Flutter

[bd32322]

Funny things happen when parts of the airframe break the speed of sound as well.

Yeah thats the whole reason for sweepback in wings .. The velocity component perpendicular to the wing span is reduced to increase the margin from supersonic speeds

[bd32322]

Interesting link discussing aerodynamic flutter:

 

http://www.cs.wright.edu/~jslater/SDTCOutreachWebsite/aerodynamic_flutter_banner.pdf

Very informative article and just as complicated !

[Alan Fox]

Yeah thats the whole reason for sweepback in wings .. The velocity component perpendicular to the wing span is reduced to increase the margin from supersonic speeds

Sweeping the wings is to increase lift by aerodynamically increasing the chord , without actually building a bigger wing....More chord equals more lift.....

[bd32322]

Sweeping the wings is to increase lift by aerodynamically increasing the chord , without actually building a bigger wing....More chord equals more lift.....

You cannot increase average chord without a bigger wing. You just increased chord at the root but not at the tips. Total lift stays the same I would think as a rectangular wing with average chord.

[N601RX]

In addition to the Mooney I also have a Zenith 601 xl experimental. 6 of them had the wings fold up in flight due to flutter. The ailerons did not have any counterbalance on the and it had a "springy" pull/pull cable system. In addition to the normal cable stretch the ribs the pulleys were attached would flex a little. The FAA grounded all the factory ones registered as LSA and refused to issue any new airworthenss certs for the experimemtal ones. A lot of both flight test as well as ground viberation test were ran both here and in Europe. The Manufacturer came up with a fix in the form of a kit that added counterbalances and stiffened up the ribs that were flexing as well as stiffened up the center pass through section of the wing. For the Ground viberation test the plane was suppended from the ceiling of a hanger and all the control surfaces and wings were fitted with accelerometers and a data acquisition system. Electrical viberators were then hooked to different places and the viberations were swept across the frequency that the plane would see in flight to make sure there were no undampend viberations present. That was about 3 years ago and there have not been any other wings folding up since. Before the upgrade it was hapening at a rate of abot 2 a year.

[jlunseth]

There is an excellent book called Flying High Performance Twins and Singles I believe, that delves into all the "math" about airplanes.  Flutter is true airspeed, not indicated.  The best way this fits into my limited cranial computing capacity, is that indicated airspeeds are based on pressure, and pressure is what makes the wing fly and lets the control surfaces control.  It does not make any difference if the wing is flying at sea level or at FL200, it needs "x" amount of pressure around the wing to produce "y" amount of lift or performance.

 

Flutter, on the other hand, is not directly a function of pressure.  It is a function of the energy of the air moving over the control surface. Even though there are fewer air molecules in the higher altitudes, e=mv² and at some point the control surface is unable to withstand or satisfactorily react to the energy being imparted to it. 

 

My simple minded attempt to grasp a complex subject not in my field.