In Sept. 2014, the Central Intelligence Agency declassified a number of stories from their in-house publication Studies In Intelligence. While many are about intelligence gathering, a few break out of that mold. Rotor-heads will enjoy a great description of the agency’s efforts to develop a quieter helicopter.
The program began in 1970, and was run by the CIA’s Technical Services Division. The Hughes OH-6 was selected as the test bed for the project, based upon previous work by ARPA. The aircraft was named the Cayuse by the military but for the average grunt, it was the “Loach.”
Gear-Heads and Spies
The first issue that the engineers faced was reducing the noise produced by the engine. Drawing boards were warmed up and a five-blade rotor was designed to replace the original four-blade. This allowed the aircraft to operate with a lower engine RPM yet still achieve desired lift.
The gear boxes on the copter were modified by the manufacturer. Silicone inserts were added in the main ring gear. This both reduced noise and also improved the gear box’s ability to handle the increased horsepower from a modified engine.
General Motors produced the Allison C-18 engine for the Loach. It was designed for 317 horsepower but was derated to 250 hp for the copter. GM was extremely concerned about any engine modifications but eventually assigned a troubleshooter to the team at the CIA. He suggested a third party vendor that he knew was rebuilding C-18s and increasing their power. By selecting the top rated engines over time, and replacing the old with the new rebuilds, the team assembled six engines with significant increases in power.
As major noise sources were muffled, others that would not normally be heard came to the fore. Bendix designed and replaced the relief valve in the fuel supply system. Solid state parts would replace rotating inverters. The team was even successful in obtaining an exhaust muffler. One Lockheed engineer, working at home on his own time, designed the item.
An auxiliary fuel system was installed, based upon the foam filled tanks used in automobile racing. A “quick dump” feature was added to allow the copter pilot to reduce the aircraft’s weight quickly enough to permit auto rotation in the event of an emergency.
The ‘Quiet’ Loach
The “quiet” Loach was at its quietest at a speed of 85 knots. At 110 knots, the aircraft achieved its best fuel per mile usage. Mission planning would be key to ensuring that the copter was as quiet as possible throughout the length of the tasking.
The aircraft also benefited from the development of the first high performance Forward Looking Infra-Red system. Two young engineers at Hughes took some ideas and made them happen in less than 60 days. The CIA piece called this product a “ten-year jump in the technology”, and “instantly rendered just about every other idea or system of this type obsolete.”
The article continues with details of the testing program and a compilation of technological and intelligence results. Very little of the piece has been redacted and it is an enjoyable read.
