14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 62 



respectively. The discrepancy is i per cent only and is about the 

 precision of the measurements. The comparison is best brought out 

 by eliminating reference to angle of attack as the effect of the change 

 in tail angle appears to be mainly to move the curves of L and D, 

 plotted on i, to the right or left. 



Figure 5 shows the ratio L/D for the model for cases I, II, and III, 

 plotted on L in pounds as abscissae. For small values of L and angles 

 of incidence between —2° and +2°, corresponding in practice to 

 high-flight velocity, the curves are practically identical. For angles 





3^ y ^ s 7 -s ^ ya ^./ 



/^ xj y^f- /s y6 y.z 



Fig. 5. — Curves of L/D plotted on L for three tail settings. 



of incidence near 8°, the L/D ratio for case III is 8.6, while it is 8.2 

 for case II, and 8.0 for case I. 



It appears, therefore, that changing the angle of tail surface has 

 but sHght efifect on the lift and drift of the aeroplane. The actual 

 aeroplane should have the same maximum and minimum speeds in 

 any case since the maximum lift and minimum drift are about the 

 same regardless of angle of tail surfaces. 



The statical stability against longitudinal pitching is, however, 

 very different for the three cases. Thus the pitching moments (ob- 

 served about the spindle and converted to pitching moments about 

 the assumed center of gravity) are as follows, in pounds-inches on 

 the model at 30 miles per hour. Positive angles and positive moments 

 are stalling angles and stalling moments respectively. 



