MODEL AEROPLANE NOTES

Dominion, Volume 29, Issue 133, 29 February 1936, Page 6

MODEL AEROPLANE NOTES

Details of Construction TAIL UNIT DESIGNING

(By

Aileron.)

In these notes 1 propose to give some more detail concerning tail units aud their usual dimensions and settings. Before considering this we must know something about the "angle of incidence.” You Have seen that nil the models yet described iu these notes have had their wings placed not flat iu relation to the thrust line (the line of rubber), but have had the leading edges placed a little above the trailing edges.. This angle that a wing makes with the thrust Hue is called tho “angle of incidence.” In the case of a wing, where the leading edge is raised, we sny that there is a "positive” angle of incidence. Should the leading edge be lowered below the trailing edge, as sometimes in tailplanes and rarely in wings, there is it “negative” angle of incidence. Most tailplanes, .being set in the same line as the thrust line have “no angle of incidence.” IVhile on this, it may he said that, most tailplanes are set at ub angle of incidence, and most wings at a small positive angle of incidence (a rise' of about l-Bin. for each four iuches of cord). Why do we need a tailplane and rudder? The answer is that without one the model would never recover from any gust of wind, would be almost uncontrollable, and would be altogether too delicate of adjustment. Any sort of a tailplane will not do. It must be of a certain area compared -to that of the wing, this area being governed by the speed of the model and the distance of the tail from the wing. I have already shown a means to ensure that the tail is as far from the wing at, possible. The reason for this is seen when we know that the tail is a sort of force acting at the end of a lever (the fuselage), holding a model on its course. In other words, the further, the tail of a model is from the wing the better is the action of the lever, of which it is a part. Generallj' speaking, this will prove a satisfactory guide when designing a tail for a model: Take tbe centre of the wing and the centre of the tailplane. (You will know roughlj’ where the wing will be placed.) If the distance between these two is about three times the chord (length of rib) of the wing, the tail should have an area about one-third of that of the wing. If this distance is less the area should be a little more; if more, the area may be a little less. In uearlj' all cases, the rudder may be half the urea of tbe tailplane used. The shape of the tail is important. If made too much like a wing it will be too weak or too heavy; if made too short and wide it will not do its work well enough. A good principle is to make the length four times the width. ' A tailplane which should have about 24 square inches area could be made roughly lOia. long by 2}iu. wide. . . Usually tails are not cambered like a wing. A simple flat shape is probably as good as anything. Any complications, such as making the ribs in the.tail the same shape as the ribs in the wing, will meke the model harder to fly. Jn conclusion, do not be riiislcd by the shapes and proportionate sizes of tailplanes on full-size aeroplanes. The tailplane of the average model is better fitted to do its work fhun the’ tailplane of a “real” aeroplane. Only by following model principles will you obtain satisfactory flying.