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djpacro

A Little Bit of a History of Flutter in Aerobatic Aeroplanes

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There are quite a few aeroplanes derived from the Stephens Akro. The Akro itself was derived from a Formula 1 racer, the Shoestring. The structure comprised a steel tube fuselage and a wooden wing. More power was used so the aeroplanes went faster. The competition sequences became more difficult so the aeroplanes were worked harder. Pilots wanted to roll faster so bigger ailerons were fitted. Quite easy to accomplish modifications in Experimental category as no-one is looking over your shoulder to the extent of ensuring compliance with any airworthiness regulations.

 

The wing came off one aeroplane in a vertical dive. Some evidence suggested that the cause was a failure in a propeller blade. Some evidence suggested overload of the structure as this wing had been modified to reduce weight. No-one knew anything about the flutter characteristics of this aeroplane so some good work was then done in this area. Ground Vibration Tests (GVT) and some detailed calculation of flutter modes and damping. This provided some evidence that the cause may have been flutter. There were a number of factors which would be different between individual aeroplanes such as an aileron servo tab, stiffness of wing attachment fittings and aileron span. The flutter analyst recommended a severe speed restriction until the wing was modified. The modification comprised an additional mass balance for the aileron which could be incorporated into an aileron aerodynamic balance – also known as a “spade”.

 

Several of my friends had already built aeroplanes. One had the lightened wing structure and, like others, developed significant structural damage. Ours was under construction.

 

One was flying after just being completed but didn’t have the additional mass balance initially. A run along the runway to show people the new aeroplane was the last. A little bit too fast and there were many witnesses to the aileron flutter.

 

Back in those days in Australia there was no Experimental Category so any modifications had to be approved as if it was a factory-built aeroplane. A GVT is normally done prior to flight testing to substantiate adequate structural damping and freedom from flutter up to the design dive speed. We conducted the flutter testing up to about 235 kts with the usual attempts to excite any flutter or vibration with the stick. VNE was about 200 kts.

 

There were some who didn’t like the feel of the ailerons with the additional mass balance so they chose to use a lighter mass balance. I guess they applied some sort of rationale that the interim speed restriction was conservative so the actual flutter speed was quite a bit higher. Then I guess they assumed a linear relationship between mass balance and flutter speed and told themselves that they wouldn’t fly any faster than that. That approach concerned me so I did some additional calculations based on the GVT data. The interim speed restriction was indeed conservative but these are only calculations so it was appropriate to apply such conservatism. The recommended mass balance seemed to have totally eliminated what we now knew to be an instantaneous catastrophic flutter mode. With a reduced mass balance the flutter mode reappeared. There certainly was not a linear relationship between flutter speed and weight of the mass balance. I could see a big risk in using a small mass balance without repeating the flight flutter tests although there would be a risk that flutter would be encountered while doing so. Time to stop modifying.

 

One of my friends bought a Pitts project recently. It has much bigger ailerons than what it left the factory with. The seller told him to be wary about aileron flutter. My friend asked me about that as I had been involved with similar modifications myself on a couple of Pitts. One had an interesting aeroelastic issue under certain conditions where the ailerons on the upper and lower wings would move in opposite directions. I wouldn’t contemplate any such modifications without first undertaking a GVT and analysis.

 

One aeroplane that I got involved with was one of those types which is of similar configuration to a Piper Cub. Aileron flutter encountered during initial flight testing at about 90 kts. That was before I became involved with it so I was certainly wary of the limits associated with the aileron balance. Flutter can occur at low speeds too.

 

In another thread some-one mentioned flutter of the elevator trim tab if the trim cable breaks. In the case of a Pitts the elevator tab will flutter. It is a high frequency vibration to full deflection. Fortunately the frequency is so high that it does not force or excite any motion of the elevator, tailplane or fuselage structure. In cases that I am familiar with there was no structural damage as the speed was well below VNE however it was very exciting for the pilot.

 

These days in some GA aeroplanes with recent certification you may notice that tabs will have their own mass balance or perhaps a second push-pull tube to eliminate the possibility of tab flutter.

 

It is good to have Experimental Category in Australia these days but it is still important to be mindful of the standard airworthiness requirements such as FAR 23 as they generally arose from problems encountered with aeroplanes over the years. These days you can buy fancy aerobatic aeroplanes as a kit or as a certified factory built aeroplane. It is worthwhile checking out the extent of engineering substantiation of an aeroplane in Experimental category before flying it. Aerobatic pilots just seem to love more power, higher speeds, higher g’s and bigger ailerons. It is good to see that even the builders of aerobatic kit aeroplanes are actually putting in the appropriate engineering effort. As for the one-off modifications to the Pitts that my friend bought – as the seller said, he must be wary of aileron flutter. More later.

 

 

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Interesting post - my only experience of flutter was fortunately second hand, watching flight testing of an experimental type in the circuit. To this day I am not convinced that the noise associated with a trim tab fluttering, wasn't in fact the pilots sphincter, on a harmonic frequency!

 

 

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