AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



APPENDIX A.* 



The wind tunnel of the Massachusetts Institute of Technology was built in 

 accordance with that of the National Physical Laboratory, Teddington, England, 

 with the exception of several changes of an engineering nature introduced with 

 a view to a more economical use of power. An increase of the maximum wind 

 speed from 34 to 40 miles per hour was thus obtained. 



Upon completion of the tunnel an investigation was made of the steadiness 

 of flow. It appeared that the variation of velocity with time and from point to 

 point of the cross-section was not more than one per cent. 



The wind tunnel proper is a square trunk 16 square feet in section and 53 

 feet in length (Plate 3). Air is drawn through an entrance nozzle and through 

 the tunnel by a propeller driven by a lo-horse-power motor. Models under test are 

 mounted in the middle of the tunnel on the arm of a delicate balance. 



The air entering the mouth passes through a honeycomb made up of a nest 

 of 3-inch metal conduit pipes. This honeycomb has an important eflfect in straight- 

 ening out the flow and in preventing swirl. 



Passing through the square trunk and past the model under test, the air is 

 drawn past a star-shaped longitudinal bafile into an expanding cone. This cone 

 expands in 1 1 feet to a diameter of 7 feet. The velocity of the air is reduced in 

 passing through the cone and has its pressure increased in accordance with a well- 

 known hydraulic principle. 



The propeller is made of black walnut with four blades (Plate 6). It works at 

 the large end of the cone and discharges into the dififusor. The latter is built of 

 wood grating with holes closely spaced except on the side facing the propeller, which 

 has no openings. The propeller race is stopped by this wall, the velocity of the air 

 destroyed and the pressure raised. The air then escapes through the holes in 

 the dififusor into the room. The current is thus turned through 90° and brought 

 nearly to rest. 



The propeller was designed on the Drzwiecki system, which assumes that 

 each blade section is an aeroplane wing moving through the air in a spiral path. 

 In order to keep down turbulence, a very low pitch and a broad blade were used. 

 To gain efficiency the blades were made thin and, therefore, weak. To prevent 

 fluttering of the blades, the blade sections were so arranged that the centers of 

 pressure of all sections lie on a radial line drawn on the face of the blade. This 

 artifice seems to have prevented the howling at high speeds commonly found with 

 thin blades. 



The propeller is driven by a "silent" chain from a lO-horse-power inter-pole 

 direct-current motor. The propeller and the motor are mounted on a bracket 

 structure fixed to a concrete block, and are hence independent of the alignment of 



♦Abstract from "Scientific Aeronautical Research," J. C. Hunsaker, Science Conspectus M. I. T., Vol. 

 V, No. 1. 



