John Wallinger's letter (Flight International, 12-18 November) "If it looks good it flies well" has its origins in that 1930s adage that "if it looks right, it flies right." However, if we add in bad-weather characteristics, there are contradictions in the correlations.
It's all a matter of pitch stability, aspect ratio, dihedral, end-plate fin-area effect, thrust lines and centres of polar inertia.
That is why, as he notes, short stubby types like the Airbus A319 and Boeing 737-200 suffer in bad weather. Adding length helps, cutting length does not - hence the extra fin area added to the shortened Boeing 707-138. The McDonnell Douglas DC-8 was aerodynamically cleaner than the 707 and had more dihedral - so it was more stable and less of a yaw merchant. Yet the beautiful Vickers VC10, with minimal dihedral, was rock solid in turbulence - the large tail area did catch crosswinds, however.
The Airbus A310 and Boeing 747 are so long that each end of the fuselage can encounter different gust angles and loadings - so when it comes to wandering in the wind, the rules are there to be bent.
Lance Cole Swindon, UK
Mr Wallinger raises an interesting question when he asks whether there are any ratios that could explain why one aircraft rides better in turbulence than another. There are such ratios, such as wing loading, aspect ratio and (albeit not a ratio) wing sweep. The higher the wing loading, the lower the aspect ratio and the higher the wing sweep, the smoother the ride. An exceptionally clear presentation of this can be found in Flightwise, Principles of Aircraft Flight, Volume 1, pages 388-393, by Chris Carpenter (Airlife, England, 1996).
Clemens Zehnder Pfaffhausen,Switzerland
Source: Flight International
