In the late 1950s and early 1960s, several aircraft manufacturers developed Vertical Take Off and Landing (VTOL) aircraft under military research contracts. One unexpected VTOL program participant was the Curtiss-Wright Corporation, which had abandoned its aircraft division in 1952 after failing to earn any significant post-war military contracts. However, engineers in the company's propeller division discovered an innovative approach to the development of practical VTOL aircraft - the radial force principle. The subsequent research program resulted in the X-100 and proved the validity of the "tilt-propeller" VTOL aircraft, but it also revealed that the innovative technology required considerable investment to perfect.

Henry Borst, chief aerodynamicist for the Curtiss-Wright propeller division, proposed a VTOL aircraft based on the radial force principle. This states that as a propeller is inclined towards the vertical from the horizontal, the resultant of the propeller's thrust and the pressure of the relative wind acting on the rotating disk is a force with an additional lift component in the vertical axis. The penalty of the additional lift is increased drag, but this was not an issue because only slow-speed take-offs and landings required the added lift component, as small conventional wings provided most of the lift in cruise. Borst realized that short propellers with wide blades magnified the radial force effect by increasing the surface area of the propeller disk without the compressibility issues of longer rotor blades. This offered a potential advantage over other tilt-rotor models, such as the Bell XV-3, with longer and narrower blades that did not have sufficient surface area to take advantage of the phenomenon. The added lift generated from the radial force permitted an aircraft built with the specially constructed lifting propellers to have smaller wings, which decreased weight and high-speed drag. This resulted in the most aerodynamically efficient of all of the VTOL designs. An added benefit of the smaller propellers was a reduction in noise levels over other VTOL models.

Borst had a proven record of applying revolutionary propeller designs to innovative aircraft such as the Lockheed XP-88 and the Convair XFY Pogo (see NASM collection). Based on his research, the president of the company, Roy Hurley, felt that their ability to apply the radial force principle gave them an advantage over their competitors. On November 20, 1957, Curtiss-Wright reentered the aircraft business. On February 20, 1958, construction began on a flying demonstrator, designated the X-100.

The ungainly appearance of the X-100 was due to its hodge-podge of disproportionate components. The two thin and stubby wings each mounted a sleek propeller nacelle that hinged at the leading edge of the wingtips and were capable of pivoting from vertical to twelve degrees above horizontal. The relatively wide aluminum and fabric-covered fuselage accommodated two persons. The center of the fuselage contained a single Lycoming YT53 turboshaft engine, with its large rectangular intake mounted behind the cockpit. The X-100 relied on a fixed tricycle landing gear with two main wheels and a small tailwheel.

The propellers were the heart of the project and were remarkable in appearance and construction. The blades had a much longer chord than was normal, with a high degree of twist. They consisted of a foam core mounted on a steel shank with a fiberglass covering.

In hovering or slow-speed flight, pressurized engine exhaust, ducted through a series of vent doors, controlled pitch and yaw. A boxy and cumbersome assembly, known as the jetivator, mounted at the rear of the squared-off fuselage, controlled the exhaust vents. When the engine operated at full power, this system could supply 64 kg (140 lb) of thrust for pitch and 18 kg (40 lb) for yaw. The pitch of the propellers varied differentially to control roll in slow-speed operations. In cruise flight, conventional ailerons, elevators, and rudders provided control. During the transition from hovering to cruise flight, the pilot toggled a switch on the control stick, which rotated the pylons towards horizontal, ten degrees at a time. For each 10-degree increment of forward propeller inclination, the X-100 gained approximately 32 kph (20 mph) in increased airpseed.

On December 22, 1958, Curtiss-Wright unveiled the aircraft for the first time, and the X-100 began tethered hovering tests on April 20, 1959. Then, on March 29, 1960, test pilot Bill Furlich made the first rolling takeoff, with the propellers inclined to take advantage of the radial force principle. He successfully made the first and only transition from vertical flight mode to high-speed flight mode on April 13. The flight tests verified the validity of using the radial force effect in VTOL designs, but the X-100 proved to have a number of flaws that required correction before Curtiss-Wright could produce a practical aircraft.

One of the most challenging tasks in flying the X-100 occurred when the high velocity downwash from the propellers created turbulence as the aircraft entered ground effect just before landing. Slow throttle response, and the jetivator, further complicated this unstable condition. The reduction of engine power before touchdown also minimized the thrust available to maintain a nose-level pitch attitude, which frequently resulted in a sudden drop of the tail. If the pilot over-corrected, he could potentially strike the nose. To prevent such an occurrence, engineers added two wheels to the front fuselage.

Another flaw with the tilt-propeller design was that the aircraft would have been impossible to land safely in case of engine failure. The small propellers were too highly loaded to perform a helicopter-style autorotation and the small wings were not sufficient to allow an airplane-style power-off glide at a safe speed.

Nevertheless, by October 1960, Curtiss-Wright had verified the validity of its VTOL concept and transferred the X-100 to the National Aeronautics and Space Administration (NASA). NASA was keenly interested in VTOL aircraft programs and began its own year-long series of tests with the aircraft at its Langley Field facilities. However, these tests did not explore the capabilities of the aircraft, but instead focused on the effects of high velocity prop-wash on varied types of landing surfaces, such as packed dirt, grass, snow, and pavement. The X-100 only flew in vertical flight mode during the NASA evaluation and spent most of its time tethered to the ground. An accident on October 5, 1961, attributable to a flight control malfunction resulted in the X-100 rolling onto its left pylon and sustaining moderate damage, which effectively ended its flight test program. The aircraft had accumulated a total of fourteen hours of flight time.

While testing of the X-100 was under way, Curtiss-Wright began work on a much-improved version, designated the X-200. The new design utilized an elegant fuselage similar to that of the Aero Commander with four lifting propellers attached at the tips of two sets of small wings placed in tandem on the upper fuselage. In forward flight, elevators and ailerons mounted on the aft wing section controlled pitch and roll, while a conventional vertical stabilizer and rudder provided yaw control. The forward wing had conventional flaps on its trailing edge. In slow flight and VTOL mode, differential pitch and propeller rpm provided control about all the axes. This system, demonstrated previously on Curtiss-Wright's VZ-7 "Flying Truck" that developed concurrently with the X-100, proved much more practical than the jetivator. After considerable effort, Curtiss-Wright convinced the military to fund development of the X-200, subsequently redesignated as the X-19, under a tri-service agreement between the Army, Navy, and the Air Force, which managed the project.

Curtiss-Wright began construction on two X-19s, but lacked the funds to adequately test and refine essential components, such as the gearboxes, which resulted in the potentially disastrous combination of new technology and untested parts. Flight tests began on November 20, 1963, with the first X-19. While hovering, the aircraft suffered some of the same propeller downwash and slow throttle response problems experienced by the X-100. Parts failures and technical problems delayed the first hover-to-cruise transition attempt of the X-19 until August 25, 1965, but a gearbox failure combined with pilot error resulted in the loss of the aircraft before the crew could attempt the transition. Fortunately, both crewmembers ejected safely.

The loss of the X-19, combined with the Air Force's general lack of enthusiasm for VTOL research programs, quickly led to a cancellation of the program before the second X-19 had an opportunity to fly. The X-100/X-19 project along with other VTOL aircraft programs demonstrated the practicality of VTOL aircraft, but also illustrated that substantial government interest and financial support were necessary to overcome the technical hurdles.

The X-100 and X-19 were the last aircraft built by Curtiss-Wright and ended that company's long tradition of innovative aircraft development. In 1969, Curtiss-Wright donated the X-100, as an example of VTOL technology, to the Smithsonian Institution.

Rotor Diameter:3.05 m (10 ft)

Length:8.64 m (28 ft 4 in)

Height:3.28 m (10 ft 9 in)

Weight:Empty, 1,481 kg (3,265 lb)

Gross, 1,691 kg (3,729 lb)

Engine:Lycoming YT53-L-1, 825 SHP

References and Further Reading

Miller, Jay. "The X-Planes: X-1 to X-45." Hinckley, England: Midland Counties

Publications, 2001.

X-100 curatorial file, Aeronautics Division, National Air and Space Museum

R.D. Connor

In the late 1950s and early 1960s, several aircraft manufacturers developed Vertical Take Off and Landing (VTOL) aircraft under military research contracts. One unexpected VTOL program participant was the Curtiss-Wright Corporation, which had abandoned its aircraft division in 1952 after failing to earn any significant post-war military contracts. However, engineers in the company's propeller division discovered an innovative approach to the development of practical VTOL aircraft - the radial force principle. The subsequent research program resulted in the X-100 and proved the validity of the "tilt-propeller" VTOL aircraft, but it also revealed that the innovative technology required considerable investment to perfect.

Henry Borst, chief aerodynamicist for the Curtiss-Wright propeller division, proposed a VTOL aircraft based on the radial force principle. This states that as a propeller is inclined towards the vertical from the horizontal, the resultant of the propeller's thrust and the pressure of the relative wind acting on the rotating disk is a force with an additional lift component in the vertical axis. The penalty of the additional lift is increased drag, but this was not an issue because only slow-speed take-offs and landings required the added lift component, as small conventional wings provided most of the lift in cruise. Borst realized that short propellers with wide blades magnified the radial force effect by increasing the surface area of the propeller disk without the compressibility issues of longer rotor blades. This offered a potential advantage over other tilt-rotor models, such as the Bell XV-3, with longer and narrower blades that did not have sufficient surface area to take advantage of the phenomenon. The added lift generated from the radial force permitted an aircraft built with the specially constructed lifting propellers to have smaller wings, which decreased weight and high-speed drag. This resulted in the most aerodynamically efficient of all of the VTOL designs. An added benefit of the smaller propellers was a reduction in noise levels over other VTOL models.

Borst had a proven record of applying revolutionary propeller designs to innovative aircraft such as the Lockheed XF-88B and the Convair XFY Pogo (see NASM collection). Based on his research, the president of the company, Roy Hurley, felt that their ability to apply the radial force principle gave them an advantage over their competitors. On November 20, 1957, Curtiss-Wright reentered the aircraft business. On February 20, 1958, construction began on a flying demonstrator, designated the X-100.

The ungainly appearance of the X-100 was due to its hodge-podge of disproportionate components. The two thin and stubby wings each mounted a sleek propeller nacelle that hinged at the leading edge of the wingtips and were capable of pivoting from vertical to twelve degrees above horizontal. The relatively wide aluminum and fabric-covered fuselage accommodated two persons. The center of the fuselage contained a single Lycoming YT53 turboshaft engine, with its large rectangular intake mounted behind the cockpit. The X-100 relied on a fixed tricycle landing gear with two main wheels and a small tailwheel.

The propellers were the heart of the project and were remarkable in appearance and construction. The blades had a much longer chord than was normal, with a high degree of twist. They consisted of a foam core mounted on a steel shank with a fiberglass covering.

In hovering or slow-speed flight, pressurized engine exhaust, ducted through a series of vent doors, controlled pitch and yaw. A boxy and cumbersome assembly, known as the jetivator, mounted at the rear of the squared-off fuselage, controlled the exhaust vents. When the engine operated at full power, this system could supply 64 kg (140 lb) of thrust for pitch and 18 kg (40 lb) for yaw. The pitch of the propellers varied differentially to control roll in slow-speed operations. In cruise flight, conventional ailerons, elevators, and rudders provided control. During the transition from hovering to cruise flight, the pilot toggled a switch on the control stick, which rotated the pylons towards horizontal, ten degrees at a time. For each 10-degree increment of forward propeller inclination, the X-100 gained approximately 32 kph (20 mph) in increased airpseed.

On December 22, 1958, Curtiss-Wright unveiled the aircraft for the first time, and the X-100 began tethered hovering tests on April 20, 1959. Then, on March 29, 1960, test pilot Bill Furlich made the first rolling takeoff, with the propellers inclined to take advantage of the radial force principle. He successfully made the first and only transition from vertical flight mode to high-speed flight mode on April 13. The flight tests verified the validity of using the radial force effect in VTOL designs, but the X-100 proved to have a number of flaws that required correction before Curtiss-Wright could produce a practical aircraft.

One of the most challenging tasks in flying the X-100 occurred when the high velocity downwash from the propellers created turbulence as the aircraft entered ground effect just before landing. Slow throttle response, and the jetivator, further complicated this unstable condition. The reduction of engine power before touchdown also minimized the thrust available to maintain a nose-level pitch attitude, which frequently resulted in a sudden drop of the tail. If the pilot over-corrected, he could potentially strike the nose. To prevent such an occurrence, engineers added two wheels to the front fuselage.

Another flaw with the tilt-propeller design was that the aircraft would have been impossible to land safely in case of engine failure. The small propellers were too highly loaded to perform a helicopter-style autorotation and the small wings were not sufficient to allow an airplane-style power-off glide at a safe speed.

Nevertheless, by October 1960, Curtiss-Wright had verified the validity of its VTOL concept and transferred the X-100 to the National Aeronautics and Space Administration (NASA). NASA was keenly interested in VTOL aircraft programs and began its own year-long series of tests with the aircraft at its Langley Field facilities. However, these tests did not explore the capabilities of the aircraft, but instead focused on the effects of high velocity prop-wash on varied types of landing surfaces, such as packed dirt, grass, snow, and pavement. The X-100 only flew in vertical flight mode during the NASA evaluation and spent most of its time tethered to the ground. An accident on October 5, 1961, attributable to a flight control malfunction resulted in the X-100 rolling onto its left pylon and sustaining moderate damage, which effectively ended its flight test program. The aircraft had accumulated a total of fourteen hours of flight time.

While testing of the X-100 was under way, Curtiss-Wright began work on a much-improved version, designated the X-200. The new design utilized an elegant fuselage similar to that of the Aero Commander with four lifting propellers attached at the tips of two sets of small wings placed in tandem on the upper fuselage. In forward flight, elevators and ailerons mounted on the aft wing section controlled pitch and roll, while a conventional vertical stabilizer and rudder provided yaw control. The forward wing had conventional flaps on its trailing edge. In slow flight and VTOL mode, differential pitch and propeller rpm provided control about all the axes. This system, demonstrated previously on Curtiss-Wright's VZ-7 "Flying Truck" that developed concurrently with the X-100, proved much more practical than the jetivator. After considerable effort, Curtiss-Wright convinced the military to fund development of the X-200, subsequently redesignated as the X-19, under a tri-service agreement between the Army, Navy, and the Air Force, which managed the project.

Curtiss-Wright began construction on two X-19s, but lacked the funds to adequately test and refine essential components, such as the gearboxes, which resulted in the potentially disastrous combination of new technology and untested parts. Flight tests began on November 20, 1963, with the first X-19. While hovering, the aircraft suffered some of the same propeller downwash and slow throttle response problems experienced by the X-100. Parts failures and technical problems delayed the first hover-to-cruise transition attempt of the X-19 until August 25, 1965, but a gearbox failure combined with pilot error resulted in the loss of the aircraft before the crew could attempt the transition. Fortunately, both crewmembers ejected safely.

The loss of the X-19, combined with the Air Force's general lack of enthusiasm for VTOL research programs, quickly led to a cancellation of the program before the second X-19 had an opportunity to fly. The X-100/X-19 project along with other VTOL aircraft programs demonstrated the practicality of VTOL aircraft, but also illustrated that substantial government interest and financial support were necessary to overcome the technical hurdles.

The X-100 and X-19 were the last aircraft built by Curtiss-Wright and ended that company's long tradition of innovative aircraft development. In 1969, Curtiss-Wright donated the X-100, as an example of VTOL technology, to the Smithsonian Institution.