By Nathan Pfau, Army Flier Staff WriterMarch 29, 2018
FORT RUCKER, Ala. -- When it comes to Aviation, higher and faster is typically the name of the game, and one aircraft was trying to make its mark by flying faster than any rotor-wing aircraft at the time.
In 1962, the Lockheed Aircraft Company came out with the XH-51A, which was the second in a series of rotor-wing aircraft and was an evolution of the CL-475 aircraft, which utilized a revolutionary rigid-rotor system, according to Bob Mitchell, U.S. Army Aviation Museum curator.
Although the XH-51's looks aren't very striking, the helicopter was able to achieve speeds that few rotor-wing aircraft could attain -- close to 300 knots -- with average speeds of helicopters at the time reaching 80-90 knots, said Mitchell.
"Once Lockheed had proofed the concept of the rigid rotor with the CL-475, they wanted to develop a second vehicle with that same rotor design that could achieve the speed that they were looking for -- speeds upwards in ranges of 300 knots," he said. "They came out with the XH-51, which was essentially a conventional helicopter that was fully aluminum in design and had a turbo-shaft engine that drove a three-bladed rotor system."
Initial tests of the three-bladed rotor system found that there was a considerable amount of vibration coming from the system itself, so the company decided to utilize a four-bladed system instead, which reduced the amount of vibration and stabilized the aircraft, said the museum curator.
By utilizing an extra rotor, coupled with the rigid-rotor design, the aircraft was able to achieve speeds not possible with previous rotor designs, such as the under-slung rotor system that the Hueys of the time were using, that would essentially cap the top speed of the aircraft due to a phenomenon called the dissymmetry of lift, said Mitchell.
"If you look at the rotor system on the Huey, which uses a two-bladed, under-slung rotor system, that system is only able to produce 'X' amount of lift, and when you try to go even faster, what you get into is a situation called retreating blade stall," said the curator. "If you look at the rotor system turning, the advancing half of the rotor system, which is moving forward, is producing more lift than the retreating blade that is going backward. That retreating blade has to flap down and take a bigger bite of air to compensate for what we call the dissymmetry of lift."
Mitchell said that at a certain point, the rotors can no longer compensate for the dissymmetry and the aircraft will begin to buffet violently.
"(To compensate), on the XH-51, they added not only one, but two more blades, so we now have a system that is producing a lot more lift," said Mitchell. "The dissymmetry of lift still exists, but (the system) capable of producing more without the buffeting and the vibration. The fewer the moving parts, the less vibration (in the system), so we were able to get a lot more speed out of it."
With the new design, during testing, the aircraft was able to achieve a top speed of nearly 270 knots, one of the fastest speeds for a rotor-wing aircraft during that time, said the curator, adding that the XH-51 never was able to achieve the desired speed of 300 knots, despite its impressive performance.
Although a successful aircraft, the issues with the XH-51 were less about performance and more about perception, said Mitchell.
"The problem with this aircraft was that it was such a radical design that people just didn't trust it," he said. "This thing came out and (the military) was looking at (other aircraft), so were already committed, so where there could have been a market for this, it was eclipsed by other aircraft that were coming down the line."
The XH-51's evolution didn't stop with the A-model, however, and eventually evolved into the XH-51 Compound, which added stub wings and a J60-2 engine, which then ultimately paved the way for the AH-56 Cheyenne attack helicopter.
The current model XH-51A that sits in the museum's collection is a three-bladed system, which Mitchell says is a rare variation of the aircraft, and just returned from a 10-month restoration. Although the aircraft has returned to the museum's floor, the restoration isn't yet complete.
The restoration process included buffing out all of the glass, cleaning out rusted parts and repainting the entire aircraft. What remains is re-fabrication of the seats, populating the instrument panels and putting the rotor system back on the aircraft. "It's got some work left, but we're almost there," said Mitchell.