50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



The forces and moment measured in the first test are called 



Rx, drift component, 



Ry, cross wind component, 



Mz, yawing moment ; 

 and in the second case, 



Rx, drift component, 



Rz, lift component, 



Aly, pitching moment. 



Forces are measured directly in pounds and moments in pounds- 

 inches on the model for a wind velocity of 30 miles per hour. Density 

 of air is 0.07608 pound per cubic foot. 



For any angle of pitch we may substitute for the lift, drift, and 

 pitching moment a resultant force vector defined as that force which 

 is the mechanical equivalent of these. It is to be noted that we here 

 deal only with forces in the plane of symmetry of the aeroplane. 



Similarly in the second test, where the angle of pitch was kept 

 constant at 1.5 degrees, the drift, cross wind force, and yawing 

 moment are represented by a vector in the horizontal plane passing 

 through the intersection of the body axis and vertical spindle. 



These two resultant force vectors may be called for convenience 

 pitching resultant and yawing resultant. They are shown in posi- 

 tion, magnitude, and direction on the two views of figure 13. 



The artifice of representing a force and a moment by a vector 

 makes it immaterial where the axes of support of the model were 

 originally taken. In aeroplane design, the axis of the propeller 

 usually is made to pass near the center of gravity, and it is hence 

 necessary to locate the center of gravity at such a point that angular 

 deviations from normal flight attitude will produce moments about 

 the center of gravity tending to restore the original attitude. 



The usual location of the center of gravity well forward in the 

 body will insure that the pitching resultants pass to the rear of the 

 center of gravity and that the pitching is stable. See figure 13, side 

 elevation, where the pitching resultants are shown graphically. 



For convenience, the resultant force on the model is given on figure 

 14 in terms of lift and drift components marked Rg and Rj. The reso- 

 lution of the forces is shown on the upper figure. It appears from 

 figure 15 that the drift R^ is practically constant from +2 degrees to 

 — 2 degrees pitch, but the lift Rz is zero only for i^ degrees pitch. 

 It would be of some advantage to fly the machine at full speed with 

 the body " tail heavy " i^ degrees. 



