XV-15 Tiltrotor Research Aircraft

The following titles link to pages describing suggested tiltrotor applications:

I. Introduction to the XV-15 tiltrotor research aircraft

Two of the most versatile aircrafts ever designed are being tested in a joint NASA-Army research program. The XV-15 Tiltrotor Research Aircraft combines standard aircraft cruise flight with vertical takeoff and landing (VTOL) and short takeoff and landing (STOL) capabilities.Two of these vehicles are now being tested. One aircraft is engaged in a flight research program at NASA-Ames Research Center in Mountain View, CA. The other aircraft is being operated by the Bell Helicopter Company in Arlington, Texas and is being used for tiltrotor development and military and civil demonstrations.

The unique feature about these aircrafts is the two large, three bladed proprotors mounted at the tips of the wings. For takeoff, the proprotors and their engines are rotated to the straight-up position where the lift developed is entirely propulsive. The XV-15 then climbs vertically into the air like a helicopter. In this helicopter mode (VTOL), the vehicle can lift off at its design mass of 6,000 kilograms and hover for approximately one hour.

Once off the ground, the XV-15 has the ability to fly in one of three different modes. It can fly as a helicopter, in the partially converted airplane mode. Also, the XV-15 can convert rapidly from the helicopter mode to the airplane mode. This is accomplished by continuous rotation of the proprotors from the helicopter rotor position to the conventional airplane propeller position. During the ten to fifteen second conversion period, the aircraft speed increases and lift is transferred from the rotors to the wing. Operating now as a conventional airplane, the XV-15 can cruise for more than two hours. To land, the proprotors are rotated up to the helicopter rotor position and flown as a helicopter to a vertical landing.

The ability of the XV-15 to rotate its proprotors and engines to different angles also makes it possible to operate as a short takeoff and landing aircraft. Proprotor tilt angles of 60 to 70 degrees produce lift from both the proprotors and wings. After rolling a short distance along the runway, the XV-15 is airborne. In this STOL mode heavier payloads can be lifted than in the VTOL mode.

The advantages of the tiltrotor concept are many. The ease with which the aircraft can be converted from one flight mode to another enhances its maneuverability and permits the aircraft to be configured to meet mission requirements. Also, the unique tiltrotor flight mode provides a flight envelope that overlaps the helicopter and airplane flight envelopes, increasing mission flexibility. Operating as a VTOL aircraft, it can take off like a helicopter and deliver payloads on half the amount of fuel consumes by a helicopter when traveling distances greater than 185 kilometers. Takeoff and landing terminals can be small, making tiltrotor aircraft ideal for intercity commuter travel. In the STOL mode, tiltrotor aircraft are ideal for long distance transport of heavy cargos into remote areas. Only short runways are necessary.

The XV-15 NASA/Army Tiltrotor Research Aircraft represents a unique technology for meeting the aeronautical needs of the future.

II. History of tiltrotor technology

Research into tiltrotor technology began in the 1940s. A commercial tiltrotor would be a direct descent of the XV-3, XV-15, and V-22.

XV-3. Built in 1953, this experimental aircraft flew until 1966, proving the fundamental soundness of the tiltrotor concept and gathering data about technical improvements needed for future designs.

XV-15. 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. The XV-15s have demonstrated excellent handling, low pilot workload, and good ride qualities; they continue to be used as experimental testbeds.

V-22. In 1981, using experience gained from the XV-3 and XV-15, Bell Helicopter Textron and Boeing Helicopters began developing the V-22 "Osprey", a twin-turboshaft military tiltrotor aircraft. Six flying full-scale development aircraft are to be built; four had flown at year-end 1990.

III. Background information on XV-15 and V-22

Tiltrotor aircraft combined features of helicopters and fixed-wing aircraft. They have the vertical takeoff and landing ability of the helicopter and the cruise speed, range, and fuel economy of fixed-wing aircraft.

Tiltrotors achieve this by the use of rotors that operate like helicopter rotors during takeoff and landing, then tilt to horizontal thrust to act like turboprop propellers during cruise.

The military V-22 Osprey tiltrotor, on which this study was based, is the result of more than 30 years of tiltrotor development, starting with the Bell XV-3. In 1977, two XV-15's were built as proof of concept prototypes.

Funded by NASA and the Army, the XV-15's have accumulated more than 800 hours of testing, and they continue to serve as testbeds to refine tiltrotor concepts, prove new components and systems, and demonstrate the controllability, performance, and community compatibility of tiltrotors.

The V-22 Osprey is now full scale development, with its first flight scheduled for mid-1988.

The V-22 Osprey has these features:

• articulating wingtip rotors and nacelles
• composite airframe
• advanced cockpit and avionics
• fly-by-wire control systems
• folding rotors and wings
• fixed-wing maintenance concepts

IV. Vehicle design guidelines for the Civil Tiltrotor

• V-22 derivative or technology base:
  • twin engines
  • composite airframe
  • tilting wingtip mounted rotors
  • fly-by-wire
  • advanced cockpit displays
• 600 nmi design range, vertical takeoff with one engine inoperative (OEI hover)
• 800 nmi design range with rolling takeoff from 750 ft filed (STOL)
• Commuter mission profile with FAR reserve fuel requirements
• All federal aviation regulations met for safety, including Category A operations
• Ramp self-sufficiency: airstairs, APU, powerback
• Helicopter NPRM for 30 sec emergency power rating assumed
• Pressurized fuselage
• Normal passenger accommodations and amenities:
  • Seating at 30 inch pitch
  • Lavatories and galley
  • Full cabin heating and air conditioning
  • Pressurization desired

V. SimLab's contributions to tiltrotor technology

Tiltrotor development

The development of the XV-15 tiltrotor research aircraft was initiated in 1973 with joint Army/ NASA funding as a "proof of concept", or "technology demonstrator" program, with two aircraft being built by Bell Helicopter Textron (BHT). Aircraft development, airworthiness testing, and the basic "proof of concept" testing were completed in September 1979. In October 1980, the Government took delivery of one aircraft (N703) for continued research testing, to include detailed documentation of the aircraft and further research on the impact of automatic flight control systems on the aircraft performance, stability and control, handling qualities, and aeroelastic stability characteristics. The other aircraft needed further development and participation in military applications demonstrations.

The original program was conceived as a minimum cost development through "proof-of-concept" testing. This philosophy limited efforts in systems development to baseline (proposed) systems designs, with changes permitted only if flight safety were an issue. This resulted in a conservative Automatic Flight Controls System (AFCS) design philosophy, which did not take full advantage of the unique, generally uncoupled, response characteristics of the tiltrotor concept.

Simulation work is needed

1. To better understand pilot workload and aircraft response to a one engine inoperative condition during takeoff, landing, and transition, and to better formulate proper power margins.
2. To develop plans to validate acceptable technical and operational criteria for civil tiltrotor transport service. CTR will require flight validation to show proper integration of the technologies of powered lift, vertical takeoff, and transition to fixed-wing operation.

SimLab's contributions to the tiltrotor technology

• Early 70's. Simulation evaluation and downselect to Bell for the XV-15 (FSAA)
• Mid 70's. Design and development prior to first flight (FSAA)
• Late 70's, early 80's. Continued XV-15 engineering development and beginning operational use evaluations (FSAA/ VMS)
• Early 80's. Joint Services Vertical (JVX) program leading to V-22 program (VMS)
• Mid 80's. Army LHX evaluations. (VMS)
• Late 80's. Tactical development (Marine Air Combat) of V-22. (VMS)
• 1989 --- Beginning of CTR series. (VMS)
• 1994. XV-15 return to flight -- flight test safety support (VMS)

VMS role in the civil tiltrotor development

Six primary civil tiltrotor simulation experiments have been conducted in the Ames Research Center Vertical Motion Simulator (VMS) since 1989. Following earlier efforts using a simulation model of the XV-15, the CTR-1 experiment began our investigations using a large, 40,000 pounds, tiltrotor model. The next pair of experiments explored issues of steep approaches, beginning the development of appropriate cockpit displays and guidance. The fourth experiment featured the development of terminal area operations suitable for current aircraft designs, with a particular concern for single engine failures. The fifth experiment more explicitly investigated one engine inoperative terminal area operations. This topic was further investigated with the Sikorsky Variable Diameter Tiltrotor concept. Finally the most recent NASA CTR experiment investigated more complex approach profiles intended for noise abatement.

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