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Aviation History
1941
1941 - 1588.PDF
FLIGHT July jylh, 1941. TAIL UNIT DESIGN (Above) THE BOEING FLYING FORTRESS is a successfulexample of a large aeroplane with single fin and rudder. The tailplane is set well forward. (Right) THE CONSOLIDATED PB2Y in its original andfinal forms indicates a change from the single tail, with auxiliary fin surfaces, to the more modern twin type. as is ever used, and that of the Wellington is very similar. The increase of fin area by adding it in the manner adopted on the Boeing Stratoliner interferes as little as possible with the structural design, but the aerodynamic efficiency of the new area must be very low. Its "aspect ratio," if such a quantity for the added area can be spoken of, is so low as to be fractional, and, furthermore, the moment arm about the centre of gravity is very much smaller than that of the original fin. On both counts its effectiveness is low. To some eyes, however, the " fairing- in effect" to the fuselage will look "right," but actually there appears to be no need for any such thing since the only change of direction to which air flowing along the top of the fuselage is subject is in the horizontal plane. Therefore, no vertical "fairing" is needed. The Welling- ton and the Sunderland testify to this. How Many Fins ? In seeking a high aspect ratio one has to be careful not to make the fin too high and thin, or it would be too flexible and might cause serious control or nutter troubles. A designer is very rarely free to choose his fin and rudder shape without restrictions. To give it high aspect ratio increases its height, and the height of the hangar door, particularly for nosewheel craft such as the Douglas DC-4, may impose a limit. For end plate fins the lower ends must be sufficiently high for ground clearance or, in the case of flying boats, so that they will be above the worst of the spray. But neither of these restrictions is likely to dictate the adoption of a very low aspect ratio fin and rudder and the reasons for some shapes seen are not at all apparent. Research has shown, however, that if a fin and rudder are of the end-plate type the relative pro- portions above and below the tailplane do not affect the aerodynamic efficiency of the tailplane or rudder control. The question of single or twin fins and rudders has not been settled and gives no indication that it is likely to be settled in the near future. The answers to it are as diverse as the leviathan four-engined Douglas B-19, with a single fin and rudder, and the single-engined General Aircraft Cygnet with a twin tail. (Incidentally, the Cygnet has the fin interchangeable with the rudder, probably a unique arrangement.) On the question of single v. twin there are certain considerations which must be borne in mind by the designer in making his selection, but there is cer- tainly no unanimity of opinion throughout the world on this subject. • If the aircraft is a military one, a single fin and rudder gives less reduction of the field of fire of the rear gunner, and this consideration generally overrides everything else in fighter and bomber design. But civil craft do not have this hampering restriction, and the twin tail is much more general though by no means universal. The latest Curtiss- Wright transport is a notable newcomer with a single tail, but Lockheed, a firm distinguished for the versatility and soundness of its work as well as the liberality with which it makes its information available to all, always goes in for twin tails. (No firm which continues its design work through the years ever goes on entirely trouble-free ; the early Lockheed 14, civil forerunner of the Hudson, was involved in a bad crash when a North West Airlines trans- port lost some parts of its tail, probably through ice deposi-' tion. This happening caused the tail design to be altered.) The Lockheed company has been very faithful to its own TWO ENGINES CUT. The original Boeing307 Stratoliner in flight on only two engines and held straight by plenty of rudder. Morefin area was found necessary and this was added experimentally as shown, but theproduction version was faired in with curves.
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