2 AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



A detailed description of the laboratory is given in the Appendix. Suffice it 

 to say here that it consists of a square duct 4 feet on a side through which a 

 current of air is drawn at uniform velocity with a balance under it which supports 

 the models to be tested and weighs the different forces acting on them (Plate 3). 



The method of carrying out the observations was to set the sail with the 

 plane of the boom and mast parallel to the center line of the wind tunnel, then by 

 turning the boom upstream to known angles with the center line to measure the 

 torque and the moments of the thrust, in the direction of the wind and at right 

 angles to it. A plane passed through the boom and mast was considered the plane 

 of reference for all settings. The twisting moment or torque about the mast was 

 measured by the torsional distortion of a calibrated wire in the extension of the 

 axis of the mast (see Plate 4). The balance which was used for this work (Plate 

 5) is a copy of the one in use in the National Physical Laboratory in Great 

 Britain, and was built under their direction by the Cambridge Scientific Instrument 

 Company especially for the Institute. It is a remarkable combination of ingenuity 

 and extraordinary instrument-making skill, and has been used extensively for the 

 regular aerodynamical experiments carried out in the laboratory. The observations 

 for thrust and torque were taken for each angle with the speed of the wind main- 

 tained constant at 1 5. 1 miles per hour. When the observations of thrust had been 

 completed for all angles the settings were repeated at the same wind velocity for 

 measurements of torque or moment about the mast, as both thrust and moment can- 

 not be measured simultaneously on the balance. 



In order to determine the effect of varying wind intensity some of the runs 

 were repeated for winds of approximately 10 miles and 20 miles per hour. It was 

 thought that intermediate points could be filled in by cross fairing from the sim- 

 ilar curves derived for 15 miles per hour. 



Observations were taken of the moment of the component of the wind pres- 

 sure on the sail acting along the wind (d) and of the moment of the component 

 acting across the wind (c) and of the moments of the resultant or total pressure 

 about the mast. This gave four equations (i, 3, 5, 6, Plate 4) which, however, 

 contained five unknowns. By repeating the (c) and (d) observations, with the 

 length of the vertical arm shortened 6 inches, two more equations (2 and 4) were 

 added without increasing the number of unknowns, which resulted in six equations 

 involving five unknowns. The unknowns were: first, the transverse component of 

 the wind pressure (c) ; second, the down-stream component (d) ; third, fourth and 

 fifth, the coordinates of the center of pressure x, y and 2, measured respectively 

 along the wind, across the wind, and vertically, origin at the intersection of the bot- 

 tom of the boom with the center line of the mast as shown on Plate 4. The known 

 quantities were the weights on the scale beams, the torque and the length of the 

 weighing arms. 



