Spycraft - Melton [139]
“First we slowed the tip speed down on the main rotor,” said Jack Knight, the TSD officer who headed the project. “That required we change the rotor, so we made a five-blade version rather than four to get the same lift pattern. We could move the same amount of air at a lower RPM and maintain the same lift. We also changed out the tail rotor, going from two blades to four.”
The engine noise presented a different set of problems. An initial attempt with a muffler failed when a contractor designed one weighing nearly 400 pounds, far too heavy for the OH-6. However, Knight had heard that a commercial aircraft manufacturer was running a program to quiet its jet aircraft and paid the company a visit. “There was a guy working on a ‘quieting program’ for a long haul passenger plane and we went to talk to him,” said Knight. “We asked to borrow him for a few months, but the company had other plans for him. So that was a disappointment. But on the way out the door, he handed me a business card with his home number written on the back. I called that night and he said he’d work on the project during his off hours. He eventually designed an engine quieting system that weighed about thirty pounds, a perfect fit for us.”
What the engineer did, explained Knight, was identify the sound frequencies coming from the engine making the most noise and attacked them by creating a series of sophisticated acoustic chambers. Just as high-end audio speakers are designed internally to acoustically enhance certain frequencies, the engineer’s design performed the opposite function, trapping the sound waves in a carefully constructed series of baffles.
With the rotors and engine silenced—or at least quieted—Knight and his team next targeted the noises coming from the chopper’s other moving parts. First, the transmission was quieted, and then they turned their attention to the converters (small generators) that provided auxiliary power, which were found to be extremely noisy. Their studies eventually led to converting the OH-6 over to solid-state electronics that required less power and smaller, quieter generators. “Then all of a sudden we found a real noisy valve in the fuel control system,” remembered Knight. “You never hear it in a normal helicopter, but it was a screamer. I took it back to the manufacturer and told them to quiet it down. They looked at me like I was a nut case. But they did it using silicone insets to cushion the moving parts.”
From start to finish, the project was completed and delivered in less than two months. The result was an OH-6, operating in “quiet mode,” that could not be heard on the ground as it passed over at 500 feet. Flying at the optimum “quiet” speed of 85 knots, the helicopter was less fuel-efficient, while higher speeds increased the noise but improved fuel efficiency.
Richard Helms, the DCI at the time, followed the progress of the silenced helicopter with great interest. He called Knight in for personal briefings on the project as it moved through various stages, the conversations frequently focusing on the difference between “quiet” and “silent.” One day, Lawrence Houston, Helm’s senior lawyer, called Knight. “I want to go to California and hear this thing,” Houston stated.
Knight obliged, taking Houston out to the Culver City Airport late at night to stand in the center of a darkened runway. Knight ordered a fly-by of a standard OH-6, which was audible from one end of the runway to the other. Houston and Knight stood on the runway tarmac as the sound faded and then vanished altogether. After a few minutes, Houston asked when the quiet helicopter would be coming. “It just did,” Knight replied, and then radioed the pilot to make another pass and illuminate the helicopter when overhead.
“That sonofabitch is quiet!” Houston exclaimed. His report to Helms settled