Early helicopter engineers were looking for ways to increase power available and decrease weight. Placing the propulsion system on the tips of the rotor blades eliminated the need for a power consuming anti-torque rotor (tail rotor) and a heavy transmission. Although, propellers mounted on the blade were tried it was the ramjet developed during World War II that launched a major effort to build a successful rotor tip driven helicopter.
In 1946, McDonnell developed a single-seat, 285 pound (empty) helicopter for the U.S. Air Force. Called Little Henry, it used two ramjets (one on each blade tip) producing about 10 pounds of thrust each. A fuselage mounted tank supplies fuel to each engine through a rotating seal in the rotor hub. After a long flight test program, the Air Force decided not to purchase the helicopter.
Hiller Aircraft also developed a ramjet powered helicopter. The XHJ-1 Hornet’s larger engines developed 40 pounds of thrust each. Flight test began in 1950 and the U.S. Military expressed interest provided the helicopter and its engines received FAA certification. Hiller worked hard to overcome several design challenges like excessive centrifugal loads acting on the engines and high fuel consumption. Ramjets are also noisy and need a substantial airflow to start, requiring a small ground engine to spin the rotor system. Although Hiller eventually received certification for the engines, the FAA would not sign off on the high level of drag caused by the engines during autorotation. Without the Military, Hiller ended the project.
A Dutch company solved the drag issue during autorotation with a larger high inertia rotor system. The company’s NHI H-3 Kolibrie (Hummingbird) received certification in 1958.