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 tail-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-tail, 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 tail rotor
can’t.”
The VTDP also has fixed wings that work in concert with the tail
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 for 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 not 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 program.