Snap roll vs. Va

Hi Dave,

I thought the V-g diagram typically refers to a "symmetric loading,"
i.e.: no rolling as the g is applied.

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?

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.

Thanks,

Rich
http://www.richstowell.com


"Dave" wrote in message ...
"Rich Stowell" wrote in message
om...
Most things in aviation are related to the wings-level, 1-g stall
speed, Vso. The maneuvering speed, Va, is actually the stall speed of
the airplane at the design limit, and it is related to Vso by the
square root of the g-load. (Of course, all of these are CAS, so you
may have to do some massaging through the airseed calibration data to
convert back and forth between IAS and CAS to find the numbers you
must read on the airspeed indicator.)

For example, in aerobatic airplanes like the Citabria which were
certificated at +5.0 g's (at max. gross), Va = 2.24 x Vso. In
aerobatic airplanes certificated at +6.0 g's (at max. gross), Va =
2.45 x Vso.

In terms of the snap roll entry speed (and snap rolls are really
accelerated stall/spins), the speed will naturally fall somewhere
between Vso and either 2.24 or 2.45 x Vso.

In Eric Muller's book, Flight Unlimited, he recommends intially
practicing snap rolls at 1.5 x Vso, so there's a starting point. In my
experience, I'd recommend around 1.6 x Vso as a good "recommended"
snap roll speed, which translates into a 2.5-g pull to stall/spin the
airplane at that speed. The MAXIMUM snap roll speed should probably be
no greater than about 1.7 to 1.8 x Vso...

Hope this helps (and HI Ken!),

Rich
http://www.richstowell.com

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