The advantage of being able to twist the blade (within limits) on the axis is one not to be underestimated in experimental work.
Fig. 34.—The Author's Propeller Testing Apparatus.
With a view to ascertain some practical and reliable data with respect to the dynamic, or actual thrust given when moving through free air at the velocity of actual travel, the author experimented with the apparatus illustrated in Figs. 34 and 35, which is so simple and obvious as to require scarcely any explanation.
The wires were of steel, length not quite 150 ft., fitted with wire strainers for equalising tension, and absolutely free from "kinks." As shown most plainly in Fig. 35, there were two parallel wires sufficiently far apart for the action of one propeller not to affect the other. Calling these two wires A and B, and two propellers x and y, then x is first tried on A and y on B. Results carefully noted.
Fig. 35.—Propeller Testing.
Showing distance separating the two wires.
Then x is tried on B and y on A, and the results again carefully noted. If the results confirm one another, the power used in both cases being the same, well and good; if not, adjustments, etc., are made in the apparatus until satisfactory results are obtained. This was done when the propellers "raced" one against the other. At other times one wire only was made use of, and the time and distance traversed was noted in each case. Propellers were driven through smoke, and with silk threads tied to a light framework slightly larger than their disc area circumference. Results of great interest were arrived at. These results have been assumed in much that has been said in the foregoing paragraphs.
Fig. 36.—One Group of Propellers Tested by the Author.