Aeroplane Construction and Operation

by John B. Rathbun

According to a number of experiments on full size biplanes averaging 1900 pounds, it has been found that the interplane resistance (Struts, wires and fittings) amounts to about 24 per cent of the total parasitic head resistance of the entire machine, the drag of wings not being included. The maximum observed gave 29 per cent and the minimum 15 per cent. The resistance of the interplane bracing of speed scouts will be considerably less in proportion, as there are fewer exposed struts and cables on this type, the resistance probably averaging 15 per cent of the total head resistance. Based on these figures the resistance of the interplane bracing can be expressed by the following formula, in which I = resistance of interplane bracing in pounds, and V = translational speed in miles per hour:

I = 0.009 V² (For two-place biplanes weighing 1900 pounds).

I = 0.0054V² (For biplane speed scouts or racing type biplanes).

Strut Resistance. The above estimate includes wiring, strut fittings, etc., complete, and also takes the effect of the slipstream into consideration. A more accurate estimate can be made on the basis of strut length. To obtain this unit value we have recourse to the B. E.-2 tests. The translational speed in 60 miles per hour (88 feet per second) and the slipstream is taken at 25 feet per second. This gives a total velocity in the slipstream of 113 feet per second. The struts are 1% inches wide, and vary in length from 3’ 0" to 6’ 0". In the slipstream the increased velocity increases the resistance of the items by 64 per cent.

Total running length = 110’–0". Total resistance = 10.81 pounds. The resistance per foot = 10.81/110 = 0.099 pounds.

Resistance of Wire and Cable. In this estimate we will take the resistance given in the B. E.-2 tests, since values are given in the slipstream as well as for the outer portions. In the translational stream there is 240’ 0" of cable, 70’ 0" of No. 12 solid wire, and 52 turnbuckles, the total giving a resistance of 38.10 pounds. In the slipstream there is 50’ 0" of cable and 30’ 0" of solid wire with a resistance of 11.00 pounds. The total wire and cable resistance for the wings is therefore 49.10 pounds. The resistance of the wire and cable combined is 0.127 pounds per running foot.

Summary of Interplane Resistance. The total interplane resistance includes the struts, wires, cables and turnbuckles, a portion of which are in the slipstream. Since the total head resistance of the entire machine (B.E.-2) is 140 pounds at 60 M. P. H., and the interplane resistance = 10.81 + 49.10= 59.91 pounds, the relation of the interplane resistance to the total resistance is 43 per cent. This is much higher than the average (24 per cent), but the B.E.-2 is an old type of machine and the number of struts and wires were much greater than with modern aeroplanes.

Control Surface Resistance. The resistance of the control surfaces is a variable quantity, since so much depends upon the arrangement and form. Another variation occurring among machines of the same make and type is due to the various angles of the surfaces during flight, or at least during the time that they are used in correcting the attitude of the machine. With the elevator flaps or ailerons depressed to their fullest extent, the drag is many times that with the surfaces in "neutral," and as a general thing the controls are depressed at the time when the power demand is the greatest—that is, on landing, flying slow, or in "getting off."

Ailerons "in neutral" can be considered as being an integral part of the wings when they are hinged to the wing spar. In the older types of Curtiss machines the ailerons were hinged midway between the planes and the resistance was always in existence, whether the ailerons were in neutral or not. Wing warping, in general, can be assumed as in the case where the wings and ailerons are combined. With ailerons built into the wings, the resistance of the ailerons, and their wires and fittings, can be taken as being about 4 per cent of the total head resistance. With the aileron located between the two wings, the resistance may run as high as 20 per cent of the total.

Like the ailerons, the elevator surfaces and rudder are variable in attitude and therefore give a varying resistance. In neutral attitude the complete tail, consisting of the rudder, stabilizer, elevator, fin and bracing, will average about 15 per cent of the total resistance, it being understood that a non-lifting stabilizer is fitted. With lifting tails the resistance will be increased in proportion to the load carried by the stabilizer. In regard to the tail resistance it should be noted that these surfaces are in the slipstream and are calculated accordingly, although the velocity of the slipstream is somewhat reduced at the point where it encounters the tail surfaces. The total tail resistance of the B. E.-2 is given as 3.3 pounds.