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Converting A Citation Business Jet to a military trainer
Aircraft Design 1 (1998) 51—60
Converting A Citation Business Jet to a military trainer James W. Lyle Jr* The Cessna Aircraft company, A Textron company, 1 Cessna Blvd, Wichita, Kansas 67215, USA
Abstract The United States Navy needed a replacement for aging T-39 Naval Flight Officer (NFO) training aircraft. NFOs perform radar and navigation functions on Navy aircraft. It was decided that conversion of FAA certified business aircraft would be the most economical approach. They also upgraded the groundbased training systems. The Cessna Aircraft Company won the competition, proposing the Citation Model 550 which had to be heavily modified to meet the rigorous training requirements, including high G air intercept maneuvers and high-speed low-level flight. Wing, tail, tailcone, and windshield beef-up and higher thrust engines resulted in a new FAA certification and a new model number assigned to the aircraft as well as the Military T-47A designation. The interior of the aircraft was changed to accommodate an instructor, two students in the cabin and one in the copilot position. The copilot instrument panel was dominated by the radar display similar to Navy attack aircraft and the airplane was flown single pilot. Cessna, on its own initiative, performed a full-scale fatigue life test and gathered field service data to prove the design was satisfactory. The Navy declared the T-47A training system was the most successful during the T-47A tenure. ( 1998 Elsevier Science Ltd. All rights reserved.
In the popular movie Top Gun, Goose is Maverick’s back seat partner in the operation of one of the US Navy’s awesome weapon platforms. In Navy terminology, he is a Naval Flight Officer, or for short, an NFO. NFOs not only perform Goose’s Radar Intercept Officer (RIO) combat role, in other airplanes they act as Tactical Navigators who use ground radar returns to find their way to targets where the normal radio navigation aids have been shut down. They also perform
About the author. Mr Lyle was the Project Engineer responsible for coordinating the design of the T-47A. He has over 35 years exeperience in advanced design and in design and development of numerous Business and Special Mission aircraft. He has been with The Cessna Aircraft Company for 20 years. *Tel.: 001 316 831 2803; fax: 001 316 831 2828. S1369-8869/98/$—see front matter ( 1998 Elsevier Science Ltd. All rights reserved. PII: S 1 36 9- 88 6 9 (9 8 )0 0 00 6 - 8
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
high-speed low-level over-water navigation, and/or regular cross-country navigation utilizing TACAN, VOR, ADF, and the other conventional navigation systems. Undergraduate Naval Flight Officers (UNFO) are students in the Navy’s intensive and difficult NFO training course. A number of years ago, UNFOs were being trained in 20# aging and difficult to support T-39 Saberliners with obsolete radars. It made more economic sense for the Navy to replace the systems rather than try to refurbish the existing ones. They also took advantage of computer technology to upgrade ground training system capabilities. A competition was held for a FAA Part 25 certified airplane to replace the T-39s that, when modified, would be a good transition to the high-performance fleet airplanes. Cessna as the successful bidder, provided the upgraded UNFO training system consisting of aircraft, radars, full-up representative ground simulators, total system support, and instructor pilots. From contract award to initial training capability, Cessna had 14 months to demonstrate the first airplane. The normal no-nonsense schedule for the design and certification task was 22—24 months. Twelve to fifteen Citations were sufficient to replace the much larger fleet of Saberliners because of the higher Citation reliability and a support system patterned after the commercial Cessna Citation support system. The UNFO Citation was designated the T-47A and was also FAA certified as the Cessna Citation Model 552, a new model. Fig. 1 shows the final configuration of the airplane.
1. Design requirements The design requirements specified that the airplanes, including the radar and other systems, with the support system could achieve a 95% mission completion rate while flying up to 17,000 training hours per year as defined by the contract. Each airplane was required to perform any of the UNFO training missions; tactical navigation, over-water jet navigation, radar intercept, and navigation. The airplane had to be capable of maneuvering with three students, a Navy instructor, and the Cessna instructor pilot to at least 3.5 Gs in Radar intercept training, achieve roll rates of at least 90° per second, and fly at 350 kts at 500 ft above terrain while withstanding the low level pounding and defeating any birds that might be in the way. A general Navy performance requirement specified that the airplane had to be able to perform a sustained 2G turn at 20,000 ft without losing altitude. A 300 lb combination fire-control radar modified for air to ground with its 22 in antenna occupied the nose of the airplane. Two student training consoles, two instructor stations with appropriate avionics and flight instrumentation, and the cockpit right seat training station were also required. A number of other factors drove the design of the airplane. Each crew/student seat had to accommodate a 230 lb 41 in (105 kg, 104 cm) sitting height student, a sitting height that normally occurs in a 6 ft 8 in (2 M) or taller individual. The Navy had also commented during pre-request for proposal demonstration flights that they were concerned that pilot fatigue would be a factor in the Cessna Citation because the RIO maneuver’s aileron control forces were higher than the airplanes they were flying. They expected these forces to be reduced in the airplanes they received. The airplane had to be new, FAA certified, and be a modified commercial off-the-shelf model. Since some of the maneuvers required in the training exceeded the FAA Part 25 criteria, those flight maneuvers were to be evaluated to FAA Part 23 acrobatic requirements.
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
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Fig. 1. Twelve to fifteen reliable Cessna Citation T-47A aircraft replaced more than 20 US Navy T-39’s while providing more available training flight hours.
Training was at Pensacola, Florida, adjacent to the Gulf in a moderate salt air environment. It was also anticipated that the low level over water flights would subject the airplane to some additional salt exposure. 1.1. General design approach Many of these requirements were not inherent in the basic FAA Certified Citation proposed for the program. Consequently, many design changes had to be made to the airplane; including but not limited to the following: Higher thrust JT15D-5 engines. Wings shortened by approximately six feet. Aileron boost system. New nose and radome. New design seats with five point shoulder harness. Reinforced seat track structure.
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
Two cabin student training consoles and one cockpit training station. Extended tracking for student cockpit seat. Tracking instructor seat with integral folding work table. Overhead observation window. 350 KTAS bird-proof windshield. Strengthened and more fatigue resistant wing, horizontal stabilizer, and vertical stabilizer. Strengthened and more fatigue resistant tailcone. New elevator with dynamic balancing. Integration of radar. Avionics. Full VOX intercom system Military avionics Basic Flight/Navigation information at student stations The aircraft chosen to be modified was the Citation SII, an upgrade of the Cessna Citation II (Model 550). Fig. 2 shows many of the design changes that were made to the Citation SII to arrive at the T-47A.
Fig. 2. The changes made to meet the US Navy’s flight officer training requirements yielded an airframe which successfully passed over 90,000 h of flight training fatigue life test.
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
55
The modifications to the aircraft were so extensive and the characteristics were so different that a new model number 552 was assigned for FAA certification. The certification basis was FAA Part 25 and, as noted above, acrobatic maneuvers from other parts of the regulations were utilized where FAA Part 25 regulations did not have pertinent requirements. The high-speed low-level flight required in training was a new environment for the Citation so it was decided that a full-scale cyclic test article would be built and tested to identify any fatigue characteristics and to verify the analytical life predictions. The test article would accumulate equivalent flight hours sooner than the operating aircraft by a significant margin so there would be plenty of time to implement any necessary corrective action. Since the test load spectrum was based on environment predictions supplied by the Navy, it was decided that the first six aircraft should be instrumented to confirm the load spectrum was representative of the actual training environment. Flight data recorded included C.G. acceleration and structural strain at selected locations. The program was set up so the data collected could be matched to the type of training flight that was being conducted. The data downloaded was entered into a computer program for tabulation and comparison of the actual loads to those predicted. Six months into the training and again in another year, a formal comparison was made. Both checks showed that the actual loads during each kind of training flight and the test load spectrum were equivalent. The cyclic test was continued on the original plan until over 90,000 equivalent flight hours of tests were completed. Realizing that the mission mix flown during training would vary among the aircraft and since mission mix is a critical variable in the prediction of airframe life, an (individual aircraft tracking) (IAT) program was also implemented. In this program, the pilot filled out a form for each flight that identified the mission parameters and unusual loads or conditions. This data was entered into a computer program for tabulation and summarization. Information could then be generated such that the maintenance organization could adjust the aircraft assignments as required to equalize the severity of flights among the airplanes. 1.2. Aerodynamics The derivative Citation SII was designed with a relatively straight, high aspect ratio wing which contributes to the good stability of the aircraft. Analysis showed the wing needed to be shortened in order to achieve the required 90° per second roll rate. This was accomplished by removing the 33 in of wing outboard of the ailerons and adding a new end cap. Wing skin thickness was increased to meet lightening strike requirements. The reduction in span also reduced the wing stress and improved the ride in low-level training flights. Both a servo tab similar to that used on the Cessna A-37 and a powered aileron system used on the Cessna Citation Model 650 were considered to achieve the deflection required for the 90° per second roll rate at low enough forces while maintaining control force harmonization. The Aileron boost system was selected and modified to physically interface with the T-47A configuration. With the shorter wing, the thrust required for maintaining a sustained 2G turn at 20,000 ft maneuver was higher than the JT15D-4 engines provided. The JT15D-5 engines, which was an upgrade of the JT15D-4, had sufficient thrust and could be installed with relatively minor modifications to the aircraft.
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
An earlier Citation aircraft program had installed an F-16 nose mounted radar antenna for use on another government use aircraft. The UNFO radar antenna was able to fit in this radome which only needed some thickness changes to be compatible with the training radar frequency. This radome increased the length of the nose approximately 12 in and changed the shape. As expected from the results of the previous program, analysis and flight test showed no significant change to the handling characteristics or speed. The elevators of the baseline aircraft were primarily balanced with a weight in a horn. Dynamic analysis showed that a distributed balance was required. The same elevator form was retained and the hinge line was moved aft to provide space for the balance weight. The horizontal stabilizer was raised approximately 5 in which gave the bell crank approximately the same space from the vertical stabilizer rear spar and maintained the same tail arm. This change resulted in less elevator authority due to horizontal stabilizer shrouding. 1.3. Structural design Two of the UNFO training sorties imposed the highest structural loads on the aircraft. The high G Radar Intercept Officer (RIO) training with up to 3.5G turns and multiple lesser maneuvers imposed the highest stress. The low-level overwater jet navigation (OJN) mission imposed lower stresses but a much higher number of cycles due to the gust environment. The RIO training is normally accomplished around 20,000 ft altitude. The training involves detection of a target aircraft with the radar at distances of 20 miles or more. Once the target is identified, the student commands the maneuvers required to position the aircraft for a forward quadrant intercept. The student continues to make corrections as the two aircraft close. After making the head-on intercept and achieving a radar lock-on and simulated firing, the aircraft is maneuvered for a tail lock-on and simulated tail shot. These maneuvers are frequently done at the maximum capability of the aircraft and a number of times with up to three students during each training sortie. This frequency of the maximum 3.5G acceleration and the 90° roll rate varies depending on the proficiency of the student and, later in the training, the evasive actions of the target aircraft. An overhead window, similar in shape to the cabin windows, is used by the pilot and the student to keep the target airplane in sight during the passing maneuvers when radar lock may be lost. OJN training is accomplished at altitudes as low as 500 ft above terrain. Even though most of the training is over the Gulf training area where turbulence is less than over land, the low-level thermals and normal turbulence are still significant. (The occupants are sensitive to first and second acceleration derivatives, and on a sunny day the green/brown field thermal roughness can induce nausea in even an experienced flight crew.) A durability and damage tolerance assessment (DADTA) was conducted and showed the OJN to have the greatest negative affect on the life of the structure. On the other hand, the RIO maneuvers imposed high tail loads and tailcone torsion loads. Seat track loads were also increased due to the 235 lb crew requirements and complicated by the flight loads of the training missions. The FAA speed limit at altitudes where bird strikes are likely is 250 knots. The baseline Citation uses a 0.875 in thick multi-layer Plexiglas windshield which satisfies the FAA requirement. The bird impact energy dissipation required for the 350 knot OJN mission is a function of the square of the
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
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velocity. A polycarbonate windshield capable of defeating the bird is about the same thickness as the standard Plexiglas window so it could be installed in the same location without physical interference and with similar optical qualities. The structural design to accommodate the polycarbonate was still challenging because of the higher loads and because the polycarbonate deflects more on impact than the Plexiglas. The deflection results in a different distribution of the structural loads. The overhead window also changed all of the load paths. The design passed the FAA bird shot tests. Structural modifications included a structural beef-up of the horizontal tail, higher moment of inertia tailcone stringers, increased wing skin thickness, stainless-steel straps which replaced the aluminum wing spar caps, reinforced aileron cable pulley brackets, seat track support structure, and the new windshield and windshield support structure combined with the overhead window. Selected lower wing, horizontal tail, and engine beam fastener holes were cold worked to improve the fatigue life. Cold working is a process where the fastener holes are drilled, reamed to size, and then a mandrel is drawn through a sleeve placed in the hole to condition the metal in the hole. It is an economical way to improve fatigue life while keeping the lowest practical weight. To assure long-term structural integrity, all Citations receive multi-process corrosion protection consisting of chemical film, epoxy coatings, fay and fillet sealing, and internal and external top coats. The stainless-steel straps were epoxy coated, bonded, and assembled with wet fasteners where appropriate. Special attention was given to all of the above and a special in-service corrosion prevention program was implemented because of the location of the training base. The changes made to improve fatigue life resulted in an airframe which was capable of 4.25Gs. This bonus higher G capability provided an extra margin of safety when a pilot tried to make up for an untimely student command; something which happened more than once. As noted above, the structural testing eventually extended to more than 90,000 equivalent flight hours. Only one minor change to the structure was needed as a result of the testing. After cyclic testing was terminated the maximum load anticipated in an aircraft’s life was successfully simulated on the test article. Fig. 2 summarizes the structural changes to the aircraft.
2. Systems design The key to achieving the 95% mission completion rate required by the contract was the reliability of the radar. The maintenance program planned would make the airplanes available, but the systems had to last through all of the rigorous training missions in the North Florida environment adjacent to the Gulf of Mexico. Temperatures in the 90—100°F (32—38C) and 90% salty humid air were common. A vapor cycle air conditioning system was installed and its primary function was to cool the radar. The nose compartment where the radar was located was insulated to control the temperature in the optimum range; cool enough to reduce the stress on the electronic parts and warm enough to prevent damaging condensation after descent from a high-altitude cold-soaked flight. The avionics control boxes mounted in the nose also benefited from this cooler and drier environment.
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Extended seat tracks were provided on the right cockpit (student) seat. Since students exchange positions in flight, this extra seat range made it easier for students to get in and out of the seat, diminished the chance of damage to the console controls, and reduced the possibility of interference with the flight controls. The seat bottoms were redesigned to provide the required 41 in sitting height. This design resulted in a comfortable seat that was much firmer and which proved to be superior to the standard business seats in the high G maneuvers. All seats were provided with a five-point restraint system for proper positioning of the students and instructors for the training missions and to prevent contact with the console controls in most minor emergencies. Interior photograph (Fig. 3) shows the cockpit training station and Fig. 4 shows the cabin student consoles. The aft instructor seat was mounted on the cabin centerline between the two student consoles and slightly higher so he could observe both students. He was provided with failure simulation switches to provide necessary and timely challenges to students. The forward instructor seat was
Fig. 3. Radar Intercept training’s primary position was in the right seat so the student could make visual as well as radar contact. Photo courtesy of Flying Magazine.
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
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Fig. 4. T-47A Student Console — Radar Navigation students primary training stations were in the cabin. Flight instrumentation helped provide air sense. Photo courtesy of Flying Magazine.
mounted immediately aft of the pilot. It could track fore, aft, and inboard to provide visibility to the cockpit student and also move clear for student changes.
3. Ground simulators The initial pre-flight radar training and practice during flight training was accomplished on computer controlled fixed based simulators which duplicated the cabin student stations including the hardware and function. The same radar equipment was used in the simulators as in the aircraft. Aircraft instrumentation utilized aircraft cases and displays driven by the computer. Four air-to-air and four air-to-ground training stations were available 24 hrs a day and six and one-half days a week. These stations enabled the students the opportunity to sharpen their radar tuning and flying response skills.
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4. Results of the design The training system performed flawlessly. The fleet flew up to 17,000 hrs per year with mission completion rates as high as 99%. The system failed to meet the 95% mission completion rate only once when a bad batch of radar components and their even poorer replacement parts decided to fail almost simultaneously. Even with the radar component problem, the training system still performed at better than a 90% mission completion rate. There were no major structural failures. There were multiple bird strikes, only two of which penetrated the structure. All aircraft returned safely and there were no injuries attributable to the aircraft design. The Navy, during a major training meeting, declared the Cessna Citation Training System to be the most successful training system in the Navy.
Converting A Citation Business Jet to a military trainer James W. Lyle Jr* The Cessna Aircraft company, A Textron company, 1 Cessna Blvd, Wichita, Kansas 67215, USA
Abstract The United States Navy needed a replacement for aging T-39 Naval Flight Officer (NFO) training aircraft. NFOs perform radar and navigation functions on Navy aircraft. It was decided that conversion of FAA certified business aircraft would be the most economical approach. They also upgraded the groundbased training systems. The Cessna Aircraft Company won the competition, proposing the Citation Model 550 which had to be heavily modified to meet the rigorous training requirements, including high G air intercept maneuvers and high-speed low-level flight. Wing, tail, tailcone, and windshield beef-up and higher thrust engines resulted in a new FAA certification and a new model number assigned to the aircraft as well as the Military T-47A designation. The interior of the aircraft was changed to accommodate an instructor, two students in the cabin and one in the copilot position. The copilot instrument panel was dominated by the radar display similar to Navy attack aircraft and the airplane was flown single pilot. Cessna, on its own initiative, performed a full-scale fatigue life test and gathered field service data to prove the design was satisfactory. The Navy declared the T-47A training system was the most successful during the T-47A tenure. ( 1998 Elsevier Science Ltd. All rights reserved.
In the popular movie Top Gun, Goose is Maverick’s back seat partner in the operation of one of the US Navy’s awesome weapon platforms. In Navy terminology, he is a Naval Flight Officer, or for short, an NFO. NFOs not only perform Goose’s Radar Intercept Officer (RIO) combat role, in other airplanes they act as Tactical Navigators who use ground radar returns to find their way to targets where the normal radio navigation aids have been shut down. They also perform
About the author. Mr Lyle was the Project Engineer responsible for coordinating the design of the T-47A. He has over 35 years exeperience in advanced design and in design and development of numerous Business and Special Mission aircraft. He has been with The Cessna Aircraft Company for 20 years. *Tel.: 001 316 831 2803; fax: 001 316 831 2828. S1369-8869/98/$—see front matter ( 1998 Elsevier Science Ltd. All rights reserved. PII: S 1 36 9- 88 6 9 (9 8 )0 0 00 6 - 8
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
high-speed low-level over-water navigation, and/or regular cross-country navigation utilizing TACAN, VOR, ADF, and the other conventional navigation systems. Undergraduate Naval Flight Officers (UNFO) are students in the Navy’s intensive and difficult NFO training course. A number of years ago, UNFOs were being trained in 20# aging and difficult to support T-39 Saberliners with obsolete radars. It made more economic sense for the Navy to replace the systems rather than try to refurbish the existing ones. They also took advantage of computer technology to upgrade ground training system capabilities. A competition was held for a FAA Part 25 certified airplane to replace the T-39s that, when modified, would be a good transition to the high-performance fleet airplanes. Cessna as the successful bidder, provided the upgraded UNFO training system consisting of aircraft, radars, full-up representative ground simulators, total system support, and instructor pilots. From contract award to initial training capability, Cessna had 14 months to demonstrate the first airplane. The normal no-nonsense schedule for the design and certification task was 22—24 months. Twelve to fifteen Citations were sufficient to replace the much larger fleet of Saberliners because of the higher Citation reliability and a support system patterned after the commercial Cessna Citation support system. The UNFO Citation was designated the T-47A and was also FAA certified as the Cessna Citation Model 552, a new model. Fig. 1 shows the final configuration of the airplane.
1. Design requirements The design requirements specified that the airplanes, including the radar and other systems, with the support system could achieve a 95% mission completion rate while flying up to 17,000 training hours per year as defined by the contract. Each airplane was required to perform any of the UNFO training missions; tactical navigation, over-water jet navigation, radar intercept, and navigation. The airplane had to be capable of maneuvering with three students, a Navy instructor, and the Cessna instructor pilot to at least 3.5 Gs in Radar intercept training, achieve roll rates of at least 90° per second, and fly at 350 kts at 500 ft above terrain while withstanding the low level pounding and defeating any birds that might be in the way. A general Navy performance requirement specified that the airplane had to be able to perform a sustained 2G turn at 20,000 ft without losing altitude. A 300 lb combination fire-control radar modified for air to ground with its 22 in antenna occupied the nose of the airplane. Two student training consoles, two instructor stations with appropriate avionics and flight instrumentation, and the cockpit right seat training station were also required. A number of other factors drove the design of the airplane. Each crew/student seat had to accommodate a 230 lb 41 in (105 kg, 104 cm) sitting height student, a sitting height that normally occurs in a 6 ft 8 in (2 M) or taller individual. The Navy had also commented during pre-request for proposal demonstration flights that they were concerned that pilot fatigue would be a factor in the Cessna Citation because the RIO maneuver’s aileron control forces were higher than the airplanes they were flying. They expected these forces to be reduced in the airplanes they received. The airplane had to be new, FAA certified, and be a modified commercial off-the-shelf model. Since some of the maneuvers required in the training exceeded the FAA Part 25 criteria, those flight maneuvers were to be evaluated to FAA Part 23 acrobatic requirements.
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
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Fig. 1. Twelve to fifteen reliable Cessna Citation T-47A aircraft replaced more than 20 US Navy T-39’s while providing more available training flight hours.
Training was at Pensacola, Florida, adjacent to the Gulf in a moderate salt air environment. It was also anticipated that the low level over water flights would subject the airplane to some additional salt exposure. 1.1. General design approach Many of these requirements were not inherent in the basic FAA Certified Citation proposed for the program. Consequently, many design changes had to be made to the airplane; including but not limited to the following: Higher thrust JT15D-5 engines. Wings shortened by approximately six feet. Aileron boost system. New nose and radome. New design seats with five point shoulder harness. Reinforced seat track structure.
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
Two cabin student training consoles and one cockpit training station. Extended tracking for student cockpit seat. Tracking instructor seat with integral folding work table. Overhead observation window. 350 KTAS bird-proof windshield. Strengthened and more fatigue resistant wing, horizontal stabilizer, and vertical stabilizer. Strengthened and more fatigue resistant tailcone. New elevator with dynamic balancing. Integration of radar. Avionics. Full VOX intercom system Military avionics Basic Flight/Navigation information at student stations The aircraft chosen to be modified was the Citation SII, an upgrade of the Cessna Citation II (Model 550). Fig. 2 shows many of the design changes that were made to the Citation SII to arrive at the T-47A.
Fig. 2. The changes made to meet the US Navy’s flight officer training requirements yielded an airframe which successfully passed over 90,000 h of flight training fatigue life test.
J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
55
The modifications to the aircraft were so extensive and the characteristics were so different that a new model number 552 was assigned for FAA certification. The certification basis was FAA Part 25 and, as noted above, acrobatic maneuvers from other parts of the regulations were utilized where FAA Part 25 regulations did not have pertinent requirements. The high-speed low-level flight required in training was a new environment for the Citation so it was decided that a full-scale cyclic test article would be built and tested to identify any fatigue characteristics and to verify the analytical life predictions. The test article would accumulate equivalent flight hours sooner than the operating aircraft by a significant margin so there would be plenty of time to implement any necessary corrective action. Since the test load spectrum was based on environment predictions supplied by the Navy, it was decided that the first six aircraft should be instrumented to confirm the load spectrum was representative of the actual training environment. Flight data recorded included C.G. acceleration and structural strain at selected locations. The program was set up so the data collected could be matched to the type of training flight that was being conducted. The data downloaded was entered into a computer program for tabulation and comparison of the actual loads to those predicted. Six months into the training and again in another year, a formal comparison was made. Both checks showed that the actual loads during each kind of training flight and the test load spectrum were equivalent. The cyclic test was continued on the original plan until over 90,000 equivalent flight hours of tests were completed. Realizing that the mission mix flown during training would vary among the aircraft and since mission mix is a critical variable in the prediction of airframe life, an (individual aircraft tracking) (IAT) program was also implemented. In this program, the pilot filled out a form for each flight that identified the mission parameters and unusual loads or conditions. This data was entered into a computer program for tabulation and summarization. Information could then be generated such that the maintenance organization could adjust the aircraft assignments as required to equalize the severity of flights among the airplanes. 1.2. Aerodynamics The derivative Citation SII was designed with a relatively straight, high aspect ratio wing which contributes to the good stability of the aircraft. Analysis showed the wing needed to be shortened in order to achieve the required 90° per second roll rate. This was accomplished by removing the 33 in of wing outboard of the ailerons and adding a new end cap. Wing skin thickness was increased to meet lightening strike requirements. The reduction in span also reduced the wing stress and improved the ride in low-level training flights. Both a servo tab similar to that used on the Cessna A-37 and a powered aileron system used on the Cessna Citation Model 650 were considered to achieve the deflection required for the 90° per second roll rate at low enough forces while maintaining control force harmonization. The Aileron boost system was selected and modified to physically interface with the T-47A configuration. With the shorter wing, the thrust required for maintaining a sustained 2G turn at 20,000 ft maneuver was higher than the JT15D-4 engines provided. The JT15D-5 engines, which was an upgrade of the JT15D-4, had sufficient thrust and could be installed with relatively minor modifications to the aircraft.
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J.W. Lyle Jr / Aircraft Design 1 (1998) 51—60
An earlier Citation aircraft program had installed an F-16 nose mounted radar antenna for use on another government use aircraft. The UNFO radar antenna was able to fit in this radome which only needed some thickness changes to be compatible with the training radar frequency. This radome increased the length of the nose approximately 12 in and changed the shape. As expected from the results of the previous program, analysis and flight test showed no significant change to the handling characteristics or speed. The elevators of the baseline aircraft were primarily balanced with a weight in a horn. Dynamic analysis showed that a distributed balance was required. The same elevator form was retained and the hinge line was moved aft to provide space for the balance weight. The horizontal stabilizer was raised approximately 5 in which gave the bell crank approximately the same space from the vertical stabilizer rear spar and maintained the same tail arm. This change resulted in less elevator authority due to horizontal stabilizer shrouding. 1.3. Structural design Two of the UNFO training sorties imposed the highest structural loads on the aircraft. The high G Radar Intercept Officer (RIO) training with up to 3.5G turns and multiple lesser maneuvers imposed the highest stress. The low-level overwater jet navigation (OJN) mission imposed lower stresses but a much higher number of cycles due to the gust environment. The RIO training is normally accomplished around 20,000 ft altitude. The training involves detection of a target aircraft with the radar at distances of 20 miles or more. Once the target is identified, the student commands the maneuvers required to position the aircraft for a forward quadrant intercept. The student continues to make corrections as the two aircraft close. After making the head-on intercept and achieving a radar lock-on and simulated firing, the aircraft is maneuvered for a tail lock-on and simulated tail shot. These maneuvers are frequently done at the maximum capability of the aircraft and a number of times with up to three students during each training sortie. This frequency of the maximum 3.5G acceleration and the 90° roll rate varies depending on the proficiency of the student and, later in the training, the evasive actions of the target aircraft. An overhead window, similar in shape to the cabin windows, is used by the pilot and the student to keep the target airplane in sight during the passing maneuvers when radar lock may be lost. OJN training is accomplished at altitudes as low as 500 ft above terrain. Even though most of the training is over the Gulf training area where turbulence is less than over land, the low-level thermals and normal turbulence are still significant. (The occupants are sensitive to first and second acceleration derivatives, and on a sunny day the green/brown field thermal roughness can induce nausea in even an experienced flight crew.) A durability and damage tolerance assessment (DADTA) was conducted and showed the OJN to have the greatest negative affect on the life of the structure. On the other hand, the RIO maneuvers imposed high tail loads and tailcone torsion loads. Seat track loads were also increased due to the 235 lb crew requirements and complicated by the flight loads of the training missions. The FAA speed limit at altitudes where bird strikes are likely is 250 knots. The baseline Citation uses a 0.875 in thick multi-layer Plexiglas windshield which satisfies the FAA requirement. The bird impact energy dissipation required for the 350 knot OJN mission is a function of the square of the
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velocity. A polycarbonate windshield capable of defeating the bird is about the same thickness as the standard Plexiglas window so it could be installed in the same location without physical interference and with similar optical qualities. The structural design to accommodate the polycarbonate was still challenging because of the higher loads and because the polycarbonate deflects more on impact than the Plexiglas. The deflection results in a different distribution of the structural loads. The overhead window also changed all of the load paths. The design passed the FAA bird shot tests. Structural modifications included a structural beef-up of the horizontal tail, higher moment of inertia tailcone stringers, increased wing skin thickness, stainless-steel straps which replaced the aluminum wing spar caps, reinforced aileron cable pulley brackets, seat track support structure, and the new windshield and windshield support structure combined with the overhead window. Selected lower wing, horizontal tail, and engine beam fastener holes were cold worked to improve the fatigue life. Cold working is a process where the fastener holes are drilled, reamed to size, and then a mandrel is drawn through a sleeve placed in the hole to condition the metal in the hole. It is an economical way to improve fatigue life while keeping the lowest practical weight. To assure long-term structural integrity, all Citations receive multi-process corrosion protection consisting of chemical film, epoxy coatings, fay and fillet sealing, and internal and external top coats. The stainless-steel straps were epoxy coated, bonded, and assembled with wet fasteners where appropriate. Special attention was given to all of the above and a special in-service corrosion prevention program was implemented because of the location of the training base. The changes made to improve fatigue life resulted in an airframe which was capable of 4.25Gs. This bonus higher G capability provided an extra margin of safety when a pilot tried to make up for an untimely student command; something which happened more than once. As noted above, the structural testing eventually extended to more than 90,000 equivalent flight hours. Only one minor change to the structure was needed as a result of the testing. After cyclic testing was terminated the maximum load anticipated in an aircraft’s life was successfully simulated on the test article. Fig. 2 summarizes the structural changes to the aircraft.
2. Systems design The key to achieving the 95% mission completion rate required by the contract was the reliability of the radar. The maintenance program planned would make the airplanes available, but the systems had to last through all of the rigorous training missions in the North Florida environment adjacent to the Gulf of Mexico. Temperatures in the 90—100°F (32—38C) and 90% salty humid air were common. A vapor cycle air conditioning system was installed and its primary function was to cool the radar. The nose compartment where the radar was located was insulated to control the temperature in the optimum range; cool enough to reduce the stress on the electronic parts and warm enough to prevent damaging condensation after descent from a high-altitude cold-soaked flight. The avionics control boxes mounted in the nose also benefited from this cooler and drier environment.
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Extended seat tracks were provided on the right cockpit (student) seat. Since students exchange positions in flight, this extra seat range made it easier for students to get in and out of the seat, diminished the chance of damage to the console controls, and reduced the possibility of interference with the flight controls. The seat bottoms were redesigned to provide the required 41 in sitting height. This design resulted in a comfortable seat that was much firmer and which proved to be superior to the standard business seats in the high G maneuvers. All seats were provided with a five-point restraint system for proper positioning of the students and instructors for the training missions and to prevent contact with the console controls in most minor emergencies. Interior photograph (Fig. 3) shows the cockpit training station and Fig. 4 shows the cabin student consoles. The aft instructor seat was mounted on the cabin centerline between the two student consoles and slightly higher so he could observe both students. He was provided with failure simulation switches to provide necessary and timely challenges to students. The forward instructor seat was
Fig. 3. Radar Intercept training’s primary position was in the right seat so the student could make visual as well as radar contact. Photo courtesy of Flying Magazine.
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Fig. 4. T-47A Student Console — Radar Navigation students primary training stations were in the cabin. Flight instrumentation helped provide air sense. Photo courtesy of Flying Magazine.
mounted immediately aft of the pilot. It could track fore, aft, and inboard to provide visibility to the cockpit student and also move clear for student changes.
3. Ground simulators The initial pre-flight radar training and practice during flight training was accomplished on computer controlled fixed based simulators which duplicated the cabin student stations including the hardware and function. The same radar equipment was used in the simulators as in the aircraft. Aircraft instrumentation utilized aircraft cases and displays driven by the computer. Four air-to-air and four air-to-ground training stations were available 24 hrs a day and six and one-half days a week. These stations enabled the students the opportunity to sharpen their radar tuning and flying response skills.
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4. Results of the design The training system performed flawlessly. The fleet flew up to 17,000 hrs per year with mission completion rates as high as 99%. The system failed to meet the 95% mission completion rate only once when a bad batch of radar components and their even poorer replacement parts decided to fail almost simultaneously. Even with the radar component problem, the training system still performed at better than a 90% mission completion rate. There were no major structural failures. There were multiple bird strikes, only two of which penetrated the structure. All aircraft returned safely and there were no injuries attributable to the aircraft design. The Navy, during a major training meeting, declared the Cessna Citation Training System to be the most successful training system in the Navy.