From: cdenk@ix.netcom.com Date: Tue, 29 Dec 1998 17:02:47 -0600 (CST) Subject: COZY: Flutter????? Flutter airspeed is a function of Indicated airspeed. Right?? At high altitude are there any minor or other effects ?? Date: Wed, 30 Dec 1998 10:58:09 -0500 From: bil kleb Subject: Re: COZY: Flutter????? cdenk@ix.netcom.com wrote: > > Flutter airspeed is a function of Indicated airspeed. Right?? yes, for "slow" speeds. > At high altitude are there any minor or other effects ?? no. i asked a local aeroelasticity human, he confirmed my thoughts; "Except for really nonlinear stuff, classic flutter = f(q,Mach). For most GA piston planes Mach << 1 so flutter = f(q)" where q is the ram, or dynamic pressure, q = 1/2*[air density]*[velocity]^2. dynamic pressure is exactly the quantity that the airspeed indicator measures. thus, at the same _indicated_ airspeed you have the same flutter margin (in q units, anyway) regardless of _true_ airspeed. however, if you are talking about compressible flow, i.e., Mach numbers greater than, say, 0.4, then all bets are off as indicated above. -- bil Date: Thu, 31 Dec 1998 10:13:59 -0500 From: David Domeier Subject: Re: COZY: Flutter????? Carl, re "Flutter airspeed is a function of Indicated airspeed. Right??" I think not. At high altitudes, limit speeds are Mach not IAS. If the Cozy limit speed of 190 KIAS were imposed on at say 29,000 feet, I think the airplane would not sustain the forces on it whereas at 8,000 feet we know it will. Not being an expert on such matters, I can not explain the why of it, but I suspect "flutter" limit speed may well be a function of true airspeed, not IAS. I say this from the perspective that a jet can be flown without buffet at 330 knots IAS at 12000 feet, but at 39,000 feet, the airplane will "buffet" at a much lower IAS (260 or so) due to pushing Mach limit. All the while the true airspeed may be nearly constant. Certainly much more so than IAS. dd From: mikefly@juno.com Date: Fri, 1 Jan 1999 14:03:44 -0600 Subject: Re: COZY: Flutter????? HAPPY NEW YEAR to ALL !!! I'm no expert either, but I have spent some time in Lear Jets. Mach buffet and flutter are two different things. Flutter is a function of IAS. (VNE) Mach buffet is caused by a shock wave. (sound barrier) Mach limits are at slower IAS's at high altitude because the speed of sound is slower. Not exact , but the speed of sound at sea level is around 750 mph. and 500 mph. at 40,000 ft. Exceeding mach limits does not usually cause structural failure or catastrophic flutter. It does cause control loss which can result in a dive that can exceed the structural speed limit. (VNE) This dive is sometimes called a mach tuck and is usually preceded by a buffet or aileron buzz. In Lear's, at low altitude, you can not exceed max mach (MM) without first exceeding VNE. At normal cruise altitude, (FL 410) you can not exceed VNE with out first exceeding max mach (MM) Indicating mach .80 the TAS, depending on OAT will usually work out to around 440 kts. with a IAS of around 210 kts. VNE on 20 series Lear's is 301kts. IAS or 350 kts. IAS depending on model. All have a .82 MM. Mike Bowden > re "Flutter airspeed is a function of Indicated airspeed. >Right??" > > I think not. At high altitudes, limit speeds are Mach not IAS. > > Not being an expert on such matters, I can not explain the why of >it, but I suspect "flutter" limit speed may well be a function of >true >airspeed, not IAS. I say this from the perspective that a jet can be >flown without buffet at 330 knots IAS at 12000 feet, but at 39,000 >feet, >the airplane will "buffet" at a much lower IAS (260 or so) due to >pushing Mach limit. All the while the true airspeed may be nearly >constant. Certainly much more so than IAS. > >dd > > > >