The Bell XV-15 was an American tiltrotor VTOL aircraft. It was the second
successful experimental tiltrotor aircraft and the first to demonstrate
the concept's high speed performance relative to conventional helicopters.
Development = Early VTOL rotor aircraft=
The idea of building Vertical Take-Off and Landing aircraft using
helicopter-like rotors at the wingtips originated in the 1930s. The first
design resembling modern tiltrotors was patented by George Lehberger in May
1930, but he did not further develop the concept. In World War II, a German
prototype called the Focke-Achgelis FA-269 was developed starting in 1942,
but it never flew. Two prototypes that made it to flight
were the one-seat Transcendental Model 1-G and two-seat Transcendental Model 2,
both powered by single reciprocating engines. Development started on the
Model 1-G in 1947, and it flew in 1954. The Model 1-G flew until a crash in
Chesapeake Bay on 20 July 1955, destroying the prototype aircraft but
not seriously injuring the pilot. The Transcendental 1-G is the first
tiltrotor aircraft to have flown, and it accomplished most of a
helicopter-to-aircraft transition in flight to within 10 degrees of true
horizontal aircraft flight. The Model 2 was developed and flew shortly
afterward, but it did not fly much beyond hover tests. The United States
Air Force withdrew funding in favor of the Bell XV-3.
The Bell XV-3 was first flown in 1955. Like its predecessors, the XV-3 had the
engines in the fuselage and drive shafts transferring power out to tilting
wingtip rotor assemblies. Other rotor concepts, such as slowed
rotors, stopped rotors, folding rotors, and variable-diameter rotors, were
investigated for subsequent designs, but Bell engineers Kenneth Wernicke and Bob
Lichten deemed them unfeasible compared to experience with the XV-3 project.
= Technological advances= One of the major problems with the early
tiltrotor aircraft designs was that the driveshafts carrying power from the
fuselage out to the wingtip rotors, along with the gearbox and tilting
mechanisms at the wingtips, had substantial loads placed upon them and
were heavy. They were transferring large amounts of power and torque long
distances for an aircraft power transmission system.
The XV-15 experimental aircraft introduced a major design concept
advance: instead of engines in the fuselage, the XV-15 moved the engines
out to the rotating wingtip pods, directly coupled to the rotors. The
normal path for power was directly from the engine into a speed-reduction
gearbox and into the rotor/propeller without any long shafts involved. There
was still a driveshaft along the wings for emergency use to transfer power to
the opposite rotor in case of engine failure, but that shaft did not normally
carry any power loads, making it lighter.
The tilting engine concept introduced complexities in the design of the
engines and engine pods to be able to shift from operating horizontally to
operating vertically. Those problems were addressed fairly early in the XV-15
program. In the late 1960s and early 1970s, NASA
and other researchers worked extensively on theoretical and wind tunnel tests of
various rotor pods. Two companies were involved in the research and proposing
designs: Bell Helicopter and Boeing-Vertol. The focus was on tilt
rotor pods, integration of the tilting rotors with the wings and fuselage of
the aircraft, and studying the airflow as the rotors tilted. Tilt rotors with
fixed rotors and with folding rotors were investigated.
= XV-15 project= What was to become the XV-15 program was
launched in 1971 at NASA Ames Research Center. After preliminary work, a
competition was held to award two $0.5 million research and development
contracts for prototype designs. Companies that responded included
Sikorsky Aircraft Corporation, Grumman Aircraft, Boeing Vertol, and Bell
Helicopter. R&D contracts were issued to Bell
Helicopter and Boeing-Vertol on 20 October 1972. The two companies' design
proposals were delivered on 22 January 1973.
Competing designs Boeing proposed a design, Model 222, in
which the engines were in fixed pods at the end of each wing, and a small,
rotating pod with the rotor was slightly closer to the fuselage on the wing. This
design simplified the engine design by keeping it horizontal at all times
without having very long driveshafts to the tilting rotors.
In the Bell design, Bell Model 301, the whole wingtip pod rotated between
horizontal and vertical, with the engine and rotor assembly fixed together within
the pod. This simplified the power transmission, but it had more
complicated requirements for the engine design and was probably slightly heavier
than the Boeing proposal. After a review of both proposals, NASA
selected the Bell 301 for further development, and a contract for further
R&D was issued on 31 July 1973. Extensive engineering and testing took
the next four years to complete the development of the aircraft. The first
of two Bell XV-15s, tail number N702NA, first flew on 3 May 1977. After minimal
flight tests at the Bell test facility, the aircraft was moved to Ames Research
Center in Mountain View, California, where it was then mounted in the large
Ames wind tunnel and tested extensively in various simulated flight
environments. For the U.S. Department of Defense
Joint-service Vertical take-off/landing Experimental aircraft program, Bell
Helicopter and Boeing Vertol teamed to submit a bid for an enlarged version of
the XV-15 in 1983. The Bell Boeing team received a preliminary design contract
that year, which led to the Bell Boeing V-22 Osprey.
Flight testing Following wind tunnel and flight testing
by Bell, the aircraft was then moved to NASA Dryden, which is at Edwards Air
Force Base in the California High Desert. The XV-15 flight testing
continued expanding its flight envelope. It was able to successfully operate in
both helicopter and normal aircraft flight modes and smoothly transition
between the two. Once the aircraft was considered sufficiently tested, it was
returned to Ames Research Center for further testing.
The XV-15s were deemed sufficiently tested, and one aircraft was taken to
the 1981 Paris Air Show for demonstration flights. The New York
Times praised its performance. "And if ever there was a lovable plane, it is
the Bell XV-15 ... The machine, the hit of the show, performed a series of
maneuvers including bowing to the crowd." Over the next year, Senator
Barry Goldwater, Navy Secretary John Lehman, and other officials were offered
guest co-piloting flights to promote tiltrotor technology for military
development. The XV-15s were a standard demonstration
in the annual summer airshow at the co-located Moffett Field Naval Air
Station for several years during the 1980s. Both XV-15s were flown actively
throughout the 1980s testing aerodynamics and tiltrotor applications
for civilian and military aircraft types that might follow, including the V-22
program. Fédération Aéronautique Internationale
classifies the XV-15 as a Rotodyne, and as such it holds the speed record of 456
kilometres per hour, and the 3 km and 6 km time-to-climb.
= Further testing= The first XV-15 prototype aircraft,
N702NA, was transferred back to Bell for company development and demonstration
use. On 20 August 1992, the aircraft crashed while being flown by a guest
test pilot. He was lifting off for a final hover when a bolt slipped out of
the collective control system on one pylon, causing that rotor to go to full
pitch. The aircraft rolled upside down out of control and crashed inverted.
While significantly damaged, the aircraft was largely structurally intact
and both the pilot and copilot escaped with only minor injuries from the crash.
The cockpit of the aircraft was salvaged and converted for use as a flight
simulator. The second XV-15 prototype, N703NA, was
used for tests to support the V-22 Osprey military tiltrotor program and
Bell/Agusta BA609 civilian medium tiltrotor transport aircraft. It
continued in primarily NASA test operations until September 2003. The
shortest take-off distance was achieved with the nacelles at 75 degrees angle.
After N703NA was retired from test operations, it was donated to the
Smithsonian National Air and Space Museum in Washington, D.C. The XV-15 was
flown cross-country from Fort Worth, Texas to the museum before being
decommissioned for display. It is now on display at the Steven F. Udvar-Hazy
Center at Washington Dulles International Airport.
Specifications Data from NASA SP-2000-4517
General characteristics Crew: Two
Length: 42 ft 1 in (12.83 m Wingspan: 57 ft 2 in with turning
rotors) Rotor diameter: 25 ft
Height: 12 ft 8 in Airfoil: NACA 64A015
Empty weight: 10,083 lb Max. takeoff weight: 13,000 lb
Powerplant: 2 × Avco Lycoming LTC1K-4K turboshaft engine, 1,550 shp each
Engine power ratings: 1,550 shp normal takeoff power
1,802 shp emergency power Fuel weight: 1,436 lb
Performance Maximum speed: 300 knots
Stall speed: 100 knots when in airplane mode
Range: 445 nmi Service ceiling: 29,500 ft
Disc loading: 13.2 lb/ft² Power loading: 1.35 kg/kW
Hovering altitude: 8,800 ft out of ground effect
See also Related development
Bell XV-3 Bell Boeing V-22 Osprey
AgustaWestland AW609 Aircraft of comparable role,
configuration and era Canadair CL-84
Sikorsky S-69 Sikorsky S-72
Related lists List of VTOL aircraft
References Notes
Bibliography External links
Image of XV-15 in hovering flight