yd SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



always been a machine whose lateral sta1)ility is neutral so that it will 

 not be thrown about by the wind. 



The Curtiss type military tractor tested by us in a manner identical 

 wnth that described in this paper, was found at high speed to have 

 resistance derivatives of the same order of magnitude as the Clark 

 tractor, except that a large rudder and deep rectangular body make 

 A'^■ twice as large for the Curtiss, and there being no high fin surface 

 Li- for the Curtiss is small. As would be expected the spiral motion 

 is slightly unstable, tending to double itself in 28 seconds. The 

 " Dutch roll " is very stable, having a period of 5.25 seconds and 

 damping to half amplitude in 1.77 seconds. The machine in flight at 

 high speed should then have the characteristics of the Bleriot and be 

 steady and easily controlled. This is, in fact, the general reputation 

 of this type of aeroplane. 



At low speed, matters are not so favorable. We have no data for 

 the Bleriot at slow speed, but the Clark model is seen to become 

 spirally unstable to such an extent that an accidental deviation doubles 

 itself in 7.2 seconds. 



The " Dutch roll " for the Clark model remains stable at low speed, 

 but is somewhat less strongly damped than at high speed. The period 

 is 12 seconds damped to half amplitude in 6 seconds. This motion 

 should be not uncomfortable. 



The Curtiss, at low speed, due to falling ofT of Nv. and marked 

 increase in Lv, becomes spirally stable. The spiral motion is a sul^- 

 sidence damped 50 per cent in 3.3 seconds. The wings had no 

 dihedral angle. A separate test ' made on a single wing without 

 body or tails showed a small rolling moment for an oblique wind 

 indicating a small and positive Li,. At large angles of incidence this 

 effect was considerably magnified. The decrease in Nv (or in the 

 weather helm) at large angles of incidence cannot be laid to the 

 straight wings. Tests on a wing alone show a small negative A'„ 

 which is not changed at large angles of incidence. 



The increase in Lv and decrease in A'^i, for the Curtiss aeroplane, 

 favorable to stability of the spiral motion, are unfavorable to stability 

 in the " Dutch roll.'' Furthermore, A^'p increases from zero at high 

 speed to +38 at the low speed, and Lp decreases from —314 to —78. 

 These changes are very unfavorable and, as we should expect, the 

 " Dutch roll " for the Curtiss is unstable. The natural period is 

 about 5.7 seconds and any initial ami)litu(l(* is doubled in 7.66 seconds. 



^ Smithsonian Misc. Coll., Vol. 6j, No. 4. " Experiments on a Dihedral Angle 

 Wing," J. C Hunsakcr and D. W. Douglas. 



