Lift Question

[Macgyver]

Hey guys,

The Lift vector is technically defined to act perpendicular to the relative airflow (and not perpendicular to the wing's chord line).

However, in a book I am currently reading, the author states that when considering supersonic flight, lift actually acts perpendicular to the wing itself, thus causing "wave drag" at high AOAs.

Does anyone have an explanation as to why this is? I just don't fully understand the difference as to why the definition of lift must change.

[RYR-738-JOCKEY]

First thing that comes to mind is that for a supersonic wing profile, there is virtually no camber... and because of the high speed when increasing AoA you will get a resultant force pointing more backwards(because of wave drag) than for a normal profile.
I think what you describe is only a practical sollution to simplify a bit, I don't think this is actually theoretically accurate.

[FakePilot]

Is it because the "static" lift (?), i.e. the force of the air hitting the wing becomes a bigger factor when supersonic vs. "conventional" lift? By "static" I mean more like a water ski then pressure differential.

And no, I have no idea what I'm talking about, so take it easy on me - I'm just trying to understand.

[barit1]

The net force vector on any wing is up and aft (assuming positive lift...) It makes no difference super- or subsonic.

That net vector is resolved into two perpendicular components: One parallel to the incoming airflow, and one at right angle to that. The first is called the drag vector, the second is lift.

Again, these are the vector components regardless of Mn.

[The African Dude]

barit1 got there first.

The lift resulting from the wing aerodynamics acts perpendicular to it. At a given angle of attack, this resultant is angled slightly backwards.

The vertical component of the resultant, in addition to the resultant vertical force of the engine thrust at a given positive angle of attack, is what keeps the aircraft aloft. The horizontal component causes a drag force, adding to the other drag forces already affecting the aircraft.

In supersonic flight shock waves form due to compressibility effects. Energy of the airflow is absorbed by these in various ways and add to the drag force.