This Kaman K-225 was the first helicopter to fly with a gas turbine driven transmission. Turbines offered important advantages for helicopters including reduced weight, improved reliability, easier maintenance and higher power-to-weight ratios, which allowed for larger useful loads, increased safety and lower operating costs. In 1949, Kaman built the K-225 commercial model, primarily for use as a crop-duster. The Navy ordered two to evaluate the advantages of the intermeshing rotor system and the novel blade mounted servo-flap control system.

In 1951, Kaman replaced the reciprocating engine that originally powered this K-225 with a Boeing 502-2 gas turbine to demonstrate the potential of jet-powered helicopters to the Navy. The K-225 served as the prototype for Kaman's successful HOK series of military helicopters, which incorporated a cabin in place of the open cockpit. The engine currently on the aircraft is not original.

Kaman K-225

Modern helicopters have established themselves as an essential component in a variety of operations, some of which involve flight over treacherous terrain or enemy territory while carrying heavy loads of personnel, weapons, or equipment. The turbine engine with its high degree of reliability, and large shaft-horsepower ratings have made the helicopter safe enough to operate in environments in which piston engines would have difficulty in providing the necessary degree of safety, reliability, and performance. The Kaman K-225, the world's first helicopter with a turbine-driven transmission, represents the transition from piston engines that made a number of modern designs possible.

Charles H. Kaman began the Kaman Aircraft Company in late 1945 to develop an intermeshing twin-rotor helicopter, known as a synchropter, for the civilian market. Kaman felt that counter rotating rotors were more efficient than the tail rotor designs pioneered by Igor Sikorsky during World War Two. The advantage of Kaman's synchropter configuration was that the counter-rotating blades negated torque, which in turn meant that there was no need for a tail rotor that robbed engine power from the main rotors. Additionally, the design would save weight by doing away with most of the tail boom. The synchropter had originally been pioneered by in World War Two Germany by Anton Flettner, whose Fl 282 helicopter saw limited military service. Kaman's first two designs the K-125 and K-190 validated his concepts, and quickly surpassed the Fl 282 in performance. The K-190 was marketed as an open cockpit crop-duster and was able to demonstrate aerobatic maneuvers that were beyond the capability of most helicopters at that time.

The K-190 did not sell, but it served to drum up interest in Kaman's helicopters, while the company moved to create a version with more appeal to civil and military markets. This improved version began production in 1949 at Kaman's Bradley Field factory in Connecticut as the K-225. The decision was made to lease the K-225s coming off the assembly line for crop dusting, so that the operators of the helicopters could see its advantages, and would not be burdened by maintenance concerns and other difficulties that could result in dissatisfaction with the struggling company. The K-225 proved to be an excellent crop duster because the strong rotor downwash spread the insecticide more evenly and thoroughly than aircraft sprayed crops.

The airframe of the K-225 consisted of a very open steel tube structure with tandem seats in front of the masts supporting the side-by-side twin intermeshing rotors. The only concession to pilot protection from the elements was in the form of a small Plexiglas windscreen that provided marginal protection from the onrushing air during forward flight. A vertical stabilizer assisted in directional control during forward flight. Kaman's original decision to enter into the helicopter design business was based on the development of a servo-flap control system. The K-225 incorporated this system, which was integrated into each rotor blade with flap located 75% of the rotor radius outboard of the rotor hub. The servo-flaps were linked directly to the pilot's controls, instead of the traditional arrangement in which the control rods were linked to the rotor hubs. This novel control system drastically reduced control pressures, which on most helicopters up to this time were quite heavy, and very tiring for the pilot to contend with on long flights. However, an adjustable damper system had to be installed to give the pilot a realistic control feel. Tricycle landing gear was used, which was cushioned by oleo shock absorbers. The K-225 was one of the first helicopters to initially come equipped for night operations, which would not have been possible without its very open cockpit that allowed superb visibility, since instrumentation was basic, with no gyro instruments. One disadvantage of the synchropter configuration was that they were significantly slower than their single-rotor counterparts. In the case of the K-225, the never-exceed speed was only 62.6 knots (72 mph) which is over 25% less than several other helicopters from the same period.

Ultimately, the crop-dusting operators were unable to justify the higher operating costs of a helicopter over the use of fixed-wing aircraft, and Kaman had to seek other ways to market the K-225. Kaman managed to sell one to the government of Turkey, and another that was used for geological surveys in Mississippi. Military orders consisted of two for the Navy and one for the Coast Guard, all of which were used for evaluation purposes. While delivering one of the K-225s to the Navy at the Patuxent River Naval Air Test Center, company test pilot William R. Murray successfully performed the first intentional loop in a helicopter. However the open cockpit K-225 was not a suitable contender for a modern military aircraft. The company faced bankruptcy without any further buyers for the K-225, but Kaman was able to secure an order from the Navy for four prototypes of an improved model, the HOK-1, as a possible alternative to Sikorsky's S-52. However, Kaman could not afford to undertake the design of this new aircraft without an additional source of income, and after some difficult negotiations, he was able to convince the Navy that the company's survival was important for meeting the military's needs for the developing conflict in Korea. In September 1950 the Navy granted Kaman an order for the K-225 with an enclosed cockpit, which was to be known as the HTK-1 training helicopter. The cabin reduced the excellent handling qualities of the K-225, and considerably more engineering effort was required than had been originally anticipated. This same problem also caused considerable difficulties later during the design of the HOK-1. By 1953, Kaman had produced 29 HTK-1s. Due to the demands placed by the Korean War, many saw service as utility helicopters before settling down in their assigned role as trainers. The HTK-1 failed to generate much enthusiasm as a trainer because its docile handling characteristics did not adequately prepare its pupils for the quirks common to tail rotor equipped helicopters that they were likely to fly.

The K-225 would have faded into distant memory after the introduction of the HTK-1 and HOK-1, if it hadn't been for Kaman's response to a challenge put forth by some of his Navy colleagues to develop a gas turbine powered helicopter. Kaman quickly found a naval gas turbine engine, the Boeing 502-2, capable of a continuous 175 shaft horsepower to replace the 220 horsepower Lycoming O-435-A2 reciprocating engine that normally powered the K-225. For the experiment, Kaman used the same airframe that William R. Murray had looped during his Navy demonstration. The first flight of the K-225 with the Boeing engine took place on December 11, 1951, and was a dramatic success. Even though the new engine produced less power than the original reciprocating model, its weight was reduced by more than half. The turbine engine not only weighed less but it did not require a heavy cooling fan or clutch assemblies. As a result, the K-225 performed better with the new engine than it ever had before. This was especially true at higher altitudes where decreased air density creates large reductions in piston engine efficiency, while gas turbines have very little loss in performance with increases in altitude. While the French Sud Ouest SO-1120 "Ariel III" was the first helicopter utilize a turbine powerplant during its initial flight on April 18, 1951, its engine powered a compressor for the tip-jet drive. This unusual configuration allowed for a simpler design because torque compensation was not necessary, but the K-225's more efficient turbine-driven transmission became and an essential component on most modern helicopters.

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 first flew, an HTK-1 became the first twin engine turbine helicopter to fly. In that helicopter two Boeing engines, providing a total of 380 shaft horsepower, replaced its 240 horsepower reciprocating engine with no increase in weight. The first American production helicopter designed around a gas-turbine engine was the Bell XH-40, which flew in 1956. This helicopter went on to achieve considerable notoriety as the UH-1 Huey. Kaman's H-43, a further refinement of the HOK series, was provided with a turbine engine as standard equipment when it entered service with the U.S. Air Force as the H-43B. This direct descendent of the K-225 provided excellent service throughout the Vietnam era as a rescue and fire fighting aircraft.

Once the Navy had finished its analysis of the turbine-equipped K-225 it went into storage at Bradley Field, Connecticut. In 1957, the Navy donated the aircraft to the Smithsonian Institution. Unfortunately, the original turbine engine was removed after testing. A later YT50-BO-2 engine was added to the aircraft to give the appearance of the original. This engine was the type mounted on the HTK-1 when it was under going twin-turbine testing, and is simply a slightly updated version of the original. In 1985, the National Air and Space Museum loaned the K-225 to Kaman to restore it in preparation for exhibition during the company's 40th anniversary celebrations.

The addition of the turbine engine to the helicopter has not entirely done away with reciprocating power for rotary-winged aircraft. In recent years, civilian sector sales of piston helicopters have begun to overtake turbine designs. This is in large part due to the efforts of Frank Robinson whose R-22, and R-44 helicopters have firmly established a niche in the helicopter market by utilizing the cost advantages of the piston engine. These helicopters have considerable advantages in markets such as training and sightseeing where operating costs can be less than half that of turbine helicopters.

While purchase and operating costs have increased with the use of turbine engines, their safety and performance benefits ensure that the turbine engine will remain the predominant powerplant in rotary-winged aircraft. The Kaman K-225 took the development of helicopters to the next logical step, and provided an impetus for new helicopter development. Kaman abandoned the synchropter configuration after the H-43, and produced a conventional helicopter, the SH-2 Seasprite. However, Kaman returned to the synchropter with the development of the K-Max load-lifting helicopter, which entered production in the 1990s, and owes much of its development experience gained with the K-225.

Rotor Diameter:11.58 m (38 ft)

Length:6.83 m (22 ft 5 in)

Height:3.51 m (11 ft 6 in)

Weight:Empty, 816 kg (1,800 lb)

Gross, 1225 kg (2700 lb)

Engine:Reciprocating configuration, Lycoming O-435-A2, 220 h.p.

Turbine configuration, Boeing 502-2, 175 s.h.p.

Serial No.125477

References and Further Reading:

Kaman, Charles H. Kaman: Our Early Years. Indianapolis: Curtis Publishing Company, 1985.

Lambermont, Paul. Helicopters and Autogyros of the World. New York: A. S. Barnes

and Company, 1970.

K-225 curatorial file, Aeronautics Division, National Air and Space Museum

R. Connor