> >>With a flat plate, it basically stalls as soon as there is an angle
> of attack. The corrective force is fairly large at even small angles
> of attack. With an airfoil, you can get various amounts of angle of
> attack - with only slight corrective forces. <<
> When a fin (or wing) stalls it goes from producing lift to just drag. The lift
> that a fin can generate as a stability corrective force is more than it can
> produce as mere drag for any given angle of attack.
when it stalls -- it's just that there's a very sharp drop in the
lift/drag ratio. There have been a number of experiments with "stalled
flight" and the military has proposed utilizing stalled flight as a
standard maneuvering enhancement in the next generation of fighters.
A stall is generally avoided in normal flight, though, because the
turbulence over the wing produces so much extra drag, combined with a
distinct reduction in lift, that the result is usually a sharp drop in
altitude -- a very hazardous thing, given that it's most likely to occur
close to the ground during landing or takeoff.
Bill's right, though, in that a flat plate generates all it's lift in a
stalled condition. Likewise, a simple kite, such as the ordinary
diamond kits most kids fly at one time or another, normally flies in a
stalled condition, even though its airfoil is somewhat more complex than
a flat plate -- the drag of the stalled sail helps keep the kite line
taut, even as the lift holds the kite and line off the ground.
It was my understanding at one time that an airfoil, even one with zero
camber (i.e. "symmetrical"), would generate more lift at low angles of
attach than a flat plate; I've been corrected on this, and now
understand that the lift is almost the same in unstalled flight, but the
streamlining delays the stall, while reducing induced drag, making it
more desirable in most cases.
> better stability than a flate plate fin.
flights of Long Shots where the upper stage fails to ignite, with the
rocket often ejecting the D booster motor; in most of these cases,
instead of the lawn dart one might expect, the rocket will tailslide,
and fall into a flat horizontal descent, with all fins stalled -- and
thus avoid serious damage. That's good for the specific case, but
stalled fins in normal flight are generally bad -- they slow the rocket,
but apply their restoring force mostly opposite the direction of travel,
instead of always at right angles to the fin surface; that means (in
most cases) that less restoration is available because only a fraction
of the drag acts to restore the rocket to pointing in the direction of
There are times when even the Weaver of Skeins can make an awful tangle
of a perfectly simple tapestry. -- M. A. R. Barker, _The Man of Gold_
Donald Qualls, aka The Silent Observer NAR # 70141-SR Insured
Rocket Pages http://members.aol.com/silntobsvr/launches.htm
Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.