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#3545868 - 03/27/12 09:37 AM Lift theory. Were we wrong all along???  
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This may not be news to some of the rivet counters that inhabit these parts, but apparently the generally accepted theory of lift (using the bernoulli principle) is a complete fallacy.

Don't take my word for it. NASA says so.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

As I (don't) understand things, it seems that flow turning (is this the same as the coanda effect??) is now considered the accepted theory for how an aerodynamic surface creates lift. Or is NASA going crackers? As a teacher I HATE teaching the wrong thing, and teaching my students the various theories is out of the question.

Cheers


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#3545873 - 03/27/12 10:17 AM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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I don't think they're saying that Bernoulli's principle is wrong. Really all Bernoulli's principle says is that increasing the speed of a fluid decrease's its pressure.

I read it as taking issue with texts that claim airfoils generate lift by having the top surface longer than the bottom surface. They're pointing out symmetrical airfoils (same length top and bottom) can generate lift as well as designs where the bottom surface is longer.

Of course, I'm still half asleep right now, so I could be misreading something. reading

#3545875 - 03/27/12 10:23 AM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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Agreed. The issue here isn't Bernoulli's principle, it's the theory ((longer path/equal transit theory) which uses bernoulli's principle) to explain lift that NASA is debunking. This theory is THE theory that almost everyone learns, it's in just about every sim manual I've read and the number 1 theory carried by websites and encyclopedias.

What I'm interested to know is which theory is now considered to be the 'correct theory'. From a teaching perspective coanda effect is infinitely neater and simpler to teach aswell as being fun to demonstrate.

Cheers


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#3545895 - 03/27/12 11:47 AM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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The 'debunking' is a bit of a nit pick. Although the original theory of air travelling faster over the top to meet the air coming from the bottom has been extremely widespread I don't remember specifically learning that when I studied Aeronautical Engineering at college in the 80s. In fact we used a rotating cylinder to represent an aerofoil in most theoretical situations.

The nit pick comes from the fact that they are now saying that the air going over the top doesn't meet up exactly with the air coming over the bottom, so that's not the exact reason that the air goes faster.

Basically the air goes faster over the top because it's got further to go than the air at the bottom, that's still true, they're just saying that it's not racing to catch up with the air on the bottom.

Hope that makes sense, typing in a bit of a hurry frown

#3545915 - 03/27/12 12:30 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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You can continue to teach based on Bernoulli's principle as it remains perfectly valid, just drop the additional assumption of equal transit time on top and bottom (for which there quite simply is no justification). As Ant says, the latter merely provided a convenient, though unscientific, explanation of WHY the flow on the airfoil suction side is faster but has no basis in fact - it does not follow from any established physical principle (including Bernoulli's).

So the air DOES go faster on top and you CAN calculate the correct lift force from the resulting pressure difference according to Bernoulli, but equal transit time is NOT the reason why said difference in velocity manifests.

#3545922 - 03/27/12 12:46 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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The use of Bernoulli's theory is inappropriate for explaining the creation of lift, such as why the suction surface is *so* much stronger than the lower surface, AoA effects, assumption of equal transit times, not to mention the lack of viscous effects.

AD, as you mention, it IS the *turning* of air (or rather, accelerating it around the curvature of the aerofoil) which creates lift through changing the surface pressure. If you think back down to Newton's laws, F=ma - by turning the mass of air, you are accelerating it in a different direction, which exerts pressure on the aerofoil surface, resulting in lift.

If it was as easy as applying Bernoulli, we wouldn't need (so many) supercomputers to analyse the aerodynamic design of aircraft.

#3545923 - 03/27/12 12:48 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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I have not believed the oficial old theory since i heard it for the first time, some 35 years ago.

But then gases can be compressed and thinned out. Think about it.
The classical form is good for letting the air flow as undisturbed and turbulence-free as possible, and thinning the air at the upper side.
You need energy. "With enough energy, even a barndoor will fly" (Ernst Heinkel)

#3545935 - 03/27/12 01:01 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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I don't know why this would be a "new" theory. Wolfgang Langewiesche wrote what makes lift work (Newtonian physics) in Stick and Rudder back in 1944.

What makes the air flow around and then downward on the trailing edge of the wing (and forcing the plane "up" on the airfoil) is aided by Bernoulli, but I always knew the whole "sucking the airplane into the sky" thing didn't make sense long before I read the science on it.


The opinions of this poster are largely based on facts and portray a possible version of the actual events.

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#3545950 - 03/27/12 01:27 PM Re: Lift theory. Were we wrong all along??? [Re: Dart]  
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Originally Posted By: Dart
I don't know why this would be a "new" theory. Wolfgang Langewiesche wrote what makes lift work (Newtonian physics) in Stick and Rudder back in 1944.

Mathematical physicists had a decent and general formula long before then as well, so none of the underlying knowledge is new at all. I blame media ignorance for this whole Bernoulli thing.

#3545958 - 03/27/12 02:11 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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Does this mean my airplane will no longer fly ?


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#3545959 - 03/27/12 02:14 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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Call me dumb if you wish, but as a question for lift ... Would not angle of attack have a considerable bearing of contributing to such as well as airfoil shape and camber?

Only asking this as I have built numerous wings for model aircraft and all work with consideration that remaining forces required for flight is within scope. (Drag, thrust, weight)

Not trying to deter from this subject in any way, but you can have an airfoil equal in surfaces and lift can still be achieved.

And I probably don’t know anything anyway…. Just my thoughts.


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#3545965 - 03/27/12 02:34 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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Yep, your AoA will increase your lift, to a point (generally 10-15 degrees).
Your basic aerofoil shape determines the basic capability of the wing. Increasing the AoA linearly results in a proportional increase in lift, until the air starts separating/stalling.

#3545969 - 03/27/12 02:47 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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So does a plane with symmetrical aerofoils (for example a stunt plane) have to fly at a positive AoA to generate lift?

Cheers


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#3545977 - 03/27/12 03:05 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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Originally Posted By: AD
So does a plane with symmetrical aerofoils (for example a stunt plane) have to fly at a positive AoA to generate lift?

Yup.
Keep in mind that aeroplane wings may already be "rigged" at a positive AoA, so that a nose-level attitude may by default put the wings at a positive AoA to the oncoming air.

#3545983 - 03/27/12 03:19 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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AD: Yep.

As Dart said, it's all to do with Newtonian physics. (I'm about to explain the 'downwash' theory of lift. Feel free to debunk Smile2 )

(1) Force = mass x acceleration (F = ma.)

(2) Every action has an equal and opposite reaction.

In this case, the airfoil simply changes the direction of the airmass, providing an acceleration. (Note the acceleration occurs because velocity is actually made up of two bits - direction and speed. Acceleration is simply the rate of change of velocity, so, if you change the direction of the airflow, you accelerate it just as if you changed its speed.)

So we've got an acceleration of the airmass. Now when it's the right way up, the airfoil, due to shape and angle of attack, accelerates the airmass down.
By accelerating the airmass downwards, we must be exerting a downwards force on the airflow, since F = ma. That last sentence is the key. It brings us to (2): Since every force has an equal and opposite reaction, the airflow creates an equal and opposite force acting upwards on the airfoil.

Of course, that upward force is LIFT biggrin and it's the reason noble knights of the air such as ourselves exist.

Cheers

R

P.S. This explains why symmetrical airfoils must fly at a positive angle of attack; at zero angle of attack, they don't throw the air down, so they produce no lift. But cambered (curvy, non-symmetrical) airfoils do: so they can fly at zero angle of attack. But cambered airfoils normally produce more drag than symmetrical ones, so you don't get them on fast movers. And, of course, they don't work nearly as well upside down as they do the right way up, which is why we select symmetrical airfoils for aerobats.



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So the Thunderhawk would come screaming out of the sky at mach ohmygod! and as soon as the struts hit the ground they would rip off.


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You're trying to apply scientific principles to 40k. Do you not see the problem with that?
#3545985 - 03/27/12 03:24 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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#3546006 - 03/27/12 04:13 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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I always thought the old "the air wants to meet at the same time" line sounded a bit off.

#3546018 - 03/27/12 04:30 PM Re: Lift theory. Were we wrong all along??? [Re: oldgrognard]  
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Originally Posted By: oldgrognard
Does this mean my airplane will no longer fly ?


hahaha

This nacent revisionism will void all previous theories of flight (however anal it may be), whilst Bournelli turns over his grave etc. There will also be global delays at the airports as well while passengers wait for the new and improved theory to take over. Airlines will seek government assistance as a result. eek


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#3546019 - 03/27/12 04:32 PM Re: Lift theory. Were we wrong all along??? [Re: AD]  
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..and more importantly..will the airplane still lift off from the treadmill..??

popcorn



#3546023 - 03/27/12 04:40 PM Re: Lift theory. Were we wrong all along??? [Re: BeachAV8R]  
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Originally Posted By: BeachAV8R
..and more importantly..will the airplane still lift off from the treadmill..??


African or European? And will coconuts be involved?

popcorn


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