At 12:36 07 November 2008, Doug Hoffman wrote:
Martin Gregorie wrote:
On Thu, 06 Nov 2008 16:59:54 -0800, Papa3 wrote:

I think that shock absorbtion is a good idea. I'm not so worried
about
the load on the spar or even on the wing skin under the cuff (pretty
insignificant) as I am about two scenarios:

I'd add a third:

3. Personally, I'd be more worried about the twisting effect that will

drive the wing LE against the fuselage and possibly damage the pickup
pins and/or upset their shims if the wheel hits anything. The loads
applied at this point will be FAR greater than any forces at the tail
dolly or on the far wingtip.

Thats why you NEVER push a glider by its wingtips.

I'd suggest that your best protection against these forces is
twofold:

1) use the biggest possible bicycle wheel on your tip dolly. A big
wheel

will ride over a higher obstacle without catching on it than a smaller

one.

2) take care not to run the tip wheel over anything that it won't ride

over easily.

My personal practice is to *never* exceed 5 mph when towing. I read an
article some time ago (Soaring magazine?) where an insurance company
reported that most damage incidents while towing were due to towing too
fast. Or the damage could have been avoided by towing more slowly.

Never stop quickly (don't ask).

The other thing I do is to use small/short screws for the wing cuff
clasp attachment. Any excessive loads will result in the screws letting

go: in effect they are like shear pins, so the wing is protected. I
have "tested" this safety when attempting to tow with a flat glider
main
wheel (a bad idea!). The safety worked.


- If you carefully align the tip dolly wheel so its parallel with the
fuselage centre line it will never drag sideways. Its axle is so
close
to being inline with the main wheel axle that there's never a
noticeable
side force on a properly aligned dolly wheel even when you pivot the
glider round its main wheel.

Agreed. My practice is to frequently glance at the wheel while towing.
If it starts to skip or lean then I stop the car, get out, and
straighten the wheel. With practice one quickly learns how to put the
wheel on straight at first try.

Regards,

-Doug

Hi Doug,
I have used a trailing arm design successfully for over 12 years
on various gliders . There attribute: it makes the wing /
wheel combination very stable even in rough grass. Here is a picture.
http://www.ssa.org/myhome.aspmbr=825...5&show=gallery
Udo