On 11/22/2007 09:10 AM, Jon S. Berndt wrote:
> At this time, JSBSim zeroes out the wind velocities while at rest on
> the ground, to prevent slippage.
Aha. Thanks for the info.
FWIW that sounds like *two* bugs to me.
1) Maybe we need a better model of static friction and quasi-static
rolling friction so as to properly handle the behavior of tires
gripping the ground.
2) We do need the real wind and real airspeed vector while on the
ground. There are many reasons for this, including things like
the weathervaning tendency, which is an important part of real
crosswind takeoffs and landings.
> At this time, JSBSim zeroes out the wind velocities
How strongly time-dependent are these bugs? The workaround (below)
is not particularly arduous, but nobody wants to implement a workaround
if the bugs are going to be fixed Real Soon.
=============================
LeeE: It is not hard to work around these bugs.
If you want the slip angle (which may or may not be everything
you want), all it takes is ye olde wind-triangle calculation.
Start by getting the true wind vector components from
/environment/wind-from-north-fps
/environment/wind-from-east-fps
I assume the FDM is not messing with the environment's copy
of this vector.
Rotate that into the aircraft frame using the usual rotation
operator. The rotation angle is the true heading.
Add that vector to the groundspeed vector
u = /fdm/jsbsim/velocities/u-fps
v = /fdm/jsbsim/velocities/v-fps
The resulting sum is the airspeed vector, in the aircraft
frame, i.e. relative to the aircraft axes.
If desired, take the atan2(v, u) as previously described.
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