FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1941
1941 - 1586.PDF
aircraft, with tail surfaces designed according to widely accepted formulae based on previously successful designs, were unstable longitudinally. In some cases the machine was quite stable with engines off, but with engines on developed instability which could not be tolerated. The explanation appeared to be that unpredictable slipstream effects were set up by the flow of air round the nacelles when the power was on. This trouble was widespread and resulted in a considerable change of the established ideas 'on tailplane and elevator area. Later, the fin and rudder have caused more grey hairs for the designers. The Boeing 314, the now famous Pan Amercan Clipper, had only a single fin and rudder when first flown. This was inadequate to provide directional stability, and the tail was redesigned with twin fins and rud- ders of end-plate type substituted. But this arrangement also had to be changed by the addition of a central fin before the boat became the great success which it is to-day. This article is a non-technical survey of the state of knowledge in one branch of aeronautical design, not a harping on designers' mistakes. The lot of the aircraft designer is a hard one. He works in a science in which knowledge is far from complete, and, furthermore, he works FLIGHT, July 17th, 1941 THE BOEING 314 CLIPPER appeared first with a single fin and rudder. This was later altered to a twin tail, but the addition of still more fin area was required, and the final version now flies the Atlantic with three fins in the broad light of day compared with many other professional men. If he produces a new design which is not quite like any other aeroplane that has yet been built, and the tail turns out to be inadequate, the fact is widely known. The designer has not the possibilities of concealment available to the surgeon or physician, or the smoke screen of bewilderment with which the unscrupulous lawyer can blind his client, for a ten- or twenty- ton aeroplane or thirty-ton flying boat is not easy to conceal. The purpose of a tail is to make the aircraft controllable and stable. It is controllable if the pilot can manoeuvre it as he desires by means of elevator and rudder (and, of course, ailerons, though they do not enter this discus- sion). Stability is a different charac- teristic. An aeroplane is said to be stable if, after receiving some small disturbance from equilibrium in steady flight, it returns to this same steady condition of flight without any aid from the pilot. The "small disturb- ance" may be an up-gust under the tail which puts the nose down and so causes an increase of speed. If the aeroplane then tends to return to its original level attitude and to its for- mer speed without help from the pilot, it is said to be statically stable. But it may " over correct " itself and, com- ing out of the dive, may climb and climb until its speed has dropped much below the original value. Then it may start to dive again and repeat these actions with greater and greater magnitude until it either dives into the ground on one of its wild swoops or loses so much speed in the climb that it stalls. Such an aeroplane, though statically stable, is dynamically unstable ; the diagram is a repre- sentation of its mad frolics after it has been disturbed in straight and level flight and left by the pilot to right itself. But what is required is an aeroplane which is both statically and dynamically stable ; in other words, one which behaves as in the second diagram. This shows the aero- plane to have been trimmed to fly straight and level at 120 m.p.h. A disturbance, such as the nose having been raised by a gust or by a backward movement of the stick, has reduced the speed to 100 m.p.h. (represented by point A) Left to its own devices, the aeroplane puts its nose down and the speed increases until it reaches 120 m.p.h. again. The increase continues but does not reach 140, and then decreases again. So the oscillations continue, but always dying down, until the aeroplane is again flying straight and level at its original speed of 120 m.p.h. Twin fins and rudders were adopted for the first version olthe tail of the De Havilland Albatross
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
