How Airplane Wings REALLY Generate Lift

[Silent Film]

 

[Pavlin_33]

Haven't seen the video, but I would say that angle of attack generates lift.

[AndyJWest]

51 minutes ago, Pavlin_33 said:

Haven't seen the video, but I would say that angle of attack generates lift.

Just think of the time, effort, and expenditure that could be saved if aircraft builders didn't have to install wings at all, and just installed an angle instead! 

[Dragon1-1]

X-24 would like a word...  

[-0303-]

On 4/12/2025 at 1:00 PM, Pavlin_33 said:

Haven't seen the video, but I would say that angle of attack generates lift.

It's a good thorough video. Didn't know, never seen a complete explanation before. He even goes down to the atomic level, explaining the boundary level, very interesting and not nearly as complicated as it may sound (*1). But he says that part is not required to understand the whole video. I think video could be significantly shorter if he repeated himself less.

There's three things that all add lift (had just a vague understanding, 3rd factor I didn't know at all):

1. Angle of flat underside imparts force by redirecting a vector of the air 90 degrees downwards. Imagine the incoming air split into two vectors, one of which 90 degree down. Then Newtons law of action / reaction.
2. The Bernoulli thing, kind of, except not really. Higher pressure compressed air in front of the wing leading edge leads to a lower pressure above the wing and hence higher pressure below the wing adds lift.
3. Air stream adheres to the wing topside (and underside for that matter)via the boundary layer. Since the typical wing shape includes a downward slope of the topside, the top air stream departs the trailing edge angled a bit downwards. Again split this into two vectors, one of which is 90 degrees down. Again, Newtons law action / reaction.

A flat two dimensional wing angled into the wind would indeed add lift and fly but not as efficient as the classical wing shape with a  curved topside, flat underside.

But all airplanes do fly with the flat underside angled versus the oncoming air stream. The angle depends on speed, may be very slight at high speeds and larger angle at slow speeds in a intuitively easy to understand  relationship.

What happens in a stall, if I understand it correctly, the air stream departs it's orderly flow from the topside, completely loosing component 3 and also component 2(?) of lift.

 

*1)

Boundary layer description reminded me of the "wind gradient", how wind speed decreases rapidly close to the ground. Stole illustration from here. My glider flying textbook makes the same comparison I noticed picking it up.

The reason why there's a rule of thumb formula, stall speed x 1.5 + 0.5 x wind speed (formula from glider textbook) to add landing speed depending on wind. Or else, as the image intuitively conveys, having a good stall speed margin at 10 meters (assume 30 knots at 10 meter ) could mean unpleasantly stalling at 5 meters.

ASK13 stalls at 31 knots. Applying the formula: 31 x 1.5 + 0.5 x 30 = 61.5. So indicated airspeed at final should be ~62 knots in this situation. If  only 46 knots at 10 meters, it will be at stall speed at 5 meters (assuming 15 knot wind at 5 meter). An embarrassing, or worse, day for our intrepid aviator).

Air flow boundary layers at atomic level looks like this except at nanometer, micrometer, millimeter scale. Atoms in contact with the wing doesn't move at all, slightly further away move a little etc.

Edited April 19 by -0303-

[AndyJWest]

It should be noted that it is entirely possible for an aircraft with a 'flat bottomed' aerofoil profile (e.g. the classic Clark Y) to fly inverted. 

I've learned over the years to largely ignore explanations of lift that treat it as some sort of magical attribute of aerofoil profiles, or indeed of 'wings'. The simple fact is, if you have relative motion between any solid object and a fluid it is immersed in, and the object is asymmetrical perpendicular to the flow direction, there is liable to be a net force generated at right angles to the flow. Wings are just particular shapes chosen to exploit this as 'lift, while minimising 'drag', a force exerted on the object in the direction of flow.