Air University Review, March-April 1982

V/Stols

a myth or a promise

Vertical/short takeoff and landing (V/STOL) aircraft have interested the military for several decades. While the classical vertical takeoff and landing aircraft (VTOL), the helicopter, has already firmly established its role in all the services, there has also been great interest in aircraft that combine the best features of both the helicopter and the airplane. Helicopters are great for takeoffs, landings, and hovering but are severely limited in missions calling for high forward speeds, substantial range, or long-duration flight. Through the years many concepts have been proposed to overcome the limitations of the conventional helicopter. However, except for the U.S. Marine Corps’s AV-8A Harrier, none have gone much farther than the prototype stage.

Past V/Stol concepts have included aircraft with lift fans embedded in their wings, planes with ducted propellers that tilted, aircraft that took off and landed on their tails, and planes with rotors or propellers that could be tilted from horizontal to vertical position.

The tilt rotor, which tilts the lifting rotors to become propellers, was initially the most commonly tried approach.* For example, first-generation tilt rotor aircraft included the Vertol VZ-2, the Hiller X-18, the Bell XV-3, and the Curtiss-Wright X-100. All these flew with varying degrees of success, met overall goals of showing feasibility, and provided much information and experience for the more advanced V/STOLs to come. The second generation, designed so they could carry more significant payloads and thus show the operational suitability of V/STOLs, included not only tilt rotors but tilt-wing aircraft in which the entire wing tilted, engines and all. These second-generation aircraft included the LTV XC-142A and the Canadair CL-84, as well as the tilt rotor Curtiss-Wright X-19A. These aircraft all flew, but none progressed beyond the exploratory development stage. However, this is the story of V/STOLs: lots of designs, prototype construction, and testing, but no operational hardware.

*TiltRotor is a Bell Helicopter trademark label. Tilt rotor is also an accurately descriptive term.

Several rotary-wing-based V/STOL aircraft are currently being developed by the Army, Navy, and National Aeronautics and Space Administration (NASA). Again, these developments are not specifically aimed at designs that would go directly into production. Their purpose is to prove new concepts and advance V/STOL technology.

•XV-15, TiltRotor. This aircraft, being developed by Bell Helicopter Textron, represents a third-generation tilt-rotor V/STOL. The 42-foot long, 13,000-pound craft is powered by two 1500-horsepower turbine engines located in the wing tip nacelles that rotate with the rotors. The XV-15 program has been under way for a considerable period of time, the original NASA/Army contract dating back to 1973. One reason for the delay is the program’s low-level of funding. Starting with hovering flights in 1977, the XV-15 has now been flown successfully throughout its entire flight envelope.

• XH-59A, Advancing Blade Concept (ABC). One way to eliminate the problem of stalling and reverse rotor blade flow that limits the forward speed of a helicopter is to use two contra-rotating rotors. This is the principle behind the XH-59A, built by Sikorsky Aircraft in a jointly funded Army, NASA, and now, Navy program. The XH-59A program started in 1971; by 1973, two XH-59As were ready for flight-testing. However, one aircraft was lost during a hard landing. Flight-testing of the XH-59A as a pure rotary-wing craft had been completed by 1977. The XH-59A’s extreme agility and maneuverability were even more than had been expected. This maneuverability comes about because of the very stiff rotor blades used on the XH-59A, making the ABC an ideal candidate for a combat aircraft. The stiffer blades are also more rugged and thus more likely to survive encounters with tree limbs and hits from small arms fire. In 1978, the high-speed test program was started with the addition of two auxiliary 3000-pound thrust jet engines mounted on the fuselage.

•X-Wing. The U.S. Navy is quite interested in the X-wing concept as a way to combine the characteristics of the helicopter and the airplane. During takeoffs, landings, and slow-speed flight, the X-wing’s four-bladed rotor operates like a conventional rotor. Once relatively high speeds (say 200-230 knots) are reached, the rotors would be locked into place and would function like normal wings. The X-wing could be operated at speeds up to those found in today’s subsonic airliners. Yet there is more to the X-wing concept than just the rotor-wing idea. There is the circulation control rotor (CCR), for example. With the CCR, the lift of the rotor blade is controlled by blowing compressed air through the leading edge or also the trailing edge of the rotor blade. This principle greatly simplifies the flight-control system during the helicopter mode. So far a Navy SH-2 helicopter has been equipped with a OCR system by Kaman Aerospace, and flight-testing of this craft is nearing completion. To test the X-wing in both the helicopter and stopped rotor, fixed-wing mode, a single-seat demonstrator model is planned. This aircraft will be powered by two jet engines that will turn the rotor, provide the compressed air, and supply forward thrust.

The V/STOL will probably never replace the helicopter or fixed-wing airplane where these aircraft have firmly established roles. However, V/STOLs can fill the gap between the two where speed and endurance must be combined with helipad or short-field deployment. For example, the Army is interested in V/STOLs since they could be stationed near the forward edge of the battle area (FEBA), ready to make high-speed penetration deep into enemy territory to strike enemy targets or gain reconnaissance information. The excellent nap-of-the-earth (NOE) flying characteristics of V/STOLs would allow on-the-deck flights to ensure penetration without detection. The Army is also considering V/STOL aircraft for special electronic missions. Potentially, the Navy could use V/STOLs for many missions now calling for aircraft launched from aircraft carriers, such as antisubmarine warfare, search and rescue, airborne early warning, and vertical on-board delivery. However, V/STOLs would allow operations from the decks of even much smaller ships. Likewise, V/STOL capability would enable the Marines to mount airborne assaults from smaller ships located farther from shore. Because of the agility and maneuverability of V/STOLs, they may even be used for air-to-air combat against Soviet helicopters like the Hind attack helicopter, which can outrun current U.S. choppers. The Air Force could use V/STOL aircraft for long-range and speedy rescue of downed aircrews, long duration forward air controller missions, and for operation from battle-damaged airfields.

With all these advantages, why are V/STOLs not operational in any sizable numbers with United States forces? There are many reasons that mainly consist of priorities and money. With so many things to do and so little to do it with, the "nice-to-do" cannot be done, and even many of the necessities are neglected. Since the V/STOL role lies between the operating regimes of two proven types of aircraft, it has been difficult to justify it in an austere environment. Thus, to meet mission requirements, rather than fully develop a new concept, the capabilities of the helicopter have been stretched, fixed-wing aircraft have been functioning at the lower end of their operating envelopes, and tactics have been changed to match available capabilities.

Another problem facing the V/STOL is that most projected missions are of the special-category type calling for at most about one-hundred aircraft of a given configuration, surely not enough to amortize the entire engineering and development costs for a V/STOL concept. Traditionally, such special requirements have been satisfied by modifying an existing airframe or commercial aircraft. Then why not find several of these special types of missions and build a common airframe that can be modified during assembly to meet different and unique requirements? In the past, multimission aircraft have met with limited success; they seem to do many things, but none exceptionally well.

The way for the V/STOL to get into an operational role is for it to compete with other concepts in satisfying "mainline" military missions. For example, the V/STOL could be a viable contender for the next-generation ground support attack platform to follow the USAF’s A-10 or the Army’s AH-64 Apache Attack Helicopter. V/STOLs would thus be a solution to military requirements rather than a solution looking for a mission, as has really been the situation to date. Meanwhile, V/STOL technology demonstration programs like the XV-15, XH-59A, and the X-wing should continue at full speed so that all the homework will be done; then, the V/STOL will be ready to compete with all the necessary experience and test results to support its advocacy. Particular areas of importance are the operational, maintenance, logistics, and tactics aspects of V/STOL deployment. The V/STOL will have to compete with the helicopter and fixed-wing aircraft, which are mature, seasoned weapon systems. Thus, V/STOL demonstrations must include vigorous field and operational tests to prove they can make it in a real-world combat environment.

V/STOLs have proved they can combine the advantages of both the helicopter and the fixed-wing airplane. Nowthey must prove that they are operationally sound. There is still time to do it if steady, adequately funded technology and demonstration programs are continued for the next few years.

Colonel William D. Siuru, Jr. (B.S., Wayne State University; M.S., AFIT; Ph.D., Arizona State University), is Commander, Frank J. Seiler Research Laboratory (AFSC), USAF Academy, Colorado. He has held a variety of technical and management positions in Air Force Systems Command, including assignments at the Aeronautical Systems Division, Foreign Technology Division, Space and Missile Organization, and Rocket Propulsion Laboratory, and in the Department of Engineering at the U.S. Military Academy. Colonel Siuru has written extensively on aerospace subjects and is a previous contributor to the Review.

Disclaimer

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.

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