n8 AEROPLANE PERFORMANCE CALCULATIONS 



that for a given value of X the gliding angle at any altitude is 

 the same as in standard density air. 



To find the best gliding angle in still air, plot the last column 

 against X and note the minimum value : this is the tangent of 

 the best gliding angle in still air. Note also the value of X to 

 which it refers, as from this, by a reference to the machine per- 

 formance curve of the First Method for standard density air, the 

 best gliding speed can be noted. At an altitude the best gliding 

 angle is the same as that found above, but the machine speed 

 (which is the pilot's guide) is higher at a greater altitude. 

 Fortunately, however, the Air Speed Indicator Reading suffers the 

 same correction, so that the best gliding speed at any altitude is 

 obtained by holding the machine on an Air Speed Indicator 

 Reading equal to the best gliding speed as found above. 



Gliding in a Wind. 



v is the wind speed. 



<f) is the gliding angle relative to the ground. 



W is the total weight of the machine in pounds. 



Tan has already been found for each value of X in the im- 

 mediately preceding piece of work. 



First write down the numerical values of W, R, and a (taking 

 the last two from the immediately preceding work), and decide 

 on a wide series of values of v (say v lt v^ v^, etc.) in standard 

 density air over which to carry the investigation. Note that the 

 range should extend to negative values of v. 



Then construct a table in the following form : 



