A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Aerobatics
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Snap roll vs. Va



 
 
Thread Tools Display Modes
  #11  
Old December 1st 03, 01:19 AM
Rich Stowell
external usenet poster
 
Posts: n/a
Default

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

Ads
  #12  
Old December 1st 03, 03:26 AM
Greg Esres
external usenet poster
 
Posts: n/a
Default

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.




  #13  
Old December 1st 03, 05:19 PM
Rich Stowell
external usenet poster
 
Posts: n/a
Default

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.

  #14  
Old December 1st 03, 05:53 PM
Greg Esres
external usenet poster
 
Posts: n/a
Default

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.
  #15  
Old December 2nd 03, 02:02 AM
Rich Stowell
external usenet poster
 
Posts: n/a
Default

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.

  #16  
Old December 2nd 03, 10:27 AM
Dave
external usenet poster
 
Posts: n/a
Default

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



  #17  
Old December 2nd 03, 12:14 PM
Dave Pilkington
external usenet poster
 
Posts: n/a
Default

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

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
rec.aviation.aerobatics FAQ Dr. Guenther Eichhorn Aerobatics 0 November 1st 03 07:27 AM
rec.aviation.aerobatics FAQ Dr. Guenther Eichhorn Aerobatics 0 October 1st 03 07:27 AM
rec.aviation.aerobatics FAQ Dr. Guenther Eichhorn Aerobatics 0 September 1st 03 07:27 AM
rec.aviation.aerobatics FAQ Dr. Guenther Eichhorn Aerobatics 0 August 1st 03 07:27 AM
Barrel roll And g's Quest. Robert11 Aerobatics 6 July 16th 03 02:51 PM


All times are GMT +1. The time now is 06:48 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright 2004-2019 AviationBanter.
The comments are property of their posters.