Further to Tom Batot Frazier's "Bird lessons" (Flight International, 30 March-4 April): the Chinese developed the Soviet Antonov An-2 biplane under the designation Y-5 and the Y-5B model features wing-tip "feathers".
The An-2 can fly very slowly, perhaps the feathers help it to hover?
Officially, these increase the climb rate by about 20% and reduce fuel consumption and raise the gliding ratio by about 15%.
Robert Ruffle
Russian Aviation Research Group Shoreham-By-Sea, UK
Feather tips were tried and tested many years ago. In Darrol Stinton's excellent book The Design of the Aeroplane, there is a chapter dedicated to wing tips, with illustrations of, among other ideas, Spillmans "sails" installed on a Piper PA25 Pawnee: five in this case. So, as usual, there is nothing new under the Sun.
Peter Gambardella
Farnborough, Hampshire, UK
Source: Flight International
