At 11:18 06 January 2011, ProfChrisReed wrote:
Thanks Doug (am happy to learn from City as well as my own
institution!) and twocoolgliders.

So, if I understand you both correctly, the glider climbs on both
winch and aerotow because there is a force *pulling* it in (roughly)
the direction it is pointing, i.e. above horizontal. Once in a steady
climb, the lift generated by the wings balances the weight of the
glider + any other downward forces.

In a winch launch there are substantial downward forces from the
weight of the cable and the downward vector of the direction of pull.
Thus lift is higher than in steady free flight, and AoA is higher.

On aerotow the only additional downward force is from half the weight
of the towrope (pretty small), so the lift required is similar to that
in steady free flight (and in fact a little lower for other reasons).

_____________

This means that there are only two possible explanations for the
phenomenon on slow tow where the glider feels as if it is close to the
stall. Either:

1. It really is close to the stall, which means that the AoA is
greater than above, which means it must be flying in a continuous
downdraft (Andreas's explanation); or

2. Its AoA is as above, and the phenomenon has some other cause (such
as vortices acting on different parts of the wing) which replicate the
symptoms of approaching stall but do not in fact herald it.

Presumably we could test which is correct by taking a slow tow and
deliberately stalling the glider, monitoring the airspeed at which the
stall occurs. Volunteers to perform this experiment might be hard to
find!

Is there anyone who has actually stalled on tow unintentionally and
noted the airspeed when the stall occurred? I'd guess not, as the
pilot's attention would probably be elsewhere..

Chris

yes, that's about it. The danger on a winch launch is that although the
wing lift is much greater than the weight, the accelerations felt by the
pilot once in the climb are very small - so you've no physical indication
of a potential overstress.

As ever, it's a bit of both - there is a downdraft behind the tug, but if
this was constant over the whole span you would end up at the same AoA
*relative to the local airflow* as before, but at a higher pitch attitude
*relative to the tug flightpath* - so you might feel uncomfortably
nose-up, but shouldn't be any closer to the stall.

The explanation that I and others here favour is that you get closer to
the stall, and have poor aileron control, because the downdraft is not
constant in magnitude or direction - but varies from downwards over the
centre section of your wing to upwards over your tips, leading to a
different stall behaviour from free flight.
I did a calculation on a Discus2-like wing at 50knots, at which speed at
which the wing lift coefficient was pretty constant at about 1.1 across
the span in free flight. Put this wing behind a Pawnee and the lift
coefficient changes to about 0.9 at the root and 1.4 at the tip. Put in a
bit of aileron or bank angle and you've potential for early stall and wing
drop.

(not sure about the occasional report of reduced elevator authority though
... will have to think further on this one!)

We know the upwash is really there because flight tests (and watching the
birds) have shown you can get a significant reduction in power (= fuel
flow) required for cruise by flying just outboard of the tip vortices of
another aircraft. NASA did this with a couple of F18s - migrating birds
do it all the time. Interestingly, everyone wins in this scenario,
because the lead aircraft gets a push from the trailing aircraft - people
have looked seriously at flying airliners in formation across the Atlantic
to save fuel, but I'm not sure what ATC would have to say about it!