Snap roll vs. Va

Comments edited into the text.....

"Rich Stowell" wrote in message
om...
Hi Dave,

I thought the V-g diagram typically refers to a "symmetric loading,"

Agreed, I ought to have said "symmetrical loading" rather then "stall" - but
I think this is just terminology.


If the airlpane is subject to a rolling pull-out, for example, then
the structural design limit is derated by 2/3. The added twisting
moment present during a rolling pull-out, therefore, could lead to
structural damage with as little as 4-g's in the aerobatic airplane,
whereas 6-g's would be available with a straight (symmetric) pull.
I wasn't aware that the 2/3 factor also applied to symmetric vs.
asymmetric stalling -- can you point me toward a reference for that?

I wish I could find the reference but I've had a quick look around and
failed, maybe we've got an aeronautical engineer reading this who can
provide a pointer...?


True, some (most?) apply aileron as part of the snap roll process.
However, properly done in most aerobatic airplanes, only rudder and
elevator actions are necessary (ailerons neutral). I suppose that the
application of aileron as part of the snap roll might then qualify as
a "rolling pull" in which case, the 2/3 factor might apply.

From memory, the derating to 2/3 occurs because of torsional effects AND
lift asymmetry - the lift asymmetry is present without any aileron input
but, as you say, many of the more experienced aero pilots use aileron to
accelerate the snap (called a flick roll in the UK) once it's started. I
generally teach a basic snap without aileron and then bring it in to
demonstrate how it can be used to vary the rotation.

Dave

"Dave" wrote in message
-
....snipped

Don't forget that the structural g limit is for a symmetrical stall and
is
reduced to 2/3 for an asymmetric stall - therefore the absolute max snap
roll speed at MAUW for a 6g airframe is 2xVso.
Also, this speed should decrease at lighter weights by the ratio of the
square roots of the weights. Vso at weight w = Vso x sqrt(w)/sqrt(MAUW),
this can make a 10% difference to Vso so could easily affect the max
snap
speed by 10kts or more.

Dave Sawdon