X-14: A Little Testbed That Could

On May 24, 1958, Bell test pilot David Howe completed a vertical take­off followed by conventional flight, a transition, and a vertical landing during testing at Niagara Falls Airport, NY. His short foray was a mile­stone in aviation history, for the flight demonstrated the practicality of using vectored thrust for vertical flight. Howe took off straight up, hov­ered like a helicopter, flew away at about 160 mph, climbed to 1,000 feet, circled back, approached at about 95 mph, deflected the engine
thrust (which caused the plane to slow to a hover a mere 10 feet off the ground), made a 180-degree turn, and then settled down, anticipat­ing the behavior and capabilities of future operational aircraft like the British Aerospace Harrier and Soviet Yak-38 Forger.

The plane that he flew into history was the Bell X-14, a firmly sub­sonic accretion of various aircraft components that proved to have sur­prising value and utility. Before proceeding with this ungainly creature, company engineers had first built a VTOL testbed: the Bell Model 65 Air Test Vehicle (ATV). The ATV used a mix of components from a glider, a lightplane, and a helicopter, with two Fairchild J44 jet engines attached under its wing. Each engine could be pivoted from horizontal to vertical, and it had a stabilizing tail exhaust furnished by a French Turbomeca Palouste compressor as well. Tests with the ATV convinced Bell of the possibility of a jet convertiplane, though not by using that particular approach, and the ATV never attempted a full conversion from VTOL to conventional flight. Accordingly, X-14 differed from all its predeces­sors because it used a cascade thrust diverter, essentially a venetian – blind-like vane system, to deflect the exhaust from the craft’s two small British-built Armstrong-Siddeley Viper ASV 8 engines for vertical lift. Each engine produced 1,900 pounds of thrust. Since the aircraft gross weight was 3,100 pounds, the X-14 had a thrust to weight ratio of 1.226. Compressed-air reaction "controls” kept the craft in balance during take­off, hovering, and landing, when its conventional aerodynamic control surfaces lacked effectiveness. To simplify construction, the X-14 had an open cockpit, no ejection seat, the wings of a Beech Bonanza, and fuse­lage and tail of a Beech T-34 Mentor trainer.[1430]

Early testing revealed that, as completed, the aircraft had numer­ous deficiencies typical of a first-generation technological system. After Ames acquired the aircraft, its research team operated the X-14A with due caution. Not surprisingly, weight limitations precluded installation of an ejection seat or even a rollover protection bar. The twin engines imparted strong gyroscopic "coupling” forces, these being dramatically illustrated on one flight when the X-14’s strong gyroscopic moments gen­erated a severe pitch-up during a yaw, "which resulted in the aircraft
performing a loop at zero forward speed.”[1431] The X-14’s hover flight – test philosophy was rooted in an inviolate rule: hover either at 2,500 feet, or at 12-15 feet, but never in between. At the higher altitude, the pilot would have sufficient height to recover from a single engine fail­ure or to bail out. At the lower altitude, he could complete an emergency landing.[1432] Close to the ground, the aircraft lost approximately 10 per­cent of its lift from so-called aerodynamic suck-down while operating in ground effect. During hover operations, the jet engines ingested the hot exhaust gas, degrading their performance. As well, it possessed low control power about all axes, and the lack of an SAS resulted in marginal hover characteristics. Hover flights were often flown over the ramp or at the concrete VTOL area north of the hangar, and typical flights ran from 20 to 40 minutes and within an area close enough to allow for a comfortable glide back to the airfield. Extensive flight-testing investi­gated a range of flying qualities in hover. Those flights resulted in cri­teria for longitudinal, lateral, and directional control power, sensitivity, and damping.[1433]

By 1960, Ames V/STOL expertise was well-known throughout the global aeronautical community. This led to interaction with aeronauti­cal establishments in many countries pursuing their own V/STOL pro­grams, via the North Atlantic Treaty Organization’s (NATO) Advisory Group for Aeronautical Research and Development (AGARD).[1434] For example, Dassault test pilot Jacques Pinier flew the X-14 before fly­ing the Balzac. So, too, did Hawker test pilots Bill Bedford and Hugh Merewether before tackling the P. 1127. Both arrived at Ames in April 1960 for familiarization sorties in the X-14 to gain experience in a "simple” vectored-thrust airplane before trying the more complex British jet in VTOL, then in final development. Unfortunately, on Merewether’s sortie, the X-14 entered an uncontrolled sideslip, touching down hard and breaking up its landing gear, a crash attributed to low roll control power and no SAS. "Though bad for the ego,” the British pilot wrote good-naturedly later, "it was probably a blessing in

disguise since it brought home to all and sundry the perils of weak reaction controls.”[1435]

Later that year, Ames technicians refitted the X-14 with more pow­erful 2,450-pound thrust General Electric J85-5 turbojet engines and modified its flight control system with a response-feedback analog com­puter controlling servo reaction control nozzles (in addition to its exist­ing manually controlled ones), thus enabling it to undertake variable stability in-flight simulation studies. NASA redesignated the extensively modified craft as the X-14A Variable Stability and Control Research Aircraft (VSCRA). In this form, the little jet contributed greatly to under­standing the special roll, pitch, and yaw control power needs of V/STOL vehicles, particularly during hovering in and out of ground effect and at low speeds, where conventional aerodynamic control surfaces lacked effectiveness.[1436] It still had modest performance capabilities. Even though its engine power had increased significantly, so had its weight, to 3,970 pounds. Thus, the thrust to weight ratio of the X-14A was only
marginally better than the X-14.[1437] For one handling qualities study, researchers installed a movable exhaust vane to generate a side force so that the X-14A could undertake lateral translations, so they could study how larger V/STOL aircraft, of approximately 100,000 pounds gross weight, could be safely maneuvered at low speeds and altitudes. To this end, NASA established a maneuver course on the Ames ramp. The X-14A, fitted with wire-braced lightweight extension tubes with bright orange Styrofoam balls simulating the wingspan and wingtips of a much larger aircraft, was maneuvered by test pilots along this track in a series of flat turns and course reversals. The results confirmed that, for best low-speed flight control, V/STOL vehicles needed attitude-stabilization, and, as regards wingspan effects, "None of the test pilots could perceive any effect of the increased span, per se, on their tendency to bank during hovering maneuvers around the ramp or in their method of flying the airplane in general.”[1438]

Attitude control during hover and low-speed flight was normally accomplished in the X-14A through reaction control nozzles in the tail for pitch and yaw and on each wingtip for roll control. Engine compres­sor bleed air furnished the reaction control moments. For an experi­mental program in 1969, its wingtip reaction controls were replaced temporarily by two 12.8-inch-diameter lift fans, similar to those on the XV-5B fan-in-wing aircraft, to investigate their feasibility for VTOL roll control. Bleed air, normally supplied to the wingtip reaction control nozzles, drove the tip-turbine-driven fans. Fan thrust was controlled by varying the pressure ratio to the tip turbine and thereby controlling fan speed. Rolling moments were generated by accelerating the rpm of one fan and decelerating the other to maintain a constant net lift.[1439]

A number of "lessons learned” were generated as a result of this handling qualities flight-test investigation, as noted by project pilot Ronald M. Gerdes. The fans were so simple, efficient, and reliable that the total bleed air requirement was reduced by about 20 percent from that required
using the tip nozzles. As a consequence, the jet engines produced about 4 percent more thrust and could operate at lower temperatures during vertical takeoffs. Despite this, however, during the flight tests, control sys­tem lag and increases in the aircraft moment of inertia caused by place­ment of the fans at the tips negated the increased roll performance that the fans had over the reaction control nozzles and resulted in the pilot having a constant tendency to overcontrol roll-attitude and thus induce oscillations during any maneuver. The wingtip lift-fan control system was thus rated unacceptable, even for emergency conditions, as it scored a Cooper Harper pilot rating of 6% to 7% (on a 1-10 scale, where 1 is best and 10 is worst). Finally, Gerdes concluded: "This test also demonstrated a principle that must be kept in mind when considering fans for controls. Even though the time response characteristics of a fan system are capa­ble of improvement by such means as closing the loop with rpm feed­back, full authority operation of the control eliminates the fan speed-up capabilities provided by the closed loop, and the fans revert to their open- loop time constants. In the case of the X-14A, its open – and closed-loop first-order time constraints were 0.58 and 0.34 seconds, respectively.”[1440]

The X-14A flew for two decades for NASA at the Ames Research Center, piloted by Fred Drinkwater and his colleagues on a variety of research investigations. These ranged from evaluating sophisticated electronic control systems to simulating the characteristics of a lunar lander in support of the Apollo effort. In 1965, it was configured to enable simulations of lunar landing approach profiles. The future first man on the Moon, Neil Armstrong, flew the X-14A to evaluate its con­trol characteristics and a visual simulation of the vertical flightpath that the Apollo Lunar Module would fly during its final 1,500-foot descent from the Command Module (CM) to a landing upon the lunar surface.[1441]

Another study effort examined soil erosion caused by VTOL oper­ations off unprepared surfaces. In this case, a 5-second hover at 6 feet
resulted in chunks of soil and grass being thrown into the air, where they were ingested by the engines, damaging their compressors and forcing subsequent replacement of both engines.[1442]

In 1971, under the direction of Richard Greif and Terry Gossett, NASA modified the X-14A a third time, to install a digital variable sta­bility system and up-rated GE J85-19 engines to improve its hover per­formance. It was redesignated as the X-14B and flown in a program "to establish criteria for pitch and roll attitude command concepts, which had become the control augmentation of choice for precision hover.”[1443] Unfortunately, in May 1981, a control software design flaw led to satu­ration of the VSCS autopilot roll control servos, a condition from which the pilot could not recover before it landed heavily. Although NASA con­templated repairing it, the X-14B never flew again.[1444]

As a personal aside, having had the opportunity to fly the X-14B near its final flight, I was impressed with its simplicity.[1445] For example, one of the more important instruments on the airplane was a 4-inch piece of yarn attached to a small post in the center of the front windshield bow. You never wanted to see the yarn pointed to the front of the airplane. If you did, it meant you were flying backward, and that was a real no-no! The elevator had a nasty tendency to dig in and flip the aircraft over on its back. We aptly named the flip the "Williford maneuver,” after J. R. Williford, the first test pilot to inadvertently "accomplish” it. The next most impor­tant instrument was the fuel gauge, because the X-14 didn’t carry much gas. In retrospect, I consider it a privilege to have flown one of the most successful research aircraft of all time, one that in over 20 years contrib­uted greatly to a variety of other VTOL programs in technical input and piloting training, and to the evolution of V/STOL technology generally.