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#11
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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 |
#12
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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
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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
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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
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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
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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
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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 |
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