Special thanks to Raymond L Puffer - HistorianBy Assistant Webmaster
David de Botton - Flash@F-111.net
Presented at the
FLIGHT TEST REPORT - F-111
On October 15 1964 the roll out ceremony of the F-111 was the public unveiling of a new concept in aircraft design. Introduced on this occasion was an airplane with the capability of spectacular performance over the widest range of altitude and Mach numbers ever realized in any operational aircraft. Having a take off and landing performance comparable to the slow speed aircraft of yester year the F-111 is able to sustain supersonic speeds from sea level to altitudes above 60,000 feet. Though it's maximum speed is only a step beyond the capabilities of today's fighters in the present day inventory the major advance of the F-111 is an operational flexibility and mission capability never previously achieved.
Designed and manufactured under the unique arrangement of a fixed price contract with guaranteed performance, reliability, and maintainability, the F-111 is the newest and one of the most interesting airplanes ever developed. Presented herein is a thumb nail sketch of some of the more interesting features of the F-111 and a brief report on the high lights of it's initial flight test phase conducted from December 1964 to September 1965.
From the very beginning the F-111 program had many unique requirements, and one of the most difficult to satisfy was the retention of a full performance spectrum simultaneously maintaining the capability of performing the missions of the two services, the Air Force and the Navy. In the interest of reducing production and development costs maximum Air Force and Navy commonality was also a very important criterion. As a means of accelerating operational availability and in-order to achieve maximum reliability "off-the-shelf" sub-systems and existing construction techniques were used where ever possible. On first examination of the Air Force and Navy missions, the task was impossible. The primary mission of the Air Force airplane was that of a fighter bomber with a secondary air to air capability for air superiority. The navy's requirements were just the reverse. The primary mission of the navy was a long endurance air superiority capability with a secondary air to ground role.
Further consideration of the task revealed that, with a single exception, the airframe performance requirements of both services were quite similar. A dispersal requirement of the Air Force to operate in relatively short and unprepared landing fields coincided with the navy's carrier operation requirements. The Navy's requirement to loiter on station for maximum duration was quite similar to the Air Force maximum range requirement for global deployment. Both services had similar performance requirements for air superiority in the high Mach high altitude envelope. The single exception was the requirement of the Air Force to have the capability of sustained supersonic operation at very low altitude.
This wide variation in the performance envelope was the requirement that dictated the unique feature of variable sweep. In keeping with the program philosophy of maximum commonality the Air Force version (designated F-111A) and the navy version (designated F-111B) became 85% common by actual part numbers. The major difference between the two versions are only those equipment and requirements that are both essential and unique to the operation of each service. A comparison of the two versions quickly reveals almost complete similarity in appearance with two minor differences; a shorter nose length and longer wing span. The wing span of the F-111A varies from 63 feet in the most forward wing sweep angle to 32 feet in the most aft wing sweep angle while the F-111B wing span varies from 70 feet to 34 feet for the same wing sweep angles. The F-111B wing is an F-111A wing with 3.5 foot sections added to each wing tip.
Because of the different avionics the nose section forward of the cockpit is different, resulting in 73.5 foot length for the F-111A and approximately 67 feet for the F-111B. Both airplanes have a side by side seating arrangement for the crew of two pilots for the F-111A and one pilot plus a missile control officer for the F-111B. Both airplanes carry fuel in the wing and fuselage integral fuel tanks; have common hydraulic, electrical and flight control systems; and are powered by two TF30 after burning turbo fan engines. Both airplanes carry missiles in a fuselage missile bay and on a swiveling pylons externally mounted below the wing. In the interest of saving weight the landing gear on each airplane is slightly different being designed to specifications unique to each service. The F-111B for high sink rate landings, and the F-111A for low foot print pressures.
Other minor differences exist but to the casual observer the only apparent differences are the nose and the wing span. With it's wings swept forward for landing and take off the airplane assumes the proportions of being very large as a fighter aircraft. Though eight to fourteen feet longer than two other supersonic fighters presently in the USAF inventory the F-111A with the wings swept to 72 degrees has a wing span some three to seven feet shorter than these same supersonic fighters. In addition to the variable wing sweep which allows an optimum cruise configuration for each Mach number the feature of the F-111 that contributes the most to range performance is the propulsion system. The F-111 is equipped with two Pratt & Whitney TF-30 afterburning fan engines. Not only does this engine allow exceptional long range cruise economy in the afterburning range of operation it also has a very good specific fuel consumption through the major portion of the afterburning range of operation. The afterburner can be modulated from 20% to 100% afterburner thrust which allows selection of the most efficient cruise power for extended supersonic operation and provides high thrust for short take-off and high rates of acceleration during flight. One of the attractive features of variable seep is that it allows maximum use of high lift devices to achieve maximum take-off and landing performance. The F-111 takes full advantage of this feature and has full span flaps and leading edge slats, but it does not have boundary layer bleed. Both the slats and flaps are manually operated by the pilot. The slats are actuated electrically while the flaps are actuated hydraulically. Flap and slat operation is limited to wing-sweep angles forward of 26 degrees. For wing-sweep positions greater than 26 degrees sweep the inboard section of the flap is retracted into the wing sweep well and thus does not allow flap or slat operation. An asymmetric device locks the slats and flaps in place in the event the left and right flaps or slats positions do not coincide within 5 degrees tolerance.
The flight crew of the F-111 have a shirt sleeve environment through the use of a crew escape module which is an integral part of the airplane. This module which consists of the cockpit area plus a portion of the wing glove area (for stability) separates from the aircraft through the use of primer cord and a 20,000lb/sec rocket motor. The module has a zero-altitude, zero-airspeed escape capability and contains flotation bags and survival gear to allow global survival on land or in the water. The module may be fired by either aircrew member.
The flight control system is an irreversible system powered by two separate hydraulic systems. Each system is powered by two engine driven pumps (one for each) and consequently the loss of either engine reduces the flow rate to each hydraulic system but does not affect hinge moment capability. The horizontal stabilizer has both differential and symmetrical motion to provide control in pitch and roll. The F-111 does not have ailerons but when the wings are swept forward of 45 degrees spoilers along the top of the wing provide additional roll control. These spoilers are commanded linearly with lateral stick motion. In addition these spoilers are extended symmetrically during landing to increase landing performance. (It should be noted that the airplane does not have a drag chute). Directional control is provided through the use of a conventional rudder. The stability argumentation system contains fixed gains in the yaw channel but has a self adaptive gain changing system in pitch and roll. In addition, a command augmentation system is used in pitch and roll to provide relatively constant stick force per "G" characteristic in low speed flight. Because of the large authority of dampers and the stability augmentation has triple redundancy in all three axes as protection against a single hard over failure. Any single malfunction will be switched out by a logic voting circuit and will result in no degradation of performance. The wing-sweep mechanism is powered by both hydraulic systems of the airplane and is capable of operation with either system operating. The wings are swept by individual irreversible actuators which mechanically interconnect to prevent asymmetrical wing sweep. In the event of a failure of either actuator the mechanical interconnection prevents movement of the opposite actuator and thus prevents an asymmetric wing condition. Wing sweep actuation is commanded by a sliding pistol grip handle mounted under the left canopy rail. It's motion is related to performance so that forward motion configures the airplane for higher speed. As a result, it presently commands aft-wing-sweep by forward motion of the handle. In the event of a failure with the wings swept in the most aft position a landing can be accomplished at a touchdown speed slightly below 200 knots.
Though other systems and features of the F-111 are worthy of mention, the ones described have been mentioned as features of primary interest. Of greater interest is how well the systems and airplane perform in flight. On 21 December 1964 the F-111A achieved it's first flight. On this flight the wings were fixed at 26 degrees wing sweep and no wing sweep operation was made because of a delay in final qualification tests of the wing-sweep mechanism. The primary mission of this flight as in all first flights was to demonstrate flight, check systems operation, and to evaluate low speed handling characteristics . A failure of the flap asymmetry device prevented flap and slat retraction otherwise all systems operated satisfactorily including landing gear operation.
Some 16 days later on 6th January the second flight was launched with complete success and during this flight wing sweep was demonstrated from the full forward to the full aft sweep position. As was predicted, longitudinal trim change was negligible, being much smaller in magnitude than the trim changes that occur with landing gear of flap extension on most aircraft. On completion of this flight the airplane seem well on it's way to achieving it's primary mission of conducting flutter investigation. Flutter analysis studies before the initial flight has imposed a temporary speed restriction of 400 knots and 1.2 Mach which could only be extended by flutter investigation flights. But before any significant progress could be made towards the extension of this envelope an engine stall problem was encountered. This problem was soon identified as an engine inlet matching problem and was primarily a result of the distortion tolerance of the engine not being compatible with the distortion characteristics of the inlet. In retrospect we now know that we had some indication of the potential problem during our wind tunnel testing programs. However, this was not brought home to us until we begun to conduct our actual flight test program. To make a long story short, we have been able to reduce the distortion of the inlet substantially and Pratt & Whitney has improved the tolerance of the engine by a considerable degree. We have enlarged our flight envelope to above mach 2 and altitudes above 50,000 feet. At present time it appears wee have the solution in sight, if not actually in hand. In thinking about this problem it's important that we keep in mind that this particular engine and inlet combination must function and be effective throughout a flight spectrum that ranges from 100 knots to Mach 2.5. In our case, that is the inlet and the duct, the solution consisted of a relatively simple vortex generator pattern coupled with the refinement of the ducts interior lines and an improvement in our fuselage boundary layer bleed. The combination of these measures reduced the most critical distortion pattern from about 12.5% to less than 6% On February 25th 1965, the second airplane made it's initial flight immediately begun it's scheduled test program of stability and control.
Though limited severely by the current flutter envelope, significant progress was achieved in the refinement of the flight control system. One of the most important achievements was improvement in operation of the adaptive gain system. Early flights had revealed the gain system was responsive to turbulence causing the gains to reduce to a minimum level. In such conditions the signal to the dampers was so small that for all practical purposes the dampers were disengaged. By increasing the threshold of the adaptive gain computers and making some minor changes to the gain computer clock, successful gain scheduling was achieved through all flight conditions including turbulence. An early evaluation of the handling characteristics in roll revealed the spoiler scheduling versus stick motion in roll commands contained small discontinuities during initial spoiler motion. The obvious result was an inability to make precise roll commands in the stick motion regions when these discontinuities existed.
Rescheduling spoiler motion allowing a more nearly asymptotic approach to the fully retracted position resulted in a completely smooth roll rate versus lateral stick position and or lateral stick force curve. Most importantly this program gave opportunity to verify the predicted excellent handling characteristics. In all flight conditions with "SAS on", damping is well within specification requirements. Airplane response about all axes, very good in all cruise conditions, is also impressively good in the power approach configuration. Nevertheless there still remain improvements to be completely resolved. Trimmability, initially rather poor because of high trim rates in both roll and pitch, has been improved but needs further development. The effect of yaw damper operation on turn co-ordination in the landing configuration also requires some additional improvement; and most significantly, there still remains a large portion of the flight envelope to envestigate. But within the flight envelope tested, the overall handling characteristics have been exceptionally good.
Concurrent with the stability and control program of the second F-111A the first F-111B began it's stability and control program in May 1965. The initial mission of this airplane was to conduct stability and control investigation of the low speed envelope. It should be noted that no significant differences were reported in regards to the handling characteristics of the F-111B. One of the primary purposes of this airplane is to clear the flutter envelope of the Navy configuration and these tests will commence on the completion of the current Navy Pilot Evaluation. As a means of allowing early develpoment of the high lift devices through flight test the 4th airplane was equipped with a boiler plate configuration of the fixed flaps and slats. The extended wing tips of the F-111B configuration were also added to allow early evaluation of the Navy configuration.
Because spin studies and spin tunnel tests predicted a divergance in yaw at the stall the airplane was equipped with with a spin chute before commencing it's stall program. Though this program is only in it's initial phase, stall speeds well below 100 knots have been achieved at relatively high gross weights and initial evaluation of the stall characteristics have been performed. Mild buffet occurs approximately five knots before stall; and at the stall, the aircraft nose drops and rather slowly diverges in yaw. Airplane controllability is quite good and recovery is immediate when back pressure on the stick is relieved. This program is scheduled to continue for the refinement of the high lift configuration as well as to obtain preliminary landing performance data.
In addition to the above programs, two other aircraft are in the process of conducting developing tests of the Navigation and Attack Radar system (including the terrain following radar) , and the mission and traffic control systems. We have been most pleased with both the functioning and the reliability of the systems. For example,our navigation system has indicated accuracies that are better than spec requirements by a factor of two. This same system in a total of 169 flight hours has incurred only three failures. The terrain following radar with the exception of a small bias error has demonstrated similar accuracies and reliability. The mechanical, hydraulic, and electrical systems have been most completely free of discrepancies or maintenance problems and we have demonstrated that the airplane can be quickly turned around. For example, our stability and control test airplane has flown as many as five flights over a three day span.
Though the engine inlet problem which I have mentioned caused some delay in the flight test program we are catching up fast. Recent flights in excess of Mach 2.0 give indication that this problem is well on it's way to final resolution and other test programs can now progress as scheduled. Despite delays a number of significant achievements have been accomplished during this intitial flight test phase. These achievements are; #1 - Demonstration of variable sweep. #2 - Operation of the stability augmentation system in automatic gains. #3 - Intial evaluation of the F-111B. #4 - Flutter investigation in excess of Mach 2.0 #5 - Initial tests on the Navigation and Attack Radar. #6 - Demonstration of Terrain Following Radar. #7 - Demonstration of good handling characteristics. #8 - Initial stall tests.
Though there satisfaction in the contemplation of past achievements it is the challenge of new problems and goals that inspire man to progess. For the F-111 the best is yet to come.
David de Botton - Flash@F-111.net
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