CHAPTER VI 



BALANCING THE AERODROME 



By " balancing " I mean such an adjustment of the mean center of pressure 

 of the supporting surfaces with reference to the center of gravity and to the line 

 of thrust, that for a given speed the aerodrome will be in equilibrium, and will 

 maintain steady horizontal flight. " Balance " and " equilibrium " as here used 

 are nearly convertible terms. 



Lateral Stability 



Equilibrium may be considered with reference to lateral or longitudinal sta- 

 bility. The lateral part is approximately secured with comparative ease, by im- 

 itating Nature's plan, and setting the wings at a diedral angle, which I have usu- 

 ally made 150°. Stability in this sense cannot be secured in what at first seems 

 an obvious way — by putting a considerable weight in the central plane and far 

 below the center of gravity of the aerodrome proper, for this introduces rolling. 

 Thence ensues the necessity of carrying tbe center of gravity more nearly up 

 to the center of pressure than would otherwise be necessary, and so far introduc- 

 ing conditions which tend to instability, but which seem to be imposed upon us 

 by the circumstances of actual flight. With these brief considerations concern- 

 ing lateral stability, I pass on to the far more difficult subject of longitudinal 

 stability. 



Longitudinal Stability 



My most primitive observation with small gliding models was of the fact 

 that greater stability was obtained with two pairs of wings, one behind the 

 other, than with one pair (greater, that is, in the absence of any instinctive 

 power of adjustment). 



This is connected with the fact that the upward pressure of the air upon 

 both pairs may be resolved into a single point which I will call the "center of 

 pressure," and which, in stable flight, should (apart from the disturbance by the 

 propeller thrust) be over the center of gravity. The center of pressure in an 

 advancing inclined plane in soaring flight is, as 1 have shown in " Aerodynam- 

 ics," and as is otherwise well known, always in advance of the center of figure, 

 and moves forward as the angle of inclination of the sustaining surfaces dimin- 

 ishes, and, to a less extent, as horizontal flight increases in velocity. These facts 

 furnish the elementary ideas necessary in discussing this problem of equilibrium, 

 whose solution is of the most vital importance to successful flight. 



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