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Aviation History
1941
1941 - 1587.PDF
FLIGHT, July 17th, 1941 c TAIL UNIT DESIGN This is an example of longitudinalstability, the characteristic which it is the function of the tailplane to pro-vide. But there is also directional stability, a question of fin design, andthis has been the biggest millstone, round designers' necks in the last fewyears. The centre of pressure of a body such as a fuselage is ahead ofthe centre of gravity, in fact, it is near the nose. So if a slight disturb-ance yaws the aeroplane, the air pressure, applied at the centre of pressure, tends to turn the craft still further. This,of course, means that it is unstable. But air pressure on the fin provides a moment opposing this effect and keepingthe aeroplane nose into wind. But how big shall the fin be made?—that is the question to which the present stateof aeronautics does not give completely satisfying answers. Naturally, the longer the fuselage the smaller may thefin be, since with a longer fuselage, it exerts its force with a longer moment arm. In this connection it is noteworthythat the short fuselage of the little Stinson 105 calls for a large fin and rudder. In past years fin and rudder areas were fixed to a largeextent by consideration of wing area, but wing loading has now increased so much that the wing area has decreasedgreatly. But the instability of the fuselage remains and a new criterion for fin area has been proposed by F. L.Thompson at the 1940 S.A.E. Aeronautic Meeting. This takes account only of the fuselage diameter (D), length(L), and length of tail (1). He considers that adequate fin 0.4 D2Larea is given by the relation : Fin area = —-. It may sound strange to talk of "vertical surface stall-ing," but this can happen, though it is an unusual pheno- menon, and may be accompanied by reversal of rudderforce. This, of course, is most disconcerting to the pilot and may be very dangerous, since it may involve loss ofcontrol. Stalling of the fin may occur at about 15 deg. angle of yaw.This is a large angle of yaw,but can occur with a multi-engined craft climbing veryslowly at a steep angle at fulltake-off power if partial enginefailure happens. The BoeingStratoliner pho- tograph shows Mz '10the great SIX TWINTAILS repre- sentative ofBritish, German and Americandesign. The Me no and theDH Flamingo are of the mostmodern practice. The small piecesof tailplane out- side the fin onthe Hampden and Hudson areof very doubtful efficiency. TheWhitley fin is placed suffici-ently close in to allow it to bestrutted for rigidity . . . but though the twin type was retained, the hns and rudders were shifted outwards to act as end plates, a muchmore efficient arrangement. amount of rudder which must be applied to hold a largeaeroplane straight when both engines on one side are " out." The change which was made on the De Havilland Alba-tross illustrates very well some of the principles of tail design. Both original and final designs are of the twin-tail type, but the second is very much the better. Instead of the fins sitting on the tailplane at about the half points,they are arranged as " end plates " to the tailplane, thereby making it much more efficient since they reduce the "endloss" which always occurs at any wing tip. It will be noted, too, that the area of the fins has been increased. "End loss" raises the question of aspect ratio, a quan-tity which seems to be forgotten frequently, judging by the strangely stubby shapes of tail surfaces which are some-times seen. A circular shape for fin and rudder is some- times adopted, but who would use such a shape for wingplan form, with its aspect ratio of one? Yet the same con- siderations apply to the side view of fin and rudder as towing plan form. Aerodynamic efficiency is just as import- ant for stability and control surfaces as for wing surfaces.With greater efficiency the surface can be smaller and, therefore, lighter. The fin and rudder of the Sunderlandare beautifully designed, with about as high an aspect ratio D.H. Flamingo H.P. Hampden
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