NO. 5 STABILITY OF AEROPLANES HUNSAKER AND OTHERS 1/ 



Such stability is " inherent " in the design of the aeroplane and 

 depends wholly on the location of the center of gravity and setting of 

 the stabilizer. No automatic devices are required which may or 

 may not function in an emergency. The inherent stalnlity here shown 

 is static only. Later w^e will investig'ate the effects of inertia and 

 damping involved in dynamical inherent stability. However, dy- 

 namical stability is impossible unless there be statical stal)ility, and 

 before undertaking' a study of the former property, we were obliged 

 to provide a reasonable righting moment to oppose diving and 

 stalling-. 



§7. PERFORMANCE CURVES 



In the design of this aeroplane, the resistance, and hence the speed 

 for given power, was estimated from tests on wings, body, struts, 

 wires, etc., considered separately. The test results were corrected 

 and expanded to full speed full size, using reasonable corrective 

 factors. As is well known, the resistance of many parts does not 

 increase so rapidly as the square of the speed, on account of skin 

 friction. Making all allowances a speed of over 85 miles per hour 

 was predicted for no brake horse-power. 



If w^e use the lift and drift observed on the model (-y full size) 



at 30 miles per hour and convert to full size by assvnning the " law 

 of squares,'' the performance is not quite so favorable and a maxi- 

 mum speed of but 75 miles per hour is indicated. 



P"or a stability investigation we are little concerned with the exact 

 speed, and for simplicity, the L and D from the wind tunnel test on 

 the complete model of figure i are converted to full size by multiply- 

 ing by the squares of speed and scale. 



A total weight of 1600 pounds is assumed, corresponding to tanks 

 half full. For any speed [' the lift is a function of speed and atti- 

 tude and must equal the weight JV. 



By the " law of squares " 



Force on Model _ / 30 

 Force on Aeroplane \26^^ 

 hence : 



V 



26V L ' 



where L is lift on model at 30 miles per hour. 



For a series of values of L, corresponding to a series of attitudes 

 or angles of incidence, the required speed V was computed. The 



