I have hijacked Mustang's posting and placed it here for discusson
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Quote:
Originally Posted by mustang
The recent series of articles on flutter in the replica aircraft thread has ceased for the time so I thought it might be timely to comment on a personal experience which had tragic results and invite further comments from members who may have had similar experiences.
I spent some time as a production test pilot on the GAF N22 and N24 Nomad, a small twin-engined STOL aircraft designed for outback operations on bush airstrips. The N24, the stretched fuselage version, exhibited on test longitudinal instability problems in some configurations of flap and power settings. The reasons were complex and I won't go into them in this post.
One of the "fixes" tried was to attach Al alloy "tee" strips to the trailing edge of the tailplan in incremental lengths, Initial tests showed some improvement and further tests were scheduled, progressively increasing the length of the strips until a satisfactory result was obtained, or some other solution to the problem had to be considered.
Accordingly, a stip of increased length was added and another flight test scheduled. The result was disastrous. As the aircraft passed through about 90Kts on take-off violent flutter developed in the tailplane causing severe structural damage and loss of pitch control.
The subsequent crash killed the senior experimental test pilot and the design engineer in the right hand seat. The flight test engineer survived, as a paraplegic. Flutter can be deadly and any aircraft which exhibits the slightest indications that it may develop should be either grounded or speed restricted until an aerodynamic and aeroelastic analysis is done and certainly no mass should be added to any control surface aft of the hinge line without expert engineering advice
Any pilot who ignores the initial symptoms of the onset of flutter will need to ensure that his will and insurance are up to date.
Mustang
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As I understand it, the "T" shaped extrusion trick on the trailing edge of a control surface is used where there a wide deadband in a control surface effectiveness caused by a thick boundary layer. Obviously, riveting in a strip of this extrusion into the trailing edge of a control surface moves the C.G back quite a bit and would require adjusting the static balance of the control surface by adding additional counterweight. Perhaps the people running this test didn't appreciate how much they were affecting the static balance of the elevator.
Ideally, you would want every inch of a control surface balanced so that accelerations would not displace the control surface and would not twist the control surface. In practice, this degree of counterbalance distribution is impractical, and designers cluster counterbalance mass in a few "horns" forward of the hinge line on the control surface. When you do this, each balanced section of the control surface can torsionally oscillate relative to the adjacent counterbalanced sections. These torsional oscillation frequencies need to be evaluated for potential of causing flutter.
A crop-duster/FBO/IA Mechanic/sometimes ferry pilot told me of a scare he had ferrying one of Cessna's popular 4 seaters. The plane had been sitting unloved in South Florida for a considerable period and had then been sold. My friend was hired to ferry the bird and he checked for the obvious things to watch out for, particularly nests, and things that might affect the powerplant.
Very shortly after takeoff the ailerons began flailing wildly. He slowed to minimum airspeed, brought the plane around the pattern and landed.
The aileron counter weights were long strips of lead sandwiched between aluminum strips and riveted to the leading edge of the aileron lower skin. Dissimilar metal corrosion had caused the attaching rivets to fail and one of the counterweights had been lost.
Proper mass balance of control surfaces is crucial.
Sid