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Aviation History
1933
1933 - 0211.PDF
51 TULY 27, 1933 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT iv;th missing slip stream. In such cases an aeroplane ,n.i\ stall by itself. i >uspect of such dangerous behaviour all those planes which show a poor elevator effect during a normal three-point landing with stopped engine, i.e., which show a tendency to pancaking suddenly if properly flattened out near the ground. This peculiarity means either a poor wing characteristic—now very rare with pianos of modern design—or a getting out of control on behalf of breaking-up of the air flow at the tailplane. During the landing manoeuvre the pilot becomes some what helpless without use of the throttle, when flying a plane of this kind. Such aeroplanes will be dangerous during a take-off or a turn near the ground with an unreliable engine. (3)—Wings with Protruding Rib Webs The necessity of an efficient lift distribution along flie span under all flying conditions, i.e., a slow decrease of the circulation towards the wing tips is well recog nised. Better aspect ratios may help. But a good aspect ratio will also create certain disadvantages in design, construction and handling. Therefore the de signer should do everything to get a wing of low aspect ratio with exceptionally good aerodynamic qualities. He will get thereby also a high maximum lift coefficient. Besides other well-known means, there seems to be a very simple possibility of obtaining an excellent lift distribution over the span, like the wing-tip discs of certain tailless planes. This is provided by wings with protruding ribs, i.e., with rib webs which protrude somewhat over the wing surface into the free air-stream. A famous fighting monoplane of 1918, the German Kondor-Parasol, " Kou E III," designed by W. Rethel, showed that peculiarity. Every pilot who tried this fast fighter agreed in the exceptionally good behaviour of this aeroplane, especially in turns. The plane showed no tendency of skidding in the steepest, improperly flown turn. It had also an exceptionally low landing speed and a very good climb. Perhaps this simple system of construction may be a valuable help in the design of a wing of poor aspect ratio, which gives good aerodynamic qualities. Con- structionally this method seems to be superior to wing- tip discs. (4)-Aeroplane Strength and the "Ideal" Polar Diagram Instead of the " brutal " dictate of a load factor for a given aeroplane structure, the German authorities iww have decided to rule individually the load factor by the known or computed values of performance and flymg qualities of the plane in question. Each design will so get its own load factor. ertainly this great progress will be a sane constraint T,t the designer to think about possibilities of lowering dynamical loads occurring in flight. If he succeeds, "'ill be able by the new airworthiness regulation to ^struct a very light and efficient plane. Now it is a well-known fact that a fast plane suffers 11 greater accelerations in flight than a slower one. j aerodynamic refinement will be punished by a -tier load factor. The considerable wing structure Rights of modern high-speed planes prove it! I'om the pilot's point of view, such high load factors "i really useless, or even pure nonsense, because "• pilot or passenger will suffer an acceleration of more trf;' Sa^' ^ **' As an aeroplane will not be built in- n'' t;onauy for carrying dead inhabitants, there will be teason to construct our planes stronger than, say, ie» times _a small factor of safety. Poj •>rrWittl a niSn-sPeed plane, there will often be the ',.. ; H 7 of getting unintentionally dangerously high ^s-^rations, during transport flight by sudden gusts, X| ^na& service flight by sharp turns and aerobatics. e are remedies for lowering these unwelcome •I orations. Let us consider some of them. The most important case of loading, and a practical one, is the abrupt flattening out from a terminal nose dive. For that loading three factors are governing, namely, the terminal diving speed, the resulting air force coefficient at the stalling angle (angle of incidence of maximum lift), and the pitching time during a pitch ing movement of the plane, ranging from the angle of no lift to the incidence of maximum lift. The last factor is a function of the elevator effect. If we suppose an optimum elevator operation, i.e., a pitching time equal zero, the greatest possible load factor will be 2 KL • S • p • V / T T, max ' «• L.r. = 2 • W where V(, = the terminal velocity of dive. Now, may it be possible to diminish the terminal diving speed? Certainly it is! But nobody seems to pay attention to that possibility. It will not be difficult to invent and to incorporate simple arrangements for obtaining automatically an enlarged drag as soon as the speed of the plane increases over a certain definite limit. Moreover, certain aeroplanes and even wing sections exist already which prove—wholly unintentionally—that peculiarity aimed at, namely, those planes famous or ill-famed for a slow diving speed. Good examples are the Fokker Triplane and the Fokker D VII of war fame. It is not always the parasitic head resistance which causes a low diving speed. Apparently the aerodynamical refinements of to-day do not allow for a low diving speed, especially with a high wing loading. But we should regain it. An automatically functioning air brake causing a high drag during the beginning of a steep glide, especially a power glide, may be ever possible without complicating the design. This simple consideration leads to a thought of a more general character: What characteristics does a high-performance aeroplane need which shall not suffer from high accelerations? In order to get a survey in that direction, it may be useful to compare the well-known polar diagram of an ordinary aeroplane (Fig. 1) with an " ideal " polar curve for such a high-performance plane, giving low accelerations (Fig. 2). With the " Ideal polar curve for flight " (Fig. 2), two points are striking. At first the drag coefficient for the incidence of disappearing lift is very much greater than the smallest drag coefficient. This results in a lowered diving speed. Secondly, the maximum lift co efficient remains small and does not surpass markedly ki t • "' the value belonging to the optimum —— feD And what of the landing? the reader may ask. We have already laid it down that this polar curve shall only be valid for the condition of flying itself—not for start and landing. For these extraordinary flying conditions we have possessed for many years certain well-proved installa tions in order to produce a high maximum lift coeffi cient at high angles of incidence, like slotted wings, variable camber wings, wing flaps, split flaps and other useful things. Why not use these? But no aeroplane designer should forget that the corresponding increase of lift has to be used exclusively during starting and landing, never during ordinary flight. Certainly one has often overlooked this necessity, and several some what inexplicable disasters may be caused by that reason. For high-speed flying a great lift coefficient results only in very undesired loads and in airsickness too. Our " ideal " polar curve for flight testifies against automatic regulating wings, i.e., against those wings having any automatic regulations for gaining a high lift coefficient with increasing angles of incidence, like autoslots not fixed in ordinary flight, wings with fixed 754 k
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