Do Pilots Understand How An Airfoil Generates LIft? (Survey Says...)

[Capt. Geoffrey Thorpe]

I the past, I have made the assertion that most pilots don't understand how a wing generates lift. And that the common "explanations" have evolved like a bad game of telephone...

But, I really don't know how many is "most" - if you could be so kind as to answer above...
And, if none of the above seem to be true, what is your understanding?

T.I.A.

 

[PeterNSteinmetz]

My take on this is that the following:

The actual explanation is that the wing changes the pressure distributions in a way best described by fluid mechanics.

That is complicated so as instructors we have simplifications such as the Bernoulli effect or momentum changes which are valid in different regimes of flight.

And that most people not recognizing the first point think that their preferred simplification is completely valid and all others are invalid.

 

[PeterNSteinmetz]

I would argue that an explanation that violates the laws of physics is not a "simplification" - it's more of a fairy tale.
Yes, the math is complicated if you want to do detailed calculations of pressures and velocities. Way complicated.
But...
The basic concept of air flowing along the top and bottom surfaces which causes a net acceleration of the air downwards requires low pressure on top and a relatively higher pressure underneath to make the air accelerate is pretty darn straightforward and takes less than 10 minutes.

Agree that such simplifications should agree with the laws of physics.

I think even this video has a few issues. Firstly the embarrassing correction of Newton’s third law to first law, but that is not a big deal.

The bigger issue in my view is his statement that the pressure differences arise from the fact that the air is accelerated (@5:52). The acceleration is caused by the force, which he does describe elsewhere. And the force is simply the integral of the pressure difference over an area, so neither causes the other, they are the same thing. So his statement of causality in that order , acceleration -> force is not technically correct. Rather, pressure difference integrated over area = force -> acceleration and lift.

 

[PeterNSteinmetz]

And, where does the pressure difference come from?

To accelerate a bit of air around a curve, you have to apply a force. That force is expressed as a pressure when dealing with fluids. So, to "pull" the air over the top of an airfoil, there has to be a low pressure at the surface that results from the fact that the air follows the surface.

I agree that the pressure distribution or difference is in some sense primary. But in the video he mis-speaks a bit and says the acceleration causes it.

Does one normally explain Newton’s laws by saying that the acceleration of an object causes the the force on it? Or the other way around?

If he had said the wing pushes the air up and around it, I think that would have been better as it is fundamentally the presence of the wing in the fluid that gives rise to the changes in the pressure distribution.

To some extent this is a problem with using words to describe fluid mechanics. Which is why I think it best to always precede such explanations with the note that this is fundamentally a fluid mechanics problem, which is just Newton’s laws applied to fluids, and let’s talk about some simplifications.

Overall I think his presentation in the video is a rather good one, but it would have benefited from writing out in advance the exact words to be used at certain points. That would have avoided the whole 3rd law / 1st law correction. Indeed, it would be very nice if he would revoice the video!

 

[PeterNSteinmetz]

Was thinking about this a bit more and of course this will always depend on the audience.

With a 14 year old taking lessons to fly gliders, I think even the pressures creating forces to accelerate parcels of air may be a bit much. That is where the simple demos may be better or just saying there is a higher and lower pressure and you will learn more about it later may work better.

 

[PeterNSteinmetz]

You are falling into a common trap many do when thinking about subsonic aerodynamics. It is not a good mental model to think of causality in this way. Your statement basically boils down to "Does the pressure gradient cause the flow turning/streamline curvature or does the flow turning/streamline curvature cause the pressure gradient?" The answer is there is not arrow of causality here, but rather a compatibility of the resultant flow pattern/solution with the governing equations and the boundary conditions imposed by the wing. While with time and experience one can develop deep physical intuition for these types of systems and flows, thinking of it causally like this often leads one into circles because as I said, there isn't a direct arrow of causality but rather compatibility. As to *why* the flow/streamlines must curve, see my next post.

Agreed. That was sort of my point relative to the video under discussion. It does not make sense to say that the acceleration causes either the force or the pressure difference, as the video does.

 

[PeterNSteinmetz]

If you tie a weight to a string and swing it around overhead, does the acceleration of the weight cause the force in the string or does the force in the string cause the acceleration?

Seems pretty clear to me that the force on the string accelerates the weight and that without that force the weight continues in a straight line.

You might be alluding to the “centrifugal force” apparently pushing things out in the accelerated rotating reference frame.

But many people complain about that sort of explanation as well and it acts in the opposite direction. I don’t think in the video he was referring to a virtual force felt in the reference frame of the accelerated air.

Overall I thought it was a fairly good video - but would benefit from a new audio track with some corrections.