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.


A winch launch is very different because (a) the angle between the cable
and the direction of motion of the glider is large, and therefore unlike a
tow the downwards component of the cable tension is no longer negligible,
and (b) the motion is not steady.

In this case the lift is greater than the weight because it is partially
counteracting the cable tension and weight. The precise balance depends
on pilot and winch driver technique. Even so, it is still the forward
component of the cable tension force that is doing the work required to
raise the glider to its release height.

The one (illegal in the UK I believe?) situation in which the glider wing
lift is doing the work would be a kiting launch - and here you are in fact
extracting the energy required from the wind ... something that a number of
researchers are trying to do

http://www.ampyxpower.com/PowerPlane.html