NASA's Contributions to Aeronautics, Volume 2 by National Aeronautics & Space Administration. - HTML preview

CASE

14

On the Up and Up: NASA Takes on V/STOL

G. Warren Hall

The advent of vertical flight required mastery of aerodynamics, propulsion, and flight control technology. In the evolution of flight characterized by progressive development of the autogiro, helicopter, and various convertiplanes, the NACA and NASA have played a predominant role. NASA developed the theoretical underpinning for vertical flight, evaluated requisite technologies and research vehicles, and expanded the knowledge base supporting V/STOL flight technology.

Case-14 Cover Image: Three important NASA research aircraft representing different approaches to V/STOL flight pass in review over NASA’s Ames Research Center. Left to right: the deflected lift QSRA, the tilt rotor XV-15, and the vectored-thrust Harrier. NASA.

One of the major accomplishments in the history of aviation has been the development of practical Vertical Take-Off and Landing (VTOL) aircraft, exemplified by the emergence of the helicopter in the 1930s and early 1940s, and the vectored-thrust jet airplane of the 1960s. Here indeed was a major challenge that confronted flight researchers, aeronautical engineers, military tacticians, and civilian planners for over 50 years, particularly those of the National Aeronautics and Space Administration (NASA) and its predecessor, the National Advisory Committee for Aeronautics (NACA). While perhaps not regarded by aviation aficionados as being as glamorous as the experimental craft that streaked to new speeds and altitudes, early vertical flight testbeds were likewise revolutionary at the other end of the performance spectrum, in vertical ascents and descents, low-speed controllability, and hover, areas challenging accepted knowledge and practice in aerodynamics, propulsion, and flight controls and controllability.[1]

The accomplishment of vertical flight was as challenging as inventing the airplane itself. Only four decades after Kitty Hawk were vertical takeoff, hovering, and landing aircraft beginning to enter service. These were, of course, the first helicopters: successors to the interim rotary wing autogiro that relied on a single or multiple rotors to give them Vertical/Short Take-Off and Landing (V/STOL) performance. Before the end of the Second World War, the helicopter had flown in combat, proved its value as a lifesaving craft, and shown its adaptability for both land- and sea-based operation.[2] The faded promises of many machines litter the path to the modern V/STOL vehicle. The dedicated research accompanying this work nevertheless led to a class of flight craft that have expanded the use of civil and military aeronautics, saving the lives of nearly a half million people over the last seven decades. The oil rigger in the Gulf going on leave, the yachtsman waiting for rescue, and the infantryman calling in gunships to fend off attack can all thank the flight researchers, particularly those of the NACA and NASA, who made the VTOL aircraft possible.[3]

Helicopters matured significantly during the Korean war, setting the stage for their pervasive employment in the war in Southeast Asia a decade later.[4] Helicopters revolutionized warfare and became the iconic image of the Vietnam war. On the domestic front, outstanding helicopter research was being carried on at NASA Langley. Of particular note were the contributions of researchers and test pilots such as Jack Reeder, John P. Campbell, Richard E. Kuhn, Marion O. McKinney, and Robert H. Kirby. In the late 1950s, military advisers realized how much of the Nation’s defense structure depended on a few large airbases and a few large aircraft carriers. Military interests were driven by the objective of achieving operations into and out of unprepared remotely dispersed sites independent of conventional airfields. Meanwhile, commercial air transportation organizations were pursuing ways to cut the amount of real estate required to accommodate new aircraft and long airstrips.[5]

Since NASA’s inception in 1958, its researchers at various Centers have advanced the knowledge base of V/STOL technology via many specialized test aircraft and flying techniques. Some key discoveries include the realization that V/STOL aircraft must be designed with good Short Take-Off and Landing (STOL) performance capability to be cost-effective, and that, arguably, the largest single obstacle to the implementation of STOL powered-lift technology for civil aircraft is the increasingly objectionable level of aircraft-generated noise at airports close to populated areas.

But NASA interest in fixed wing STOL and VTOL convertiplanes predates formation of the Agency, going back to the unsuccessful combined rotor and wing design by Emile and Henry Berliner tested at College Park Airport, MD, in the early 1920s. In the late 1930s and early 1940s, NACA researcher Charles Zimmerman undertook pioneering research on such craft, his interest leading to the Vought V-173, popularly known as the “Flying Flapjack,” because of its peculiar near- circular wing shape. It led to an abortive Navy fighter concept, the Vought XF5U-1, which was built but never flown. The V-173, however, contributed notably to the emerging understanding of V/STOL aircraft challenges and performance. Aside from this sporadic interest, the Agency’s research staff did not place great emphasis upon such studies until the postwar era. Then, beginning in the early 1950s, a veritable explosion of interest followed, with a number of design studies and flight-test programs undertaken at Langley and Ames laboratories (later the NASA Langley and Ames Research Centers). This interest corresponded to rising interest in the military in the possibility of vertical flight vehicles for a variety of missions.

For example, the U.S. Navy sponsored two unsuccessful experimental “Pogo” tail-sitting turboprop-powered VTOL fighters: the Lockheed XFV-1 and the Convair XFY-1. Only the XFY-1 subsequently operated in true VTOL mode, and flight trials indicated that neither represented a reasonable approach to practical VTOL flight. The Air Force developed a pure-jet equivalent: the VTOL delta-winged Ryan X-13. Though widely demonstrated (even outside the Pentagon), it was equally impracticable.[6] The U.S. Army’s Transportation and Research Engineering Command sponsored ducted-fan flying jeep and other saucerlike circular flying platforms by Avro and Hiller, with an equivalent lack of success. Overall, the Army’s far-seeing V/STOL testbed program, launched in 1956 and undertaken in cooperation with the U.S. Navy’s Office of Naval Research, advanced a number of so-called “VZ”-designated research aircraft exploring a range of technical approaches to V/STOL flight.[7] NATO planners envisioned V/STOL close-air support, interdiction, and nuclear attack aircraft. This interest eventually helped spawn the British Aerospace Harrier strike fighter of the late 1960s and other designs that, though they entered flight-testing, did not prove suitable for operational service.[8]

NACA–NASA and Boundary Layer Control, Externally Blown Flap, and Upper Surface Blowing STOL Research

Short Take-Off and Landing flight research was primarily motivated by the desire of military and civil operators to develop transport aircraft with short-field operational capability typical of low-speed airplanes yet the high cruising speed of jets. For Langley and Ames, it was a natural extension of their earlier boundary layer control (BLC) activity undertaken in the 1950s to improve the safety and operational efficiency of military aircraft, such as naval jet fighters that had to land on aircraft carriers, by improving their low-speed controllability and reducing approach and landing speeds.[9] Indeed, as NACA–NASA engineer-historian Edwin Hartman wrote in 1970, “BLC was the first practical step toward achieving a V/STOL airplane.”[10] This research had demonstrated the benefits of boundary layer flap-blowing, which eventually was applied to operational high-performance aircraft.[11]

NASA’s first large-aircraft STOL flight research projects involved two Air Force–sponsored experimental transports: a Stroukoff Aircraft Corporation YC-134A and a Lockheed NC-130B Hercules. Both aircraft used boundary layer control over their flaps to augment wing lift. The YC-134A was a twin-propeller radial-engine transport derived on the earlier Fairchild C-123 Provider tactical transport and designed in 1956. It had drooped ailerons and trailing-edge flaps that deflected 60 degrees, together with a strengthened landing gear. A J30 turbojet compressor provided suction for the BLC system. Tested between 1959 and mid-1961, the YC-134A confirmed expectations that deflected propeller thrust used to augment a wing’s aerodynamic lift could reduce stall speed. However, in other respects, its desired STOL performance was still limited, indicative of the further study needed at this time.[12]

More promising was the later NC-130B, first evaluated in 1961 and then periodically afterward. Under an Air Force contract, the Georgia Division of Lockheed Aircraft Corporation modified a C-130B Hercules tactical transport to a STOL testbed. Redesignated as the NC-130B, it featured boundary layer blowing over its trailing-edge flaps (which could deflect a full 90 degrees down), ailerons (which were also drooped to enhance lift-generation), elevators, and rudder (which was enlarged to improve low-speed controllability). The NC-130 was powered by four Allison T-56-A-7 turbine engines, each producing 3,750 shaft horsepower and driving four-bladed 13.5-foot-diameter Hamilton Standard propellers. Two YT-56-A-6 engines driving compressors mounted in outboard wing-pods furnished the BLC air, at approximately 30 pounds of air per second at a maximum pressure ratio varying from 3 to 5. Roughly 75 percent of the air blew over the flaps and ailerons and 25 percent over the tail surfaces.[13] Thanks to valves and crossover ducting, the BLC air could be supplied by either or both of the BLC engines. Extensive tests in Ames’s 40- by 80-foot wind tunnel validated the ability of the NC-130B’s BLC flaps to enhance lift at low airspeeds, but uncertainties remained regarding low-speed controllability. Subsequent flight- testing indicated that such concern was well founded. The NC-130B, like the YC-134A before it, had markedly poor lateral-directional control characteristics during low-speed approach and landing. Ames researchers used a ground simulator to devise control augmentation systems for the NC-130B. Flight test validated improved low-speed lateral- directional control.

For a corresponding margin above the stall, the handling qualities of the NC-130B in the STOL configuration were changed quite markedly from those of the standard C-130 airplane. Evaluation pilots found the stability and control characteristics to be unsatisfactory. At 100,000 pounds gross weight, a conventional C-130B stalled at 80 knots; the BLC NB-130B stalled at 56 knots. Approach speed reduced from 106 knots for the unmodified aircraft to between 67 and 75 knots, though, as one NASA report noted, “At these speeds, the maneuvering capability of the aircraft was severely limited.”[14] The most seriously affected characteristics were about the lateral and directional axes, exemplified by problems maneuvering onto and during the final approach, where the pilots found their greatest problem was controlling sideslip angle.[15]

Landing evaluations revealed that the NC-130B did not conform well to conventional traffic patterns, an indication of what could be expected from other large STOL designs. Pilots were surprised at the length of time required to conduct the approach, especially when the final landing configuration was established before turning onto the base leg. Ames researchers Hervey Quigley and Robert Innis noted:

The time required to complete an instrument approach was even longer, since with this particular ILS system the glide slope was intercepted about 8 miles from touchdown. The requirement to maintain tight control in an instrument landing system (ILS) approach in combination with the aircraft’s undesirable lateral-directional characteristics resulted in noticeable pilot fatigue. Two methods were tried to reduce the time spent in the STOL (final landing) configuration. The first and more obvious was suitable for VFR patterns and consisted of merely reducing the size of the pattern, flying the downwind leg at about 900 feet and close abeam, then transitioning to the STOL configuration and reducing speed before turning onto the base leg. Ample time and space were available for maneuvering, even for a vehicle of this size. The other procedure consisted of flying a conventional pattern at high speed (120 knots) with 40° of flap to an altitude of about 500 feet, and then performing a maximum deceleration to the approach angle-of-attack using 70° flap and 30° of aileron droop with flight idle power. Power was then added to maintain the approach angle-of-attack while continuing to decelerate to the approach speed. This procedure reduced the time spent in the approach and generally expedited the operation. The most noticeable adverse effect of this technique was the departure from the original approach path in order to slow down. This effect would compromise its use on a conventional ILS glide path.[16]

Flight evaluation of the NC-130B offered important experience and lessons for subsequent STOL development. Again, as Quigley and Innis summarized, it clearly indicated that

The flight control system of an airplane in STOL operation must have good mechanical characteristics (such as low friction, low break-out force, low force gradients) with positive centering and no large non-linearities.

In order to aid in establishing general handling qualities criteria for STOL aircraft, more operational experience was required to help define such items as:

(1) minimum airport pattern geometry,

(2) minimum and maximum approach and climb-out angles,

(3) maximum cross wind during landings and take-offs, and

(4) all-weather operational limits.[17]

Overall, Quigley and Innis found that STOL tests of the NC-130B BLC testbed revealed

(1) With the landing configuration of 70° of flap deflection, 30° of aileron droop, and boundary-layer control, the test airplane was capable of landing over a 50-foot obstacle in 1,430 feet at a 100,000 pounds gross weight. The approach speed was 72 knots and the flight-path angle 5° for minimum total distance. The minimum approach speed in flat approaches was 63 knots.

(2) Take-off speed was 65 knots with 40° of flap deflection, 30° of aileron droop, and boundary-layer control at a gross weight of 106,000 pounds. Only small gains in take-off distance over a standard C-130B airplane were possible because of the reduced ground roll acceleration associated with the higher flap deflections.

(3) The airplane had unsatisfactory lateral-directional handling qualities resulting from low directional stability and damping, low side-force variation with sideslip, and low aileron control power. The poor lateral-directional characteristics increased the pilots’ workload in both visual and instrument approaches and made touchdowns a very difficult task especially when a critical engine was inoperative.

(4) Neither the airplane nor helicopter military handling quality specifications adequately defined stability and control characteristics for satisfactory handling qualities in STOL operation.

(5) Several special operating techniques were found to be required in STOL operations:

(a) Special procedures are necessary to reduce the time in the STOL configuration in both take-offs and landings.

(b) Since stall speed varies with engine power, BLC effectiveness, and flap deflection, angle of attack must be used to determine the margin from the stall.

(6) The minimum control speed with the critical engine inoperative (either of the outboard engines) in both STOL landing and take-off configurations was about 65 knots and was the speed at which almost maximum lateral control was required for trim. Neither landing approach nor take-off speed was below the minimum control speed for minimum landing or take-off distance.[18]

During tests with the YC-134B and the NC-130B, NASA researchers had followed related foreign development efforts, focusing upon two: the French Breguet 941, a four-engine prototype assault transport, and the Japanese Shin-Meiwa UF-XS four-engine seaplane, both of which used deflected propeller slipstream to give them STOL performance. The Shin-Meiwa UF-XS, which a NASA test team evaluated at Omura Naval Air Base in 1964, was built using the basic airframe of a Grumman UF-1 (Air Force SA-16) Albatross seaplane. It was a piloted scale model of a much larger turboprop successor that went on to a distinguished career as a maritime patrol and rescue aircraft.[19] However, the Breguet 941 did not, even though both America’s McDonnell company and Britain’s Short firm advanced it for a range of civil and military applications. A NASA test team was allowed to fly and assess the 941 at the French Centre d’Essais en Vol (the French flight-test center) at Istres in 1963 and undertook further studies at Toulouse and when it came to America at the behest of McDonnell. In conjunction with the Federal Aviation Administration, the team undertook another evaluation in 1972 to collect data for a study on developing civil airworthiness criteria for powered-lift aircraft.[20] The team members found that it had “acceptable performance,” thanks largely to its cross-shafted and opposite rotation propellers. The propellers minimized trim changes and asymmetric trim problems in the event of engine failure and ensured no lateral or directional moment changes with variations in airspeed and engine power. But they also found that its longitudinal and lateral-directional stability was “too low for a completely satisfactory rating” and concluded, “More research is required to determine ways to cope with the problem and to adequately define stability and control requirements of STOL airplanes.”[21] Their judgment likely matched that of the French, for only four production Breguet 941S aircraft were built; the last of which was retired in 1974. Undoubtedly, however, it was for its time a remarkable and influential aircraft.[22]

Another intriguing approach to STOL design was use of lift- enhancing rotating cylinder flaps. Since the early 1920s, researchers in Europe and America had recognized that the Magnus effect produced by a rotating cylinder in an airstream could be put to use in ships and airplanes.[23] Germany’s Ludwig Prandtl, Anton Flettner, and Kurt Frey; the Netherland’s E.B. Wolff; and NACA Langley’s Elliott Reid all examined airflow around rotating cylinders and around wings with spanwise cylinders built into their leading, mid, and trailing sections.[24] All were impressed, for, as Wolff noted succinctly, “The rotation of the cylinder had a remarkable effect on the aerodynamic properties of the wing.”[25] Flettner even demonstrated a “Rotorschiff” (rotor-ship) making use of two vertical cylinders functioning essentially as rotating sails.[26] However, because of mechanical complexity, the need for an independent propulsion source to rotate the cylinder at high speed, and the lack of advantage in applying these to aircraft of the interwar era because of their modest performance, none of these systems resulted in more than laboratory experiments. However, that changed in the jet era, particularly as aircraft landing and takeoff speeds rose appreciably. In 1963, Alberto Alvarez-Calderon advocated using a rotating cylinder in conjunction with a flap to increase a wing’s lift and reduce its drag. The combination would serve to reenergize the wing’s boundary layer without use of the traditional methods of boundary-layer suction or blowing. Advances in propulsion and high-speed rotating shaft systems, he concluded, “indicated to this investigator the need of examining the rotating cylinder as a high lift device for VTOL aircraft.”[27]

In 1971, NASA Ames Program Manager James Weiberg had North American-Rockwell modify the third prototype, YOV-10A Bronco, a small STOL twin-engine light armed reconnaissance aircraft (LARA), with an Alvarez-Calderon rotating cylinder flap system. As well as installing the cylinder, which was 12 inches in diameter, technicians cross-shafted the plane’s two Lycoming T53-L-11 turboshaft engines for increased safety, using the drive train from a Canadair CL-84 Dynavert, a twin-engine tilt rotor testbed. The YOV-10A’s standard three-bladed propellers were replaced with the four-bladed propellers used on the CL-84, though reduced in diameter so as to furnish adequate clearance of the propeller disk from the fuselage and cockpit. The rotating cylinder, between the wing and flap, energized the plane’s boundary layer by accelerating airflow over the flap. The flaps were modified to entrap the plane’s propeller slipstream, and the combination thus enabled steep approaches and short landings.[28]

Before attempting flight trials, Ames researchers tested the modified YOV-10A in the Center’s 40- by 80-foot wind tunnel, measuring changes in boundary layer flow at various rotation speeds. They found that at 7,500 revolutions per minute (rpm), equivalent to a rotational speed of 267.76 mph, the flow remained attached over the flaps even when they were set vertically at 90 degrees to the wing. But in the course of 34 flight-test sorties by North American-Rockwell test pilot Edward Gillespie and NASA pilot Robert Innis, researchers found significant differences between tunnel predictions and real-world behavior. Flight tests revealed that the YOV-10A had a lift coefficient fully a third greater than the basic YOV-10. It could land with approach speeds of 55 to 65 knots, at descent angles up to 8 degrees, and at flap angles up to 75 degrees. Researchers found that

Rotation angles to flare were quite large and the results were inconsistent. Sometimes most of the sink rate was arrested and sometimes little or none of it was. There never was any tendency to float. The pilot had the impression that flare capability might be quite sensitive to airspeed (CL)[29] at flare initiation. None of the landings were uncomfortable.[30]

The modified YOV-10A had higher than predicted lift and downwash values, likely because of wind tunnel wall interference effects. It also had poor lateral-directional dynamic stability, with occasional longitudinal coupling during rolling maneuvers, though this was a characteristic of the basic aircraft before installation of the rotating cylinder flap and had, in fact, forced addition of vertical fin root extensions on production OV-10A aircraft. Most significantly, at increasing flap angles, “deterioration of stability and control characteristics precluded attempts at landing,”[31] manifested by an unstable pitch-up, “which required full nose-down control at low speeds” and was “a strong function of flap deflection, cylinder operation, engine power and airspeed.”[32]

As David Few subsequently noted, the YOV-10A’s rotating cylinder flap-test program constituted the first time that: “a flow-entrainment and boundary-layer-energizing device was used for turning the flow downward and increasing the wing lift. Unlike all or most pneumatic boundary layer control, jet flap, and similar concepts, the mechanically driven rotating cylinder required very low amounts of power; thus there was little degradation to the available takeoff horsepower.”[33]

Unfortunately, the YOV-10A did not prove to be a suitable research aircraft. As modified, it could not carry a test observer, had too low a wing loading—just 45 pounds per square foot—and so was “easily disturbed in turbulence.” Its marginal stability characteristics further hindered its research utility, so after this program, it was retired.[34]

NASA’s next foray in BLC research was a cooperative program between the United States and Canada that began in 1970 and resulted in NASA’s Augmentor Wing Jet STOL Research Aircraft (AWJSRA) program. The augmentor wing c