26 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 62 



The aeroplane may ])itch about its normal attitude. At any instant 

 the angle of pitch is the angle B between the normal attitude axis of 

 X and the new position of x. The axes, of course, pitch with the 

 aeroplane. The axes are fixed by the equilibrium conditions and 

 differ for each speed since each speed requires a different attitude. 



-^' O' -^^' Z"^' ^<5- /^a- t/O' Z'/^' X/^ 



X^y/i" t>/'>*^yy7^ ^y^e>^o^.^ ' ^^c^ir//o y^A:yi'^y7^/ir^ <S> 



Fig. 8.— -V, Z, and M for / = o°. 



z/*^' r/s- 



It was convenient to measure in the wind tunnel the lift and drift 

 on the model referred to axes always vertical and horizontal. The 

 corresponding forces along the moving axes x and z are readily 

 obtained from : 



Z' = L cos ^ + Z) sin ^, 



X' = Dcos^-Lsin^. 



Here L and D are })Ounds on model, 6* is angle of pitch, and Z' and X' 

 are pounds force along the moving axes. A'' and Z' are then con- 



