By Nathan Pfau, Army Flier Staff WriterMay 17, 2018
FORT RUCKER, Ala. -- Today's modern helicopter fleet is a testament to the evolution of the aircraft that came before, and one helicopter proved that experimentation is a necessary process in that evolution.
The Lockheed XH-51 Compound helicopter was the next step in a series of aircraft that helped to shape today's U.S. Army Aviation attack helicopter fleet, according to Bob Mitchell, U.S. Army Aviation Museum curator.
An evolution of the XH-51A aircraft, the XH-51 Compound was the next step in achieving greater speed in rotor-wing aircraft and did so by not only utilizing a four-bladed, rigid-rotor system, but by adding stub wings with an auxiliary 2,900-poundforce J60-2 engine mounted on one side for added thrust.
"This was just to get it past the speed range they wanted, so they could prove that the rigid-rotor system would in fact work at extreme speeds," said the museum curator.
With the added thruster, the aircraft first flew in 1965 and by 1967 it had reached a speed of 302 miles per hour during a shallow descent, a record for rotor-wing flight that stood for decades, said Mitchell. The aircraft was also able to achieve a top level-flight speed of 257 mph, with a cruising speed of 160 mph.
Comparatively, today's AH-64 Apache helicopters have a top speed of around 182 mph.
Being able to reach these kinds of speeds in a helicopter was a marvel, said the curator, considering rotor-wing aircraft speeds are limited due to dissymmetry of lift, which occurs when the retreating blade can no longer achieve lift due to the aircrafts forward speed.
"The rigid-rotor system was gyro-stabilized, so it was better able to accommodate and handle that difference in lift," said Mitchell, adding that the additional rotor blades also helped to alleviate the phenomenon. "But you eventually get to a point where you can no longer compensate and it goes too fast, and the aircraft will just roll."
The museum currently boasts in its collection the only XH-51 Compound in the world, which is undergoing restoration. The aircraft was, unfortunately, neglected and mishandled over the years, requiring a substantial amount of work, said the curator.
Despite the amount of work involved, the restoration so far has been a success, according to David Williams, senior project manager for the restoration.
"We've been working on this for about a month and a half so far, and at least another couple of weeks to get it wrapped up," said Williams. "We're knocking out the dents, replacing some panels and doing some cover patches to some damage that was done to it," but the saving grace is that the aircraft was, for the most part, intact, so no parts other than some glass panels had to be fabricated, he added.
It's with restoration projects like the one on the XH-51 Compound that will allow future generations of Aviators to be able to enjoy the artifacts from the past and learn from them, said Mitchell.
The XH-51 Compound was an experimental aircraft, it existed for the sole purpose of proving that the rigid-rotor system was viable at high speeds, said the curator, and once it was proven, the next evolution of the aircraft began -- the AH-56 Cheyenne attack helicopter.
Although the Cheyenne program was eventually dropped, it was an important stepping stone in developing today's attack helicopter fleet, Mitchell said.