can be used explain the same thing, but for very different purposes.
__________

Gyroscopic precession and 'so-called' aerodynamic precession are two quite different activities. For example; envision a teetering rotor in a helicopter without any mass. Gyroscopic precession can no longer explain the rotor's activities, whereas aerodynamic precession can.

(The consideration of coning angle is excluded from the above because both centrifugal force and gross weight will be zero.)

[ 08 August 2001: Message edited by: Dave Jackson ]

[imabell]

bernoulli states that if you increase the speed of a fluid that the pressure drops this is basic fluid dynamics. he was a preety smart fellow for his time.

on an aeroplane wing this theory is present in the form of an "aerofoil" shape. this can create lift. an easy example of this is to tear a strip of paper off one inch wide by about six inches long hold it between your thumb and forefinger and gently blow across the top of it. as the speed of the air increases the strip of paper will rise. it's a good domonstation of the principle.

most helicopter blades have an "aerodynamic" shape in as much as they are designed to pass through the air smoothly with little pressure change.(hunting etc)

this gives rise to newton stepping in on the game and it's a fair arguement i think. action reaction.
;) :cool:

[MightyGem]

Of course, all us rotorheads are aware of the "wing pushes the air down" thoery because of induced flow and rotor downwash. Aren't we?
:cool:

[heedm]

Dave said, "For the brash fun of it, I will suggest that this is not always so. Newtonian physics and quantum mechanic can be used explain the same thing, but for very different purposes."

Don't get me started ;-)


And, "...envision a teetering rotor in a helicopter without any mass. Gyroscopic precession can no longer explain the rotor's activities, whereas aerodynamic precession can."

If you apply a force to an object with no mass, you get infinite acceleration. Don't attempt to prove your point by moving into a realm which doesn't exist.

Matthew.

[Arm out the window]

A flat plate rotor might be easier to make, but I bet it would have some crappy flying characteristics.

Airflow behaviour approaching the stall would probably be fairly 'yucky' (scientific term there).

If that's wrong, I bet all those scientists who have spent hours watching wind tunnels to see how their latest aerofoil designs are working will be kicking themselves for wasting their lives, when they could have just drawn a flat plate on the design sketches and gone down the pub!

[Dave Jackson]

Heedm;

>Don't get me started ;-)<

OK. - but, for the fun of it -


>If you apply a force to an object with no mass, you get infinite acceleration.<

The mass may be gone but the aerodynamic drag is still there, Ain't it?

[heedm]

Dave, if the net force is anything other than zero, that force will accelerate the blade. If the blade's mass is zero, the acceleration would be infinite (F=ma). This only happens for photons, but we agreed not to bring quantum mechanics into this discussion.

If the net force on the blade is zero, then the blade will not change it's motion at all.

Matthew.

[ShyTorque]

vfrpilotpb,

"Flat plate theory" was originally used in the earliest days of aviation. I have a book dated 1911, that uses this, despite curved aerofoils being in use by then. Some early aircraft did indeed use flat aerofoils, as a follow on from kites but they weren't very efficient.

Then some sharper builders realised that birds have curved wings, not flat ones!

It was very quickly realised that a curved wing of the same size as a flat one provides more lift (that's also how control surfaces and flaps work).

Different aircraft require different wing characteristics and so many different profiles have been developed over the years.

One notable exception are aerofoils designed for supersonic flight. Many of these are angular and look rather like flat plates. The Lightning (twin jet) and the Starfighter were two very good examples. The Starfighter had sharp leading edges that needed protective strips on the ground.

ShyT

[Dave Jackson]

heedm;

Your uses of (F = m * a) is not quite fair.

My proposition was that the aerodynamics be included and the mechanic excluded.
You are discussing the mechanics and excluding the aerodynamics.

Just trying to 'drag' out the answer ~ aerodynamically speaking :)

[heedm]

Dave,

By making some small, apparently valid assumptions I can prove to you that 1=2, that you have enough money to buy Microsoft, that Elvis lives, and that Kennedy wasn't even shot.

You're right that without mass, there is no moment of inertia so apparently would be no gyroscopic precession. The only place that we can confirm this would be at the event horizon of a black hole, so we'll just take it as fact.

As far as applying aerodynamics without any mass to the system, the infinite acceleration is only one of your problems. Centrifugal force is used in that discussion and that of course is the realization of conservation of linear momentum in a non-inertial reference frame. No mass...no momentum (except photons, etc.). No momentum...no centrifugal force.

To be honest, I'm not even sure what point you're trying to make. We both know that all of the rotor systems out there are not massless, so does it really matter how a massless one would work?

Matthew.

[Dave Jackson]

heedm

I'm not even sure what point you're trying to make.
.....infinite acceleration is only one of your problems.

The current point is that there cannot be infinite acceleration when there is profile drag and its exponential growth.

By making some small, apparently valid assumptions I can prove to you that 1=2, that you have enough money to buy Microsoft, that Elvis lives, and that Kennedy wasn't even shot.

Don't tell me; the king is dead. :(

It can also be shown that nothing can be proven.
Science is based on math.
Math is based on the concept of 1 (unity).
The concept of unity is based on philosophy.
and the whole darn thing is man made; including the primary concept of [b]1[/i]
But back to mortal problems, such as Lu's :)

[heedm]

If you add up all the forces, including drag, and you end up with something other than zero then that force will cause an acceleration. If your mass is zero then that acceleration will be infinite.

This was just getting fun, why did you bring Lu into this?

Matthew.

[Dave Jackson]

heedm;

Lu is easier to pick on. Just kidding.


Lu, and anyone else who's interested;

A gift. Two textbooks for free.

Helicopter Aerodynamics http://www.cybercom.net/~copters/helo_aero.html


Applied Aerodynamics: A Digital Textbook http://www.desktopaero.com/appliedaero/index.html