NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 4I 



is the force in pounds on i square foot due to a wind of i mile per 

 hour, of air of standard density {i. e., .07608 pound per cubic foot). 



The " center of pressure " is also shown on figure 10 in terms of 

 distance from leading edge in fraction of chord. 



On the same sheet are shown the experimental points published by 

 the National Physical Laboratory for this wing, using the same size 

 model and the same speed, i. e., 29.85 miles per hour. 



It is seen that there is only slight discrepancy between the lift 

 observations up to an incidence of 14 degrees, the useful range in 

 aviation. The English points lie from i to 3 per cent higher than the 

 corresponding points for model B, and coincide with those of 

 model A. 



Similarly for the drift observations there is very good concord- 

 ance up to 14 degrees. 



The curve of center of pressure coefificient is in practical coinci- 

 dence with the English observations. 



It appears that undetected differences in workmanship and finish 

 between two models may cause a change in coefficients of not more 

 than 3 per cent. Actual observations are precise within one-half of 

 I per cent. Consequently, our results may be considered sufficiently 

 precise for purposes of aeroplane design. 



VI. STABILITY OF STEERING OF A DIRIGIBLE 

 By J. C. HUNSAKER 



When floating in the air with no way on, a dirigible takes an atti- 

 tude such that the center of gravity lies on the vertical passing 

 through the center of buoyancy of the envelope or gas bag. The 

 ship is then in stable equihbrium. When under way, the thrust of 

 the propellers is balanced by the resistance of the air. In order that 

 the attitude shall not be changed, the moment of the propeller thrust 

 and the moment of the air resistance taken about any point must 

 balance one another. It is convenient to take axes of coordinates, 

 vertical, transverse, and horizontal, located at the center of buoyancy. 

 If it be assumed that the ship is in equilibrium on her course, then the 

 component forces along and moments about the three axes through 

 the center of buoyancy are each zero. 



This is equivalent to the statement that the motion of the ship is 

 one of pure translation in the direction of the fore and aft axis of 

 the envelope, which axis is horizontal. 



