SPECIAL PROBLEMS: POWER MODELS ‘GOING FLAT’

Free flight duration engine-powered models sometimes instead of climbing steeply and fast, ‘go flat’ and fly very fast, more or less he rizontally or at a shallow angle of descent, to hit the ground.

It seems very likely that the cause is the same as that of the ‘tucking-under’ glider. These models frequently have the static margin reduced by aft centre of gravity location. Under power, a static margin that may be adequate for gliding or climbing may be reduced so that the model verges on neutral stability. Some flights then may succeed but on occasions a minor variation in launching technique may bring the model to a dangerous condition.

12.16 SPECIAL PROBLEMS: VERY SMALL MODELS

Very little is known about airflows at Reynolds numbers verging on, or below, the critical. where the flow tends to separate completely from the wing. In some small modefs it seems the flow may change abruptly from one condition to the other more than once during a flight. This seems particularly likely with ‘chuck’ gliders which are launched fast and are at relatively safe Re number during the initial stages of a flight, but which slow down fairly rapidly and may then fall into a sub-critical state. Once in this condition they may not recover, in which case a very poor flight results. Associated with the flow break away there is a change of pitching moment which upsets the normal stability. The likely solution to such problems is to increase the wing chord and use an aerofoil which is not badly affected by low Re numbers. Probably the closer the profile comes to a flat plate section the more consistently it will behave, although performance in the absolute sense is likely to sufTer.

Quite different stability problems appear with indoor, microfilm covered models. These are so flimsy in structure that distortion under flight loads is commonplace. The uneven unwinding of rubber motors also can cause serious shifts of centre of gravity position during a flight, which upsets both balance and stability. Humidity and air temperature also make differences which affect other models very much less. It becomes practically impossible to work out static margins or even trimming angles, since in flight these change. Even with all these effects, it still seems that a centre of gravity that is at least in the right place to start with will improve stability and hence consistency. There is no advantage, apparently, in using ‘lifting’ tailplanes of large area, when, by adding the excess area to the wing and moving the c. g. forward, stability would be improved without loss of aerodynamic efficiency.