i 4 8 AEROPLANE PERFORMANCE CALCULATIONS 



back one a depth of 5 inches, and a beam of I foot 6 inches. 

 We find, however, we suppose, that the steps project to form a 

 wide " mudguard," so that the sum of the areas of the steps when 

 found by the planimeter comes out at I 79 square feet. 



We now turn to page 83 and refer to the formula r = 2ga 

 there given. Therefore the resistance of the steps is 29 x 1 79 

 = 5 1 '9 pounds at 100 miles per hour. 



Therefore the resistance of the hull, including steps but with- 

 out cockpit, etc., is given by 



r = 14*0 + 51*9 = 65*9 pounds at 100 miles per hour. 



Example (5). Resistance of a Pontoon Float. Consider a 

 float 3 feet 6 inches beam by 2 feet 6 inches deep at the maxi- 

 mum cross section. Then the maximum cross sectional area is 

 875 square feet. 



.-. r = 6 x 875 = 5 2 '5 pounds at 100 miles per hour. 



Hxample (6). Resistance of a Complete Tail Unit. Suppose 

 the area of the tail plane plus elevators is 66 square feet and the 

 area of the fin and rudder together is 2 1 square feet. Then the 

 resistance of the complete tail unit is given by 



r = 78 x 66 + "58 x 21 = 51-5 + 12*2 

 = 637 pounds at 100 miles per hour. 



Example (7). Resistance of Struts. Take the case of 20 

 feet of strut i^ inches wide across the wind direction, 20 feet 

 2-J inches wide and 14 feet I inch wide. 



. .*. a = 30 + 50 + 14 = 94 inch-feet units. 



We will suppose that all the struts have cross sections geo- 

 metrically similar to Fig. I, page 149. 



First round off the tail a bit and we get Fig. 2. Note the 

 fineness ratio of Fig. 2, which is 3*69. 



Then expand Fig. 2 to 4 inches by I inch and compare with 

 shapes A and B of page 80. For convenience these shapes 

 are repeated in Figs. 3 and 4 respectively, the position taken up 

 by the traced contour being shown dotted. 



On considering Figs. 3 and 4, remembering that after all it 

 is only tails that count, we conclude that the strut we are dealing 

 with is \ the way from shape B towards shape A. With this 

 fact and the fineness ratio of 3^69, we get from the curve of 

 page 80 



x = -268. 



.*. r = 94 x -268 =25*2 pounds at 100 miles per hour. 



Example (8). Interference of Struts, Suppose that there 

 are on each side of the machine a pair of struts 5 feet long, of 



