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Greg Esres
November 29th 03, 05:06 AM
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.

Ken Ibold
November 29th 03, 04:54 PM
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.
>
>
>

Greg Esres
November 29th 03, 06:24 PM
<<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 ?

Ken Ibold
November 29th 03, 07:55 PM
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 ?
>
>

Greg Esres
November 29th 03, 09:43 PM
<<Yes.>>

Thank you.

Rich Stowell
November 30th 03, 01:57 AM
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 >...
> 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.

Greg Esres
November 30th 03, 02:38 AM
<<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.

Rich Stowell
November 30th 03, 05:20 PM
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.

Dave
November 30th 03, 07:10 PM
"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

I am not a number
November 30th 03, 09:31 PM
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

Rich Stowell
December 1st 03, 01:19 AM
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

Greg Esres
December 1st 03, 03:26 AM
<<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
December 1st 03, 05:19 PM
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.

Greg Esres
December 1st 03, 05:53 PM
http://naca.larc.nasa.gov/reports/1951/naca-tn-2525/naca-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.

Rich Stowell
December 2nd 03, 02:02 AM
Got it -- thanks Greg!


Greg Esres > wrote in message >...
> http://naca.larc.nasa.gov/reports/1951/naca-tn-2525/naca-tn-2525.pdf
>
> The above link should take you directly to the specific report that I
> mentioned.

Dave
December 2nd 03, 10:27 AM
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

Dave Pilkington
December 2nd 03, 12:14 PM
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/1951/naca-tn-2525/naca-tn-2525.pdf

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