Aviation firm aims to stir interest in compound helos.

Proponents of compound helicopters are hopeful that the Defense Department will consider including these aircraft in the mix of rotary-wing military platforms. Compound helicopters have been around for at least 35 years, but have not caught on for various reasons. Advocates now believe that the time is right to reintroduce this technology, as a low-cost alternative to buying new helicopters and as a way to improve the speed and range of current helicopters.

A compound is a modified conventional helicopter that has an auxiliary propulsion system to provide additional thrust--beyond the thrust already produced by the rotors. The extra thrust helps unload the rotor in forward flight and increase forward speeds.

The only compound helicopter concept currently funded by the Defense Department is called the Vectored Thrust Ducted Propeller (VTDP). This compound helicopter is a modified Sikorsky H-60 airframe. A technology demonstration for the VTDP is under way at the Naval Air Systems Command, with funding from the Office of Naval Research.

The VTDP has fixed wings and a rail-mounted ducted propeller. It was designed by Piasecki Aircraft, in Essington, Penn.

The company's director of business development, John Piasecki, describes the VTDP as "an evolutionary step for the helicopter."

The tail-mounted ducted propeller, called the ring-rail, provides auxiliary thrust and control in forward flight, Piasecki explained. While a conventional helicopter gets all its lift and propulsion out of the main rotor, he said, "this tail helps to unload the main rotor from its propulsion responsibilities, because it can provide forward thrust, whereas the conventional rail rotor cant.

The VTDP also has fixed wings that work in concert with the rail to unload the rotor during lift. "By unloading the rotor from its lift and propulsion responsibilities, you can delay the onset of retreating blade stall, which limits the speed of helicopters," said Piasecki. The wings can fold and can be removed for stowage.

According to a Navair spokesperson, "the Navy is evaluating the VTDP/compound concept as a technology demonstration." Piasecki Aircraft received a $26.1 million contract in October 2000 to design, modify and test the VTDP technology on a YSH-60F aircraft.

Program officials involved in the demonstration declined to be interviewed. Through the Navair spokesperson, a program official said, "Results on the potential for the technology will be forthcoming after we complete analysis of flight test results in early 2005." Asked about the Navy's interest in this technology, the official said, "Piasecki Aircraft Company has designed a vectored thrust ducted propeller, which they claim has the potential for increased efficiency."

The closest the Defense Department came to buying compound helicopters was in the 1960s, when Lockheed built the AH-56 Cheyenne, which won a U.S. Army 1966 competition fur an airborne fire support system. The company made 10 pre-production units and expected that the Army would ultimately order 375 aircraft. But the Army later decided to buy Cobra gunships and cancelled the Cheyenne procurement in 1969.

Piasecki said the Office of Naval Research had shown interest in the compound helicopter as an option for a Navy airborne mine-hunting platform.

To hunt mines, the helicopter has to tow a sensor. According to Piasecki, the extra weight, on many helicopters, can make the nose tip forward, "raising safety concerns." A compound, he said, "could provide more stability." But that capability has not yet been tested, he noted.

It appears, however, that the Navy now is not convinced that it needs a compound helicopter for the mine-hunting mission, said an official from the office of the defense secretary. He requested that he nor be quoted by name. "They decided that the current helo [the MH-60S] can do that job and don't need the [compound] capability," he told National Defense.

Nevertheless, he added, "there is a broader need for this technology, other than for countermine operations." The ability of the compound aircraft to fly long distances at speeds of 230 knots makes the VTDP technology a viable option for upgrading the Army's Black Hawk fleet, for example, said the official. "There should be a broader basis than just countermine." Most conventional helicopters fly no faster than 140 knots.

The OSD official stressed that even though the military services abandoned the compound helicopter decades ago, it is worth considering its reintroduction, because the Defense Department needs to upgrade aging aircraft and does not have enough money to replace every outdated helicopter with a brand-new one.

Joe Horn, a helicopter expert at Pennsylvania State University, said that the VTDP could provide a "low-cost alternative to the tilt-rotor." He noted that the additional weight of the compound creates problems in low-speed flying and hovering. "But it's viable for forward flight," when high speed is the top priority.

The Navair official noted that compound helicopters have "potential advantages in speed and range over conventional helicopters, but have disadvantages in hover, cost and complexity. A suitable application would be one in which the speed and range advantage outweighed the disadvantages.

Piasecki is hoping to interest the Army special operations forces in the compound helicopter, as an option to upgrade the MH-60 Black Hawk and the MH-47 Chinook.

The company, meanwhile, submitted a proposal to the U.S. Air Force to upgrade the H-60 for its combat search-and-rescue fleet. An analysis of alternatives by the Air Force, however, noted that the compound aircraft may not be ready on time to meet the desired schedule for the CSAR pro gram.