NO. 5 STARILITY OF AEROPLANES HUNSAKER AND OTHERS 77 



The motion is a swaying of the aeroplane of increasing amphtnde and 

 intensity. However, we must ahvays point out that an alert pilot 

 with powerful controls can check the natural motion of the aeroplane 

 before it has became violent and so maintain his equilibrium. 



The increase in A'p at low speed or rather large angle of incidence 

 is due to the steeper drift curve for a wing at large angles. As the 

 aeroplane rolls, the downward moving wing has its drift relativelv 

 more increased as the normal flight attitude requires a larger angle of 

 incidence. 



The drop in Lp is due to the less steep lift ciu've at high angles of 

 incidence. As the aeroplane rolls, the increase in angle of incidence 

 of the downward moving wing gives very little increase in lift on 

 that wing if the wing be already near its angle of maximum lift. We 

 might imagine an aeroplane flying at an angle of incidence giving the 

 maximum lift. Any increase in incidence can produce no additional 

 lift. In most aeroplane wings, an increase in incidence beyond the 

 optimum angle causes the wing to lift less at the same air speed. 

 Now if the aeroplane in such an attitude roll, the increased angle of 

 incidence of the downward moving wing gives no more lift on that 

 wing and hence the rolling is unresisted. The damping of the roll 

 will be zero, or even negative. In the Curtiss aeroplane, the low speed 

 chosen required an incidence of I5°5. very near the "burble point," 

 or angle of maximum lift for the wings. The small value —78 of Lp 

 appears to-be one of the principal causes of the instability. In the 

 Clark model, the wing loading is smaller and an equal speed about 

 44 miles per hour is obtained for an incidence of only 6°, giving 

 Lp= — 319. The lowest speed of the Clark model is taken as about 

 ^y miles per hour where an incidence of but 12° is needed. Lp at this 

 angle is —224. 



It appears that lateral dynamical stability is incompatible with a 

 high wing loading which requires a large angle at landing speed. 

 The analysis of longitudinal stability led to a similar conclusion. 



If we turn to practical aviation we observe that aeroplanes which 

 are noted for their steadiness at low speeds are the light Antoinette, 

 Farman, and the various German Taubes derived from the Etrich. 

 All these aeroplanes have large wing area and light loading, probably 

 between 3 and 4 pounds lift per square foot. The light loading 

 enables these aeroplanes to gain a safe low speed without having the 

 angle of incidence near the angle of maximum lift. 



In the Clark model the loading is about 3.55 pounds per square 

 foot, while it is 5.2 in the Curtiss type discussed. More recently the 



