The Adoption of the Controllable Pitch Propeller by the Outside World



Canada is not exactly known for having produced several ground-breaking inventions or discoveries in her time. However, the period of rapid technological advancement that she incurred during the third period of the history of engineering in Canada brought with it several important engineering inventions which had their roots in Canada. The creation of the controllable pitch propeller was one such invention which was perfected in Canada and was so successful that this primarily Canadian development spread throughout the world.

Wallace Rupert Turnball lived in Rothesay and it was there that he carried out his experiments in aeronautical theory beginning in 1902. His specialty was that of dihedrals which he studied in a wind-tunnel. He looked at water borne hydroplanes propelled by motor-driven airscrews. An airscrew the Great Britain term for a propeller. A standard propeller consists of anywhere from two to four blades each a section of a helix, the geometric form of a screw thread, hence the term airscrew. The first plane had two air-screws on each side whereas the second one had only one, more highly efficient propeller located at the rear end of craft, near the pilots seat. However, both had an uneven torque of engine that was in fact destructive to the efforts of the propeller. Turnball experimented with all different types of air-screws; some with a 30 gauge track that were 300 long for truck. With each air-screw he tested, he recorded the propeller thrust, rpm and the forward speed. What determines the forward speed is the distance that a propeller will move in the forward direction when the shaft of the propeller is rotated 360o. Assuming that there is no slippage, this distance is termed the geometric pitch. The propellers that Turnball tested had diameters ranging from 1.5 up to 3.5, all different dimensions and shapes.

Upon his return to Rothesay in 1918, after the war, he dove into his research and experimentation on a possible controllable pitch propeller, an idea that he had been developing since the autumn of 1916. He ran several tests using rotating electric motor apparatus in order to spin the blades of his propeller. The finished product was a propeller whose pitch can be adjusted by the pilot, at different angles, during flight giving the pilot the ability to command the optimal combination of torque and speed for the situation at any given moment from his aircraft. By means of a small electric motor mounted just in front of the propeller, the pitch of the propeller itself could eventually be adjusted which makes for more efficient take-offs and regular flight than what would be achieved with an everyday fixed blade propeller incapable of any pitch change.

Under the supervision of both the Ontario government and the Canadian Air Force, a ground test was run in 1923 on Avro aircraft at Camp Borden, Ontario only to conclude that more research and experimentation was necessary. Four years later, on June 6, 1927, again at Camp Borden on Avro Biplane, Flight Lieutenant G.G. Brookes took Turnballs controllable pitch propeller for its first air test. Funding was granted immediately to perfect the invention it was such a success.

The news of the Canadian invention spread rapidly. Turnball wrote a treatise based on his discoveries and new found technology called The Efficiency of Aerial Propellers which was published in the Scientific American on April 3, 1909. His second and third publications on the subject were entitled Laws of Air-Screws and appeared in The Aeronautical Journal, in the October 1910 and January 1911 issues. For his studies and discoveries, Turnball was awarded the Bronze Medal of Royal Aeronautical Society and was, in addition, elected a Fellow. Come 1914, Turnball had published several scientific articles and found himself one of the worlds authorities on the subject. He sold the patents to the controllable pitch propeller in December of 1929. The Curtiss Wright Corporation won the American rights and the Bristol Aeroplane Company, the English rights. In 1935, the Norseman, the most highly successful bush plane in the world at the time, was designed in Canada by Robert Noorduyn, an aviation engineer trained in Holland. The Norseman quickly caught the attention of the entire