At 18:54 05 December 2003, Todd Pattist wrote:
(Fred the Red Shirt) wrote:

Now, I'm confused! An airfoil cannot produce thrust
-- only 'lift.' But if an airfoil has a reverse counterpart,
the two are joined at the center and rotated about
an axis in a vertical plane, they are then a propeller;
this produces 'thrust.'

Well if that last statement is true then the earlier
one
is false. Do propellers produce thrust?

Back to definitions again. 'Lift' is defined perpendicular
to the path of the airfoil through the air. 'Thrust'
is
typically parallel to that path. However, when we
have a
moving airfoil on an aircraft (rotating propeller),
there
are two 'paths' that are relevant. One is the path
of the
aircraft (this path defines the AOA of the wing), and
the
other is the spiral path of the rotating airfoil (this
path
defines the AOA of the prop blades). The propeller's
airfoil produces 'lift' perpendicular to the spiral
path.
It produces 'thrust' when considered relative to the
airplane's path.

IOW, at any instant, the prop is mostly moving at 90
degrees
to the path of the airplane. If it produced both lift
and
thrust relative to the same path, then the prop would
turn
itself :-)

Todd Pattist - 'WH' Ventus C
(Remove DONTSPAMME from address to email reply.)

Okay, it promises to be a long winter; I'll take my
tongue out of my cheek!