Out of the Box: V/STOL Configurations

International interest in Vertical Take-Off and Landing (VTOL) and Vertical/Short Take-Off and Landing (V/STOL) configurations escalated during the 1950s and persisted through the mid-1960s with a huge num­ber of radical propulsion/aircraft combinations proposed and evaluated

throughout industry, DOD, the NACA, and NASA. The configurations included an amazing variety of propulsion concepts to achieve hover­ing flight and the conversion to and from conventional forward flight. However, all these aircraft concepts were plagued with common issues regarding stability, control, and handling qualities.[476]

The first VTOL nonhelicopter concept to capture the interests of the U. S. military was the vertical-attitude tail-sitter concept. In 1947, the Air Force and Navy initiated an activity known as Project Hummingbird, which requested design approaches for VTOL aircraft. At Langley, dis­cussions with Navy managers led to exploratory NACA free-flight studies in 1949 of simplified tail-sitter models to evaluate stability and control during hovering flight. Conducted in a large open area within a build­ing, powered-model testing enabled researchers to explore the dynamic stability and control of such configurations.[477] The test results provided valuable information on the relative severity of unstable oscillations encountered during hovering flight. The instabilities in roll and pitch were caused by aerodynamic interactions of the propeller during for­ward or sideward translation, but the period of the growing oscilla­tions was sufficiently long to permit relatively easy control. The model flight tests also provided guidance regarding the level of control power required for satisfactory maneuvering during hovering flight.

Navy interest in the tail-sitter concept led to contracts for the devel­opment of the Consolidated-Vultee (later Convair) XFY-1 "Pogo” and the Lockheed XFV-1 "Salmon” tail-sitter aircraft in 1951. The Navy asked Langley to conduct dynamic stability and control investigations of both configurations using its free-flight model test techniques. In 1952, hovering flights of the Pogo were conducted within the huge return passage of the Langley Full-Scale Tunnel, followed by transition flights from hovering to forward flight in the tunnel test section during a brief break in the tunnel’s busy test schedule.[478] Observed by Convair

personnel (including the XFY-1 test pilot), the flight tests provided encouragement and confidence to the visitors and the Navy.

Without doubt, the most successful NASA application of free-flight models for VTOL research was in support of the British P.1127 vectored – thrust fighter program. As the British Hawker Aircraft Company matured its design of the revolutionary P.1127 in the late 1950s, Langley’s senior man­ager, John P. Stack, became a staunch supporter of the activity and directed that tests in the 16-Foot Transonic Tunnel and free-flight research activi­ties in the Full-Scale Tunnel be used for cooperative development work.[479]

In response to the directive, a one-sixth-scale free-flight model was flown in the Full-Scale Tunnel to examine the hovering and transition behavior of the design. Results of the free-flight tests were witnessed by Hawker staff members, including the test pilot slated to conduct the first transition flights, were very impressive. The NASA researchers regarded the P.1127 model as the most docile V/STOL configuration ever flown during their extensive experiences with free-flight VTOL designs. As was the case for many free-flight model projects, the motion-picture segments showing successful transitions from hovering to conventional flight in the Full-Scale Tunnel were a powerful influence in convincing critics that the concept was feasible. In this case, the model flight dem­onstrations helped sway a doubtful British government to fund the proj­ect. Refined versions of the P.1127 design were subsequently developed into today’s British Harrier and Boeing AV-8 fighter/attack aircraft.

The NACA and NASA also conducted pioneering free-flight model research on tilt wing aircraft for V/STOL missions. In the early 1950s, several generic free-flight propeller-powered models were flown to eval­uate some of the stability and control issues that were anticipated to limit the feasibility of the concept.[480] The fundamental principle used by the tilt wing concept to convert from hovering to forward flight involves reorienting the wing from a vertical position for takeoff to a conventional position for forward flight. However, this simple conversion of the wing angle relative to the fuselage brings major challenges. For example, the

wing experiences large changes in its angle of attack relative to the flight path during the transition, and areas of wing stall may be encountered during the maneuver. The asymmetric loss of wing lift during stall can result in wing-dropping, wallowing motions and uncommanded transient maneuvers. Therefore, the wing must be carefully designed to minimize or eliminate flow separation that would otherwise result in degraded or unsatisfactory stability and control characteristics. Extensive wind tunnel and flight research on many generic NACA and NASA models, as well as the Hiller X-18, Vertol VZ-2, and Ling-Temco-Vought XC-142A tilt wing configurations at Langley, included a series of free-flight model tests in the Full-Scale Tunnel.[481]

Coordinated closely with full-scale flight tests, the model testing initially focused on providing early information on dynamic stability and the adequacy of control power in hovering and transition flight for the configurations. However, all projects quickly encountered the anticipated problem of wing stall, especially in reduced-power descend­ing flight maneuvers. Tilt wing aircraft depend on the high-energy slipstream of large propellers to prevent local wing stall by reducing the effective angle of attack across the wingspan. For reduced-power con­ditions, which are required for steep descents to accomplish short-field missions, the energy of the slipstream is severely reduced, and wing stall is experienced. Large uncontrolled dynamic motions may be exhibited by the configuration for such conditions, and the undesirable motions can limit the descent capability (or safety) of the airplane. Flying model tests provided valuable information on the acceptability of uncontrolled motions such as wing dropping and lateral-directional wallowing dur­ing descent, and the test technique was used to evaluate the effective­ness of aircraft modifications such as wing flaps or slats, which were ultimately adapted by full-scale aircraft such as the XC-142A.

As the 1960s drew to a close, the worldwide engineering community began to appreciate that the weight and complexity required for VTOL missions presented significant penalties in aircraft design. It therefore

turned its attention to the possibility of providing less demanding STOL capability with fewer penalties, particularly for large military transport aircraft. Langley researchers had begun to explore methods of using pro­peller or jet exhaust flows to induce additional lift on wing surfaces in the 1950s, and although the magnitude of lift augmentation was relatively high, practical propulsion limitations stymied the application of most concepts.

A particularly promising concept known as the externally blown flap (EBF) used the redirected jet engine exhausts from conventional pod – mounted engines to induce additional circulation lift at low speeds for takeoff and landing.[482] However, the relatively hot exhaust temperatures of turbojets of the 1950s were much too high for structural integrity and feasible applications. Nonetheless, Langley continued to explore and mature such ideas, known as powered-lift concepts. These research stud­ies embodied conventional powered model tests in several wind tunnels, including free-flight investigations of the dynamic stability and control of multiengine EBF configurations in the Full-Scale Tunnel, with empha­sis on providing satisfactory lateral control and lateral-directional trim after the failure of an engine. Other powered-lift concepts were also explored, including the upper-surface-blowing (USB) configuration, in which the engine exhaust is directed over the upper surface of the wing to induce additional circulation and lift.[483] Advantages of this approach included potential noise shielding and flow-turning efficiency.

While Langley continued its fundamental research on EBF and USB configurations, in the early 1970s, an enabling technology leap occurred with the introduction of turbofan engines, which inherently produce relatively cool exhaust fan flows.[484] The turbofan was the perfect match for these STOL concepts, and industry’s awareness and participation in the basic NASA research program matured the state of the art for design data for powered-lift aircraft. The free-flight model results, coupled with NASA piloted simulator studies of full-scale aircraft STOL missions, helped provide the fundamental knowledge and data required to reduce

John P. Campbell, Jr., left, inventor of the externally blown flap, and Gerald G. Kayten of NASA Headquarters pose with a free-flight model of an STOL configuration at the Full-Scale Tunnel. Slotted trailing-edge flaps were used to deflect the exhaust flows of turbofan engines. NASA.

risk in development programs. Ultimately applied to the McDonnell- Douglas YC-15 and Boeing YC-14 prototype transports in the 1970s and to today’s Boeing C-17, the EBF and USB concepts were the result of over 30 years of NASA research and development, including many valu­able studies of free-flight models in the Full-Scale Tunnel.[485]