On Mon, 04 May 2020 05:23:58 -0700, stu857xx wrote:

I'm bored with cabin fever, so why not talk about how rubber meets...


All turns, whether on the ground or in the air, have one immutable
thing in common: there must be a lateral force to cause an object with
momentum in a straight line trajectory to change that trajectory...

Here's the logic I'm hearing here.
The glider has momentum which must be countered to make it change
direction.
The wing wheel provides this because there is no other place for it to
go.

Given that a glider with a castering wing wheel can still turn, perhaps
the main wheel can provide most of the turning force?

Yes, came here to say that. The two places that carry the main loads
while a glider is being towed are the main wheel and the tow ball.

Side loads when turning or towing cross-wind will be largely carried by
the main wheel while the tow ball adsorbs rotary momentum (of wings and
rear fuselage) at the start and end of a turn or when swivelling the
glider in place.

If the wing wheel is fairly close to the spanwise line through the main
wheel's contact point with the ground and its axle is parallel to the
spanwise line, then there should be little or no side force on the wing
wheel: if your wing wheel cuff can't guarantee that the main and whng
wheel axles are parallel then, or course, all bets are off.

OK, heres an analysis of a light glider (my 201 Libelle). My wing wheel's
axle is roughly under the LE when fitted, so is probably close enough
fore/aft to the main wheel's contact point for scuffing not to be an
issue. The cuff is 280mm (11") wide and a snug fit everywhere including
at LE and TE, so the wing wheel's axle will be fairly accurately parallel
to the main wheel axle. I've never seen any tendency for the wing wheel
to scuff when towing the glider on either grass or hard surfaces.

My wing-wheel assembly weighs 5.4 kg and is on the lighter wing: in still
air with the glider stationary its always the other tip that slowly drops
if I let go of the wing and during airframe weighing the other wing was
around 4kg heavier. Its quite obvious that there's very little load on
the wing wheel when towing because, on our grass airfield, running over a
small clump or a heap of cuttings will make it leave the ground an inch
or two and only briefly.


--
Martin | martin at
Gregorie | gregorie dot org