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Aviation History
1912
1912 - 0016.PDF
[/JJGHT First, as to resistance generally. This is primarily of two kinds ; in one part it is due to normal pressure caused by the wind striking against the face of aflat surface (Fig. i), in the other it is due to " skin friction " caused by the wind rubbing against the sides of a plate that is moving edge on (Fig. 2). Dr. Stanton, of the National Physical Laboratory, also various other authorities, have experimen ally established an accepted formula for such normal pressure resistance in the expression R as -003 V-, where R is in lbs. per square foot of area facing the wind and V is in miles per hour. In America, Dr. A. F. Zahm has experimentally pro vided a formula that has not been generally accepted, although one of the few that exist, in the expression R = '0000316V1 •s°/0'93 (where R = resistance of double surface per foot of span, / = chord of surface). This formula may be approximated for aeroplane wings, within the ordinary limits of modern flight speeds, by the simplified expression R ~ • 000018V- (Fig. 3). And, as the expression itself is in doubt, there is little object in being particular as to accuracy in detail at the moment. In this expression, R represents the resistance per unit of double surface moving as a plate edge on to the wind. When the surfaces are separated, as in the formation of a box or casing, where they would be measured separately, the coefficient in the above formula is halved to make it applicable to the single surface or external area. The important point to observe is that the relationship between skin friction and normal pressure is represented by the ratio of 1 to, approximately, 300. In other words, you may use 300 square feet of edge on surface to enclose 1 square foot of normal area, if you can ensure that this covering body is truly edge on in effect. Bodies of streamline form (Fig. 4), as understood in naval architecture and in fluid dynamics generally, are supposed to convert normal pressure into skin friction ; they, therefore, JANUARY 6, 1912. potentially are capable of reducing resistance witkin the limits indicated by the above figures. This always assumes of course, that Zahm's coefficient is approximately repre sentative of the true state of affairs. If it is not, then the substitution of a more accurate value will immediately show the corresponding limits of possible gain. In any case, these figures at least suggest the importance of eliminating normal pressure from aeroplane design, by the use of bodies of streamline form to enclose the larger masses on the machine. This body resistance—in which is included the resistance of the struts, wires, and all framework except that actually forming the wings—is a resistance that is proportionate to the square of velocity (according to the above expression) and is a kind of extra dead load on the machine. It bears no- relationship to the lift of the wings, and is, consequently, a detriment to efficiency. It is very important to dis criminate thus between body resistance and the resistance of the wings. The resistance of an aeroplane wing in flight is itself of two kinds, one being the above-discussed skin friction of the- surfaces, while the other is a dynamic resistance due to the creation of the aerial wave that supports the machine in flight. This latter we may call the resistance due to load, and it will be shown that it is a function of the effective angle of the plane. If the effective angle is reduced, the resistance due to load per unit of supporting area will be decreased, but in order to support the same total load the area itself must be increased, which in turn increases the resistance due to skin friction. Hence there is a relationship between the two kinds of resistance experienced by a wing in flight, which is -why the wing needs to be considered separately, and why it is not proper to include the wing surface with the body surface when calculating the skin friction resistance of the machine as a whole. MYH 10 20 304) 50 60 70 80 IOO LlFTper.scj,.ft. P/is|tM/J X?°2L 456 8 IO 12 14 IS 18 20 THRU5T per UNIT LOAD -i- T- t2LTt2/^"1"* 0072. /J!o% 456 8 10 12 M- 16 18 20 16
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