When the jet plane really came into its own after the end of World War II, a lot of aviation experts arched heir eyebrows and freely predicted: “That means curtains for the old ‘windmills’; the propeller will soon be as dead as the Dodo!” Wise money in the stock market whispered: “Unload propeller stocks. Lucky if you get out with your shirt!”
But those who would sell the “prop” short only three or four years ago must be reconsidering the aviation industry in general, and the turboprop plane in particular, with a certain amazement, if not chagrin.
Back of the scenes something like a counter-revolution has been in progress— aided and abetted by determined engineers who just wouldn’t give the propeller up. In fact they couldn’t, not as long as there were some aviation jobs that could not quite be done with pure jets, nor with souped-up versions of standard piston propeller planes.
So the answer had to be a compromise, the turboprop, mating jet power with a propeller—not exactly the kind of propeller the Wright Brothers used, and certainly a far cry from the same kind of engine.
But the turboprop is still definitely a “windmill,” with most of the range of the old propeller planes, and almost the power and something near the speed of the pure jet. In April the Navy announced successful trial flights of a patrol plane and in May of an attack plane, both powered by turboprop engines. On August 30th it was announced in San Diego that the patrol turboprop had remained in the air for eight hours and six minutes, a record for a plane of this type.
Essentially, the turboprop compresses air, mixes it with fuel, and burns the mixture in a narrow combustion chamber. Expanded gases then roar past turbine blades, which in turn operate a propeller. Instead of driving the plane by the “thrust” of escaping gases, as the jet engine does, most of the power is transmitted through reduction gears to the propeller.
Escaping gases, however, do provide some “thrust” to the turboprop, too. In fact, this tail exhaust is sometimes so strong, even with the engine idling, that the propeller blades must be feathered slightly in reverse to enable the plane to stand still on the ground.
Why has the propeller survived when almost everyone was going all-out for the pure jet? Mainly because of the need for a lot of power for the short takeoff area of a carrier, and for comparatively slow flight during approaches for carrier landings. The jet has power, and plenty, at high altitudes, but is at a distinct disadvantage at sea level. The conventional reciprocating engine could develop lots of power, too, but it was falling steadily behind in the speed parade.
The turboprop, therefore, seemed to be the logical answer for the Navy’s immediate tactical needs. In fact, it may turn out to be the answer to several prayers, for not only does it have the quick punch needed for carrier work, but it weighs less than one-half as much as a piston engine of equivalent horsepower. In addition, the turboprop engine has less vibration, is much quieter and, by employing contra-rotating propellers, has eliminated the torque which formerly caused many crashes during full-power takeoffs and landings.
With so many advantages, the natural question is: “Why hadn’t this been thought of before?” The answer is that it had. For several years the British have been experimenting with combinations of jet engines and propellers in transports, Hying boats, and carrier planes. It was a successful demonstration of what some of the British planes could do in 1949 that gave impetus to the “counter-revolution” in plane design on this side of the Atlantic.
Back in the ’30s, the Navy’s Bureau of Aeronautics was already thinking about the possibility of gearing a gas turbine to a propeller. But when it asked a gas turbine committee of the National Academy of Sciences to make a recommendation about it, the committee, whose main interest then was shipboard gas turbines, took a dim view.
Subsequent developments indicate that someone must have borrowed the wrong slide rule. Shortly after this report was submitted the Navy learned that a properly-built turboprop engine could theoretically weigh as little as ½ pound per horsepower—which is actually more than they weigh today.
Anyway it was decided to take a chance. A joint military contract was awarded to Northrop in 1941 to develop a geared gas turbine. But war intervened, and with it the necessity of supplying already developed aircraft in great numbers. Little engineering talent could be spared for new developments. When the “Turbodyne” developed by Northrop was wrecked in a test run, the contract was terminated. Here the Air Force picked up the ball, and asked Northrop to produce a pusher engine of this type, but the project was dropped when the Air Force decided not to install turboprops in the “Flying Wing.”
The Navy, however, had not completely lost interest, and when Westinghouse came forward with a proposal for a turbine driving a high-speed, contra-rotating propeller, rated at 3,000 hp at sea level, the Navy gave it a “go” signal. The first Westinghouse axial- flow turbine engine was completed in 1943, and these turbojets (not turboprops) now power the “Phantom” and “Banshee.”
Westinghouse’s engine contributed important turbine advances, but the real story of the turboprop begins with a contract awarded by the Navy in November 1944 to the Allison Division of General Motors. Allison proposed a power plant of two units, mounted side by side, driving a common reduction gear linked to the propellers. What finally evolved was the present XT-40 engine, delivering 5,500 horsepower, which is more horsepower on takeoff than any other U.S. engine now being flown.
The Chrysler Corporation and Pratt & Whitney also suggested variations of the turboprop theme in its pioneering stages.
Because the present Allison turboprop combines two complete and separate units, it has the outstanding advantage, during normal cruising, of operating on one of the twin turbines alone, thus increasing the range and economy of the aircraft, while carrying a power reserve for emergencies. A four-engine patrol plane equipped with twin turboprops is, therefore, actually an eight-engine plane when the pilot wants to go some place in a hurry or evade enemy attack. When he wants to slow down and look around (antisubmarine work) he can cut back to four engines again.
Let’s take a look at some of the other advantages. A turboprop affords a cleaner installation in the plane. It has no cooling problems at different altitudes, nor does it need inter-coolers, as does a reciprocating engine employing blowers. The nacelle in which it is housed is unusually thin, offering less air resistance, thus saving fuel. Absence of vibration means less heavy structural reinforcement in the plane.
One important advantage does not show in the blueprints, but it is most welcome to both pilots and crew, and it will be a prime selling point when turboprops enter the commercial air passenger field. Willi less engine noise and less vibration there is much less pilot and passenger fatigue.
No account of the almost story-book rise of the turboprop, however, would be complete if only the power unit and its hookup were discussed. The propeller itself is something quite special, with a “new look,” and modified materially from the prop of its piston engine predecessor.
Harnessing the turbine’s immense power to a propeller was recognized from the start as a tough problem. In fact, healthy doubt that any propeller could handle the 5,000 or 10,000 or 15,000 horsepower delivered by turbines made some aviation experts consider the turboprop not worth a lot of expensive investigation.
But three companies—Aeroproducts, Curtiss-Wright, and Hamilton Standard—agreed to turn their best engineers loose on the problem to see what could be done.
Hamilton Standard and Aeroproducts finally developed a hollow blade made of sheet steel, formed around a center spar made of steel tubing. Curtiss-Wright produced a “monocoque” design, in which two heavier steel shells are formed and welded together, reinforced by an internal rib.
To obtain the additional blade area necessary for efficient use of the vast amount of power developed by a turboprop, two three- bladed or four-bladed props were mounted on each twin-engine unit, one turning clockwise, and the other counterclockwise to eliminate torque. Propeller lips were squared.
Rear Admiral C. M. Holster, Assistant Chief of the Bureau of Aeronautics for Research, discussing new propeller possibilities in connection with turboprop engines, said he believes that the turboprop can compete eventually with the pure jet if it can increase its propeller efficiency to 70% or better. He feels that such a propeller is possible.
From the pilot’s and the flight engineer’s viewpoint the operation of a turboprop differs radically from that of any other standard type of plane. In the first place, the engine runs at nearly a constant speed. The XT-40 engine, for example, varies only from an idling speed of 12,800 rpm to 14,300 rpm at takeoff. Power to the propellers is varied through fuel How. With such a constant power output the pitch or “bite” of the propellers is varied with airspeed.
All of this would require something of a genius in the cockpit, if an ingenious electric brain did not relieve the flyer of a score of details. In actual flight the pilot sets a lever at the speed he wishes. Signals showing blade angle, rate of blade angle change, the difference between the desired turbine speed and actual speed, and the temperatures of the engines are all fed electrically into a computer. This “brain” controls various servomechanisms which make the necessary changes in the blade angle and the fuel output.
The Navy has already successfully flight- tested two different types of planes fitted with turboprop engines. One, a four-engine 60-ton patrol seaplane, the XP5Y-1, was built by the Consolidated Vultee Aircraft Corporation. On its initial flight test it was airborne after a takeoff run of only twenty seconds. The XP5Y-1 is designed for long- range sea-search missions.
The other, the XA2D, or “Skyshark,” is a carrier-type attack plane built for the Navy by the Douglas Aircraft Corporation. Roth planes use Allison turboprop engines.
The Navy has agreed to make all of its turboprop findings available for the use of commercial aviation, and already the nation’s plane builders and airline operators are making plans to take advantage of the benefits of this unique power plant.
Allison itself has purchased a Convair airliner and has installed in it two T-38 engines, the single unit of the twin-40. Lockheed has mentioned turboprop power for its Constellations, and Hoeing likewise has shown interest in the T-40 for use in the C-97 and the Stratocruiser.
The story of the Turboprop would not be complete without noting another Navy- sponsored design, the T-34 turbo-wasp engine, designed and built by the Pratt & Whitney Aircraft Company.
Pratt & Whitney installed this powerful unit as the fifth engine in the nose of a B-17 Flying Fortress, which acted as a flying test stand. So successful was the unit that it actually flew the big bomber without the aid of the four 1200-horsepower piston engines which normally power the plane.
The P & W T-34 is a single-unit high- pressure axial-flow gas turbine engine that, like the T-38 and T-40, delivers power both to the propeller and by jet thrust through a tailpipe at the rear. About 90 per cent of the engine’s 5,000 to 6,000 horsepower, however, is delivered to the propeller shaft, with the remainder in jet thrust.
Martin plans to use the T-34 in its “404” airliners, and Douglas in the C-124.
With its initial flight tests successfully behind it, the turboprop has moved into an important spot on the stage of American aviation—right in the center, it might be said, with almost the range and economy of reciprocating engines but nearer the speed of the jet. For the moment, it packs the most power in the smallest package ever developed for propeller-driven planes.