Lift Video

[Bede]

I think Photofly nailed it. I was too lazy to post all of that (and honestly, couldn't come up with ALL of the issues.)

One of the interesting things that I have found about aerodynamics is the different mathematical treatments given to it by different authors (similar to NMR-nuclear magnetic resonance). For example, in Anderson's Aerodynamics, he presents a lot of the theory of lift using volume integrals. Abbot's Theory of Wing Sections goes into circulation and Denker uses a thermodynamics approach. You read three different books and think they're discussing different phenomenon, but it's really the same thing discussed in different ways, all of which are approximations, but at least the approximations approach reality.

[flyingjerry]

Everyone here is overcomplicating the matter, there is only one explanation you will use every flight. Lift is a byproduct of money. Money is traded for dinosaur juice which is used to make fire. Fire makes plane go up.

[Chris M]

Also this one. It's huge so I won't post and take up too much thread space.

https://xkcd.com/1133/

[photofly]

People box - ROFL

[Chris M]

People box - ROFL

"This end should point towards the ground if you want to go to space. If it starts pointing toward space you are having a bad problem and will not go to space today."

[Heliian]

Lift is created by the pressure drop on the top surface of the wing. Can be altered by relative wind, aoa, airfoil shape and airspeed.

One of the easiest descriptions is to picture the air going over and below a common airfoil shape. The lower side continues at the same speed, the upper or lift producing side sees the same air but it is accelerated over the airfoil shape which in turn drops the pressure and creates lift.

Keep it simple.

You will also see that a wing creates the most lift/differential right before the stall.

This one is great!

[youtube]https://www.youtube.com/watch?v=6UlsArvbTeo[/youtube]

[Bede]

Lift is created by the pressure drop on the top surface of the wing. Can be altered by relative wind, aoa, airfoil shape and airspeed.

One of the easiest descriptions is to picture the air going over and below a common airfoil shape. The lower side continues at the same speed, the upper or lift producing side sees the same air but it is accelerated over the airfoil shape which in turn drops the pressure and creates lift.

Why does the air accelerate over the top of the wing?

[photofly]

Because the Kutta condition must be obeyed.

That’s why wings have to have a sharp trailing edge. If they don’t, they can’t produce any lift. Regardless of any cockamamy “Venturi” diagrams.

[jschnurr]

it is accelerated over the airfoil shape which in turn drops the pressure and creates lift.

It appears that there is a huge low pressure area above/behind the wing during a stall (In the area without smoke in the video) as well as a huge high pressure in front of the wing at the same time. Why again does the airfoil lose lift in the stall?

[photofly]

The low pressure on the top surface of the wing diminishes above the critical angle of attack. The airflow on top of the wing becomes turbulent and recovers the stagnation pressure.


That dark area over the wing in the video is high pressure, not low.

[Bede]

Because the Kutta condition must be obeyed.

That’s why wings have to have a sharp trailing edge. If they don’t, they can’t produce any lift. Regardless of any cockamamy “Venturi” diagrams.

Lol. That's not the answer I was looking for. What I wanted someone to say is "because the bump the air has to go faster over the top to catch up with the air on the bottom" which is of course false.

Explained here:
https://www.grc.nasa.gov/www/K-12/airplane/wrong1.html
https://www.grc.nasa.gov/www/K-12/airplane/wrong2.html
https://www.grc.nasa.gov/www/K-12/airplane/wrong3.html

And then of course, there is the mathematical treatment
https://www.grc.nasa.gov/www/K-12/airplane/right2.html
https://www.grc.nasa.gov/www/K-12/airpl ... ereqs.html
https://www.grc.nasa.gov/www/K-12/airplane/nseqs.html

[youhavecontrol]

I think the easiest way for me to describe it is absolutely NOT online or through anyone else's video, but with a whiteboard, a spoon and a sink faucet.

[photofly]

a spoon and a sink faucet.

That'll be the Coanda effect. It has nothing whatsoever to do with how a wing develops lift.

[Schooner69A]

A friend used to teach high-speed aerodynamics to budding jet instructors: he used to introduce his first lecture with "All you have to know about aerodynamics is EEE equals EMM CEE squared and you can't push on a rope". I replaced him but continued with the introduction.

And then off we'd go into MCrit and Lambda foot.

(;>0)

[aeroncasuperchief]

I always thought that lift was a factor of decreased pressure on the top AND air deflection down ( and other minor factors )

[photofly]

The downward deflection of air goes hand in hand with the low pressure on top of the wing.

You can't have one without the other, and each can be used to "explain" lift.

The low pressure explains the mechanism by which the upward force is exerted on the wing, while the downward deflection of the air explains where the upward momentum given to the wing comes from.

[sportingrifle]

My understanding is that a wing creates lift by several different means depending on angle of attack. The sum of them becomes the coefficient of lift that varies with angle of attack as each of the contributions to the wings total lift change. Why does it matter? It matters a lot in spin dynamics because a stalled wing is still producing some lift - just less of it and a lot more drag. There is a misconception that a stalled wing produces no lift - not quite, it just doesn't produce enough to support the aircraft. And this is why in spin aileron tends to aid recovery fro spins ( helps un-stall the inside wing and reduces the induced drag on the inside wing) while outspin aileron will drive the aircraft deeper in to the spin. At the end of the day, some lift is produced as merely a reaction to the air flow against the inclined surface, much as it is if you angle your hand out the car window. Even when stalled.

[photofly]

My understanding is that a wing creates lift by several different means depending on angle of attack. The sum of them becomes the coefficient of lift that varies with angle of attack as each of the contributions to the wings total lift change.

Ok, please don’t think that any more; it’s not true.

At the end of the day, some lift is produced as merely a reaction to the air flow against the inclined surface, much as it is if you angle your hand out the car window. Even when stalled.

This is another big misconception. That an angled hand produces lift by some “other” angled-plate mechanism that may or may not be part (or all) of what happens to a proper wing.

If you want lift, you must create a circulation of air around the lifting surface.

You can do this by rotating the surface (the Magnus effect) which is what golf balls and tennis balls do, as well as Flettner aircraft:
https://www.youtube.com/watch?v=iaLVFJuCuP8
https://www.youtube.com/watch?v=K6geOms33Dk

One problem with Flettner rotors is that there's no concept of Angle of Attack, you can only control the amount of lift by changing the speed of your rotors.

Alternatively (and much more conventionally) you can create lift by having a big surface with a sharp trailing edge (let's call it a "wing") and have the wing create its own circulation because the Kutta condition is obeyed. If you do this you get the built-in ability to control the amount of lift by adjusting the angle of attack. Which is helpful.

Even the trailing edge of your hand is sharp enough to generate some circulation if you offer it to the airflow at a reasonable angle of attack; and your hand gets pushed upwards.

[photofly]

What's wrong with the "when I put my hand at an angle out of the car window, air molecules hit the bottom of my hand" explanation?

Here are some ways that explanation is defective.

It ignores the fact that air molecules hit the top of your hand too. Sea level pressure is about 15 pounds per square inch; the difference in pressure between the top of your hand and the bottom is a tiny tiny fraction of 15 psi, otherwise your hand would be violently removed from your wrist. An explanation of why your hand is pushed up can't ignore the air molecules hitting the top of your hand too. If it's going to be a useful explanation it should explain why the number of air molecules hitting the bottom of your hand is in fact barely any different to the number of air molecules hitting the top of your hand. Even if you drive really fast.

"But I can feel the air hitting my hand!" - actually, no you can't. Do an experiment. Hold your hand out in front of you while sitting still. Can you feel the air hitting your hand? There's 15 psi of molecules hitting your hand all the time. Your hand is about 20 square inches in area so is subject to a constant pressure of about 300 lbs, which you can't feel. What you can feel when you hold your hand out of the window is the tiny tiny pressure difference of 0.01 psi between the top and the bottom of your hand, because your fingers are sensitive to pressure differences.

Secondly - air molecules hit each other much more than they hit your hand. The mean free path of ambient air is 68 nanometres. That's on average as far as an air molecule will travel before it hits another air molecule. Only air molecules that are closer than 68 millionths of a millimetre to your skin stand a good chance of actually bouncing off your hand. Yet the airflow is disturbed by your hand for some tens or hundreds of mm away from it. Everything of interest to do with the force exerted on your hand is caused by air molecules hitting other air molecules somewhere far away from your hand.

The best way to explain the force on your hand is to say:
1. At any reasonable scale, air is a continuous fluid. It has a pressure everywhere.
2. When you hold your hand out of the window your hand disturbs the airflow which results in a difference in the air pressure between the top of your hand and the bottom
3. The difference in pressure produces a net force on your hand.

[Beefitarian]

Reading the hand out the car window. I just realized my hand is airfoil shaped. The palm is concave and the top is convex.

Whoa.

I will have to go see if I can feel a low pressure on the top next chance I get.

Also this one. It's huge so I won't post and take up too much thread space.

https://xkcd.com/1133/

That is glorious. I was at NASA near Houston recently and got to walk around beside a Space Car 5. I wish I saw that blue print first.

It was not going to space the day we were there. It's inside a building and the big fire end is pointing at a wall.