On Mon, 15 Oct 2018 05:28:59 -0700, Retting wrote:

What would you think the increase (%) would be at start of roll thru
aceleration?
A heavy twin at 700 kg?

That would depend on the total of:

a) rolling resistance, i.e. the type of surface, how hard it is,
how long the grass, etc. is and tyre pressure.

b) the acceleration, which depends on the type of towplane, engine size
and the throttle-bending tendencies of the tuggie.

c) the slope, if any, of the runway. That, at least, is easy once you
know the slope because its the same calculation as the climbing
contribution to tow load in the air.

I have little idea of the values of parameters a and b except that the
acceleration load would be the all-up glider weight multiplied by the
acceleration expressed in G, so a 0.1G acceleration would have a tow load
of 50kg for an all-up weight of 500kg. However, the initial acceleration
is almost certainly less than 0.1G

As for rolling resistance, I suggest you fit a decent spring balance to a
short length of towline and rings. Then two or three of you can easily
measure low speed rolling resistance for a glider or two on a variety of
surfaces. It would be interesting to know how much difference there is
between rolling resistance on a tarmac surface and a dry, recently mown
grass strip. Last but not least, and it would be something to do at the
field on a non-flying day.


--
Martin | martin at
Gregorie | gregorie dot org