NO. 5 STABILITY OF AEROPLANES— HUNSAKER AND OTHERS 45 



flying- qualities, such as ease of control. In this connection it is im- 

 portant not to give too great statical stability. Safety in flight may 

 well depend more upon ease of control than upon stability. The 

 almost universal prejudice among accomplished flyers against so- 

 called " stable aeroplanes " appears to have a rational foundation. 



PART II. LATERAL MOTION 

 §1. LATERAL OR ASYMMETRICAL TESTS 

 When the aeroplane is yawed to right or left of its course through 

 an angle of yaw i/', the wind blows through the wings obliquely and 

 gives rise to a lateral force Y at right angles to the longitudinal axis x 

 of the aeroplane, a rolling moment L tending to roll the aeroplane 

 about the x axis, and a yawing moment N tending to yaw the machine 

 about the a axis. 



To measure the force V and moments L and A'' as the aeroplane 

 yaws, the model was mounted in the wind tunnel and held at various 

 Angles of yaw to the direction of the wind. At each position measure- 

 ments were made from which the component forces X, Y, Z and 

 moments L, M, N could be calculated. 



The details of the method are given in the Technical Report of the 

 Advisory Committee for Aeronautics, 1912-13, p. 128, where a de- 

 scription is found of the special apparatus reciuired. 



Briefly stated, the balance is arranged to measure the moments of 

 the air forces about axes parallel to those axes used for calculation, 

 whose origin is at the center of gravity of the aeroplane. A yawing 

 moment is measured about a vertical axis passing through the main 

 pivot of the balance. The moments of the drift and cross-wind forces 

 are measured about horizontal axes parallel and at right angles to 

 the tunnel axis and passing through the same point. In order com- 

 pletely to determine all forces and moments, a special fitting is pro- 

 vided on which three more measurements may be made. This moment 

 device measures the pitching and rolling moments about horizontal 

 axes passing through the pivot of the attachment. In addition, the 

 total Hft or vertical force is measured on the balance. We then have 

 five moment observations and one force observation, as follows : 

 Vf, measured on vertical force lever (a hft), 

 Mz, measured on torsion wire (a yawing moment), 

 Vp, pitching moment about a high point o-^, 

 Ve, rolling moment about a high point Oj, 

 Md, moment of drift force about a low point 0., 

 Mc, moment of cross-wind force about Oo. 



