Re: [Flightgear-devel] Turn ball fix & JSB notes

[Andy R. <an...@ne...>]

Tony Peden wrote:
> Andy Ross wrote:
> > But regardless, it does work, and the JSBSim code does the right thing
> > with the plane's motion.  EXCEPT when you try to interpret "Vdot" as
> > an acceleration.  There it breaks down, and actually has the effect of
> > cancelling itself out.  Think about a plane in a static turn: the
> > plane is accelerated relative to the GROUND, but its velocity through
> > the AIR is constant.  Thus, the vUVWdot vector will be zero.  But
> > that's wrong -- what if the plane is in a big honkin' 9G bank and the
> > pilot is about to black out?
> 
> Since a fair amount of "extra" (vs that required for steady, level) lift is 
> required to maintain a 9g turn and since wdot comes directly from the sum of 
> the forces, vUVWdot _will_ properly reflect the acceleration of the aircraft.
> (Yes, you must make more lift to pull more g's in a turn)

Ack, you're making the same mistake.  Take another look at my
arguments about accelerated reference frames.  But most importantly,
look at the actual behavior in the simulator.  Your statement is
demonstrably incorrect.  In a steady-state turn (that is, constant
bank angle and constant speed) vUVWdot will _not_ properly reflect the
acceleration of the aircraft.  Try it.

The flaw in your reasoning is the bit that goes: "since wdot comes
directly from the sum of the forces...".  The UVWdot value comes from
the sum of the forces PLUS another factor that I was (rather
uncharitably -- sorry) calling FunnyValue.  In a steady state
environment, this FunnyValue will be exactly opposite the force
vector.  Again, this isn't a Jedi mind trick, it's an
honest-to-goodness reproducible bug.*

You don't have to trust me.  Just try it. :)

Andy

* No, not a bug with the plane's motion.  But a bug with respect to
  returning a pilot "acceleration" that isn't what the pilot is
  feeling.

-- 
Andrew J. Ross                NextBus Information Systems
Senior Software Engineer      Emeryville, CA
an...@ne...              http://www.nextbus.com
(510)420-3126                 Why Wait?