Re: [Flightgear-flightmodel] What happens during rotation at takeoff?

[Ivan S. <iva...@sb...>]

Thanks for the detailed response Alan.

On 11/10/17 06:24, Alan Teeder wrote:
>
>
> If the resultant moment is non zero 

And let's say all other forces (lift thrust drag and gravity) add up to 0.

     A simplified scenario would be a parked plane with full thrust counterbalanced by
     surface friction of hand brakes. No lift or aerodynamic drag and the propwash
     over the elevator causing a positive pitching moment.


> the aircraft will pitch up or down. If the pivot point is fixed because the undercarriage is on the ground, then the aircraft reference point will also be forced to go up and down - just as with our seesaw.

You say the aircraft RP will "forced". Where is this force coming from then?

What I observe is the rear gear springs compressing (+ reaction force) and front gear springs
expanding (- reaction force).

(The following is supposition)

That shouldn't move the CG because the + and - reaction forces cancel.

The CG can only move if there is excess force and that can happen only if the front gear
leaves the ground and cannot provide the counterforce to the rear gear:

         aircraft weight: 6.

         rear reaction    front reaction  (due to LG [de]compresion)
         ------------------------------
         +3                +3
         +4                +2
         +5                +1
         +6                0
         ---------------------------------
         +7                0    => force on CG

I just haven't been able to observe this "experimentally" so to speak.

Maybe my gear spring constants need to be way lower than I think (or some other
components quantity is of the wrong scale). I intend to compare numerical values for
similar planes btw jsbsim and my code.