At 13:32 04 January 2011, BruceGreeff wrote:
Thanks Martin

I did use the "vector" word talking about the winch case - because the

cable has mass (our steel cable is ~150Kg so not insignificant) and
there is a pull at a downward angle. At top of launch cable angle
approaches 90 degrees to fuselage - If you want proof look at one of the

videos on you tube.

http://www.youtube.com/watch?v=v2Qh95I_YM0

http://www.youtube.com/watch?v=np8OGPZ2pvE

As Doug Greenwell points out - there is constant acceleration on winch
launch because the flight path is curved, describing a horisontal S. I
don't know how what magnitude the acceleration has, but subjectively it

is only significant in the brief rotation to steep climb, and possibly
on the level out if you are less than smooth...
Generally it is a relatively small change from a little over 1g to a
little under 1g at release.
Anyone have the maths capability to calculate for a known situation?

Cheers
Bruce

On 2011/01/04 1:43 PM, Martin Gregorie wrote:
On Tue, 04 Jan 2011 10:57:01 +0200, BruceGreeff wrote:

Of course - the angle that the flight path can make relative to the
ground is proportional to the excess power available - hence the low
rate of climb behind the cub, versus the extreme angle on a winch.

Aerodynamics guys - Am I confused?

Sounds fair to me except that you omitted two fairly significant
forces:
- the weight of the cable
- the tension in the cable.

Both will add to the load carried by the wing. The tension should add
a
fairly constant load to the wing once the glider has rotated into full
climb since the throttle setting remains fairly constant[*] from
rotation
until the glider is near the top, but the effective cable weight will
increase as more of it is lifted off the ground and then as the whole
cable gets closer to vertical.

[*] this is true on a calm day but is obviously incorrect in the
presense
of turbulence or a significant wind gradient.



--
Bruce Greeff
T59D #1771 & Std Cirrus #57


Hopefully this post will go this time ...

I did try modelling a winch launch some time ago. It's very dependent on
the assumptions you make on piloting technique and winch control (constant
power, tension, cable speed?), but in general once you've transitioned
into full climb the accelerations as felt by the pilot seem to be very
small ( 0.1g).

Hence the danger of overstressing, since you've no physical indication of
the high wing loads due to the cable tension & weight.