56 EXPERIMENTS IX AERODYNAMICS. 



direction of advance — is indicated by the order in which the dimensions are 

 stated, the first dimension being always the horizontal edge parallel to the 

 whirling arm. Thus the 24 x 6 inch plane is placed with its 24-inch edge hori- 

 zontal and parallel to the whirling arm, and the 6 x 24 inch plane is the same 

 plane placed with its 6-inch edge horizontal and parallel to the whirling arm. 

 This difference of position, then, will be uniformly spoken of as the aspect of the 

 plane. The column "pull of spring" contains the spring extensions converted 

 into pressures by means of the calibration curves, and the column "horizontal 

 pressure on plane" (i. c, the horizontal component of pressure) is obtained by 

 multiplying the spring pressure by the factor 0.524, which arises from the unequal 

 lengths of the arms of the instrument. The next column, headed " k m ," gives 

 for the observations with normal planes the computed value of the coefficient in 

 the equation P = k m F J , where Fis expressed in meters per second, and P is the 

 pressure on the plane in grammes per square centimeter. The column " k " gives 

 the corresponding value of this coefficient in English measures, the velocity being 

 expressed in feet per second and the pressure in pounds per square foot. 



SOARING SPEEDS. 



The soaring speeds determined in these two series of experiments are plotted 

 in Figs. 8 and 9, in which the abscissae are angles of inclination of the planes to 

 the horizon, and the ordinates are the soaring speeds which correspond to them. 

 Figure 8 contains the observations made with the planes that weigh 250 and 1,000 

 grammes to the square foot, and Fig. 9 those made with the planes that weigh 

 .",1)11 grammes to the square foot 1 5.382 grammes to the square meter). The 

 experiments with the first two of these classes of planes, plotted in Fig. 8, were 

 not repeated, and consequently the curves do not possess so high a quantitative 

 value as obtains in the case of most of the planes weighing 500 grammes to the 

 square foot, bul they serve to present several fundamental relations: 



First, they show quantitatively, when taken together with the curves of Fig. 

 9, the increase of velocity necessary to sustain the heavier planes (per unit area) 

 n\ er tli.it \\ hich will sustain the lighter ones, at the same angle of inclination. 



Second, the curves both of the 250 and the 1,001) gramme planes show the 

 difference due to shape and aspect, the soaring speeds, for small angles of inclina- 

 tion, being much less for those planes whose extension from front to back is small, 

 than for those in which this dimension is large, so that, in general, the planes 

 having this dimension smaller, for small angles of inclination, soar at lower 

 speeds. This result entirely accords in character with that already obtained with 

 the Plane- Dropper ; and, when freed from accidental errors, the present data are 

 of higher quantitative value, because in this apparatus then' are no guides, and 

 the plane has practically perfect freedom. 



