CHAPTER VIII. 



THE COUNTERPOISED ECCENTRIC PLANE. 



If a rectangular plane be made to move through the air at an angle of 

 inclination with the direction of advance, it was implicitly assumed by Newton 

 that the center of pressure would coincide with the center of figure. Such, how- 

 ever, is not the case, the pressure being always greater on the forward portion, 

 and the center of pressure varying with the angle of inclination. 



The object of the present chapter is to present the results of experiments 

 made to determine the varying positions of the center of pressure for varying 

 angles of inclination of a plane moved in a horizontal course through the air. 

 Drawings of the apparatus devised for this purpose are given on plate V. AA' 

 represents the eccentric wind-plane one foot square held in a brass frame about 

 £ of an inch wide and i of an inch thick. Two sliding pieces, SS', move in a 

 groove in the edge of the brass frame, and may be clamped in any position by 

 screws. Each sliding piece has a small central hole, in which fits a pivot. V. 

 The wind-plane (eccentric plane) is suspended by these pivots and swings about 

 the axis passing through them, so that by moving the plane in the sliding 

 pieces this axis of rotation can be moved to any distance up to two inches. A 

 flat lead weight, which also slides along the back of the plane, can be adjusted 

 so as to counterpoise it in any position. When the weight is adjusted, therefore, 

 the plane is in neutral equilibrium about its axis of rotation. A pencil, P, is 

 fixed on the lower part of the plane and records against a tracing board perpen- 

 dicular to it, In order to leave the position of the plane entirely uncontrolled 

 by the friction of the pencil, the registering board is held away from the plane 

 by spring hinges HH', and caused to vibrate by an electro-magnet so as to touch 

 the pencil point many times in a second. 



In the experiments the sliding pieces were set so that the axis of rotation 

 was successively inch, 0.25 inch, 0.75 inch, etc., from the center, and the 

 plane was counterpoised about this axis. When placed in rotation upon the arm 

 of the whirling-table, the moment of rotation of the plane about the axis is pro- 

 portional to the resultant wind pressure multiplied by the distance of the center 

 of pressure from the axis of rotation, and it will reach its position of equilibrium 

 when the plane has taken up such an angle of inclination that the center of 



12 (39) 



