Fig. 37.—Marking off the dynamometer. In order to ascertain the actual amount of power consumed in driving the propeller, a brake was put on in place of the screw, a weight applied, and the engine run at full speed. In this way all the uncertain and unknowable factors were eliminated.
In [Fig. 36], a, a is the body of the apparatus, partly of gunmetal and partly of wood. It is provided with a steel shaft to which the screw h, is attached, and also with a cylindrical pulley for taking the belt. The thrust of the screw pushes the shaft inwards and records the lift at o ([Fig. 32]). The corners of the aeroplane g, g, are attached by wires to the steel plate e. b, b, is a four-arm spider for holding the ends of the parallel bars c, c, and d, d, show vertical steel bars to which all devices to be tested are attached. In testing aeroplanes, weights may be placed at e, sufficient to balance the lifting effect, and then by adding the weight to the upward pull of the aeroplane, the true lift of the aeroplane is obtained. It is also possible to attach an aeroplane at e, that is, the machine was made to test superposed aeroplanes if required. In these experiments, I naturally assumed that the best position for a screw was at the rear and in the path of the greatest resistance, but as some experimenters with navigable balloons placed the screw in front in order to pull the apparatus along instead of to push it, I made experiments to see what the relative difference might be. In order to do this, I wound a large amount of rope one-half inch in diameter around the whole apparatus forward of the screw, converting it into an irregular mass well calculated to offer atmospheric resistance. Upon starting the engine, I was rather surprised to see how little retardation these ropes gave to the apparatus. It appeared to me that nearly all of the energy consumed in driving the ropes through the air was recovered by the screw. I then removed the right-hand screw and replaced it by a left-hand screw of the same pitch and dimensions ([Fig. 37a]). I then found that the blast of the screw blowing against the tangle of ropes greatly retarded the travel; in fact, with the same number of revolutions per minute, the velocity fell off 60 per cent. I think that these experiments ought to show that there is but one place for the screw, and that is at the stern, and in the direct path of the greatest atmospheric resistance.
Fig. 37a.—Right- and left-hand four-blade screws used in my experiments for ascertaining the comparative efficiency between screws placed in front and in the rear of the machine.
Fig. 38.—Apparatus for indicating the force and velocity of the wind direct without any timing, counting, or mathematical calculations.
[Fig. 38] shows an original apparatus which I designed and made for my own use; with ordinary anemometers it is necessary to count the number of turns per minute in order to ascertain the velocity of the wind. I wanted something that would indicate the velocity and the direction of the wind without any figures or formulæ. I therefore made the apparatus shown in the drawing, in which a, a, is a metallic disc 13·54 inches in diameter, giving it an area of exactly 1 square foot. This is attached to the horizontal bar b, and the whole mounted on two bell crank levers as shown. When the wind is not blowing, the long arms of these two levers assume a vertical position, and the spiral spring h, is in exact line with the pivots on which these levers are mounted, and has no effect except to hold the levers in a vertical position. As the spring has very little tension in this position, and as it requires a considerable movement in order to give it tension, the arms c, c, and the bar b, b, are very easily pushed backwards, but as the distance through which they travel increases, the angle of the lever changes and the tension of the spring increases at the same time, so that when the disc is pushed backwards to any considerable distance, a strong resistance is encountered. Had I made this apparatus so that the pressure acted directly on the spiral spring, the spaces on the index indicating low velocities would have been very near together, while those indicating high velocities would have been widely separated, but with this device properly designed, the spacing on the index became regular and even. The index being very large enabled one to read it at a considerable distance, and at the same time, it acted as a tail and kept the apparatus face to the wind. The spaces of the dial were not laid off with a pair of dividers, but each particular division was marked by an actual pull on the bar b, through the agency of a cord and easily running pulley and weight. The markings, however, were not correct, because Haswell’s formula was employed in which the pressure of the wind against the normal plane is considerably greater than with the more recent formula, which is now known to be correct. Haswell’s formula was V² × ·005 = P, and the recent formula P = 0·003 × V², where P = pressure in lbs. per square foot and V = velocity in miles per hour. In my experiments, I also employed a very well made and delicate anemometer by Negretti & Zambra.
