Lift - comments?

Capt. Geoffrey Thorpe

Capt. Geoffrey Thorpe

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Some time ago, in the distant past, for reasons that are no longer clear, I went down this rabbit hole...

Anyhow, after poking at it off and on I think I have a reasonably coherent narrative. Anyone care to proofread? Comments? Does it make sense? Reasonable to follow? Did the wheels come off the bus?

And, yes, I know I am tilting at windmills...
 

Attachments

Are you saying lift is NOT a result of scary micro skulls printed under the wing which frighten the air? I’m confused….

or…

Wow. Sold me. Looks good! Probably a bit esoteric for most students… but those that NEED to know, should comprehend that.
 
lies. LIES! stop spreading this cr@p about newtons laws. any document not titled "Bernoulli is the ONLY logical explanation of lift" is just trash and misinformation. you're like the jehovah's witness of newtons laws.
 
FancyG said:
Who is your target audience and are you an engineer of some kind?
Click to expand...
My target is pilots who would want to know how a wing works. And, yes I am an engineer. How did you ever guess? :-)

Tools said:
but those that NEED to know, should comprehend that.
Click to expand...

OK. The fact of the matter is that pilots don't NEED to know how a wing generates lift. If you require proof, it appears that many (most?) pilots seem to believe things like the fairy tales about venturi cross sections, Newton on the bottom, etc. and they manage to pilot just fine. BUT, in my opinion, there is no real reason to be teaching fairy tales when facts are really not THAT much more complicated.

I think.

Am I wrong?

And, I just get annoyed when someone who is supposed to know something spouts nonsense. An educator is supposed to educate. For example, there was a Scientific American article that claimed something like "Netwon's laws don't explain the low pressure, you have to use Bernoulli"... They need to change the name of the magazine.

duty_calls.png
 
Capt. Geoffrey Thorpe said:
My target is pilots who would want to know how a wing works. And, yes I am an engineer. How did you ever guess? :)
/QUOTE]

Right?

This is written by an engineer, for engineers. Brilliant, but mostly incomprehensible to the layman.

Considering that a pilot can be in high school, how would you relay this information to a 17 year-old?

The cube example. You're assuming the reader can create 'a virtual cube' in their mind with a grasp of masses, pressures, vectors and forces and basically animate this in their mind. Then run it over a 'flow curvature.'

That's a lot to ask of a layman.

I believe you have the correct explanation. If you'd like the flying public to benefit from your work, its gotta be understandable.

Take the original explanation of lift diagram, draw a new one with your logic and underexplain the reason your model is correct(er) and you're golden.

E7FACE67-B973-46D8-AEE4-CCB6D010F6C4.gif
Click to expand...
 
My FM 3-04.203 “Army aero for dummies” states that 75 % of lift is Bernoulli and 25 % is action / reaction. Written by the most intelligent aerodynamicists in the world. It can’t be wrong.
 
Velocity173 said:
My FM 3-04.203 “Army aero for dummies” states that 75 % of lift is Bernoulli and 25 % is action / reaction. Written by the most intelligent aerodynamicists in the world. It can’t be wrong.
Click to expand...
If you start with circulation, you can say that 100% is Bernoulli.
If you look at pressures top/bottom you get numbers like 75/25.
But the laws of physics do not change between the top and bottom of an airfoil. Not to mention experimental data that contradicts the notion that there is some magic suction generated on top. Wait. I did mention it.

Do they give a reason for having two components of lift?

Here is a real life expert (obviously not me)
 
Anybody who argues about Newton vs. Bernoulli doesn't understand either one.

The treatise looks ok, but it's too technical for the average pilot, it's more at a sophomore engineering level.
 
Can this explanation of lift be extended to include an explanation of Mach buffet?

(as a 5-time college dropout, two attempts of which were engineering, I couldn’t understand what you were trying to convey. But I’ve never crashed an airplane because of my understanding-or lack thereof-of lift.)
 
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I am less concerned about the true nature of lift than I am about the proper way to explain it to a student pilot. How would you explain lift to a student (er, learner) on their first or second lesson?
 
dmspilot said:
I am less concerned about the true nature of lift than I am about the proper way to explain it to a student pilot. How would you explain lift to a student (er, learner) on their first or second lesson?
Click to expand...
That appears to be the problem. How do you simplify enough, but not too much, and still remain within the laws of physics.

The short simple legitimate answer is that airflow around the wing is deflected downwards. Action, reaction, you get lift. I would think a balsa glider as a prop would help - hold it at some angle of attack and wave your arms around to show the flow direction before and after the wing? @FancyG had a good picture in his post...

Don't throw ping pong balls against the bottom of your clipboard please...

What does a DPE expect from a student on the oral?
 
dmspilot said:
I am less concerned about the true nature of lift than I am about the proper way to explain it to a student pilot. How would you explain lift to a student (er, learner) on their first or second lesson?
Click to expand...
skip the detailed aero engineering stuff....and jump right to....." it can stall at any airspeed".
 
I love it, it is well-written and concise and reasonable.

I also love http://www.av8n.com/how/ .

Yes, I went to engineering school.

I think they're great for the person who knows enough ask but are overkill for the average pilot.

It's hard enough explaining high and low pressure systems to my kids because of the Coriolis force.

I tell my kids that, like the high pressure inside the balloon that wants to get out, the high pressure on the bottom of the wing wants to be where the low pressure is on the top of the wing, and then I wave my hands about winglets and stall fences.

"But why is there low pressure on the top of the wing?" "It's complicated..." and then I try to change the subject by mentioning that the propeller is also an airfoil and so are the control surfaces and we should really be talking about forces and moments instead...

@Mtns2Skies / @nauga, how do you teach this?

Introduction said:
On the other hand, rigorous explanations of lift tend to dive into vectors, layers of math or even partial differential equations which may be a bit of overkill for the average pilot.
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You then go on to talk about the Kutta condition... :)
 
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You can make a barn door fly with enough thrust. Change the shape of the barn door to an airfoil and it will fly more efficiently. Maximum coefficient of lift (CL) about twice as much, usually. Saves on fuel.

airfoils.png
 
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Capt. Geoffrey Thorpe said:
My target is pilots who would want to know how a wing works. And, yes I am an engineer. How did you ever guess? :)



OK. The fact of the matter is that pilots don't NEED to know how a wing generates lift. If you require proof, it appears that many (most?) pilots seem to believe things like the fairy tales about venturi cross sections, Newton on the bottom, etc. and they manage to pilot just fine. BUT, in my opinion, there is no real reason to be teaching fairy tales when facts are really not THAT much more complicated.

I think.

Am I wrong?

And, I just get annoyed when someone who is supposed to know something spouts nonsense. An educator is supposed to educate. For example, there was a Scientific American article that claimed something like "Netwon's laws don't explain the low pressure, you have to use Bernoulli"... They need to change the name of the magazine.

duty_calls.png
Click to expand...


If a man says something in the woods, and nobody hears him, is he still wrong?
 
Velocity173 said:
L = Cl • 1/2 p • S • V2
Click to expand...

I prefer the simplification when in the linear part of the lift curve of
L=1/2 * rho * V^2 * Cl_alpha * alpha

It makes the relationship to angle of attack clearer.

GeorgeC said:
Yes, I went to engineering school.


"But why is there low pressure on the top of the wing?" "It's complicated..." and then I try to change the subject by mentioning that the propeller is also an airfoil and so are the control surfaces and we should really be talking about forces and moments instead...
Click to expand...

As with others here, so did I. I never got an intuitive understanding of the why and after many years of frustration I came to the conclusion that I don't care what physically causes it. All that I care about is that I can generate and use mathematical models to predict it and control it. Whether that's the simple lift equation above, using circulation, using CFD models, etc.
 
skier said:
I prefer the simplification when in the linear part of the lift curve of
L=1/2 * rho * V^2 * Cl_alpha * alpha
Click to expand...

For that you have to measure alpha from the zero lift angle, not the chord line as usually given.

The lift coefficient slope is generally about 2*pi per radian of AOA. Camber changes the y-intercept, and thickness distribution and other things change how high AOA can get before stall, but the slope doesn't change much.
 
Dana said:
The lift coefficient slope is generally about 2*pi per radian of AOA. Camber changes the y-intercept, and thickness distribution and other things change how high AOA can get before stall, but the slope doesn't change much.
Click to expand...

Leading edge slots can change them slope by a fair amount according to the charts in Abbott and Von Doenhoff Theory of Wing sections. Aspect ratio can also impact it.
 
IK04 said:
Can lift work without gravity?
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My reference says outside the atmosphere there is no lift and no drag: those are aerodynamic (requiring air) forces it says. So if you're in an atmosphere you must have gravity otherwise the air would escape. Quibbling over terms I know, but words are important.
 
dmspilot said:
I am less concerned about the true nature of lift than I am about the proper way to explain it to a student pilot. How would you explain lift to a student (er, learner) on their first or second lesson?
Click to expand...
A tangential question, what does a student pilot do differently by understanding how lift works?
 
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