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

[Tony P. <ap...@ea...>]

On Monday 27 August 2001 04:16 pm, you wrote:
> 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.

I believe it is.

> The flaw in your reasoning is the bit that goes: "since wdot comes
> directly from the sum of the forces...".  

Granted, there will be a term there due to the pitch and yaw rates.

>The UVWdot value comes from
> the sum of the forces PLUS another factor that I was (rather
> uncharitably -- sorry) calling FunnyValue.  

Yes, and that term is what corrects our values derived from an accelerated 
and rotating reference frame back to a so-called "inertial" one.  

> 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.

-- 
Tony Peden
ap...@ea...
We all know Linux is great ... it does infinite loops in 5 seconds. 
-- attributed to Linus Torvalds