On Apr 14, 2:23*pm, "BT" wrote:
I am trying to find (remember) the standard formula for tow pressure on the
tow line in a standard smooth tow.
I forget if it is weight of the glider divided by L/d or a percentage of
L/d.

Well, yes, for straight and level flight at constant speed it would be
the weight of the glider divided by the L/D of the glider at that
speed. So if it's 400 kg and 40:1 then the tow force is 10 "kg" (not a
proper SI unit of force), or about 100 N.

But if you're climbing then you also need to look at the climb angle.
Technically you'll want the sine of the climb angle times the weight
of the glider but for real world tugs it'll be plenty close enough to
use:

(rate of climb / tow speed) * weight of the glider

Making sure of course that the first two are in the same units e.g.
both in knots or both in fpm or whatever.

So, for example if you tow the above 400 kg glider at 70 knots at a
rate of climb of 700 fpm (7 knots) then you've got another 40 kg or
400 N of tension on the rope from that, for a total of 50 kg of
tension.

Plus, as you point out, if you're accelerating then you need to take
that into account as well, but once you're into a steady climb you're
obviously not.