NO. 5 STABILITY OF AEROPLANES— HUNSAKER AND OTHERS 75 



motion called " Dutch roll " will probably be unstable at low speeds 

 where Np becomes great. 



For high speed, if both A^",, and Np are zero, the lateral motion should 

 be stable regardless of the magnitude of the other derivatives. 



With the yawing moment due to rolling as measured by Np increas- 

 ing from zero at high speed to +57 at low speed, it would seem that, 

 at the maximum speed, any reasonable aeroplane will be stable so far 

 as the " Dutch roll " is concerned, but at low speed it may become un- 

 stable in this particular motion. 



In general, for high speed, considering the two possible kinds of 

 lateral instability, it is believed that very slight modifications in fin 

 disposition will suffice to render any ordinary aeroplane laterally 

 stable. Likewise, at high speed, longitudinal stability is easily 

 secured. At low speed, the longitudinal motion tends to become un- 

 stable as well as one or the other kind of lateral motion. 



§14. COMPARISON WITH OTHER AEROPLANES 



Any stalMlity discussion is much more suggestive if several aero- 

 j)lancs can be analyzed in parallel. The only published information 

 on lateral stability is Bairstow's investigation of the Bleriot mono- 

 plane used al)ove in connection with the longitudinal stability discus- 

 sion. This monoplane had only a very small rolling moment due to 

 side sli|) /., = .83 as against Li; = 3.o6 for the Clark aeroplane for high 

 speed. The coefficient Nv, yawing moment due to side slip, is not 

 greatly difi^erent in the two machines. The other coefficients are of 

 the same order of magnitude, except Lp, the damping of a roll, which 

 is small in the monoplane on account of the small wings of short span. 



Without further knowledge, we should expect the Bleriot to be 

 stable on the " Dutch roll " on account of the small Lv. Bairstow 

 finds a period of 6.5 seconds damped to half amplitude in 1.65 second. 



On the other hand, the small Lv would lead us to suspect the sta 

 bility of the spiral motion, especially as Lp is also small. In fact, the 

 coefficient E.. was found to be slightly negative and the aeroplane, 

 in consequence, spirally sHghtly unstable. The motion is a slow 

 divergence which doubles itself in 68 seconds. This is an extremely 

 slow change and should give no trouble to a pilot. Indeed, the w^ell- 

 known steadiness in flight of this famous aeroplane is in full agree- 

 ment with the theoretical conclusions. The Bleriot makes no claim to 

 lateral stability, but is essentially a steady aeroplane easily controlled. 

 In the " Dutch roll " the Bleriot is very strongly damped and hence 

 very stable. The spiral motion is not damped, but is so slow that the 

 stability mav be called neutral. The aim of the French school has 



