Ramjet Helicopters

After I posted that old newsreel footage from 1957 earlier, I decided to look up ramjet-powered helicopters, seeing if there was any information out there. Not only is there information, there is a surprise: the Hiller company actually produced an operational ramjet helicopter designated the HOE-1 Hornet and the US military bought and operated a number of them. 

Hiller and a team consisting of Robert Wagner, chief engineer, Elbert Sargent, chief of propulsion, Harvey Holm, project engineer, and hanger supervisor Edward Bennett, began to develop a small, simple helicopter powered by ramjets that would be easy to fly and maintain, and ground-transportable. The design used no heavy components such as tail rotor assemblies, drive shafts, main rotor clutches, transmissions, or engine cooling blowers. In 1948, he started building the HJ-1 Hornet single-seat sport helicopter. The most difficult challenge was to engineer the ramjet engines. Virtually no one had attempted to put these devices to practical use, but they appeared to be far easier to incorporate into an aircraft than jet turbine engines. The ramjet consists of little more than fuel nozzles and an ignition device mounted inside a metal pipe internally shaped to help pressurize high-speed airflow. When air flows through the pipe with sufficient velocity, both pressure and temperature increase. Introduce fuel and ignite it and the ramjet becomes the simplest jet engine. Both the ramjet and the jet turbine engine produce thrust by generating hot gases but the ramjet has no moving parts and is considerably lighter and more reliable. Unlike the turbine, the ramjet must be accelerated to a comparatively high-speed before it can begin to operate. The ramjet also consumes much more fuel than the turbine engine.

Hiller experimented with a version of the ramjet engine called the pulse-jet but quickly discarded this approach in favor of the pure ramjet. During World War II, the German's used pulse-jet engines to power V-1 robot cruise missiles (see NASM collection). In 1949, Hiller introduced an improved ramjet engine that weighed only 5 kg (11 lb) and produced about 14 kg (31 lb) of thrust. He installed two of these motors on each blade tip of the HJ-1 main rotor. At maximum operational speed, the ramjets moved through the air at 207 m/sec (680 ft/sec) and the rotor turned at 550 rpm. The two ramjets produced a total of approximately 27 kg (59 lb.) of thrust, a miniscule amount but more than adequate to rotate the two engines and the small rotor. The rotor freewheeled and generated no torque so a tail rotor was not necessary. Hiller used a tiny 1 hp gasoline engine to turn the rotor fast enough (50 rpm) to generate the high-speed airflow required to start the ramjets. The ramjets were not at all particular about their fuel. They would operate satisfactorily on gasoline, kerosene, even fuel oil and the less-volatile fuels reduced the dangers of explosion and fire. A "flame-holder" inside each ramjet ignited the fuel and ensured re-ignition if the engine flamed out.

Hiller referred to the first single-seat HJ-1 as the utility model. It consisted of an open steel and aluminum tube frame left exposed to facilitate the tweaks and component adjustments that most prototypes frequently require. Although additional yaw control was theoretically unnecessary because the main rotor did not generate torque, in practice the fuselage wandered from side to side, particularly at low speeds and a rudder was installed to beef-up directional control. This design first flew in 1950. The next HJ-1 featured side-by-side seating for the pilot and a passenger inside a cozy cabin made of fiberglass. This marked one of the first applications of this new composite material in the aviation industry. Hiller used this aircraft to launch a marketing campaign for the ramjet helicopter and to fly certification tests for the Civil Aeronautics Authority.

There were still others developed and tested and the French produced one called the Djinn. I've never been all that big a helicopter buff, so all this was news to me. So there's today totally useless factoid and historical oddity.

Incidentally, given today's mania for political correctness, would the HOE-1 name be allowed? I'm betting not.

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One Response to Ramjet Helicopters

  1. NortonPete says:

    Not a useless factoid rather food for thought. The trade off in torque from driving the rotor is increased centripetal force from the weight out at the end of the blade. I’m not an engineer so I don’t know how much stress that would create long term.
    F=mv2/r.
    There are many great aviation designs abandoned in prototypes. Some simply because of a lack of engineering or materials.

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