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Aviation History
1928
1928 - 0514.PDF
SrPPLKMIHT TO FLIGHT SO JUNE 21, 1928 THE AIRCRAFT ENGINEER assistance is given to the normal wing surface by such agencies as the under side of the aircraft other than wings ; the effect of the thrust of the propeller and the cushioning effect of the air between the lower wing and the ground if the results are given for an aircraft flying near the ground. Corrections to kL nmx. are of an empirical nature, and it u suggested that for obtaining an idea of the probable full scale fa max. the following known values in Table II might be referred to and the nearest shape of aerofoil taken as a guide, paying particular regard, not to the external shape of the aerofoil so much as to the shape of the median line of the section. era r\ Corrections are also necessary for aspect ratio, , ~, andchord stagger, and these are graphically represented in diagrams 3, 4 and 5. Table II K.A.F. Aerofoil Model Full-scale biplane ^"L max. "L max. 0-59 0-54 0-46 0-47 0-52 0-54 0-60 0-63 0-66 0-65 0 • 62 1415 25 26 27 28 30 31 32 33 34 0-544 (mono) 0-500 (biplane) 0'427 (mono) 0-461 „ 0-420 „ 0-517 „ 0-458 „ 0-535 „ 0-658 „ 0-620 „ 0-510 „ Derivation of Individual Upper and Lower Wing Characteristics from Biplane Results For stressing the wing system of a biplane aircraft it is useful to know the characteristics of the individual wings so that full advantage may be taken of the load distribution to give the most economical structure. The corrections so far applied relate to the biplane as a whole, and it next behoves the stress calculator to deduce the contribution of each wing to the forces in the wing structure. The theoretical aspect of this section has not been developed to any great extent, so that all that can be recommended is an empirical method based on experimental evidence and further supported by successful application to actual designs. For the equal wing biplane it is suggested that the K.A.F. 15 biplane experiments [4] might be taken as indicative <>f what is likely to happen. (For unequal wing biplanes a special treatment is suggested later.) The application of this data is as follows :— Equal Wing Biplane. (See Fig. 6.) The mean ratio of fcui/^M, for the given conditions is taken, and represents the value where A"j, for the biplane — .' ki, max. We have now the conditions that, if x khL + £LL = 2 kL since the wings are equal or (12) The stress calculator is interested primarily in the aerofoil characteristics up to the stall, hence for his purpose it will be sufficiently accurate to assume that over this range the lift curve for the lower wing can be represented by a straight CORRECTION OF KLMAV~ FROM MONOPLANE TO BIPLANE. 1-4. IS STAG6ER CORRECTION 105 1 I 00 "95 J j j 'H 1. _t- ---I" STAGGER 1 1 , I -^ "1 no. o. | -25' -20' -15' -10- -5" 10' 15' 20' 25' R.A.F. 15 BIPLANE R &M 65T & 774 k k D L •10 8 •08 -6 •Ob -4 •04 2 •02 0 IS' 20' 25' 30' 35' 0 -2 WING CHARACTERISTICS CLARK Y BIPLANE -10' 15' In passing, it should be noted that where the trailing edge of an aerofoil has been reflexed the full-scale value of kL max. is apt to change erratically, and consequently a conservative estimate should be made. Thick aerofoils, in particular thickness (i.e., when the — — exceeds, say, 0 • 13), appear to remain chord either constant for increase in wind speed or else to decrease. An exa mple in point ["] is the J linkers thick-wing monoplane, where the full-scale test shows a smaller value of kL max. which decreases as the value of VI increases. At the moment full-scale corrections can only be made b37 discretionary com- parison with known aerofoils whose kL max. is known. line. The errors so introduced will not be great, and there is experimental evidence in support of this procedure. Having now the biplane lift curve and the lower wing lift curve, it is a simple matter to plot the upper wing lift curve. An example will probably make the above quite clear. cap Suppose we have an equal wing biplane of unit —— ratio chord(A = 6) of Clark Y section [6] with a stagger of 20° to the lower wing chord, there being no relative inclination of the chords (wrongly termed decalage in this country—which means "stagger"); then for the monoplane we have (see Table I): kL = 00361 a. + 0-191 (13) 464/
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