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Aviation History
1937
1937 - 0255.PDF
JANUARY 28, 1937 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 94g (a) (b) FI6.6. (c) (d) Fig. 6.—Comparison of air flows around (a) slotted flap at iz deg. ; (b) the Nazir flap at 12 deg. ; (c) slotted flap at 20 deg. ; and (d) Nazir flap at 20 deg. the wing with slotted flap. Again the breaking of the original surface, necessary in the case of the slotted flap, appears to advance burbling in front, because the air is thereby dropped more quickly and forms eddies, whereas by keeping the whole of the top wing profile constant it has been found that the burbling is delayed until a larger angle of incidence is reached. The smooth air through the slot in Fig. 6 (a) appears to flow round the nose of the flap only (the leading edge of the slotted flap) and if optimum lift on the flap was required, the air would have to take a sharp bend, to flow round the whole of the flap (which could only be possible by sucking the flow from behind), to prevent the flap from stalling The air through the cut slot in Fig. 6 (d) flows smoothly over the entire rear wing, giving a more powerful effect even when the front part of the wing is stalled. Con sequently the slotted flap begins to stall itself much quicker when moved away from the air stream than the cut-slot arrangement. Trie Family of Stalls There appear to be five types of stalls occurring upon w mgs of aeroplanes, three of which occur in a direction a long the span, the remaining two being in direction along ™e chord. The former are :— (1) The central stall, which is formed upon the rectangular wings. (2) The tip stall, which develops upon tapered wings. (3) The intermediate stall, which takes place on moderately tapered wings. I he spanwise stalls take place both on rectangular and tapered wings. The remaining two classes of stall occur upon aerofoils either simultaneously or separately, in relation to the chord of the wing, namely :— (4) The leading edge stall. (5) The trailing edge stall. Each of these stalls has different effects upon the aero dynamic efficiency of wings. The first three types might well be classed according to the location along the span, as spanwise stalls, and the second type according to the direction along the chord, as chordwise stalls. The two families of stalls are roughly indicated in Fig. 7. On some wings they form simultaneously and quickly join together to form the second stall; on others they spread out rapidly, from one particular point, in the form of an elongated fan. The former appears to occur more generally with tapered wings, as indicated in the figure. It will be seen that both types of stall occur upon tapered wings, and quickly mingle together. This phenomenon of burbling entanglement seems to be very critical and rapid. The chordwise stall, which occupies a greater frontal area of the wing, appears to form eddies even quicker than the spanwise stall. The chordwise stall then seems to draw the spanwise disturbance which consequently occupies a greater area, resulting in the first major loss of lift. This amalgamation of turbulence appears to be greatly respon sible for the earlier stalling of tapered than of rectangular wmgs. So far as I understand, this has some relation to the ideas on which Mr. Irwin and Mr. Gray are working. Remedying the Type of Stall It will be seen that both these types are detrimental to the wing, considerably decreasing the aerodynamic effi ciency. It is, however, possible that the effect of one family may vary according to the remedy at suitable periods. My experiments indicate a limit of certain incidences within which these remedies could be called upon to counteract the disturbance ; otherwise the effect appears to be reversed, resulting in a loss of lift. For instance, the very beginning of the chordwise disturbance or burbling may first occur at say 6 deg.-8 deg. wind tunnel. (8 deg.-io deg. incidence at full scale) from the rear of the top surface, and might rapidly spread over a large area (particularly upon the tapered wing) if not checked or prevented in time. STALL -*l Fig. 7 illustrates the two classes of stall on a tapered wing joining together to form the second stall. Application of some form of remedy is therefore necessary at the rear where this formation of eddies first takes place (on the upper surface of the wing), to prevent the very formation of burbling at small angles If this turbulence is allowed to spread over a certain percentage of wing chord, it consequently develops rapidly into the second stall, as shown in Fig. 7, resulting in a greater loss of lift. It is at this stage of severe stall that the wing actually
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