NO. 5 STABILITY OF AEROPLANES — HUNSAKER AND OTHERS 3 



lence of motion may seriously impair the pilot's control and the 

 aeroplane may " take charge " at a critical time. 



However, the longitudinal motion for any particular speed of flight 

 may be made dynamically stable, while at the same time only slightly 

 stable in the static sense, by the use of a large tail surface which lies 

 very nearly in the relative wind. If the minimum of statical stability 

 be combined with the maximum of damping, the pitching will be very 

 slow and heavily damped. The longitudinal motion can then be 

 dynamically stable and yet be without violence of motion in gusty air. 



The general prejudice among pilots against " very stable " aero- 

 planes is believed to be justified. It cannot be too strongly insisted 

 upon that true dynamical stability is better given by damping than by 

 stiffness. 



Experience with rolling of vessels has led to the design of vessels 

 of small metacentric height (a measure of statical stability) fitted 

 with generous bilge-keels (damping surface) for passenger carrying. 

 An efifort is made to get away from the violence of motion associated 

 with stiffness. 



In Part II, the lateral or asymmetrical motion is investigated. The 

 necessary aerodynamical constants are determined by wind tunnel 

 tests wherever practicable and two coefficients which cannot readily 

 be found experimentally are calculated by a simple approximate 

 method. The character of the motion as indicated by the solution of 

 the determinant formed from the equations is then discussed. 



It is found that the lateral motion is a combination of a roll, yaw, 

 and side slip or " skidding." Using approximate methods, the com- 

 bined motion is resolved into three components, two of which are 

 important. 



One type of motion is a spiral subsidence if stable or divergence 

 if unstable. The Clark aeroplane becomes spirally unstable at low 

 speed. The motion is a " spiral dive " due to an overbank and a side 

 slip inwards. The aeroplane makes a rapid turn with rapidly increas- 

 ing bank accompanied by side slipping inwards. The instability is 

 such that an initial deviation from course will double itself in about 

 7 seconds. 



It is shown that the spiral motion may be made stable by adequate 

 fin surface above the center of gravity or upturned wings and by 

 reduction in " weather helm " due to too much rudder or fin sur- 

 face aft. 



