Snap roll vs. Va

For those of you who have aerobatic airplanes, I'd be interested in
data about the recommended entry speed for a snap roll vs. what Va is
in your airplane.

Thanks for any info.

In a 7GCAA, Va at max gross is 120 mph and the recommended snap roll entry
speed is 85 mph.

"Greg Esres" wrote in message
...
For those of you who have aerobatic airplanes, I'd be interested in
data about the recommended entry speed for a snap roll vs. what Va is
in your airplane.

Thanks for any info.

In a 7GCAA, Va at max gross is 120 mph and the recommended snap roll
entry speed is 85 mph.

Thank you. Can a snap roll be done at max gross ?

Yes.

"Greg Esres" wrote in message
...
In a 7GCAA, Va at max gross is 120 mph and the recommended snap roll
entry speed is 85 mph.

Thank you. Can a snap roll be done at max gross ?

Yes.

Thank you.

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




"Ken Ibold" wrote in message . com...
In a 7GCAA, Va at max gross is 120 mph and the recommended snap roll entry
speed is 85 mph.

"Greg Esres" wrote in message
...
For those of you who have aerobatic airplanes, I'd be interested in
data about the recommended entry speed for a snap roll vs. what Va is
in your airplane.

Thanks for any info.

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...

Rich:

Thanks for your reply.

The genesis for my question is that there are a number of aerodynamic
sources which discuss the concept of a "dynamic stall", where when the
a/c is rapidly rotated to a high AOA, it can generate a much higher
lift coefficient than in steady state conditions.

My thought is that a snap roll should duplicate those conditions.
However, the g-forces you're reporting don't seem to match that
theory. Bill Kershner said that he does snap rolls at 80 knots, and
has never seen more than 3 g's in his Aerobat.

Hi Greg,

You're right, dynamic stall occurs when a wing or other lifting
surface is subjected to time-dependent pitching (or other type of
time-dependent) motion, resulting in a greater effective angle of
attack than the normal, static stall angle.

But the "dynamic stall" phenomenon does not really apply to light
airplanes. It is is an unsteady stall phenomenon which can be
experienced by the retreating blade of a helicopter in forward flight
and by highly maneuverable fighter aircraft.

"Dynamic stall" means something to skydivers as well, I think when a
skydiver pendulums under the canopy too close to the ground...

Rich
http://www.richstowell.com


Greg Esres wrote in message . ..
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...

Rich:

Thanks for your reply.

The genesis for my question is that there are a number of aerodynamic
sources which discuss the concept of a "dynamic stall", where when the
a/c is rapidly rotated to a high AOA, it can generate a much higher
lift coefficient than in steady state conditions.

My thought is that a snap roll should duplicate those conditions.
However, the g-forces you're reporting don't seem to match that
theory. Bill Kershner said that he does snap rolls at 80 knots, and
has never seen more than 3 g's in his Aerobat.

But the "dynamic stall" phenomenon does not really apply to light
airplanes. It is is an unsteady stall phenomenon which can be
experienced by the retreating blade of a helicopter in forward flight
and by highly maneuverable fighter aircraft.

Rich, it's true the phenomenon is most important in helicopter flight,
but it certainly happens in airplanes as well.

I have a copy of a NACA flight test which shows a 30% increase in lift
with a rapid AOA increase, in airplanes. The increase in lift was
directly proportional to the rate of AOA increase and showed no signs
of leveling off; the test pilots just got scared, and quit. :-)

What I'm curious about is under what conditions it happens. The only
difference in a snap roll and what these pilots were doing is your
application of ruddder (as far as I can tell). Perhaps the fact that
you stall one wing earlier than the other short circuits this effect.
I'm curious.

"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