Snap roll vs. Va

A couple points to add to this discussion...

1) How does the use of rudder affect the g-loading on the wings? A
typical snap roll, in competition, anyway, is initiated by the rudder
after the pilot has loaded up the wings. This loading is less than that
required to produce a stall.

2) Snap rolls cause twisting loads and gyroscopic effects that do not
register on a g-meter. Are those loads significant? How would one
determine what those twisting limits are?

3) Reducing the aircraft's weight will reduce the load on the wings, but
it does not affect the load on non-lift-bearing parts of the aircraft,
like the propeller, engine mounts, and pilot seat.

Bottom line for me is to read the aircraft flight manual and reduce the
published limits by a percentage equal to the square root of the age of
the airplane + the cube root of the age of the pilot.

If there are no published limits, do a good preflight on your parachute
and be sure you will have an easy egress.


--
Dennis Yugo
http://www.worldpassage.net/~dyugo

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

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