Mass balancing is achieved in many different ways on full-sized aircraft. In some cases a lead weight is mounted on an arm projecting ahead of the hinge line. The arm and weight may be concealed within the wing, fin or fuselage, but sometimes this is not possible and the mass balance weight protrudes, as a source of parasite drag. In other cases, control surfaces are built with portions projecting ahead of the hinge line, with the balance weight inside. The projection may be concealed or used, for example on a Frise aileron, as a means of overcoming adverse yaw in turns, or, on elevators and rudders, as an aerodynamic balance to reduce loads for the pilot. In models, similar devices may be useful if flutter arises, though the need for aerodynamic balancing is rare. Complete mass balancing is not always necessary, since within the speed range of a particular aircraft, flutter may be a problem only for some of the controls at the highest speeds, and even these may require only partial balancing. Since mass balancing adds weight, it should not be used if not essential.
Even with the suggested precautions, most aircraft have a critical airspeed beyond which flutter of some member or other will begin. In full-sized sailplanes, for example, flutter may start sometimes even below the nominal ‘red line’ or structural ‘never-exceed – airspeed’. This can occur if the control linkages are worn with use and hence have become sloppy. If no further mass balancing or structural stiffening is practicable (if, for example, the aircraft would become too heavy after such modifications) the only solution is to fly always below the critical flutter airspeed. The modeller often does not know what this speed is until his model begins to flutter, and unfortunately, this may result in rapid loss of control or the radio gear being damaged by severe vibrations, heavy oscillating loads, etc. Once started, flutter is very hard to stop. Only a reduction or airspeed will be effective in damping the oscillations down, and if it happens to be the elevator or flaps that are involved in the fluttering, speed control may be impossible. Quite apart from the more predictable effects of poor stability, radio or servo failure, and pilot error in exceeding the structural limits of the model, some apparently inexplicable mid air break ups of model aircraft are caused by flutter. In other cases, models can be seen or heard to flutter quite violently, yet survive unharmed. Quite small variations in structure will make a difference. For example, the stiffening effect of tissue paper or silk covering as opposed to
Fig. 13.10 Mass balancing
the flexibility of plastic film is well known. A heavy piece of balsa built into a trailing edge may encourage early onset of wing flutter, when an otherwise identical model may escape, and so on. As usual, theory is useful to help explain what went wrong, but unless the model designer is prepared to spend many hours in calculations, he will have to rely on practical experience combined with an intelligent appreciation of the forces involved, when a new model is under consideration.