DESCRIPTION: These three manuscript drawings are a striking artifact of early aviation ingenuity, capturing a bold experimental idea that never left the drafting table. Their meticulously rendered telescoping blades, mechanical sections, and handwritten annotations reveal a level of detail and ambition rarely seen in surviving 1930s aeronautical work. As a display piece, it stands at the intersection of engineering art and aviation history: visually compelling, technically imaginative, and representing a line of research that was too impractical for its time or even for today. Framed, it becomes not just a engineering drawing but a cool conversation piece about the inventive spirit of early flight and the folly of some inventors.
Crowder's variable expansion propellor was designed by Cyril C. Crowder of Hollywood, Califonia. No information is found online about the designer nor is any patent found for the design.
The Crowder design differs from a variable-pitch propeller in the most fundamental way: instead of rotating each blade around its axis to change angle of attack, it changes the actual length of the blade through telescoping sections driven by racks, screws, and sliding guides. A variable-pitch propeller keeps a fixed diameter and varies only the blade’s twist, while Crowder’s mechanism enlarges or reduces the propeller disk itself. In doing so, it attempts to create a large-diameter propeller for strong takeoff thrust and a smaller-diameter propeller for cleaner cruise, something no standard pitch-control system can achieve.
Aerodynamically, the two systems address different needs. Variable pitch optimizes the angle of attack so the blade can produce efficient lift across a range of speeds, preventing stall on takeoff and reducing drag in cruise. Crowder’s concept changes thrust by altering disk area: more radius for lift at low speed, less radius for high-speed efficiency. But because it does not change blade angle, it cannot solve the aerodynamic problems that variable-pitch systems handle so cleanly. This dilemma later resurfaced in helicopter research, where engineers studied variable-diameter rotors for the same reason: large radius for hover, small radius for fast flight.
Mechanical realities ultimately favored variable-pitch systems for both airplanes and helicopters. Telescoping blades must slide under enormous centrifugal loads while staying perfectly aligned and lubricated, a challenge that defeated both Crowder’s 1935 design and later variable-diameter rotor experiments by Kaman, Sikorsky, Bell Helicopters, and others. Variable-pitch mechanisms, by contrast, contain their moving parts inside a compact hub and proved reliable by the 1930s. As a result, pitch-controlled airscrews became universal, while variable-radius concepts remained compelling on paper but impractical in real-world flight.
PUBLICATION DATE: 1935
GEOGRAPHIC AREA: United States
BODY OF WATER: N/A
CONDITION: Fair.
Uneven sides and sheet shapes. Browning. Cropping of text. Chipped edges.
COLORING: Hand color.
ENGRAVER:
SIZE:
" x
"
ITEM PHYSICAL LOCATION:
PRICE: $300
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