At 23:51 05 January 2011, ProfChrisReed wrote:
I'm still not convinced by those who propose that the wings of the
glider generate no extra lift (or even generate less lift) when
climbing on tow.

We know that on a winch launch the glider climbs because the wings
generate more lift than in level/descending flight. This must be true
because there is nothing pulling it up.

However, we are told that on aerotow the wings generate the same (or
less) lift as in level/descending flight and the tug just pulls the
glider up the slope.

Does this mean that the tug climbs in the same way, i.e. wings
generate only enough lift to carry the weight of the tug, and the prop
drags the tug up the slope? This doesn't match what I've read about
how aircraft work. L=W only in level flight. I think the tug's wings
generate more lift than its weight, and thus it climbs.

If this is true, the same must be true for the glider behind it.

Bring on an aerodynamicist to show me I'm wrong.


Chris

Yes, it does mean the tug climbs in the same way - the prop is dragging it
uphill.

In steady climb or descent the lift is actually slightly less than weight,
with the remaning very small component of the weight balanced by the drag
(in a glide) or the thrust (in a climb).

This can be demonstrated very easily with a vector diagram of the forces
acting on an aircraft, and you'll find this picture in every textbook on
aircraft performance or flight dynamics.

If you'd like to talk to an aerodynamicist, there are several
contributing to this thread - but more locally Dr Graham Dorrington in the
engineering department at Queen Mary has worked on an electric-powered
sailplane

http://www.flightglobal.com/articles...ane-flies.html

and should be able to convince you.

Doug

(aerodynamicst & flight dynamicist at City University)