No single technical development has more profoundly affected the helicopter than the shift from the reciprocating piston engine to the jet turbine engine. Turbine engines have made the helicopter safer, more reliable and easier to maintain, and greatly enhanced performance. The Kaman K-225, the world's first helicopter powered by a turbine engine, represents the initial important step in this monumental transition from piston to turbine power. This particular K-225 is also the first helicopter successfully flown through an intentional loop in March 1953.
The K-225 sprang from the fertile mind of Charles H. Kaman (pronounced like the word 'command' without the 'd'). Kaman founded the Kaman Aircraft Company late in 1945 to develop an intermeshing twin-rotor helicopter for the civilian market. In Germany during World War II, Anton Flettner had originally pioneered the intermeshing rotor configuration when he designed and flew the Flettner Fl 282 helicopter. This aircraft saw limited military service. After the war, the concept migrated to the United States and picked up the nickname 'synchropter.' Kaman believed that intermeshing, counter-rotating rotors were more efficient for certain types of helicopters than the single main/tail rotor designs pioneered by Igor Sikorsky during the war. The synchropter's counter-rotating blades eliminated torque and omitted the need for a tail rotor that contributed nothing to lifting the helicopter but only robbed engine power from the main rotors. The synchropter also saved weight and cost by doing away with most of the tail boom.
Kaman's first two designs, the K-125 and K-190, validated Flettner's concepts, and quickly surpassed the Fl 282 in performance. The K-190 was nimble enough to demonstrate aerobatic maneuvers that most other helicopters of that era could not perform but it was also underpowered and Kaman soon designed an improved model designated the K-225. It first flew in 1949 at the Bradley Field, Connecticut, factory. Kaman equipped the new helicopter with a bigger engine than the unit that powered K-190 but it otherwise closely resembled that aircraft. The K-225 consisted of an uncovered framework of welded steel tubes that supported tandem seats placed in an open cockpit ahead of the two, side-by-side rotor masts. Why Kaman insisted on retaining the open cockpit is not known but the decision had unpleasant consequences several years later.
Charles Kaman started a helicopter company because he had invented a novel servo-flap system to control the main rotor. He believed he could combine the servo-flap with the synchropter layout and produce helicopters that could successfully compete in the commercial and military markets with the established manufacturers such as Sikorsky and Bell. On the K-225, the Kaman servo-flap was installed on the trailing edge of each rotor blade at a point about 75% outboard of the rotor hub. The servo-flaps linked directly to the pilot's collective and cyclic control levers. When the pilot moved a lever, he deflected the servo-flap up or down and the flap did the heavy work of moving the rotor blade. In all other helicopters of that day, the controls were linked directly to the rotor blade. Pilots of these helicopters had to muscle the entire blade while the men and women flying Kaman helicopters moved only the servo-flap and enjoyed almost finger-tip control. Pilots who had to manhandle their helicopters often became quite fatigued during long flights. The servo-flap was so effective that Kaman had to install an adjustable damper system to increase the forces needed to move the controls. This gave pilots a more realistic feel for flying the helicopter. With this artificial enhancement, it was all too easy to pump the controls rapidly enough to overstress the rotor blades or the airframe.
A vertical stabilizer installed at the tail of the K-225 increased directional stability during forward flight. On the ground, the aircraft was supported by a tricycle landing gear and each gear leg was cushioned by oleo shock absorbers. In front of the cockpit, a small Plexiglas windscreen blocked some of the onrushing air during forward flight but otherwise the cockpit lay open to the elements. The crew could suffer in harsh weather but the arrangement had advantages. The K-225 was one of the first helicopters equipped to operate at night but this would not have been possible unless the cockpit was very open for it gave the crew superb visibility.
Kaman offered to lease the K-225s for crop dusting, an incentive that allowed customers to fly the aircraft for an extended period without bearing the maintenance burden. Kaman feared that dissatisfied operators might turn to other manufacturers but his fears were somewhat unjustified. The K-225 excelled at dusting crops. The downwash from the synchropter's two rotors angled to each side of the helicopter and dispersed the insecticide over an area considerably wider than was possible with fixed-wing aircraft. A disadvantage of the synchropter configuration also became apparent. The extra main rotor and mast generated considerable drag and the K-225 topped out about 25% slower, 112 kph (70 mph), than most single-rotor helicopters.
Neither Charles Kaman nor his struggling company had any control over the flaw that ultimately doomed the K-225 as a successful agricultural aircraft. Like the development of practical helicopters, dusting crops by aircraft was also in its infancy. There were no standards in place to determine the proper chemicals to use, the types of crops to use them on, or the correct application techniques. The crop dusting industry resembled the American West during the early 1800s, wild and often dangerous. The K-225 failed to generate production orders so Charles Kaman looked to other markets for the K-225 and managed to sell one to the government of Turkey. This was the first helicopter to fly in that country. Geological surveys were carried out from another K-225 in Mississippi. The U. S. Navy ordered two and the Coast Guard ordered one, all for evaluation trials. During the flight to delivery a Navy K-225 to the Patuxent River Naval Air Test Center, Kaman company test pilot, William R. Murray, successfully performed the first intentional loop in a helicopter in March 1950 but more than cute tricks were needed. Serious, long-term military interest in the K-225 held the greatest potential to jump-start mass production and generate desperately needed cash for the struggling company but no new orders materialized. The men who procured military rotorcraft did not want an open-cockpit helicopter.
Kaman faced bankruptcy but at the eleventh hour in July 1949, Charles talked the Navy into buying four prototypes of an improved variant of the K-225 designated the HOK-1. He touted the new model as a good alternative to the Sikorsky S-52 but he could not afford to build the new aircraft without additional income. After difficult negotiations, Charles Kaman convinced the Navy that the HOK-1 was ideal for service in Korea. Before these helicopters were built, Navy officials placed another order in September 1950 for a trainer variant of the K-225 designated the HTK-1. This aircraft had a completely enclosed cockpit but this degraded the K-225's excellent handling qualities and extensive and costly engineering was required to make the helicopter suitable for service. This same problem weighed heavily on the design and production of the HOK-1. By 1953, Kaman had produced 29 HTK-1s. Because of the demands of the Korean War, many of these aircraft served the Navy as utility helicopters before settling into the training role they were designed for. However, the helicopter failed to generate much enthusiasm as a trainer because it flew too well. Docile handling characteristics could not prepare students for the demanding quirks common to helicopters equipped with tail rotors, and it was these types that they were likely to fly after graduation.
The K-225, and the two variants developed from it, would have faded into oblivion if several colleagues in the Navy had not challenged Charles Kaman to build and fly a jet turbine-powered helicopter. According to Kaman, the challenge was presented to him during a discussion he had in Washington, D. C., with friends in the Navy. Talk centered on the impact of jet turbine propulsion on fixed-wing aircraft, and the potential benefits of jet engines installed on the rotor tips of helicopters. Kaman believed that a helicopter turbine engine should drive the main rotors in a more conventional manner, via a transmission. Tip jet propulsion was also a technical challenge requiring considerable research and development before it could be ready for production. Few helicopter manufacturers could afford such costly and time-consuming development projects (one who did, Stanley Hiller, head of United Helicopters at Palo Alto, California, developed the Hiller Hornet, powered by ramjets mounted on the tips of the main rotor, see NASM collection).
To meet the challenge, Kaman selected the same Navy airframe, Bureau of Aeronautics number 125477, that William R. Murray had looped during the delivery flight to the Patuxent River test center in March 1950. He chose a gas turbine engine developed to power small ships, the Boeing 502-2 good for 175 hp, to replace the factory-installed 220 hp Lycoming O-435-A2 reciprocating piston engine. Installing the new engine was not difficult and the first turbine-powered flight of airframe 125477 took place on December 11, 1951. It was a spectacular success. The lower output from the new Boeing power plant was more than compensated for by its dramatically reduced weight, about half the mass of the Lycoming engine. The turbine also did not require the heavy auxiliary components (a cooling fan and clutch assemblies) necessary to adapt the gasoline engine to rotorcraft. The modified K-225 performed better with the new engine than ever before, especially at higher altitudes where decreased air density rapidly chokes piston engine performance. Turbines lose performance much more slowly as altitude increases.
The turbine revolution presaged by the K-225 progressed rapidly, and Kaman continued to play a lead role in the innovations. Three years after the turbine-powered K-225 flew, an HTK-1 became the first twin-engine, turbine helicopter to fly on March 26, 1954. In that helicopter, two Boeing engines that generated a total of 380 hp, replaced the original 240 hp reciprocating engine with no increase in weight. Despite Kaman's pioneering work, the first production helicopter fitted with turbine engines was the French-built Alouette II that flew in 1955. Kaman also developed the H-43 Husky, a variant in the HOK series descended directly from the K-225 and designed from the ground up for turbine power. It served in the U.S. Air Force under the H-43B designation and provided excellent service throughout the Vietnam War era as a rescue and fire-fighting aircraft.
After the Navy finished analyzing the turbine K-225, the aircraft was placed in storage at Bradley Field, Connecticut. In 1957, the Navy donated this historic rotorcraft to the Smithsonian Institution. Unfortunately, the original turbine engine was removed after testing and a newer YT50-BO-2 engine replaced it to maintain appearances. This type of engine had also powered the first twin-turbine helicopter, the HTK-1. It was simply an updated version of the original Boeing 502-2. In 1985, the National Air and Space Museum lent the K-225 to the Kaman Aerospace Corporation and the company restored the artifact for exhibit during the firm's 40th anniversary celebrations.
Since the 1960s, turbine engined helicopters have dominated all markets but they have not made rotorcraft powered by reciprocating piston engines extinct. During the 1990s, sales of civil piston helicopters rebounded and began to overtake the turbine designs. This dramatic reversal was largely due to the brilliant work of one man, Frank Robinson, whose Robinson R-22 and R-44 family of piston engine helicopters have ignited a minor revolution by offering civil and commercial customers light helicopters that are vastly cheaper to maintain and operate than turbine rotorcraft. Robinson helicopters are used for primary flight training, tourist operations, electronic news gathering, and law enforcement where operating costs can be less than half that of helicopters equipped with turbines. Overseas sales of Robinson helicopters have been very strong.
Helicopters with turbine engines will always be more expensive to buy, maintain and operate but their safety and performance benefits offset these higher costs and ensure that the turbine engine will probably remain the predominant power plant in rotary-winged aircraft for many more years.