52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



It is also possible to resolve the force R into components X along 

 the axis of the machine and Z perpendicular thereto. Since a force 

 may be resolved at any point in its line, we choose the intersection of 

 the line of action w^ith AB. This point is a center of pressure and is 

 a distance " l^" from the nose of the machine. See upper figure of 

 figure 14. A force is completely defined by X, Z, and l^, and these 

 quantities are given in the upper table. The component X has no 

 moment about the center of gravity if the center of gravity be on 

 line AB, and if the center of gravity be a distance 3; from the nose, the 

 pitching moment is {l^ — y')Z. Upward forces are plus. In general, 

 for a center of gravity with coordinates x, y as shown on figure 14, 

 the pitching moment is : 



X,,.-Z(/,-3;). 

 By use of figure 14 and this formula, a curve of pitching moments 

 can be readily obtained for any assumed position of the center of 

 gravity. 



In a similar manner the model was held at a pitch angle of i^ 

 degrees to the horizontal wind, and placed at angles of yaw from 

 15 degrees right to 15 degrees left. Observations right and left have 

 been averaged. The resultant forces are shown in direction, magni- 

 tude, and application in the lower figure of figure 13. The cross wind 

 and drift components Ry and Rx are tabulated on figure 14. 



As above, the components along the axis of the body X, and at 

 right angles Y , are also tabulated. The intersection of the line of the 

 resultant force with the vertical plane of symmetry is taken as a 

 center of pressure and is a distance /o from the nose of the body. 

 The yawing moment about the center of gravity placed a distance y 

 from the nose is hence 



Y{U-y). 



It is apparent from the last test that the aeroplane is directionally 

 unstable, unless the center of gravity be well forward of the forward 

 passenger's seat. The addition of a propeller and the fin effect of 

 biplane struts will augment this tendency to instability of steering, 

 and require a still farther forward position of the center of gravity. 



Forces are given in pounds on the model for a wind speed of 30 

 miles. If the model is to a scale of i inch to the foot, the forces on the 

 full-size body will be 144 times as great and at 60 miles four times 

 greater. Thus the resistance of this body at 60 miles would be about 



0.137x144x4 = 79 pounds, 

 on the assumption that the resistance varies as the square of the 



