Paul Surgeon writes:

 > I don't know about everyone else's experience but I haven't found
 > one aircraft in FG that wants to sit still on the ground even with
 > the engine off.  I've never seen a stationary aircraft "weather
 > vane" into a 10 knot wind in real life.

It might be that the problem is not ground reactions but aerodynamics.
Even if the ground reactions are accurate, the aerodynamic solver
(YASim) or coefficients (JSBSim) may be producing *far* too strong
forces and moments for low relative wind speeds and/or extreme values
of alpha and beta.  That's just a guess, though.

In real life, my Warrior II (which weighs about the same as a 172)
will sit perfectly still in a 30 kt wind from any direction, with no
tie-downs and the brakes off.  I wouldn't leave it unattended that
way, of course, but it never moves during a walkaround in those
conditions (even yesterday, when the 30 kt wind was hitting it from 90
deg).  The control surfaces move in the wind, though, and sometimes
hurl the yoke into my stomach if I'm not holding it.

For a further example, the Ottawa Flying Club never leaves parking
brakes on and ties down only at night, unless the winds are very
strong.  Planes -- even light ones like Cessna 150's -- simply don't
roll around in sub-gale-force winds.

In FlightGear, on the other hand, a JSBSim or YASim plane will happily
start rolling, pitching, and yawing on the ground with sometimes only
a 5-10 kt wind.

 > Last time I flew in a PA28 she required a fair bit of throttle to
 > get her rolling even on a hard surface.

Check the tire pressure -- unlike Cessnas, Cherokees are notoriously
hard to move on the ground if the nosewheel or mains are only a few
pounds flat (i.e. too little to see).  I actually keep a bicycle
handpump and tire gauge in my plane for that purpose.


All the best,


David

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