No One Can Explain Why Planes Stay in the Air, February 1, 2020

[Mfezi]

9 hours ago, Ben Brown said:

Mfezi, thank you for the informative post! 

 

Ben

 

Thanks, Ben. We are actually namesakes (I'm also Ben). Unfortunately, my post wasn't really informative in relation to how a wing works - I'll leave it to the textbooks (or that NASA site mentioned above is a pretty good intro without trying to explain everything in one shot). I just wanted to give a bit of insight into the difference of studying something through the basics from the ground up, vs a quick explanation and how a quick explanation is sometimes not really possible. I've seen both approaches due to my studies and work on the one hand, and also doing private flying, gliding and microlight flying on the other, each of which included study material on "aerodynamics" that I personally felt was not just inadequate, it was usually wrong. As I said, I think Anderson's contribution to the Scientific American article was based on his frustration with the difficulty of explaining a fairly complex idea in sufficient detail in a few paragraphs and sketches while still keeping the physics correct. I'm sure engineers and scientists in other fields have the same frustration, and when they can't quickly explain how it works in two sentences, the interpretation is sometimes that the "scientists don't understand it either".

 

Above someone mentioned that all wings create lift by pushing air down. This is of course not wrong at all - it is exactly what happens. However, there is a bit of a chicken and egg thing going on, because is the air being pushed down because of the lift, or is the lift produced because the air is pushed down? And when you look closely, you will see that the air in front of the wing actually flows upwards relative to the wing and so does the air on the outboard side of the tip vortices. And the word "push" is misleading, because a lot of the air affected by the wing is actually above the wing, and not below it. So, how does air relatively far above the wing end up being accelerated downwards? And since air is usually considered a continuous medium (it is virtually impossible to keep track of individual molecules), which "packet" of air are we talking about? If you are satisfied by the one-liner explanation, it is fine, but personally I feel there needs to be a bit more in the explanation, because there is a lot more that actually happens around that wing. As engineers, our job is to manipulate the physics to do something for us (produce lift) in the most efficient way possible - so the one-liner explanation is of little use for that. For a pilot, it may or may not be enough: Does it explain the shape of the wing, the relation between angle of attack and lift, the relation between drag and lift, and what happens when the wing stalls - all concepts that I think he needs to understand? And this is still just aerodynamics - flight mechanics is just as important and that is an entirely separate discipline again.

 

As I said, I think the title of the Scientific American article is not completely wrong, especially if you add "in a few paragraphs", but the subtitle is completely out of place and it missed the point of what Anderson tried to convey.

[ESzczesniak]

9 hours ago, Mfezi said:

Above someone mentioned that all wings create lift by pushing air down. This is of course not wrong at all - it is exactly what happens. However, there is a bit of a chicken and egg thing going on...

That was me and I hear you. I’m trained through mechanical engineering (undergrad only).  I’m less extensively trained in fluid dynamics, but my internships were on compressor cores and of course, I love aviation. 
 

Like you said, it ain’t simple. And nothing happens in isolation. 

 

Obviously, I’m saying stuff you already know, but for interest of others...

 

The acceleration of air over a wing is in large part about the boundary later. This effectively adheres to the wing (it’s a stall when it doesn’t). So as the aft wing tapers, this essentially expands the volume of air above the wing creating lower pressure. That lower pressure accelerates the air, which then leaves the rear of the wing faster. And typically with a downward trajectory, creating some lift. The PPL book would apply Bernoulli’s principle in the opposite order, suggesting the shape of the wing accelerates the air, thus causing a lower pressing. When in reality, air actually slows to some extent ahead of the wing, but then accelerates as the airfoils tapers. 
 

But then it gets more complex, as this airstream rejoins the higher pressure stream from below. This is a vortex generator, as is air spilling over the wing to from top to bottom. And when managed well, these can generate meaningful lift as well. 
 

I had been trying to stay a little simple, but since this thread is drawing out some of us enginerds, I regress. 

Edited July 7, 2020 by ESzczesniak

[southwestforests]

And I the one who began this thread am really enjoying seeing where it goes and what is contributed.

This is fun. 🙂

 

Also, just came back here to get my 2 sources to use in a thread about boost gliders over on The Rocketry Forum and see what happens there.

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