8 AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



the tunnel. Vibration of the motor or propeller cannot be transmitted to the tun- 

 nel as there is no connection. 



In order to maintain a steady current of air, the fan must run at constant 

 revolutions per minute, but in order to allow a fine adjustment to obtain and hold 

 any speed, a direct-current motor is necessary. To run a direct-current motor at 

 constant speed requires a steady voltage. Such is not available. Consequently the 

 following procedure was adopted. A 15-horse-power induction motor is connected 

 to the alternating current power mains of the Cambridge Electric Company. This 

 induction motor is coupled directly to a 12-horse-power direct-current generator. The 

 generator supplies current to the motor which turns the propeller. For constant 

 wind speed the load is constant, and hence the induction motor will turn over at 

 constant speed since its slip is a function of load. Variation of voltage in the 

 city mains has small effect on the speed of the induction motor, which runs at a 

 speed proportional to the frequency of the supply current. The generator being 

 turned at constant speed generates constant voltage, and the propeller then runs at 

 constant speed. Due to slow changes in frequency it is necessary to provide va- 

 riable resistance in the direct-current motor field, by the use of which the wind 

 speed can be corrected from time to time. 



Any wind can be made of velocity between 3 and 40 miles per hour. 



The model of complete aeroplane, wing, tail, body or other part is mounted on 

 an aerodynamic balance constructed from the plans of the National Physical Lab- 

 oratory, England. This balance (Plate 5) consists of a cast iron pillar mounted on 

 an independent concrete block, and the balance proper. The latter is made up of three 

 arms mutually at right angles representing the axes of coordinates in space about 

 and along which couples and forces are to be measured. The model is mounted on 

 the upper end of the vertical arm which projects through an oil seal in the bottom of 

 the tunnel. 



The entire upper part of the balance rests on a steel point bearing in a steel 

 cone supported by the cast iron pillar. 



The balance is normally free to rock about its pivot in any direction. When 

 the wind blows on a model, the components of the force exerted are measured by 

 hanging weights on the two horizontal arms to hold the model in position. 



The balance is also free to rotate about a vertical axis through its pivot. The 

 moment producing this rotation is balanced by a calibrated torsion wire. 



Special attachments permit the measurement of the force in the vertical axis 

 and moments about the two horizontal axes. 



The three forces and three couples acting on any model placed in any attitude 

 to the wind can be studied at leisure. The balance is precise to one per cent. 



Velocity is measured by means of a Pitot tube which was calibrated on the 

 whirling arm at Teddington. The Pitot tube pressures are read on a Chattock 

 liquid micro-manometer. Velocity readings are precise to one-half of one per cent. 



Tests have been made to determine the lift and resistance of a model aeroplane 

 wing which had previously been tested in England. The results are in excellent 



