Snap roll vs. Va

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.

I'd be interested in the NACA report, so would you please cite it (or
can you provide an on-line link to it somewhere in the larc system?)

Also, have you tried a Google search on "dynamic stall" yet?

Rich
http://www.richstowell.com


Greg Esres wrote in message . ..
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.

http://naca.larc.nasa.gov/reports/19...ca-tn-2525.pdf

The above link should take you directly to the specific report that I
mentioned.

(If it fails for some reason, searching for "2525" on the NACA
technical report server will pull it up.)

Also, have you tried a Google search on "dynamic stall" yet?

I did in the past, but not recently. I vaguely recall that
helicopters came up the most.

My Wayne Johnson's "Helicopter Theory" discusses it in the context of
helicopters, but in the airplane case, Hoerner's "Fluid Dynamic Lift"
goes into it a bit. "Theory of Wing Sections" also discusses it, and
one or two other references in various aerodymamics books.

Got it -- thanks Greg!


Greg Esres wrote in message . ..
http://naca.larc.nasa.gov/reports/19...ca-tn-2525.pdf

The above link should take you directly to the specific report that I
mentioned.

Sport Aerobatics magazine of May 1987 noted some wind tunnel tests by
Avions Mudry - the info was vague but apparently confirmed significant
dynamic lift effects in snap rolls for the CAP 10. There are other
effects - if the flight envelope is drawn from power off stall speeds
at forward cg then its easy to get a higher load factor at the stall
than that calculated.
For the CAP 10B - Va is 146 mph and snap roll speed is 110 mph.

The reference below gives some flight data on snap rolls in a
Decathlon - not enough info for me to draw any conclusions on dynamic
lift effects but concludes that overall the loads are within the
design envelope. Some-one else may be able to analyse it - the only
relevant time history data is a positive snap roll from inverted at 80
kts giving +3g peak.

There are a number of standard design load cases - the designer must
ensure that snap rolls, at the recommended entry speed, do not exceed
the loads that they impose on the airframe - not always the same
answer for every airplane.

Reefrence: Loading Conditions Measured During Aerobatic Maneuvers by
Albert W. Hall, Langley Research Center, NASA. SAE paper 700222.



Greg Esres wrote in message . ..
http://naca.larc.nasa.gov/reports/19...ca-tn-2525.pdf