Monday, August 8, 2011

Aeromodeller 1956-10

Numerous modellers have come up against the question of how to design a model with adequate longitudinal stability, adequate, that is, for a given purpose. Some models designed for more-or-less still-air tlving, and known to be excellent performers under these conditions, simply leap from stall to stall in spite of repeated trimming, when flown in a competition in which the meteorologist�or the weather gods�did not co-operate. On the other hand, models designed for rough weather are not the best dead-air performers when it comes to clipping that odd half-inch per second from the sinking speed. The 1954 A/2 World Championships at Odense served as an excellent example of the former case (though few of the competitors would have much to say in favour of the weather there!). The writer was one of those afflicted, and upon returning home started to brood and hatch, figuratively speaking. The crux of this matter of inadequate longitudinal stability seems to be that model designers often forget that the problem of stability has two sides. But as we are model designers, i.e., scientifically-minded (or at least deem ourselves to be . . .), let us start as required in any scientific investigation by defining the problem at hand. Well: Longitudinal stability is the property of an aircraft to return to normal flight conditions after it has been displaced by some external force, a gust or the kind of towline release one only sees perpetrated by others. How do we achieve this stability?