Lockheed XH-51

infobox Aircraft
name = XH-51
type = Helicopter, compound helicopter
manufacturer = Lockheed



caption = "XH-51A following its conversion into a compound rotorcraft testbed"
designer =
first flight = 2 November 1962
introduced =
retired =
status = Retired
primary user = United States Army
more users = NASA
produced = 1962-64
number built = 3
unit cost =
developed from = Lockheed Model 86
variants with their own articles =

The Lockheed XH-51 was an experimental helicopter design by Lockheed utilizing a rigid rotor. Lockheed's design, "Model 186", was selected for a US military requirement for a high-speed, research helicopter.

History

The rigid rotor concept had been in use by Lockheed in an earlier helicopter, the CL-475, since 1949. [cite web|url=http://www.nasm.si.edu/aircraft/lockheed_cl475.htm|title=Lockheed CL-475|publisher=Smithsonian National Air & Space Museum|accessdate=2007-09-03] The choice of a rigid rotor meant that the helicopter was more agile than it would have been with a flapping rotor. The CL-475 competed against the Hughes 269 in the Army's early quest for a Light Observation Helicopter and Lockheed tested the commercial market waters without success. However, in February 1962, Lockheed's Model 186, a new design based on the CL-475's successful rigid rotor, was selected as the winner for a joint Army-Navy program to evaluate the rigid rotor for high-speed flight capability.

Two four-seat, three-bladed XH-51As were ordered and built for the program. Powered by the Pratt & Whitney Canada PT6B-9 (550-shp) turboshaft engine, the first helicopter first flew on 2 November 1962. As the aircraft began testing, the original three-bladed, rigid rotor system developed instability at higher speed ranges. Lockheed solved the problem by modifying the aircraft with a four-bladed rotor system. The second XH-51A was subsequently converted into a compound helicopter using stub wings (16.1 ft span), a 2,900 hp Pratt & Whitney J60-2 turbojet engine mounted on the left wing, and the same four-bladed rotor system to increase performance. The XH-51A Compound first flew on 10 April 1965,cite web|author=Michael J. Hirschberg|coauthors=David K. Daley|url=http://www.vtol.org/History.htm|title=US and Russian Helicopter Development In the 20th Century|date=|accessdate=2007-09-16] and on 29 November 1967 achieved a speed of 263.0 knots (302.6 mph, 486.9 km/h).cite book|last=Badrocke|first=Michael|coauthors=Bill Gunston|year=1998|title=Lockheed aircraft cutaways the history of Lockheed Martin|location=London|publisher=Osprey Aviation|id=ISBN 9781855327757]

In June 1964, NASA ordered a five-seat, three-bladed variant, the XH-51N (NASA 531) as a helicopter test vehicle.

Lockheed built two demonstrator aircraft, designated the Lockheed Model 286, to market to the public (registration numbers N286L and N265LC). These aircraft had the five-seat configuration of the XH-51N with the four-bladed rotor system of the XH-51A. The Model 286 was certificated for civil operation by the FAA on 30 June 1966 but Lockheed never sold any aircraft.

urvivors

The two XH-51A examples (Serial Numbers 61-51262 and 61-51263) are stored at Fort Rucker's Army Aviation Museum. [cite web|url=http://www.armyavnmuseum.org/museum/collection/rw4.html|title=Rotary Wing Collection|publisher= [http://www.armyavnmuseum.org United States Army Aviation Museum] ]

Variants

;XH-51A:four place, three-bladed rotor;XH-51A Compound:modified with a four-bladed rotor and stub wings and an auxiliary 2,900 hp Pratt & Whitney J60-2 engine.;XH-51N:five place, three-bladed rotor for NASA test purposes.

pecifications (XH-51A)

aircraft specifications
plane or copter?=copter
jet or prop?=prop
ref= Janes's All The World's Aircraft 1969-70 cite book |last= Taylor|first=J.W.R. (ed.)|editor= |title= Jane's All The World's Aircraft 1969-70 |year=1969|publisher= Jane's|location=London |isbn= |pages= 359-360 ]
crew=
capacity=
length main= 40 ft 9 in
length alt= 12.40 m
span main= 35 ft 0 in
span alt= 10.67 m
height main= 8 ft 2½ in
height alt= 2.50 m
area main= 962 ft²
area alt= 89.4 m²
airfoil=
empty weight main= 2,790 lb
empty weight alt= 1,265 kg
loaded weight main=
loaded weight alt=
useful load main=
useful load alt=
max takeoff weight main= 4,100 lb
max takeoff weight alt= 1,864 kg
more general=
engine (prop)=Pratt and Whitney PT-6B-9
type of prop=turboshaft
number of props=1
power main= 550 hp
power alt= 410 kW
power original=
max speed main= 151 knots
max speed alt= 174 mph, 280 km/h
cruise speed main= 139 knots
cruise speed alt= 160mph, 257 km/h
never exceed speed main=
never exceed speed alt=
stall speed main=
stall speed alt=
range main= 226 NM
range alt= 260 mi, 418 km
ferry range main=
ferry range alt=
ferry range more=
ceiling main= 16,000 ft
ceiling alt= 4,876 m
ceiling more= (hover ceiling (in ground effect))
climb rate main= 2,000 ft/min
climb rate alt= 10 m/s
loading main= 4.26 lb/ft²
loading alt= 20.9 kg/m²
thrust/weight=
power/mass main= 0.27 hp/lb
power/mass alt= 0.44 kW/kg
more performance=
armament=
guns=
bombs=
rockets=
missiles=
hardpoints=
hardpoint capacity=
avionics=

ee also

aircontent
related=
* AH-56 Cheyenne
similar aircraft=
lists=
* List of helicopters

References

External links

* [http://www.archive.org/details/1965-05-27_Space_Policeman XH-51A newsreel at archive.org]