-----Original Message-----
From: Forum for Physics Educators on behalf of Bob LaMontagne
Sent: Sat 7/2/2005 8:58 AM
To: PHYS-L@LISTS.NAU.EDU
Cc:
Subject: Re: thrust > drag
"You have probably heard of the expression "that dog won't hunt". Well, "that
plane won't fly". The lift vector is not in the opposite direction to the
weight during straight and level flight. It is tilted somewhat to the rear
of the aircraft. If the wing does not have an effective angle of attack,
there is no lift generated. Horizontally, you have thrust forward, with drag
and a horizontal component of lift acting backward. Therefore, the thrust is
always greater than the drag."
I would suggest to focus first on flight in still air. Then, by definition, the drag force is in direction opposite to velocity; in case of horizontal flight, the drag force will be horizontal. Also, I think the statement that no lift is generated without an appropriate angle of attack is too restrictive. This statement may be exactly correct in case of a kite, but in case of a plane the lift is due not so much to the angle of attack as to a very specific cross-sectoional shape of the wing. The wing is more convex at the top and less convex (and sometimes may be even concave) at the bottom, and the lift then arises from the plane's motion in air, according to Bernulli's effect.
Only in a simplified case of a flat wing, the non-zero angle of attack becomes necessary. In this case (provided the flight is horizontal) the net force produced by air on the wing has the horizontal and vertical components. The former will then contribute (together with kinematic friction, turbulence etc.) to the net drag. The latter will be the lift force. So, at least in this case, the lift force will be opposite to the weight.
If you have ever observed the operation on the wings, for instance, during the plane's landing, you must have seen the back side of the wing changing its tilt and getting steeper. This generally affects both - the drag and the lift. The drag definitely increases, which slows down the plane. The effect on the lift is more subtle - the lift force may increase due to the increased angle of attack in this part of the wing, but decrease due to decrease in the plane's speed. The plane slows down and looses its altitude.