4 SMITHSONIAN MISCELLANEOUS COLLECTIONS \OL. 62 



room, the pressure difference that the fan must maintain is thus 

 reduced. Also with a larger fan the velocity of discharge is reduced, 

 and the turbulence of the wake kept down. 



The propeller works in a sheet metal cylinder 7 feet in diameter, 

 and discharges into the large perforated diffuser. The panels of the 

 latter are gratings and may be interchanged fore and aft. The grat- 

 ings are made of li-inch stock with holes i^ by i4 inches. Each hole 

 is then a square nozzle one diameter long. The end of the diffuser 

 is formed by a blank wall. The race from the propeller is stopped by 

 this wall and the air forced out through the holes of the diffuser. Its 

 velocity is then turned through 90 degrees. The area of the diffuser 

 holes is several times the sectional area of the tunnel, and the holes 

 are so distributed that the outflow of air is fairly uniform and of low 

 velocity (pi. 2, fig. i). 



A four-bladed black walnut propeller (pi. 2, fig. 2) was designed on 

 the Drzwiecki system and has proved very satisfactory. In order to 

 keep down turbulence a very low pitch with broad blades had to be 

 used. To gain efificiency such liladcs must be made thin. It then 

 became of considerable difficulty to insure proper strength for 900 

 R. P. M. as well as freedom from oscillation. 



The blade sections W'Cre considered as model aeroplane wings and 

 their effect integrated graphically over the blade. The blade was 

 given an angle of incidence of 3 degrees to the relative wind at every 

 point for 600 R. P. ]\I. and 25 miles per hour. The pitch is thus 

 variable radially. 



To prevent torsional oscillations, the blade sections were arranged 

 so that the centers of pressure all lie on a straight line, drawn radially 

 on the face of the blade. This artifice seems to have prevented the 

 howling at high speeds commonly found with thin blades. The pro- 

 peller has a clearance of -J inch in the metal cylinder. 



The propeller is driven by a " silent " chain from a 10 H. P. inter- 

 pole motor beneath it. The propeller and motor are mounted on a 

 bracket fixed to a concrete block and are independent of the align- 

 ment of the tunnel. A'ibration of the motor and propeller cannot be 

 transmitted to the tunnel as there is no connection. 



The English plans for power contemplate a steady, direct current 

 voltage. Such is not available here. A 15 H. P. induction motor is 

 connected to the mains of the Cambridge Electric Light Company. 

 This motor then turns at a speed proportional to the frequency of the 

 supply current for a given load. F"luctuations of voltage are without 

 sensible effect, and the frequency may be taken as practically constant. 



