The development of propellers has kept pace with, in many instances exceeded, the speed with which aviation has developed since the Wright brothers' first flight. It has been a characteristic of the propeller industry, in fact, that propeller refinements contributing to superior performance have been ready anywhere from months to years before they were actually required by engine and airframe developments.
Hamilton Standard Propellers Division of United Aircraft Corporation dates back to 1909 through one of its predecessor companies, and has run the gamut of propeller types from those using wooden blades to steel to duraluminum. It has now returned to the use of steel by developing a new hollow-steel blade for larger propellers while retaining duraluminum for those under 13' in diameter.
Back in 1909, Thomas F Hamilton established a factory in Seattle, WA, building propellers for his own use and then for the Army and Navy. Following the war, Hamilton's propellers inaugurated the first continuous, scheduled air mail flight from Washington, DC, to New York with a Curtiss Jenny JN-4, and in 1919 they equipped the NC-4 used by Lieut Comdr Albert C Read in his transatlantic flight. The Army's Douglas World Cruiser, Chicago and New Orleans, used metal-sheathed wooden propellers built by Hamilton Aero, then in Milwaukee, WI. Subsequently, Hamilton's propellers flew 17,000 miles with Comdr Byrd's Fokker Josephine Ford.
Hamilton's major contribution to aviation lay in twenty years of studying wood and steel propeller designs for engines of 50 to 400 hp. When Hamilton Aero merged with Standard Steel in 1929 to become a subsidiary of United Aircraft and Transport Corporation, it was the oldest unchanged name in commercial aviation.
Standard Steel had long worked to improve all-metal propellers and to perfect adjustability. The first hollow-steel, fixed pitch design was produced in 1918, supplanting the heavy and expensive solid steel propeller being tested at the time. By 1921 a propeller with detachable, drop-forged solid duraluminum blades locked by a screw and wedge adjustment in the steel hub had been developed and the split steel hub, permitting change of blade pitch on the ground, was a reality. Within two years after the all-metal adjustable pitch propellers had proved their worth at the 1923 Pulitzer Trophy and Schneider Cup races, there was hardly an Army, Navy, mail or passenger plane flying in the high and medium horsepower class without a Standard Steel, all-metal, adjustable-pitch propeller.
In 1929, Hamilton Standard Propellers was in quantity production on the adjustable propeller. But even then, it had become apparent that the adjustable was insufficient and development had been under way for years on some method for controlling blade pitch while in flight.
The shortcomings of the adjustable centered themselves on the fact that its blades had to set at an angle that would afford the best compromise between low pitch requirements of takeoff and climb and the high pitch necessary for economical cruising.
In takeoff, the engine was incapable of turning up its full rated horsepower with the blades not set in full low pitch; and in similar fashion, the engine would race in level flight for lack of sufficient blade pitch to absorb its power. It was like driving a car locked in second gear. The engine would strain in starting, would waste gas and be inefficient on the open road because the engine's full power could not be utilized.
Heavily loaded Boeing Monomail planes struggled to get off the mile-high fields of Cheyenne and Rock Springs and flying boats labored to get off the water. Unless airplane size and engine horsepower were to be strictly limited, someone had to develop a "gear shift of the air." That came in 1930, with a simple, rugged device permitting pitch change in flight. For takeoff power, a piston attached to the propeller and operated by oil pressure from the engine twisted the blades to low pitch and the propeller revolved rapidly taking small bites of air. When the pilot was ready for cruising, he released the cockpit lever and blades were automatically pulled into high pitch by the centrifugal force on two counterweights attached to the hub and blades
Recognition of the achievement came to Hamilton Standard on May 29, 1934, when the Collier Trophy was awarded by the National Aeronautical Association for the previous year's greatest achievement in aviation "with particular credit to Frank Walker Caldwell, Hamilton Standard's Chief Engineer, for the development and demonstration of a controllable pitch propeller now in general use."
Even in 1933, Caldwell and his associates realized that the day would come when automatic control of blade pitch would replace manual control because engines with a given load have a certain speed at which they are most efficient. It was necessary, therefore, to develop a propeller which would change its blade pitch automatically to meet various flight conditions. No matter how fast a plane is flying or whether it is cruising, climbing, or diving, the propeller should be able to adjust its pitch to whatever angle is necessary to obtain the maximum forward thrust, and, at the same time, permit the engine to deliver its specified power.
Late in 1935, Hamilton Standard, in collaboration with the experienced Woodward Governor Company, designed and added a simple, constant speed control mechanism which eliminated manual control and metered the flow of engine oil to and from the propeller, adjusting blade pitch to give maximum engine efficiency under all conditions.
It had become obvious by the middle 1930's that an extra safety measure should be provided for multi-engine airplanes in the event that one engine became inoperative. Windmilling propellers created a terrific drag and by continuing to turn the engine over inflicted more damage. In 1938 this prop reached maturity when Hamilton Standard placed the quick-feathering Hydromatic propeller in production. Operating on the established hydraulic principles of the Constant Speed propeller, the new Hydromatic was feathered by pressing a button in the cockpit. Within seconds, the blades were twisted in the hub so that they became parallel to the line of flight and stationary, acting as powerful brakes on the engine, eliminating windmilling and thus providing safer and easier control of the plane in an emergency. So well-designed was the new propeller and so easy to operate that it was unnecessary to revise any of its fundamental features for seven years following the first production of this propeller.
It served through World War II on a majority of United States and British combat aircraft without a major change, contributing to the superior performance of such Allied aircraft as the Liberator, Superfortress, Flying Fortress, Mitchell, Ventura, Havoc, Dauntless, Invader, Hudson, Avenger, Catalina bombers; Mustang, Corsair and Hellcat fighters; C-47, C-53, C-54 and C-69 transports to name a few.
Today, the Hydromatic is being modified to meet the requirements of new commercial and military aircraft, incorporating the new steel blade, automatic synchronization, reversing and even more rapid feathering. Its basic operating principles are still unchanged, however. Reversing operation of propellers, hailed in recent months as a new contribution to the safety and utility of the airplane, is actually at least ten years old in the propeller laboratories.
In 1935 Hamilton Standard developed and flew a satisfactory reversing propeller of the counterweight type. For the next eight years, it lay virtually dormant, with no demand for its use.
Today, many of the first Hydromatics are still riding the world's airways packed with the experience of 12,000 to 15,000 hours — more than 2,000,000 miles of safe and dependable flight. As a result of this experience limits on the service life expectancy of the Hydromatic have been removed.
Reversing was satisfactorily accomplished again in 1941, but here again the lack of demand and increasing preoccupation with the tremendous engineering and production required for war forestalled its large-scale use. In 1943, as a result of the vast strides of wartime development, the Army and Navy began an intensive study of reversing in connection with dive-braking for fast dive-bombers.
This investigation eventually turned to ground-braking for larger airplanes requiring longer runways than those available on Army and Navy overseas bases. Airline interest increased as the vision of huge cargo and passenger aircraft in the years of peace began to materialize, and research, development and experimentation with this feature were immensely stimulated.
Relatively large braking capacity required for large airplanes can be provided by reversing propellers with the greatest possible efficiency and minimum extra weight on the airplane. The safety feature of reversing propellers in the event that wheel brakes become ineffective is also important.
In reversing propellers, it is extremely important that the blades pass through flat pitch rapidly to avoid engine overspeeding. In the Hydromatic the transition is made in about three seconds.
In view of the increasing demand for the reversing feature, Hamilton Standard propellers undertook modification of some of its latest models in 1943, and early last year the company and the Army Air Forces successfully and satisfactorily tested a prototype. Reversing Hydromatics have been flown extensively on an Army test airplane at Hamilton Standard in East Hartford for service experience during the past year.
During the same three-year period, de Havilland, Hamilton's British licensee, carried out similar modifications and flew satisfactorily reversing Hydromatics in numerous service tests. The reversing feature will be available in all of Hamilton Standard's new propeller types, including the latest improved Hydromatic models and the Super-Hydromatic.
Installation of the reversing Hydromatic requires two oil passages to the propeller through which controlled oil pressure may be directed, in contrast to the single passage for propeller oil required for normal operation of the regular Hydromatic. These passages are required in many current engines and most of the new designs.
Satisfactory control over the reversing propeller, in order to prevent inadvertent operation while in flight, is assured in Hydromatics by a safety system provided in the cockpit controls. This, together with the intermediate pitch stops, already an integral part of the propeller proper, insures the maximum safety for the reversing operation.
Propeller vibration stresses were a source of continuous trouble in the early days, be- cause there was no means for measuring them in flight until 1938, when such a method had been perfected. Carbon strips attached to the blade or hub recorded the stresses electrically on oscillograph records and revealed exactly their magnitude and character.
As a result, blades no longer had to be reinforced with extra metal at all points as a precaution against failure. Improved knowledge of stresses meant thinner, lighter and stronger blades, giving still more economical and dependable flight. Blade failure from vibration stresses is now virtually eliminated, and Hamilton Standard, proud of its discovery, offered it to the entire aviation industry. This method of determining vibration stresses has since been made a requirement of the Civil Aeronautics Administration and the Army and Navy Air Forces.
A wartime development now being incorporated into many Hamilton Standard blade models is a blade vibration absorber, known to reduce blade vibrations by almost one-half. This development permits the use of even lighter blades, which in turn permit some reduction in the weight of the hub.
During the war, Hamilton Standard extensively used the licensing and subcontracting methods for obtaining mass production of its models, while leasing three other plants in New England. Its licensees in this country and Canada were Nash-Kelvinator, Frigidaire Division of General Motors, Remington Rand and Canadian Propellers. Among them the team of Hamilton Standard propeller manufacturers produced more than 500,000 propellers and more than 2,000,000 propeller blades. Its British licensee, de Havilland, contributed more than 35 per cent of all propellers used on British combat aircraft, and Hamilton Standard's own production of Hydromatics for Britain brought the total to more than 50 per cent.
Propellers built by the US-Canada team comprised more than 75 per cent of all propellers installed on US combat planes from advanced trainers to very heavy bombers. So Hamilton propellers, which came into being almost simultaneously with the Army's first purchase of airplanes, had more than matched the flying machine's progress for almost forty years.
This article was originally published in the April, 1946, issue of Air News with Air Tech magazine, vol 10, no 4, pp 51-57.
The original article includes 31 photos, a photo montage and a cutaway diagram.
Photo credits to Underwood and Underwood, Hamilton Standard, Wide World, Grand Central Air Terminal, International, Boeing.