NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 39 



average of the measurements taken in both tests will have eliminated 

 any error due to lack of symmetry in the flow. 



Rotation or twist of the air in the tunnel has not been detected, and 

 if present must be small. The honeycomb at entrance and bafifles at 

 exit are designed to prevent the air spinning with the propeller. 



V. CHARACTERISTIC CURVES FOR WING SECTION, 



R. A. F. 6 



Bv H. E. ROSSELL, Asst. Naval Constructor, U. S. Navy, C. L. BRAND, 

 AssT. Naval Constructor, U. S. Navy, and D. W. DOUGLAS, S. B. 



In order to furnish a final check upon the calibration of instru- 

 ments, the alignment of tunnel and balance and general methods of 

 testing, it was considered desirable to repeat the determination of the 

 aerodynamical constants published by the British Advisory Com- 

 mittee for Aeronautics, Report 1912-13, for the wing profile, desig- 

 nated as R. A. F. 6. 



Two models 18 inches span by 3 inches chord were cut in brass, 

 and carefully filed and scraped to form. The surface was highly 

 polished to remove tool marks. 



Each model was mounted vertically in the wind tunnel and its 

 " lift " and " drift " forces measured on the balance for angles of the 

 chord to the wind from —4 degrees to -|-i8 degrees. 



The moment of the resultant force about the vertical axis of the 

 balance was measured on the torsion wire. It was then possible to 



determine the direction of the resultant from the ratio r^. , the 



drift 



magnitude by VLift^-f Drifts and its line of action from: 



observed m oment = jicular distance from axis, 



magnitude 



The center of pressure is usually defined arbitrarily as the inter- 

 section of the resultant force with the plane of the chord. This point 

 was calculated for each incidence. 



On figure 10 are plotted the values of lift and drift coefficients, 

 defined by : 



r^ _ Lift ^ _ Drift 



where A is the wing area in square feet, and V the velocity of the 

 wind in miles per hour. Lift and drift are in pounds force, hence Ky 



