84 AEROPLANE PERFORMANCE CALCULATIONS 



of the machine; (5) The tail plane and elevators, at the height 

 of the centre of the tail plane ; (6) The fin and rudder, at the 

 height of their approximate centre of area. 



Now let these various batches have resistances in pounds at 

 100 miles per hour, totalling respectively RI, R 2) R 3) R, R b , and 

 R 6 , and let the heights of the specified points above any con- 

 venient datum (such as the top longeron of the fuselage, if this is 

 horizontal, or the line of the propeller thrust, or even a line 

 put in on the drawing for this sole purpose) be a lt a 2 , a s , a 4 , a & , 

 and a 6 . 



Then the height of the line of action above the chosen 

 datum is 



R 



The position of the line of action is thus found. 



Note that the positive and negative signs of the a's are to be 

 retained during the calculation, of course. 



Examples. -- For numerical examples illustrating the 

 methods of this Chapter, see Chapter XV., page 147, and 

 Chapter XXIL, page 182. 



Theory. For theoretical explanations see Chapter !. 

 page 3- 



