42 AEROPLANE PERFORMANCE CALCULATIONS 



The method of finding V for this case will be given in 

 Chapter X., page 105 ; this value of V is also the top speed in 

 standard density air in this special case. 



All the above work on Full Power Flight takes, of course, 

 the same form, whatever method of finding the machine perform- 

 ance curve has been used. The accuracy of the results, however, 

 naturally depend on the accuracy of the method used in the first 

 place. 



Rate of Climb. First Approximation. If we neglect the 

 influence of the obliquity of the propeller thrust, the fact that 

 the weight is not perpendicular to the direction of flight, and the 

 additional slip stream action due to the use of full power, we can 

 take the horse-power required for mere flight from the machine 

 performance curve calculated by the Third Method, and the horse- 

 power available for mere flight plus climb from either the P T or 

 the P R curve, whichever has the lower value at the speed in 

 question (see, for instance, Fig. I and Fig. 2, page 41). 



The difference between the propeller power and the machine 

 power is the horse-power available for climbing, and the rate of 

 climb in feet per minute is this difference, multiplied by 33,000 

 and divided by the weight of the machine in pounds. 



Corresponding to each speed between the stalling speed under 

 power and the top speed, there is a definite rate of climb : at 

 some particular speed the rate of climb is a maximum, and this 

 maximum is often spoken of simply as "the climb," while the 

 particular speed is often called "the climbing speed". 



The above method applies as it stands to any altitude, merely 

 using the curves for P', P T ' and P R ' plotted on V, instead of the 

 curves of P, P T , and P R plotted on V. 



For rough work the machine performance curve can be taken 

 from the First Method instead of from the Third Method, but in 

 that case, of course, the inaccuracy is increased. 



Second Approximation. This approximation, though still not 

 perfect, goes a stage further than the First Approximation. In 

 it we assume that the forces operating relative to the flight path 

 are the same as for horizontal flight, except that we take account 

 of the propeller thrust being inclined to the horizon (being in 

 fact along the flight path), and of the weight being inclined to 

 the normal to the flight path (being in fact, vertical). That is 

 to say, we assume that the air download on the tail and the 

 propeller slip stream effect (if the Third Method or the Fourth 

 Method is being followed) are unaltered by the fact of climbing. 



