1905-1915 Wright
Bent-End Propellers

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arly in their 1904 test flights at Huffman Prairie, the Wright brothers realized that their round-end, straight propellers were not producing as much thrust as the math said they should. The props were long, slender – and flexible. When turning at speed (400 to 500 rpm), the straight blades bent slightly toward the trailing edges, assuming an S-shape.

This bending at speed put the leading edge of each blade under tension and the trailing edge under compression. This in turn caused the blade to buckle slightly, increasing the pitch – the angle at which it met the air. Consequently, the blade lost thrust at speed.

To prevent this, the Wright brothers decided to widen the blades, adding stock at the propeller's center of pressure where it was most important to keep the blade at the proper angle. The result was an elongated diamond or spade-shaped blade with squared ends. This worked, but only partly – the forces of tension and compression at the edges still caused the blade to change pitch.

Rather than strengthen the propeller yet again, the Wrights decided instead to design propellers that would balance the forces to retain the proper pitch. Experimenting with the spade-shaped blades, they added triangular pieces of wood (nicknamed "little jokers") to the trailing edges, changing the way the compressive and tensive forces were distributed. This did the trick – at speed, the prop remained at the proper pitch and produce the predicted thrust.

Next, they carved props with the little jokers incorporated into the blades. The resulting set of propellers looked as if they were pre-bent. Later, when the Wrights began to make public demonstrations, observers described these as "bent-end" propellers and the name stuck.

The Wrights carved a set of bent-end propellers for the 1905 Flyer and found that they produced 210 pounds of thrust (compared to the 120-130 pounds produced by the 1903 props). Afterwards, they used these distinctive bent-end propellers on all their aircraft up until 1916. They were able to improved the efficiency of the props from 66% in 1903 to 78% in their commercial aircraft. By comparison, modern propellers convert 82-83% of their rotational energy to thrust.

 References:

•  Orville Wright, deposition in Montgomery vs. the United States, 2 Feb 1921
• McFarland, Marvin W. (ed) The papers of Wilbur and Orville Wright. McGraw-Hill Book Co., New York, 1953, p 1214-1215.
• Lippincott, Harvey H. Propulsion System of the Wright Brothers. In Wolko, Howard S. (editor), The Wright Flyer, an Engineering Perspective. The Smithsonian Institution Press, 1987, pp 79-82; 86.

[Submitted by Joe W. McDaniel]
 

The 1904 Wright Flyer II had straight propellers with rounded ends, much like the props on the 1903 Flyer I.

The Wrights noticed that when these long propellers revolved at flying speed, they flexed and bent. This in turn caused the blades to increase pitch.

The evolution of the bent-end propeller: (A) The Wrights found their original round-end props bent and increased pitch at speed. (B) So they widened the props to make them stronger where the pressure was greatest, but this worked only partly. (C) They next added triangular pieces ("little jokers") to the trailing edges of the props to prevent to prop from changing pitch. (D) Finally, they carved a set of props with these pieces incorporated as part of each blade. Wright "bent-end" propellers became a distinguishing feature on all their aircraft produced between 1905 and 1915.

Bent-end propellers were first used on the 1905 Wright Flyer III. The blades of these props had a parabolic camber like the wings of an aircraft.

A bent-end prop installed on a Wright Model A in 1910.

The end of the line -- this bent-end propeller was designed for the Wright Model K in 1915, the last aircraft to use these distinctive props.

Student pilot Spencer Crane holds a bent-end propeller from a Wright aircraft.

The propeller-carving station in the Wright Factory in 1911.

The 1915 Model K was the only Wright aircraft in which bent-end props were used for traction – they were mounted ahead of the wings to pull the airplane through the air. In all previous Wright airplanes, the props were behind the wings, pushing the aircraft.

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