The Airplane - Jay Spenser [99]
Fully cowled radial engines had flown before—for example, the military’s experimental Dayton-Wright XPS-1 of 1922. However, nobody before Fred Weick had shown the potential benefits in terms of drag reduction. For this achievement, NACA received the Collier Trophy in 1929.
The next challenge was to achieve acceptable cylinder head temperatures within the aerodynamic NACA cowling. The champion here was Rex Beisel, a talented aeronautical engineer who would later lead the design of the Vought Corsair fighter plane of World War II fame. Heading up a four-year program of wind tunnel and flight research, Beisel learned what in aerodynamic terms was going on inside the cowling.
Beisel’s solution was to restrict total airflow through the cowling and guide this limited flow via pressure baffling to where cooling was most needed. This effort too was successful; in fact, Beisel stunned the aviation world in 1934 by achieving lower cylinder head temperatures inside a NACA cowling than in the same engine with no cowling at all.
From the 1930s onward, liquid-cooled engines would play second fiddle to radials. This would continue through World War II and all the way to the jet age.
As long as airplanes flew slowly, fixed-pitch propellers were fine. They were made of laminated wood before and during World War I, and increasingly of metal thereafter. As airplanes became faster, however, the growing difference between takeoff and cruising speeds turned fixed-pitch propellers into a frustrating liability.
A fixed-pitch prop is like a bicycle without the ability to shift gears. Depending on what fixed gear ratio exists between the rotation of the pedals and that of the rear wheel, the bicycle can be either easy to start up from a standstill or comfortable to ride along at a fast clip. Unfortunately, though, setting the ratio for the former makes the pedals turn too fast for the latter, and optimizing for the latter makes the pedals too heavy to push at start-up.
Consequently, early bicycling enthusiasts—such as the Wright brothers—rode using a compromise in-between gear ratio determined by the size of the sprocket wheels employed in the bicycle’s chain-drive transmission. This one-gear limitation remained until derailleurs were introduced, allowing bicyclists to shift between gears.
It was the same with fixed-pitch propellers. Depending on the pitch angle of the blades, a propeller could be optimized for efficient takeoffs and landings, an efficient cruise, or somewhere in between. This compromise third choice worked well enough until the disparity between the top and bottom ends of the operating speed envelope became too great for one propeller to do it all. Fortunately, help was on the way in the form of Frank Walker Caldwell.
A native Tennessean with a mechanical engineering degree from MIT, Caldwell stumbled upon his life’s work in 1912 when he joined the propeller department of the Curtiss Aeroplane Company in Buffalo, New York. Propellers fascinated this young man. Years before World War I, he presciently foresaw the need for them to evolve as airplanes became larger, heavier, and faster. In particular, he was intrigued by the notion of propellers that could change pitch in flight.
So enthralled was Caldwell with this concept that he left Curtiss for government service as a civilian employee with the Army Air Corps’ Engineering Division in 1917. Taking up residence at McCook Field, he served as head of the division’s Propeller Department. There he pioneered test practices as well as design and manufacturing innovations.
Frank Caldwell was responsible for the aviation industry moving to drop-forged metal propellers built up of separate blades and hubs. Between flights, these ground-adjustable units could be set to a different blade pitch by loosening the hub, rotating the blades to a new common angle, and then securing them again in the hub.
Ground-adjustable propellers let airplane operators set their propellers to favor either takeoff and climb or