A Brief History of Hybrid VTOL/Fixed Wing Aircraft

By Patrick J. Walsh

The potential military usefulness of vertical takeoff and landing (VTOL) capability has long been obvious to engineers and pilots alike. U.S. military planners first made use of the unique advantages of VTOL when they deployed the Sikorsky R-4 helicopter for search and rescue operations during World War II.   Shortly after the end of the war, they began the first in a long series of research and development initiatives designed to combine the vertical takeoff and landing capability of a helicopter with the speed and range of fixed-wing aircraft. The quest for a crossover aircraft initially resulted in a series of exotic-looking prototypes whose development failed to progress beyond the experimental stage.  As the research matured over the next three decades, the VTOL family tree branched out in two distinct directions: tiltrotor and directed jet thrust. These two approaches to hybrid aircraft design resulted in some interesting variations.

A V-22 Osprey (right) shares the flight deck of the USS Saipan with a CH-46 Sea Knight helicopter in January, 1999.A V-22 Osprey (right) shares the flight deck of the USS Saipan with a CH-46 Sea Knight helicopter in January, 1999.

Convertiplanes to Tiltrotors

The first major advance in what was then dubbed “convertiplane” technology was the introduction of the Bell XV-3, an experimental tiltrotor aircraft that made its initial flight on August 11, 1955. The result of a joint effort by the U.S. Army and the U.S. Air Force, the XV-3 was developed by Bell Helicopter to prove the viability of the tiltrotor approach for in-flight conversion from rotary-wing to fixed-wing mode — and back upon landing.  The 1970s successor to the long-running XV-3 program was the Bell XV-15. This model provided vital data and test flight experience that helped solve the problems of control and performance, which had long delayed the emergence of a hybrid aircraft suitable for military use. In the 1980s, a partnership between Bell and Boeing Helicopters furthered the tiltrotor approach as part of the U.S. Department of Defense’s joint-service VTOL program.  The hybrid concept reached its ultimate expression in 2007, when the U.S. Marine Corps made its first combat deployment of the Bell Boeing V-22 Osprey — the linear descendent of the XV-3 and XV-15. The versatile V-22 Osprey is also used by the U.S. Air Force for long-range special operations transport missions.

An AV-8B Harrier II demonstrates its vertical takeoff capability at Airshow '88, 
at Marine Corps Base Quantico, August 11, 1988. 


The Jump Jet

Pursuing a parallel but separate course during the post–World War II period, British aircraft research in the 1960s produced the Hawker Siddeley Harrier — the now-legendary “Harrier Jump Jet” of popular depiction.  The first-generation Harrier addressed the VTOL issues of stability and control via a vectored thrust turbofan engine. Its blend of conventional fighter capabilities and vertical/short takeoff and landing (V/STOL) made it appealing to the U.S. Marine Corps, which first employed the AV-8A Harrier in the 1970s.  For its subsequent incarnation as the AV-8B Harrier II, the initial design was thoroughly overhauled by McDonnell Douglas in the 1980s. The U.S. Marine Corps first deployed the AV-8B in 1985, and more than 300 of the successful hybrid were produced, including a Plus version that involved the remanufacture of existing aircraft to add upgraded capabilities.  Production of this model ended in 2003. Nevertheless, the AV-8B will continue to represent its generation of VTOL innovation for much of the next decade — that is, until its gradual replacement by the Lockheed Martin F-35B Lightning II V/STOL variant is completed in 2025.