F-14 Tomcat Testing

On December 30, 1970, the first aircraft crashed during its second flight. The hydraulic failure which caused the first F-14 crash, was a freak accident and-was quickly remedied.

On 21 December 1970, the first Full Scale Development (FSD) Grumman F-14A Tomcat (BuNo 157980) took off for its maiden flight from Grumman’s flight test centre at Calverton. Grumman chief test pilot Robert K. Smyth and project test pilot William Miller took off in spite of the bad weather: the poor weather conditions, however, forced the test pilots to cut the flight, consisting in a couple of visual patterns with the wings in the forward position, short. Although the flight ws shorter than initially planned, the first Tomcat flew a month ahead of the contracted data and showed the great potential of the aircraft.

The F-14 BuNo 157980 took off for the second time on 30 Decembe 1970, with y Miller in the front cockpit, since in the first flight Smyth had been in front. During this flight a chase plane noted the Tomcat leaving a trail of smoke. Soon thereafter the F-14 experienced a primary hydraulic system failure, and Miller headed back to base. While they were preparing to land, the secondary hydraulic system also failed, the pilot lost control over the aircraft, and the crew was forced to eject. The breakdown was caused by a fatigue failure of both titanium main hydraulic lines. Ironically, the F-14’s hydraulic system was fixed by changing from titanium to stainless steel hydraulic lines.

Initial flight-test evaluations of the performance of the F-14 by the Navy revealed higher drag levels at high subsonic and transonic speeds than had been predicted by Grumman wind-tunnel tests. The Navy requested Langley support in analyzing and providing solutions to the problem. Langley experience with the F-111 and other advanced fighter concepts indicated that an extremely large portion of the high subsonic and transonic cruise drag of modern twin-engine fighters is contributed by the aft end of the configuration. For example, a relatively poor aft-end design could produce almost 50 percent of the cruise drag for some configurations. In the course of the F-111 development program, Langley researchers in the 16-Foot Transonic Tunnel had developed test techniques and analysis methods to minimize this problem, and they went to work on the aft-end aerodynamic characteristics of the F-14 configuration.

Based on their extensive experience, the 16-Foot Transonic Tunnel team conducted tests in 1972 to determine the characteristics of the critical engine-fuselage fairing (pancake) at the rear of the aircraft. Several geometric variations were evaluated to determine more effective pancake shapes, with an appreciation of the trades that are necessary to minimize component interference drag while adhering to the area rule developed by Langley researcher Dr. Richard Whitcomb. In addition, certain regions had to be preserved for the F-14 fuel jettison system and the landing arrestor hook. The researchers cut away areas of the pancake, reshaped the geometry and added a "handle" in the shape of a bulbous pod at the rear of the pancake. The design recommended by the Langley tests proved extremely effective in reducing cruise drag and was incorporated into the F-14 configuration.

In addition to the pancake modification, Langley researchers recommended that generous "speed bump" (area added to shape the aircraft to comply with Whitcomb's area rule) fairings be added to the forward bottom area of the vertical tails. The suggestion was accepted by Grumman and incorporated into the production F-14 fleet.

In early 1970, initial tests conducted in the Langley 20-Foot Vertical Spin Tunnel at the request of the Navy indicated that the F-14 would exhibit two types of spins. The first spin involved relatively steep, nose-down spins from which recovery would be relatively easy for the pilot. However, the results also showed that the F-14 might exhibit a relatively flat unrecoverable spin in which the aircraft would rotate rapidly (2 sec per turn) about a vertical axis through its center of gravity, while descending vertically with the fuselage in a relatively horizontal attitude. Because of the high rate of rotation of the flat spin, the g-forces at the cockpit location would be very high (approximately 6.5 longitudinal g's outward) and would probably incapacitate the pilot if sustained for even a moderate period of time.

Under the direction of Langley researcher James S. Bowman, Jr. exhaustive spin tunnel tests were conducted to define a spin recovery procedure, but the aerodynamic control surfaces of the F-14 were ineffective at these flat attitudes. In fact, even a scaled version of a very large 35-ft diameter spin recovery parachute (the largest that could be carried by the F-14 was 21 ft) could not recover the model from the spinning motions. The only concept that provided marginal recovery was the simultaneous application of normal recovery controls, deployment of the emergency parachute, and extension of auxiliary canards on the nose of the model.

Unrecoverable flat spins have been exhibited by many fighter configurations in the Langley Spin Tunnel, and such characteristics are viewed with concern. However, additional types of model tests are required to judge the seriousness of the problem. In particular, drop-model tests are conducted to determine if the aircraft can enter the spin from initial flight conditions. Spin tunnel tests are conducted with the model launched in a flat attitude into a vertically rising airstream-conditions very favorable for the spin to stabilize. However, in actual flight conditions, many aircraft lack the control power required to reach these conditions. For example, although the F-5 aircraft exhibited a flat spin in Langley Spin Tunnel tests, it was virtually impossible for pilots to intentionally enter the flat spin.

Two F-14 drop models were under fabrication when the spin tunnel results became known. The Langley, Grumman, and Navy team had planned to equip the models with extensive instrumentation to measure flight variables for correlation with analytical studies of the spin, but the installation process would have taken several months. Because the Navy required an immediate answer about the susceptibility of the F-14 to enter flat spins, Marion O. McKinney directed the team to install limited instrumentation in one of the models, and to obtain answers as quickly as possible. With the approval of this approach by the Navy, Charles E. Libbey and his team installed a miniature movie camera in the engine inlet of the model and pointed it forward, where it could monitor the relative angle of a simple wooden angle-of-attack vane mounted on a nose boom. This innovative approach gave the Navy answers in a few weeks, rather than months.

Results of the tests showed that the model could be pitched up in an aggressive manner with no tendency to enter either the steep or flat spins. However, if the roll control (differential deflection of the horizontal tails) was used in normal fashion to pick up a down-going wing at high angles of attack, the model would depart controlled flight in a direction opposite of the intended input because of adverse yaw caused by large yawing moments produced by the horizontal tails. If the pilot held in the roll control, the model would enter a flat spin. Recovery from flat spins requires the use of an emergency parachute, special nose canards, and full differential tail deflections.

An automatic rudder interconnect (ARI) for the F-14 was implemented. The ARI system automatically phased out movement of the tails for roll control and phased in deflections of the rudders at high angles of attack. The concept was refined and matured in the simulator studies. The pilots who flew the F-14 with the ARI system were enthusiastic, and the system allowed the pilot to maneuver the aircraft without regard to angle of attack or switching from differential tails to rudders. The Grumman team regarded the ARI concept developed by Langley as a highly desirable addition to the F-14 aircraft. The production contract for the early F-14 aircraft called for the implementation of an ARI system.

Unfortunately, the early F-14 aircraft also included another late developing preproduction concept-deployable wing leading-edge maneuver slats for improved maneuverability. Early Grumman flight tests revealed that the F-14 modified with both the ARI system and the maneuver slats displayed unsatisfactory air combat maneuvering characteristics because the ARI rudder inputs aggravated lightly damped rolling oscillations (wing rock) induced by the slats during maneuvers. Because of this incompatibility, the Navy deactivated the ARI systems on all fleet F-14 aircraft.

The F-14 proved to be a relatively forgiving aircraft to fly, and pilots adapted to manually switching from using differential tails for roll control at low angles of attack to using rudders at high angles of attack. However, the F-14 fleet began to experience spin losses at the rate of about one aircraft per year. In 1978, a joint NASA, Navy, and Grumman program was initiated to develop a new ARI system to increase the spin resistance of the F-14. A new ARI that provided adequate damping of the wing rock, while retaining the spin resistance of the original ARI system developed by Langley. A flight-test F-14 was modified with a spin parachute, battery driven hydraulic pumps for emergency power, and the special foldout canards on the fuselage forebody that were recommended by the earlier spin tunnel tests. Fitted with the new ARI, flight tests were conducted over a 2-year period at NASA Dryden Flight Research Center with Langley personnel on site for the flight tests. Over 100 flights by 9 pilots were made up to low supersonic speeds.

The results of the flight-test program were extremely impressive. Wing rock was suppressed, inadvertent spins were eliminated, and the handling qualities throughout the air combat envelope were improved. Several years passed before funding constraints permitted the Navy to develop the ARI within plans to equip the F-14 fleet with a new advanced digital flight control system (DFCS). Following further refinements during Navy flight evaluations at Patuxent River Naval Air Station in Maryland, the Navy implemented the DFCS with the ARI. The first F-14 deployments with the ARI occurred during the Kosovo operations, and glowing reports from the F-14 squadrons indicated that the new system was a success.

Flutter clearance tests of the F-14 in the Langley 16-Foot Transonic Dynamics Tunnel required five entries from 1970 to 1973. The utilization of variable-sweep wings by the F-14 introduced a unique flutter problem that had been unanticipated prior to the tests. The challenge of providing an upper-fuselage covering for the variable-sweep wing panels had been addressed by Grumman with relatively flexible inner wing covers. Early in the flutter tests, large deflections and buffeting of the over wing panels were observed and viewed as a potentially serious flutter problem.

With knowledge of the Langley results, Grumman engineers designed a set of external stiffening strakes for the wing covering that eliminated the flutter problem. An additional favorable impact of the strakes was local straightening of the airflow over the upper fuselage, which resulted in performance benefits. With the strake modification, the F-14 passed flutter clearance tests in the Langley tunnel. Initially, it was proposed that this modification would only be applied to the preproduction flight-test F-14 aircraft while a redesign of the wing cover could be accomplished. However the modification proved to be extremely robust and similar strakes were incorporated in all production F-14 aircraft.

During the flutter tests, the Langley staff observed considerable buffeting of the vertical tails, particularly at moderate angles of attack. The staff of the 16-Foot Transonic Dynamics Tunnel modified the cable-mount system to permit tests at high-angle-of-attack conditions where the buffeting became more intense. Langley expressed concern that damage to the structural integrity of tails or tail-mounted avionics and antennae might be encountered, but Grumman did not accept this concern as an issue. Subsequently, the F-14 fleet experienced structural damage and the replacement of tail-mounted radar warning units. As a result, the vertical tails of F-14 aircraft were stiffened.

The experience of the engineering community with vertical-tail buffeting in the F-14 led to the development of design analysis tools and special wind-tunnel test techniques for follow-on aircraft including the F/A-18 and F-22.

In the early 1980's, researchers in the Navy community became interested in the potential benefits of using thrust vectoring for control augmentation of the F-14. A cooperative Langley and Navy piloted simulator study defined the benefits to a representative fighter aircraft of maintaining the control power required for satisfactory V/STOL flight in conventional flight. In this study, an existing Langley simulator model of the F-14 was modified to incorporate the control modifications under the leadership of Luat Nguyen. Langley and DOD pilots flew the simulated flights.

Results of the simulator study showed that the most important benefit occurred when the yaw control was augmented at high angles of attack (normally, yaw control provided by conventional rudders is markedly reduced at high angles of attack). With the increased yaw control capability, pilots could consistently win against a variety of adversaries in simulated air-to-air combat. Analysis of the desired control levels in the simulator results indicated that deflecting the engine thrust on aircraft similar to the F-14 would provide the necessary control.

To pursue the development and demonstration of the effectiveness of yaw vectoring, the Navy conducted a series of tests to evaluate the turning effectiveness and structural integrity of external vanes mounted behind the nozzles of the F-14 engines. These activities were augmented by tests in the Langley 16-Foot Transonic Tunnel. Data obtained in these tests were used to define the geometry and thrust-vectoring effectiveness for the Navy evaluations.

Full-scale vanes were fabricated and initially ground tested behind an F-14 aircraft at the Patuxent River facility. Flight tests of a modified F-14 were subsequently conducted to demonstrate the structural integrity and thrust-vectoring performance of the vane concept over a limited flight envelope.