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.

Picking up on other comments in this thread:

- a fore/aft pivot on the fork that carries the wing dolly wheel and
a gas strut suspension can't hurt, but personal observation suggests
that the load even the smallest gas strut can support is so much greater
than the weight on the wheel that the strut won't cushion anything
unless its so old and knackered that its about to collapse completely.

- 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.

- A typical tip dolly isn't particularly light if you make it with an
old touring bicycle front fork and wheel.

At a guess mine weighs 7 kg (15 lbs) or so. I don't think I've seen it
come close to lifting, even towing crosswind in a strong breeze with
the dolly on the windward side. I fly a Libelle, which isn't exactly
heavy AND has the dolly on its lightest wing. Murphy guarantees that!


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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