Aerodynamic question for you engineers

There's a physics issue here called 'frame of reference.'

Think about an external stationary (with respect to the air mass)
observer, and the airplane is made to fly a loop. The center of
rotation to that obsever is the center of the loop.


Or, if the observer tracks the airplane, or even its CG (assume it's
marked on the airplane) he'll see it move laterally at airspeed, then
at the start of the climb will probably see it dip a little bit, then
start assending.

I'm having a hard time imagining a frame of reference where the
airplane would appear to rotate about its CG, where I take 'rotate' to
mean a point about which the tail end goes down and the other end goes
up, because the CG itself will be moving, first down a little (I
think) then up. Down first because the elevator is adusted so it loses
upward lift of increases downward thrust, effectively making the
airplane heavier. As the aoa increases the wings (making a huge number
of assumptions (assume a spherical cow?) increase lift. Note also that
the 'center of lift' of a wing may change with aoa, so even that model
-- all effective lift concentrated at a fixed point -- may fail.

A more minor point, (but why not pick nits?) is that it's unlikely the
CG, center of lift of the wing, and center of lift of the elevator are
all in a straight line. In a high winged airplane the center of lift,
about a third of the way back from the front of the wing, and probably
pretty close to the wing's underside skin, is well above the CG. That
vertical displacement will not affect computing moments for horizontal
flight, but will as directions of flight different from horizontal
take place. Think for a moment or two about a helocopter in horizontal
flight transitioning to a nose up attitude. When I've seen that, it
appears the center of rotation is well above the hellcopter.


I know, there's nothing like adding some mud to the water.








flies a loop
On Jan 25, 1:22*am, Pete Brown wrote:
If a conventional aircraft is in stable level flight and the stick if the texts I have read indicate that the aircraft
pitches up, rotating through *the CG.

Is this exactly correct or is it a very useful approximation good for
all practical purposes?

Most aircraft have the CG located slightly forward of the center of
pressure ( CP or center of lift) for positive pitch stability. I was
wondering if the actual point of rotation is displaced somewhat aft of
the CG, someplace close to the CG but in fact some *small distance
towards the CP.

When the aircraft is subject only to *the force of gravity, any
displacement will cause it to rotate around the cg but in flight its
subject to gravity as well as the aerodynamic forces which act through
the CP, suggesting to me that the point of rotation is not quite on the CG..

Thanks

--
Peter D. Brownhttp://home.gci.net/~pdb/http://groups.yahoo.com/group/akmtnsoaring/

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