In 1493, Leonardo da Vinci sketched his idea for an aerial screw that he envisioned climbing vertically into the sky, powered by four men, rotating a canvas helix sail around a vertical shaft. Almost 450 years later, Igor Sikorsky flew what is widely recognized as the first practical helicopter, the VS-300, at Stratford, Connecticut. Since that time, thousands upon thousands of man-hours and many billions of dollars have been spent on improving the design. The goal: to make a vertical takeoff and landing aircraft fly larger payloads farther and faster. The engineering and aerodynamic factors limiting the capabilities of helicopters are complex and numerous, and thus the design approaches over the years have been varied and innovative. But very few have made it to production.
Why the Tilt rotor concept?
While very large, heavy-lifting helicopters such as the CH-47 can carry on the order of 26,000 pounds, most rotary-wing machines are limited to speeds of approximately150 knots at the top end and a range of around 350 nm. One of the most significant factors limiting the maximum forward speed of a helicopter is an aerodynamic phenomenon known as “retreating blade stall.” To balance the lift produced across the disc of a spinning rotor that is moving forward through the air, the advancing blade must reduce its angle of attack to maintain the amount of lift it produces as the helicopter’s forward speed is added to the rotational speed of the blade. The retreating blade must similarly increase its angle of attack, as the machine’s forward speed is removed from the blade’s rotational speed.
As the helicopter’s forward speed increases, the retreating blade will reach a point where the blade tip, the area of the blade rotating the fastest, reaches its critical angle and stalls, causing losst of lift, buffet, and vibration—and ultimately loss of control. Various attempts to expand the flight envelope of the basic helicopter have been made, such as the British Experimental Rotor Programme (BERP), which modified the blade tip to push the world speed record for helicopters to just shy of 250 mph. There were also compound helicopter research projects, including the NASA/Sikorsky Rotor System Research Aircraft (RSRA), which used stub wings and forward propulsion systems. But in the end, if the designer is relying on a rotating blade to provide the majority of lift for the vehicle, retreating blade stall will produce a barrier to top speed, efficiency, and range.
With fixed-wing aircraft making giant advances, the concept of merging the attributes of a helicopter with a fixed-wing aircraft to solve this problem might appear relatively obvious. During cruise flight, the vehicle should rely on a wing to produce the lifting force and the rotors must morph from providing all of the lift while in the hover, to producing all of the propulsive force in wing-borne cruise flight.
Finding the practical solution to the engineering difficulties faced by making this concept a reality has been one of the greatest tasks facing aeronautical engineers and aerodynamicists in the last 75 years. It might come as a surprise that the original pioneers are not household names. William E. Cobey bought Transcendental Aircraft in 1952 and developed the Model 1-G single-seat tiltrotor under an Army/Air Force contract. The centrally mounted Lycoming O-290 piston-powered 1-G crashed as a result of pilot error in 1955, but not before flights had taken place where the prop-rotor drive shafts had been tilted to within 10 degrees of the horizontal airplane mode. This vehicle is recognized as the first to explore transition from vertical flight to wing-borne airplane flight.
A founding father of Transcendental, Bob Lichten, later joined Bell Aircraft, and in 1951 designed the Bell model 200 that successfully competed for funds from the military for two “tilting thrust vector converti planes” as the XV-3. The XV-3 was similar in configuration to the 1-G, in that the Pratt & Whitney R-985 piston engine was centrally mounted in the fuselage and used drive shafts originating from a fuselage mounted transmission to turn the three-blade, 25-foot-diameter prop rotors. But with a its maximum takeoff mass of 4,700 pounds, the aircraft was almost three times the weight of the Model 1-G. The prop rotors were mounted at the end of long drive shafts, a placement that, coupled with fully articulated rotors and an unbraced wing structure, led to rotor instability and very heavy airframe vibration. These factors caused a crash of the XV-3 during a flight in October 1956. The pilot, Dick Stansbury, lost consciousness as a result of airframe vibration during an attempt to expand the flight envelope with the rotors tilted forward at an angle of 17 degrees.
The company made a number of modifications to the XV-3 that enabled test pilot Bill Quinlan to complete the first full transition from helicopter to airplane mode in December 1958.
The XV-3 was also instrumental in defining flight control systems in this new breed of flying machine. In the hover, the pilot was provided with a collective control and twist grip throttle to adjust the power and pitch of the prop rotors. To lift the machine into the hover, the collective was raised and the power increased until the lift generated matched the weight and, exactly like a helicopter, the vehicle lifted into the air.
In a conventional helicopter, the yaw pedals were connected to the tail rotor to adjust its pitch to provide yaw control. In the XV-3, the yaw pedals controlled the pitch of the prop rotors in a way very similar to how a tandem helicopter such as the CH-46 Sea Knight controlls yaw. The prop rotors are tilted laterally and differentially to provide a yawing moment and so no tail rotor or rudder is required. The aircraft flying controls remained active at all times, but as the prop-rotors are tilted forward during transition, the helicopter controls are mechanically blended out until the collective pitch control is no longer used, and the prop rotors automatically govern speed of rotation against applied power.
However, one third of the XV-3’s helicopter-style cyclic pitch control was retained in airplane mode to increase maneuverability. This was a remarkable feat of mechanical mixing and hydraulically boosted flying controls, long before stability augmentation systems were introduced. Transition to airplane flight was manually controlled by a “beep” switch located on the collective control. Tilt-rotors that followed the XV-3 broadly copied its flight control systems, albeit with more and more levels of sophisticated automation.
The XV-3 had certainly proved that the concept of the tilt-rotor was achievable, but the XV-3 only ever achieved a level flight cruise speed of 115 knots, which represented little increase over most conventional helicopters of the time. However, the flame for the tiltrotor concept had been lit, and created many devotees created, both within Bell the wider industry—and potentially most importantly, within the American military.
Moving Forwards into the 1970s
Tiltrotors could be divided into sub-categories determined by the disc loading, a measure of the overall size of the prop rotor disc divided by the all-up weight. Experiments using high disc loading, ie small prop-rotors or propellers and large maximum takeoff weights, were being carried out using a tilting wing concept. Most promising among the tilt-wings was the four-engine LTV XC-142A, but this did not progress to production despite relatively favorable test-flying-program results, probably due to the high complexity of tilting not just the rotors, but the entire wing, engines, and all the systems and structures attached or built into the wing. The tilt-wing concept also required an additional vertically mounted tail rotor to provide pitch control in the hover, which further added to the complexity and weight of the machine.
Encouraged by the XV-3 and subsequent experimentation and research into aeroelasticity by NASA Langley and Ames, wind tunnel tests of large prop-rotors by Bell Helicopter and Boeing, and further research into flight control systems by Vought Aeronautics, NASA and the Army Air Mobility Research and Development Laboratory (AMRDL) initiated the Tilt Rotor Research Aircraft project (TRRA) in 1971. The project was to consist of two phases, with phase one producing a preliminary design for a minimum size proof-of-concept vehicle and phase two, development of the vehicle. Four contractors submitted bids for phase one of the project from which Bell and Boeing Vertol were awarded a $500,000 contract. On April 12, 1973, Bell Helicopter was selected to design and build two tiltrotor research aircraft for a target cost of $26.5 million. The designation XV-15 was allocated to the project and the registrations N702NA and N703NA were reserved.
The major airframe components were received from Rockwell International in October 1975 and, one year later, the first XV-15 was officially rolled out at Bell’s Fort Worth facility at Arlington, Texas. Because of its relatively small size, the XV-15, having a wingspan of 32 feet, 2 inches “and a prop-rotor diameter of 25 feet,” the aircraft lent itself well to ground-running tests both in helicopter and airplane mode. The machine was mounted on a ground tie-down facility resembling a freeway bridge, which gave sufficient clearance for the prop-rotors to be rotated horizontally into airplane mode.
The XV-15 was initially run with a ground fuel supply and protective shields surrounding the cockpit to reduce the risk to the pilots should there be an uncontained failure of a prop-rotor at high rpm. Extensive tests were made of the transmission systems and prop-rotor integrity until, on May 3, 1977, test pilots Ron Erhart and Dorman Cannon took N702NA for its first brief hop, making a rolling takeoff and immediate landing in a profile that maintained all loads at the minimum values.
The flight envelope of the XV-15 was expanded cautiously under direction from NASA Ames. N702NA was disassembled and used for wind-tunnel tests, and N703NA carried out the first transition to wing-borne flight in July 1979.
What followed over the next 20 years was arguably one of the most successful flight-test career of any NASA X-plane. The XV-15 proved the tiltrotor concept could be turned into a practical aircraft flying off of ships, demonstrating nap-of-the-earth military flying and Coast Guard search-and-rescue capabilities, as well as operating from commercial heliports and, famously, the White House lawn. In 1981, N702NA performed at the Paris Air Show over a period of 10 days and generated industry confidence in the tiltrotor. The XV-15 impressed everyone, from civilian pilots to American senators and senior military pilots from the Army, Navy, Air Force, and Marine Corps, many of whom were to become supporters of future tiltrotor projects.
JVX to Osprey
The XV-15 had U.S. Army funding and had already caught the eye of all the other services, but it was the U.S. Marine Corps, with its requirement to replace an aging fleet of CH-46 Sea Knights and CH-53D Sea Stallions, that was most attracted to advancing the tiltrotor to an operational vehicle. The public failure of Operation Eagle Claw to rescue U.S. Embassy hostages taken by revolutionary students in Tehran in 1980 gave political impetus to developing an aircraft with VTOL capability and fixed-wing speed and range. In 1982, after a study to compare various concepts, a request for proposals (RFP) was issued under the project name Joint Services Vertical Lift and Experimental (JVX).
The service received only one proposal, from a partnership between Bell-Boeing and in April 1983, the initial contract spelled out requirements: tiltrotor able to fly at 250 knots at altitudes as high as 25,000 feet with 24 fully armed soldiers in a cabin that was also capable of carrying an F-18’s engine. The JVX was also required to operate from the deck of an amphibious assault ship, a condition that restricted the prop-rotor diameter to 38 feet and no more than 1.2 times the area of a CH-46.
The need to fit on the ships’ lift also demanded that the wings and prop-rotors of the JVX folded to limit the length of the machine to 63 feet. To the aerodynamicists in the Bell team, this meant that the disc loading, commonly 4-10 pounds/square foot on a helicopter, would be on the order of 21 pounds/square foot on the JVX. Such loading would require engines producing in the region of 10,000 hp.
The first prototype JVX, now named the V-22 Osprey, flew at Bell’s research facility at Arlington, Texas, on March 19, 1989, piloted by Bell test pilot Dorman Cannon and Boeing’s Dick Balzer. The aircraft completed the first full transition to airplane mode in September 1989.
The flight test phase of the V-22 Osprey lasted 16 years and was dogged by technical difficulties, four high-profile fatal accidents, and political budgetary machinations until, in 2005, the first deliveries were made to an operational squadron VMM-263. The aircraft has since flown more than 400,000 hours in various combat theaters including Iraq and Afghanistan and is in service with the U.S. Marines and U.S. Air Force. In 2020 it will enter service with the U.S. Navy.
Since its inception, the tiltrotor was predicted to have civilian as well as military applications, so it was no surprise that in 1996, Bell-Boeing announced its intention to produce a civil tiltrotor design, the Bell Boeing 609 (BB609) based on experience with the XV-15 and the V-22. The two-crew aircraft was designed to carry nine passengers and have a maximum speed of 275 knots over a range of 750 nm with a maximum takeoff weight of 16,000 pounds. A mock-up of the BB609 displayed at Paris resulted in 36 orders and it seemed the program was up and running.
In 1998, however, Boeing withdrew from the program citing an intention to concentrate only on military helicopters, and Agusta, a participant in the European Future Advance Rotorcraft project (EUROFAR), formed a joint venture with Bell to develop what was now renamed the BA609 (Bell-Agusta). The first prototype first flew in March 2003 at Bell’s Arlington facility piloted by Roy Hopkins and Dwayne Williams. By October 2008, two prototypes had logged 365 hours in both airplane and helicopter mode and in 2009 successfully demonstrated a dual engine failure in cruise flight.
After 2009, little obvious progress was being made on the program, and with no sign of the third and fourth prototypes, Agusta-Westland formally purchased Bell’s share in the project, now renamed the AW609. Certification was planned for 2016.
Flight testing was halted in October 2015 after the second AW609 prototype was destroyed during a high-speed test flight, killing pilots Pietro Venanzi and Herb Moran. The accident was attributed to a dutch roll condition that was not foreseen by concurrent simulator tests and led to the prop-rotors striking the leading edge of the wing, causing a hydraulic failure, flight control damage and in-flight break up.
Testing resumed in April 2016, and the third prototype joined the program in May 2016 to carry out icing trials. The fourth prototype is in final production, and with the boost of orders from the United Arab Emirates and Era group coupled with a platform development agreement with the Bristow group, the AW609 program appears to making a recovery and is aiming for certification in the second half of 2018 with deliveries hoped to commence in 2019.
Bell, having concentrated on its military programs, has now successfully flown its V-280 Valor Joint Multi Role technology demonstrator in collaboration with Lockheed Martin. The V-22 fleet is nearing 500, rapidly driving the tiltrotor to a level of maturity that was a distant dream even when the XV-15 took to the skies. And this technology, envisioned decades ago, is embracing newer technology, as around the world there exist many unmanned tiltrotor programs, such as the Bell Eagle Eye, that are furthering the technology required for the next generation of tiltrotors.
Is it possible that the 2020s will be the decade of the tiltrotor?