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Which Do You Tilt: Wing or Rotor?

By Ray Prouty | August 27, 2017
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Raymond Prouty

R.W. Prouty

As part of our celebration of R&WI's 50th anniversary this year, we are reprinting selected columns by rotorcraft aerodynamicist Ray Prouty, perhaps our most popular writer, who died in 2014. In this early 1990s column (slightly edited and updated), Ray reviews the genesis of tiltrotor and tiltwing designs and the pros and cons of each – an appropriate topic as new designs of those types prepare to take to the sky.

If you have a favorite Ray Prouty column that you would like to see again or thoughts to share on Ray's musings, let us know.

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Tiltrotor or tiltwing? It's a good question to ponder. Both represent an unusual type of aircraft. Each uses different, and innovative, engineering to achieve vertical-to-horizontal-and-back flight.

Engineering innovation plays an important role in the development of any new class of aircraft. As an industry matures, however the accumulated experiences of successes and failures tend to make innovation less and less evident. As proof of this, notice that today almost all automobiles or jet transports look about the same.

The high-speed rotorcraft industry is still immature, and so design is of not yet achieved a consensus as to the best configuration. We are still proposing different ways of achieving the same goal, as is evident in new tiltrotor and tiltwing designs [such as Bell Helicopter’s V-280 for the U.S.’ Joint Multi-Role Technology Demonstrator (JMR-TD) and Aurora Flight Sciences/Defense Advanced Research Projects Agency XV-24A LightningStrike].

The two basic requirements for any aircraft to safely take off and land vertically are: the vertical lifting force must be equal to the aircraft weight and the aircraft must be controllable in all flight conditions. If the aircraft also is required to have a high-speed capability, the designer must find some solution other than the conventional helicopter rotor, which runs into a speed barrier at about 200 knots. This challenge has inspired a number of inventors for nearly 75 years.

Boeing engineer John Schneider, in a 1983 survey of vertical-take off aircraft with high-speed capabilities, identified a rather prodigious number of prototypes that had flown or at least reached the hardware stage. They included 21 with rotors, 12 with propellers, 11 with ducted fans and 18 with jet engines.

The tiltrotor was the brainchild of Mario Guerierri, who got together with Bob Lichten (then working at Kellett Aircraft) to form Transcendental Aircraft in the early 1950s. They built and flew the Model 1-G, but after it crashed due to dynamic routers/wing problems, Lichten went to Bell, where he headed to the XV-3 tiltrotor project.

Another Kellett designer, Bill Cobey, bought out Guerierri’s interests, rebuilt the Model 1-G as the Transcendental Model 2 and flew it briefly in 1956.

At Bell, Lichten’s XV-3 went through a long development phase that culminated in full conversion to the airplane mode and eventual flights testing by the U.S. Air Force. This proved that a piston-engine helicopter could indeed be converted into a moderately fast airplane, but the price in extra weight and cost was high.

This did not completely discourage the enthusiasts at Bell. Seeing the benefit of the newly developed turbine engine for their purposes, they went on to develop the XV-15 and with it established the experience base necessary to go on to the V-22 Osprey [and the V-280].

Bell Helicopter V280 Valor

Photo courtesy of Bell Helicopter

Other companies in the exciting 1950s and 1960s had not neglected the challenge. Vertol (now Boeing Rotorcraft Systems) built the VZ-2 tiltwing testbed, which made the world’s first complete transition from hovering to forward flight 1958. (Within just a few years, there were a total of nine military-sponsored "VZ-X” contracts for small testbeds with vertical-takeoff capability.)

Vertol also carried out investigations of tiltrotors with both analysis and wind tunnel tests. This work, along with a contract to design and build composite rotor blades for the Bell XV-15, gave Boeing the background necessary to become a partner with Bell on the V-22.

Curtiss-Wright built to tiltpropeller aircraft, the twin-propeller X-100 and then the quad-propeller X-19A. Another manufacturer, Hiller, built and flew the X-18, a tiltwing modification of the Chase C-122 transport airplane. Hiller then cooperated with Vought on the much larger, four-engine XC-142A. In Canada, Canadair built and demonstrated two of its CL-84 Dynavert tiltwing aircraft.

Bell Aircraft, in upstate New York, in competition with its Texas cousin, used four big, swiveling, ducted fans on its X-22A. In forward flight, the ducts became the wings. In the hands of the Cornell Aeronautical Laboratory, this aircraft with a programmable autopilot became a valuable variable-stability research aircraft for investigating flying qualities. Also using the same technique, Ed Doak, in California, flew his VZ-4 with two ducted fans mounted on the ends of a long wing.

In the 1990s, a European consortium worked on the European Future Advanced Rotorcraft (Eurofar) tiltrotor and Japan's Ishida Group proposed the TW-68 tiltwing project.

Many of these programs were moderately successful, but none resulted in production contracts. Perhaps the time was not right.

The tiltrotor program we know best is the Bell Boeing V-22. [It has proven itself in U.S. Marine Corps and Air Force Special Operations Command combat and contingency operations around the world for more than 10 years.]

Proponents of the tiltrotor and the tiltwing have long had disagreements about the relative advantages of their particular configurations.

Historically, tiltwing aircraft with propellers have had higher disk loadings than the tiltrotor, although there is no law that says that this has to be so… . Except, argue the tiltwing proponents, one of the advantages of their configuration is that it's when produce is essentially no down load and hover, since it is always aligned with the downwash from the propellers. On the other hand, the fixed wing [of a tiltrotor] creates a substantial down load and thus requires extra rotor thrust and power. This motivated engineers on both the XV-15 and V-22 projects to investigate ways of decreasing this penalty.

As a counterargument, tiltrotor proponents will tell you that a tiltrotor's entire wing/engine/propeller assembly must be mounted on a structurally inefficient hinge – with an expected weight penalty over that of the mechanisms just required to tilt the engine/rotor assemblies [or the rotors alone, in the case of the XV-3, the proposed Eurofar and Bell’s V-280 design].

It should be recognized, however, that the military V-22 has almost the same weight penalty, since the wing assembly is mounted on a vertical pivot to allow it to be aligned with the fuselage for storage onboard ships. This would not be a feature of a civil tiltrotor.

Another argument of the tiltrotor enthusiasts is that by using the rotors for longitudinal control, their configuration does not suffer the power penalty and mechanical complexity of the horizontal ducted fan considered for the TW-68 design.

They also point out that obtaining directional control with differential longitudinal flapping is a well-established method identical to that used on tandem-rotor helicopters, whereas the tilt wings use of differential flaps in the propeller slipstream close to the ground is relatively unproven. [Aurora’s XV-24A design proposes to use propulsion units distributed throughout the span of its main wing and canard.]

In conclusion, it will be sometime before the high-speed rotorcraft industry is mature enough to converge on one mutually accepted configuration. In the meantime, designers are not restricted in their use of innovation. [This is evident by Sikorsky-Boeing’s SB>1 coaxial rigid main rotor and aft propulsor configuration for JMR-TD.]

 

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