Aileron Flutter By M. B. Taylor, EAA 14794 Box 1171, Longview, Wash.
H programs in the past and having run a number of
AVING BEEN through several FAA "certification"
"flutter" studies on light aircraft over the years, we were somewhat concerned by the story on "How To Install Aileron Counterweights" in the April, 1968, SPORT AVIATION. It would appear that the proposed installation might be on the "flimsy" side as far as accomplishing the basic purpose of balancing the ailerons. The reason for balancing control surfaces is to eliminate "flutter." There is, of course, the static balance benefit which makes the controls easier to move, but this can be accomplished aerodynamically with ease. It is quite easy to add to flutter susceptibility by having a balance weight installation which will actually contribute to the flutter installation. This is due to the attachments of the balance weight being too flexible or spongy. This situation is very easy to see if you excite the control surfaces or the wing itself with a simple eccentric weight which can be made to shake the wing at various speeds. Somewhere in the speed range you will find a speed which will excite the natural frequency of the wing just like plucking a string on a musical instrument. Similarly, the control surface has a natural frequency at which it will vibrate or "flutter." If the balance weight is not very rigidly attached to the control surface, the system composed of the balance weight and the control surface may be much more susceptible to this natural frequency excitation than were the balance weight completely eliminated. We note that the installation proposed in the story
sheet of 0.063 in. 4130 steel. The stiffness of this sheet
along this edge would be very limited, and the stiffness of the aluminum hinges would add further to the lack of rigidity. Add to this the stiffness characteristics of the link plate with the hole in it, and I doubt seriously if the installation would pass any flutter investigation. While it might counterbalance the weight of the ailerons, it certainly would be most critical from a flutter standpoint. A good way to evaluate balance weight installations and their stiffness is to strike the weight or the control surface with the butt of your hand. Do this with a sharp blow and note if the system tends to vibrate as a unit. If there is any "buzz" or excitation other than a real "solid" reaction, the system should be stiffened. There have been a number of recorded accidents with commercial lightplanes that have been traced to inadequate control surface balance weight installations. The early "Ercoupes" were noted for this, and more than one of them has lost the aileron balance weights or even lost complete control surfaces. The susceptibility of a control surface installation is very much affected by the tension on the control cables or the sloppiness of a rod control system. When testing for flutter for FAA certification on a cable control system, you must slack off on the cables to simulate a worn system; of course, in cold air cables tend to change their tension too. • We experienced aileron flutter during a test program some years ago and I, for one, can tell you that a case of aileron flutter can be a real remembrance.
supports the weight itself on the edge of a 3% in. wide
MYSTERY AIRCRAFT Answers will be found on another page of this issue.
GLASS COWLING . . . (Continued from page 11)
It is actually lightweight fence wire, galvanized, with about quarter inch mesh. It's a flat screen type of wire. Remember the plaster? Here's where you get a break.
Forget the plaster and just fashion your component, wheel pants, canopy top, or cowling to the desired contour. Put on a thin coat of glass mat, or cloth and resin, over the form. In about 40 min., presto! You have a cowl-
ing! In this process you leave the hardware cloth in your cowling; again, if you want additional strength, go on the inside and reinforce. Many builders have very elaborate 12
AUGUST 1968
and exotic ways of casting components. No two builders can agree on the best way of fiberglasing components, but few people will disagree on the simplicity of this latter method. Use polyester resin . . . it's cheaper than epoxy resin and you can buy it at any boat supply house along with the catalyst, fiberglas mat or cloth. I use one-quarter teaspoonful of catalyst with each 8 oz. of resin. Work fast, man, or you'll end up with glass marbles. Pot time, or curing time, when the temperature is about 70 deg. is about 40 min. Work outside or in a well ventilated shop, because the toxic odor is something else. ®
