ii2 AEROPLANE PERFORMANCE CALCULATIONS 



Next fill up the X column as shown, and the L/D and k c 

 columns as explained in the definitions of L/D and k c given 

 above. Then work out the columns in succession. 



The last two columns then give the values of V and P, which 

 can be used to plot the machine performance curve for standard 

 density air under the assumptions of the Third Method. 



There is a Variant of the Third Method, which is much to be 

 recommended for regular use. It takes account of slip stream 

 effect, but disregards centre of pressure. 



To use the variant, simply drop out of the work /, /', c, k c , 7, 

 and S, and put L/W = i. 



Then in the table of page ill, the third, fourth, and fifth 

 columns are not required, and L/W can be omitted from the 

 next to the last column. 



Fourth Method. 



W is the total weight of the machine in pounds. 



S is the total area of the wings in square feet (including S'). 



S' is the area in square feet of that part of the wings which 

 is subject to the action of the slip stream. S' is obtained by 

 reducing S in the ratio of the sum of the lengths of such parts of 

 the top and bottom leading edges as fall within the propeller 

 circle in front view to the sum of the lengths of the top and 

 bottom leading edges for the whole machine in front view. 



c is the chord of the wing in feet. 



/ is the horizontal distance in feet from the e.g. of the machine 

 to the c.p. of the tail plane.* 



/' is the horizontal distance in feet from the leading edge of 

 the equivalent chord f to the c.p. of the tail plane. 



d is the diameter of the propeller in inches. A reasonable 

 trial value (if not the final value) of this has already been found 



* It is sufficiently accurate for our purpose to assume that the c.p. of the tail 

 plane is at the front tail plane spar. 



f The leading edge of the equivalent chord is on the line joining the top and 

 bottom leading edges of the biplane, and at a distance above the leading edge of 

 the bottom plane of '55 of the gap. 



