A tiltrotor is an aircraft which generates lift and propulsion by way of
one or more powered rotors mounted on rotating engine pods or nacelles usually
at the ends of a fixed wing or an engine mounted in the fuselage with drive
shafts transferring power to rotor assemblies mounted on the wingtips. It
combines the vertical lift capability of a helicopter with the speed and range of
a conventional fixed-wing aircraft. For vertical flight, the rotors are angled
so the plane of rotation is horizontal, lifting the way a helicopter rotor does.
As the aircraft gains speed, the rotors are progressively tilted forward, with
the plane of rotation eventually becoming vertical. In this mode the wing
provides the lift, and the rotor provides thrust as a propeller. Since
the rotors can be configured to be more efficient for propulsion and it avoids a
helicopter's issues of retreating blade stall, the tiltrotor can achieve higher
speeds than helicopters. A tiltrotor aircraft differs from a
tiltwing in that only the rotor pivots rather than the entire wing. This method
trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.
History The idea of constructing 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, the Focke-Achgelis Fa 269,
was developed starting in 1942, but never flew.
Two prototypes which 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, though it did not fly
until 1954. The Model 1-G flew for about a year until a crash in Chesapeake Bay
on July 20, 1955, destroying the prototype aircraft but not seriously
injuring the pilot. The Model 2 was developed and flew shortly afterwards,
but the US Air Force withdrew funding in favor of the Bell XV-3 and it did not
fly much beyond hover tests. The Transcendental 1-G is the first
tiltrotor aircraft to have flown and accomplished most of a helicopter to
aircraft transition in flight. Built in 1953, the experimental Bell
XV-3 flew until 1966, proving the fundamental soundness of the tiltrotor
concept and gathering data about technical improvements needed for future
designs. A related technology development is the
tiltwing. Although two designs, the Canadair CL-84 Dynavert and the LTV
XC-142, were technical successes, neither entered production due to other
issues. Tiltrotors generally have better hover efficiency than tiltwings, but
less than helicopters. In 1968, Westland Aircraft displayed
their own designs—a small experimental craft and a 68-seater transport We
028—at the SBAC Farnborough Airshow. In 1972, with funding from NASA and the
U.S. Army, Bell Helicopter Textron started development of the XV-15, a
twin-engine tiltrotor research aircraft. Two aircraft were built to prove the
tiltrotor design and explore the operational flight envelope for military
and civil applications. In 1981, using experience gained from
the XV-3 and XV-15, Bell and Boeing Helicopters began developing the V-22
Osprey, a twin-turboshaft military tiltrotor aircraft for the U.S. Air
Force and the U.S. Marine Corps. Bell teamed with Boeing in developing a
commercial tiltrotor, but Boeing went out in 1998 and Agusta came in for the
Bell/Agusta BA609. This aircraft was redesignated as the AW609 following the
transfer of full ownership to AgustaWestland in 2011. Bell has also
developed a tiltrotor unmanned aerial vehicle, the TR918 Eagle Eye.
Russia has had a few tiltrotor projects, mostly unmanned such as the Mil Mi-30,
and has started another in 2015. Around 2005–2010, Bell and Boeing teamed
up again to perform a conceptual study of a larger Quad TiltRotor for the US
Army's Joint Heavy Lift program. The QTR is a larger, four rotor version of the
V-22 with two tandem wings sets of fixed wings and four tilting rotors.
In January 2013, the FAA defined US tiltrotor noise rules to comply with
ICAO rules. A noise certification will cost $588,000, same as for a large
helicopter. AgustaWestland says they have free-flown
a manned electric tiltrotor in 2013 called Project Zero, with its rotors
inside the wingspan. In 2013, Bell Helicopter CEO John
Garrison responded to Boeing's taking a different airframe partner for the US
Army's future lift requirements by indicating that Bell would take the lead
itself in developing the Bell V-280 Valor, with Lockheed Martin.
In 2014, the Clean Sky 2 program awarded AgustaWestland and its partners $328
million to develop a "next-generation civil tiltrotor" design for the offshore
market, with Critical Design Review near the end of 2016. The goals are tilting
wing sections, 11 tonnes Maximum takeoff weight, seating for 19 to 22 passengers,
first flight in 2021, a cruise speed of 300 knots, a top speed of 330 knots, a
ceiling of 25,000 feet, and a range of 500 nautical miles.
Technical considerations = Controls=
In vertical flight, the tiltrotor uses controls very similar to a twin or
tandem-rotor helicopter. Yaw is controlled by tilting its rotors in
opposite directions. Roll is provided through differential power or thrust.
Pitch is provided through rotor cyclic or nacelle tilt. Vertical motion is
controlled with conventional rotor blade pitch and either a conventional
helicopter collective control lever or a unique control similar to a fixed-wing
engine control called a thrust control lever.
= Speed and payload issues= The tiltrotor's advantage is
significantly greater speed than a helicopter. In a helicopter the maximum
forward speed is defined by the turn speed of the rotor; at some point the
helicopter will be moving forward at the same speed as the spinning of the
backwards-moving side of the rotor, so that side of the rotor sees zero or
negative airspeed, and begins to stall. This limits modern helicopters to cruise
speeds of about 150 knots / 277 km/h. However, with the tiltrotor this problem
is avoided, because the proprotors are perpendicular to the motion in the
high-speed portions of the flight regime, meaning that the tiltrotor has
relatively high maximum speed—over 300 knots / 560 km/h has been demonstrated
in the two types of tiltrotors flown so far, and cruise speeds of 250 knots /
460 km/h are achieved. This speed is achieved somewhat at the
expense of payload. As a result of this reduced payload, some estimate that a
tiltrotor does not exceed the transport efficiency of a helicopter, while others
conclude the opposite. Additionally, the tiltrotor propulsion system is more
complex than a conventional helicopter due to the large, articulated nacelles
and the added wing; however, the improved cruise efficiency and speed
improvement over helicopters is significant in certain uses. Speed and,
more importantly, the benefit to overall response time is the principal virtue
sought by the military forces that are using the tiltrotor. Tiltrotors are
inherently less noisy in forward flight than helicopters. This, combined with
their increased speed, is expected to improve their utility in populated areas
for commercial uses and reduce the threat of detection for military uses.
Tiltrotors, however, are typically as loud as equally sized helicopters in
hovering flight. Noise simulations for a 90-passenger tiltrotor indicate lower
cruise noise inside the cabin than a Bombardier Dash 8 airplane, although
low-frequency vibrations may be higher. Tiltrotors also provide substantially
greater cruise altitude capability than helicopters. Tiltrotors can easily reach
6,000 m / 20,000 ft or more whereas helicopters typically do not exceed
3,000 m / 10,000 ft altitude. This feature will mean that some uses that
have been commonly considered only for fixed-wing aircraft can now be supported
with tiltrotors without need of a runway. A drawback however is that a
tiltrotor suffers considerably reduced payload when taking off from high
altitude. Mono tiltrotor
A mono tiltrotor aircraft uses a tiltable rotating propeller, or coaxial
proprotor, for lift and propulsion. For vertical flight the proprotor is angled
to direct its thrust downwards, providing lift. In this mode of
operation the craft is essentially identical to a helicopter. As the craft
gains speed, the coaxial proprotor is slowly tilted forward, with the blades
eventually becoming perpendicular to the ground. In this mode the wing provides
the lift, and the wing's greater efficiency helps the tiltrotor achieve
its high speed. In this mode, the craft is essentially a turboprop aircraft.
A mono tiltrotor aircraft is different from a conventional tiltrotor in which
the proprotors are mounted to the wing tips, in that the coaxial proprotor is
mounted to the aircraft's fuselage. As a result of this structural efficiency, a
mono tiltrotor exceeds the transport efficiency of both a helicopter and a
conventional tiltrotor. One design study concluded that if the mono tiltrotor
could be technically realized, it would be half the size, one-third the weight,
and nearly twice as fast as a helicopter.
In vertical flight, the mono tiltrotor uses controls very similar to a coaxial
helicopter, such as the Kamov Ka-50. Yaw is controlled for instance by increasing
the lift on the upper proprotor while decreasing the lift on the lower
proprotor. Roll and pitch are provided through rotor cyclic. Vertical motion is
controlled with conventional rotor blade blade pitch.
List of tiltrotor aircraft AgustaWestland AW609
AgustaWestland Project Zero American Dynamics AD-150
Bell XV-3 Bell XV-15
Bell Eagle Eye Bell-Boeing V-22 Osprey
Curtiss-Wright X-19 Focke-Achgelis Fa 269
IAI Panther Bell V-280 Valor
See also Pitch drop-back
Tiltjet Tiltwing
Tailsitter VTOL
Thrust vectoring References
External links First motorized Tiltrotor
Tiltrotor and The Future - Independent 60 minute documentary video on
technological development of tiltrotor technology