The Dumb-bell/Doggybone Effect
A FEW YEARS AGO, with the arrival of Almost Ready To Fly (ARTF) model aircraft, a very odd thing started happening to the models in our Club. Prior to the ever increasing number of these ARTF models, with plastic covered foam wings, our standard .40 powered trainers flew with hardly any problems. They took off under full power and climbed to a safe height upwind, before turning, with the usual wing-rocking hardly noticed at all. These models were invariably open balsa framework covered in doped tissue or nylon and would continue to fly around with any deviation from the normal being quickly correctible..
Then came the ARTF models, some even to the same design, but their behaviour was very different. With the Centre of Gravity, dihedral and even the wing section being identical to the traditionally built model, their path to the safe height-before-turning was noticeably different. On its way up, the model was clearly gently swerving to either side, a kind of ‘Dutch Roll’ and needed constant correction. Believe it or not our answer was to put in a bigger engine, say a .56 instead of a .40, with the conviction that the model was wallowing because it was under-powered!
Now it is quite true that almost any model aircraft will fly OK, but fast, if the centre of gravity is far enough forward and there is plenty of power, but this errant behaviour needed a more scientific explanation. After a session of very wise chin-wagging and beard-tugging we tried an experiment. We went back to the original .40 powered balsa and nylon trainer and it took-off and flew OK. Then we replaced the built-up wing with the plastic covered foam wing on the same fuselage and there was the old ‘wallowing’ problem again.
We then weighed the two wings and discovered that the foam wing was nearly twice the weight of the balsa and nylon wing. Bingo! What we were seeing was a classic case of the Dumb-bell or Doggy Bone effect. The foam winged model had a lot more dead weight outboard of the C of G in the wing. Having a heavy slab of ¼ balsa as a tailplane did not help either. So if a gust disturbed the model, the inertia of that extra outboard weight exaggerated the need for correction and even caused the usual over-correction on the transmitter stick. So it was back to the traditional built-up structure for the novices while the ‘jocks’ simply poured-on some more power with their foam-winged models.
An interesting parallel to our humble Club problem could be drawn with that of the German World War 2 fighter (photo left), the Dornier D335 ‘Pfiel’. This aircraft is the classic example of poor, dumbbell, design with an engine in the nose and another one in the tail! Any design that spreads out the heavy objects from the C of G is bound to be less stable and the Pfiel was by all accounts a handful to fly. It doesn’t matter if the dead weight is outboard in the wings, or fore and aft in the fuselage, either way you end up with either lateral or longitudinal instability. Remember that classic footage of Neil Armstrong on the rocket powered wallowing Flying Bedstead? Only that he ejected from it in time he would never had made it to the Moon.