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CHAPTER IV. 



MACHINE PERFORMANCE CURVE. 



General. The machine performance curve is a curve giving P, 

 the effective horse-power required by the machine in order that 

 horizontal flight may be maintained, plotted on V, the speed of 

 flight in miles per hour as a base. 



In the first place, this curve is worked out for standard 

 density air, but it can also be found for any given altitude, in 

 which case it is a plotting of P', the effective horse-power required 

 for horizontal flight at that altitude, on a base of V, the flying 

 speed at that altitude. 



The machine performance curve can be applied directly to 

 problems of top speed, landing speed, cruising economy, length 

 of run to get off the ground, and in dealing with the rate of 

 climb of the machine. 



In determining the machine performance curve the procedure 

 consists essentially in first determining the speed for a given 

 value of the lift coefficient and then obtaining P by the relation 

 between P and the total resistance and velocity. 



There are, however, certain corrections to be taken account 

 of which together make the problem one of a somewhat formid- 

 able complexity. Fortunately it is possible to get over the 

 difficulty by splitting up the problem into a series of methods 

 of successive degrees of accuracy. The methods will be given 

 separately hereunder, as it is not always necessary to use the 

 more complex methods, and in that case easier ones can be 

 used. 



The machine performance curve will now be worked out to 

 various degrees of approximation, all the cases usually used 

 being dealt with separately. 



First Method. Assumptions. Line of flight horizontal. 

 Propeller thrust horizontal at all speeds of flight and passing 

 through the centre of head resistance. Tail air load neglected. 

 Slip stream neglected. 



Definitions : P and V have already been defined above. 



W is the total weight of the machine in pounds. 



T is the total resistance of the machine in pounds. 



