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Extreme Machine

The U.S. Marine Corps' sword gets a brand-new edge.

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Only two hours or so after getting under way, the operation began to unravel. On the evening of April 24, 1980, eight U.S. Marine Corps Sikorsky RH-53D Sea Stallions departed the USS Nimitz for a point in the Iranian desert more than 500 miles away. At a remote location designated Desert One near the Iranian town of Tabas, they would meet their strike force and six C-130 transports, some carrying fuel for the helicopters' next leg. What happened next was the first in a series of mishaps that led to a nightmare.

Aboard each RH-53D was a system called the "blade inspection method," or BIM. Each rotor blade has a hollow spar containing pressurized nitrogen gas; the pressure is monitored by a gauge wired to a warning light. If a spar cracks, the gas leaks, and the BIM gauge warns the pilot that the rotor may fail. Now one of the pilots was staring at just such a warning light. Although others later second-guessed his decision, he put the Sea Stallion down and waited while another helicopter in the formation stopped to get him and his crew.

Then another RH-53 lost some of its instruments and turned back to the Nimitz. Now they were down to six helos, the minimum required to accomplish the mission. Unforecast dust storms scattered the flight, and when the helicopters arrived at Desert One, the ground commander of the operation, Colonel Charles Beckwith, noted (as he wrote in his book Delta Force) that they seemed to come in from all points of the compass. And they were over an hour late. As Beckwith ordered his force on to the helicopters for a flight to the outskirts of Tehran, he got a final piece of bad news: another RH-53 equipment failure, this time a hydraulic system. Beckwith aborted the mission, hoping to avert losses and get his people out. But in the darkness, a hovering RH-53 struck one of the C-130s, and Beckwith heard a muffled explosion. A fuel tank had ignited.

Eight people died before the rest could escape the carnage, and 52 Americans in the occupied embassy in Tehran spent another night with their captors. President Jimmy Carter faced the public on TV the next day and took personal responsibility for approving the plan to rescue the hostages. Perhaps it is just coincidence, but about a year later, the United States took the first steps toward developing an aircraft that would become the V-22 Osprey.

To this day, when the brass briefs on the V-22, they point out how the rescue in the Iranian desert might have gone if they'd had the right aircraft.

Because of the limitations of the RH-53's range and speed, they say, the rescue had to be performed in stages. Too many stages. The plan violated the KISS Rule: Keep It Simple, Stupid. Under cover of darkness, the helicopters were to meet the C-130s, refuel, board the troops, move to a hiding place (Desert Two), and wait through the daylight hours of the second day in concealment. At nightfall, the troops would move by truck to the embassy, execute the assault-and-rescue operation, meet the helicopters, reload, then fly to an airstrip to meet some C-141 jet transports, which would speed everyone to safety. Next slide, please.

Here's the same mission but with V-22s instead of RH-53s and C-130s. One: V-22s fly nonstop from Nimitz to assault staging area near embassy. Two: Team prepares for assault. Three: Team assaults embassy. Four: Team and hostages board V-22s and fly back to Nimitz. There may be glitches, but the V-22s' backup systems do their jobs. It's over in eight hours, and who knows? Maybe Jimmy Carter gets re-elected.

But in 1980 the concept of a tiltrotor airplane, or convertiplane, that could use twin rotors to lift off vertically and then rotate the rotors so they function as propellers while wings provide lift was nothing more than a 30-year-old idea. A number of similar machines had been developed for research, mainly to evaluate schemes for controlling such an unconventional machine.

The mid-1950s through mid-1960s saw hybrids like the British Fairey Rotodyne, which combined a traditional lift rotor and a wing with propellers for forward thrust. Bell's XV-3 and X-22A were tiltrotors, the latter with ducts around the blades, while Boeing's VZ-2 and the LTV-Hiller-Ryan XC-142 were tiltwings in which the entire wing, together with engines and propellers (four of them on the -142), rotated as a unit. All these machines were conceived because the helicopter, a machine designed to hover, has an irreparable flaw when it tries to fly forward at high speed.

Hovering in still air, the helicopter is happyeverything is balanced and symmetrical, with the rotor blades creating equal lift throughout the circle of their revolution. To move forward, the pilot pushes the cyclic control, or stick, toward the nose, which causes the swash plate in the rotor head to increase the pitch of the rotors when they are swinging through the aft portion of the circle. The effective lift of the rotor system tilts forward, and the helicopter gently begins to accelerate. As its airspeed increases, the air flowing over the rotor blades begins to create an imbalance. The rotor blade that is advancing into the relative wind encounters a net gain in lift due to higher speed over its airfoil. The retreating blade encounters a net loss, and this divergence eventually increases until the retreating blade can no longer create sufficient lift and the aircraft rolls off toward the retreating blade even if the pilot tries to counter the roll with the cyclic control. This phenomenon, retreating blade stall, limits the top speed of traditional helicopters like the RH-53.

About George Larson

George Larson served as editor of Air & Space from 1985 to 2005. He is currently an inactive pilot, but holds a commercial pilot's license, with instrument and multi-engine ratings. He is between airplanes at this time, but has owned or operated a Grumman American AA-5B Tiger and a Mooney 201. He has been writing about aviation since 1972, when he joined the staff of Flying Magazine.

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