-----Original Message-----
=46rom:=09Forum for Physics Educators on behalf of Bob LaMontagne
Sent:=09Sat 7/2/2005 8:58 AM
To:=09PHYS-L@LISTS.NAU.EDU
Cc:=09
Subject:=09Re: thrust > drag
"You have probably heard of the expression "that dog won't hunt". Wel=
l, "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 atta=
ck,
there is no lift generated. Horizontally, you have thrust forward, wi=
th drag
and a horizontal component of lift acting backward. Therefore, the th=
rust is
always greater than the drag."
I would suggest to focus first on flight in still air. Then, by defi=
nition, the drag force is in direction opposite to velocity; in case =
of horizontal flight, the drag force will be horizontal. Also, I thin=
k the statement that no lift is generated without an appropriate angl=
e 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 muc=
h 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 aris=
es from the plane's motion in air, according to Bernulli's effect.=
=20
Only in a simplified case of a flat wing, the non-zero angle of atta=
ck becomes necessary. In this case (provided the flight is horizontal=
) the net force produced by air on the wing has the horizontal and ve=
rtical components. The former will then contribute (together with kin=
ematic 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 op=
posite 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 w=
ing changing its tilt and getting steeper. This generally affects bot=
h - the drag and the lift. The drag definitely increases, which slows=
down the plane. The effect on the lift is more subtle - the lift for=
ce 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 plan=
e slows down and looses its altitude.