Test Pilot Perspective: The S-3 Viking
By the late 1960s, the Soviet Union, with long range, sea launched ballistic missile-launching submarines operating in deep water, represented a menacing and very real threat to the continental United States.
To counter this increasing Cold War danger, the US Navy devised the hunter-killer team concept. By the late 1960s, this team consisted of the Sikorsky SH-3A Sea King helicopter with a dunking sonar and the Grumman S-2 Tracker piston-powered antisubmarine warfare, or ASW, platform. This team, referred to as Task Group Alpha, operated almost exclusively from Essex-class conventional aircraft carriers.
At the same time, the land-based P-3 Orion maritime patrol aircraft community was developing new high altitude passive tracking capabilities using large fields of long-life sonobuoys. With the development of new tactics and sensors, the P-3 crews were meeting with improved success at passively locating and tracking Soviet submarines.
Unfortunately, neither of these systems provided the Attack Carrier Task Group any indigenous ASW capability or defensive protection. The challenge was to be able to perform the ASW mission normally requiring a ten-man crew on the P-3 with a crew of four in an airplane that was completely carrier capable and that could integrate into the modern nuclear-powered aircraft carrier air group.
This was the genesis of the Specific Operational Requirement, or SOR, for the VSX program that was released to industry in late late1966. A formal request for proposal was answered by two industry teams in April 1968. The team of Lockheed, LTV, Sperry Univac and GE competed against Grumman, IBM, and General Dynamics. Final bids were submitted in December 1968 and on 4 August 1969, the Lockheed team was declared the winner.
One of the unique and ultimately successful features of the S-3A contract was that it was a fixed price development contract incorporating a target price as well as ceiling. It was milestone based, with fixed dates for each of some eighteen major program milestones, one of which was the first flight date of the first prototype aircraft. The production contracts were also fixed price which significantly overlapped both the development and production lots of aircraft.
To the amazement of many people in both industry and the military, the Lockheed team met or beat every single milestone in the contract and came in below the ceiling price for every phase of the development and production contract lots. Crew training started on schedule and the program met its initial operational capability, or IOC, date with the aircraft carrier deployment on schedule.
Reaching these milestones was no small task considering virtually every element of the S-3A was a new development and included the following requirements:
- Completely new airframe
- Completely new and first high bypass ratio turbofan engine in this class
- First application of an airborne central digital computer and fully integrated avionics system
- First sequenced, four-crew ejection system with zero/zero to 450 knot capability
- Single engine bolter at max carrier landing weight with gear and flaps down
- Fully Automatic Carrier Landing System, or ALCS, with autothrottle delivered at IOC
- Inflight refueling
- Ability to carry and deploy sixty search stores
- Two internal bomb bays and two wing stations capable of carrying conventional bombs, torpedoes, depth charges, and nuclear weapons
- Meet MIL-SPEC Flying Qualities requirements with no stability augmentation throughout its entire flight envelope from 90-450 KIAS at sea level
- A fully automatic or electrically selectable manual back up, or EFCS, flight control system
- Ability to descend from 30,000 feet to the surface in two minutes
- A ground and in-flight operational auxiliary power unit for self-contained operations and engine starts
- The approach speed of an A-6 with the sink rate design of the A-7 attack aircraft
- Foldable wings and vertical tail to fit an <i>Essex</i>-class aircraft carrier
- First non-paper trace system of analyzing submarine acoustic signatures on a CRT projection
Eight YS-3A prototypes built. John Christianson and I made the first flight of the first prototype (Navy Bureau Number 157992) on 21 January 1972. The aircraft was trucked from Burbank, California, where it was built. The ninety-minute flight came from Air Force Plant 42 in nearby Palmdale. Three months later, in April 1972, the Navy ordered the first thirteen production aircraft.
The first operational delivery took place to VS-41 in NAS North Island, California, in February 1974; less than two years after the first flight of the first prototype and only four-and-one-half years after the Lockheed team had been declared winners in the VSX competition. A total of 187 aircraft were built and the last was delivered to NAS North Island in August 1978.
The successful concurrency of S-3 test and production with no missed milestones and no mandatory deficiencies was an accomplishment that has not been matched since for a new start program.
From The Cockpit
The Viking’s antisubmarine warfare systems accomplishment is an incredible story within itself, but the focus here is on the aircraft’s flying qualities and performance.
In order to determine if a single crew of four could accomplish the ASW tasks of location, isolation, and destroy normally done by a P-3 crew, engineers at Naval Air Development Center Johnsville, near Warminster, Pennsylvania, were tasked with installing state-of-the-art ASW avionics suite in an A-3 Skywarrior airframe.
Pilots Cmdr. Bill Christensen and Lt. Cmdr. Day Ritt, along with Tactical Coordinator Lt. Joe Monroe were assigned to crew the A-3 test bed aircraft along with a senior AX sensor operator, to conduct the feasibility tests out of NAS Johnsville and to write the report. Based on their test results, they became the key architects of the VSX requirements document.
In the P-3, there were separate operator control panels for each type of sensor, which included active and passive acoustics (known as Julie and Jezebel); electronic support measures, or ESM; surface search radar; magnetic anomaly detection, or MAD; forward looking infrared, or FLIR. The S-3 was not going to have this luxury.
One of the key findings that emerged from project A-New (the new fixed wing ASW concept development program) in the concurrent P-3 testing in NAS Johnsville, and particularly with the A-3 feasibility crew, was that a centralized digital computer was going to be needed to control the sensors as well as process and display the sensor data to the operators.
Thus the digital computer was to become the heart of the S-3A to process all sensor data and display it on CRTs instead of paper-based recorders typically found in the early P-3s. In addition, the coordination of the placement and release of a myriad of search and kill stores with the ASW target had to be integrated with an over-water navigation solution.
This required keyboards for the control and display of this data for every crewmember, which presented its own challenges to the escape system for the rear seat crewmembers. The sensor station and Tactical Coordinator station eventually ended up with fold up trays, hinged on the outboard end that would allow them to be retracted and locked in place prior to ejection seat firing. This solution provided a free path for the ejection seat to move up the rails, yet provide a lap-located multi-key control panel to interface with the computer.
The Viking design had its roots in a number of different forces which ended up influencing the SOR. Some of the more obvious requirements made their way into the final design. Some of these requirements were never needed, yet impacted the final design in a negative way.
Manual Backup Flight Controls: A strong influencer in the S-3 design requirements was the experience of the carrier-based airplanes operating in combat in Southeast Asia where the vulnerability of fully powered hydraulic flight control systems with no manual backup had been exposed.
When ground fire or missiles took out the flight control hydraulics, the pilot would lose control of his airplane and have to eject. This was particularly disastrous when it happened over enemy territory or beyond the range of SAR assets. The manual reversion capability of the A-4 Skyhawk was proving to be a valuable asset in combat, allowing the pilot who had lost his flight control system hydraulics to get clear of the combat zone before ejecting.
This led to the requirement to have an automatic manual backup flight control system in the Viking flight control system, or FCS. The Emergency Flight Control System—EFCS—took over when hydraulic pressure was lost, leaving the pilot with conventional, trimmable rudders, elevator/horizontal stabilizer, and ailerons. The system was capable of flying anywhere in the envelope up to 250 KIAS and making a field or field arrested landing. Because it was selectable, pilots could practice flying in EFCS and then switch back to the normal system for landing.
Zero/Zero Escape System: The low altitude escape from a disabled aircraft had also become a priority, whether it was as a result of a carrier landing incident or a cold catapult shot. The downward bailout system in the A-3 would not work and the ditching option of the S-2 was considered unacceptable. For these reasons, both an automatic manual reversion flight control system and a zero-zero, fully-sequenced four-man rocket-propelled escape system found their way into the VSX requirements.
Rapid Descent Requirement: Another strong influencer was the ASW tactics being successfully employed in the P-3 community. This consisted of passive listening to a large field of sonobuoys, some of them with directional capability to detect and localize submarines. This could only be accomplished by staying at high altitude where line of sight could be maintained to the sonobuoy antennas located in the water at sea level.
However, in order to accurately localize and to ultimately attack the enemy submarine, the aircraft was required to be at low altitude where echo ranging sonobuoys and the MAD equipment could be used. Any successful attack criteria using a torpedo, depth bomb, or special weapon required a low altitude delivery after the target had been localized and tracked.
This transition from high altitude search and location to low altitude prosecution had to be accomplished quickly to keep from losing contact with the submarine. This requirement led to a design that incorporated a large top and bottom wing speed brake.
Carrier Approach Requirements: The Attack Carrier standards that had emerged during the Vietnam War also influenced the VSX requirements. The A-6 Intruder had become the standard for accident-free carrier landing operations and one of the reasons touted was its relatively slow approach speed. The sink maximum sink rate of the A-7 had become the design standard for structural design.
Lockheed partnered with LTV for the carrier suitability design aspects of the Viking. Much study had gone into defining the carrier suitability requirements any new airplane must demonstrate, using the A-6 as one of the approach speed benchmarks.
This requirement had a strong influence on the selection of the airfoil design and flap configuration of the Viking, as well as the slow stall speed at maximum carrier landing weight. Another standard was the nose-tow catapult system along with a fully automatic carrier landing system, or ACLS, with autothrottle, which by this time was well defined and became an integral part of the design-to specifications.
One of the specified requirements for VSX that proved to be extremely restrictive and compromised its very design was the maximum length and height of the airplane based on its ability to fit on the deck elevator of the 27C <i>Essex</i>-class aircraft carrier. Ironically, the S-3 never saw the decks of one of these class aircraft carriers once in service, but the aircraft carried the penalty of this restriction for its entire lifetime and compromised several aspects of its design.
In layman’s terms, the aircraft was too short; it needed a few more feet in length to contribute to its longitudinal stability, its directional control and elevator control power. Its short length drove the design to a large vertical tail and a powerful trimmable horizontal stabilizer. Because of the large size tail, it ended up requiring a folding design that added considerable weight aft of the CG and increased cost and complexity to the airframe.
Due to the requirement for the ability to fly the aircraft manually in EFCS using a conventional elevator, a conventional fully moving single slab control surface as found on carrier aircraft such as the A-7, F-4, and F-8, was ruled out.
This was complicated by the S-3’s internal bomb bay. Adding kill stores moved the CG far forward and the internal fuselage mounted sonobuoys moved the CG aft. The trim gage in the cockpit had the CG placard on the outer rim of the dial with the corresponding pitch trim setting shown on the inner dial. The range of trim far exceeded the takeoff range of settings. A similar decal was installed next to the horizontal stabilizer on the outside of the fuselage for the catapult officer to validate a trim setting in the takeoff range prior to launching to a catapult launch.
This CG range not only posed a problem for runaway trim situations, but the longitudinal trim changes from slow speed power at idle to takeoff power required almost full nose down elevator. All of these problems would have been eliminated with a fully moving stabilizer. And even the trimmable horizontal stabilizer design would have been mitigated with a longer moment arm between the CG and longitudinal control surface, i.e. longer airframe.
Enter the Test Pilot
My first assignment after flight school was briefly flying the Lockheed P2V-7 Neptune before transitioning to the P-3A Orion, the aircraft I really wanted to fly. Assigned to VP-28 at NAS Barbers Point, Hawaii, we used the breakthrough high altitude submarine hunting tactics against the newest and deadliest Soviet submarines.
Hawaii was near the transit lanes of the Russian missile boats headed to the west coast of the US, so we were tasked with tracking them across the Pacific Ocean. These new high altitude tactics proved so successful that they influenced the new VSX operational requirements.
Rather than sea duty, I was selected for Class 49 of the US Navy Test Pilot School. After several years there and as my tour as the Test Pilot School safety officer was ending, my skipper mentioned the VSX program was looking for some Navy ASW jet-qualified Aviators to beef up Lockheed’s test pilot staff for the newly awarded S-3A program.
My skipper agreed to approve my resignation assuming I would go to work for Lockheed. That was a huge presumption that Lockheed would even offer me a job, but the die was cast and I set out to land that position.
Although Lockheed had been building ASW airplanes for years, they had little experience or success with USN carrier airplanes. I finally convinced them to hire me and I went to work for them as an Engineering Test Pilot on 4 January 1971. One year and three weeks after I was hired, I flew right seat of the first flight of the YS-3A.
The year preceding first flight and many sessions afterwards, I spent hundreds of hours in the company’s flight simulator in Rye Canyon, California. I first worked on the development of the gains for the basic flight control system and later the autopilot, autothrottle, and ACLS.
Prior to first flight, in addition to the simulator, I flew the P-3A flying test bed with the S-3 avionics installed to stay flight proficient and went to Texas to get checked out in the TF-9 Cougar jet, the airplane the Navy had assigned to us to use as chase planes for the Viking program.
I flew the first prototype (Viking One as we called it) through its envelope expansion, flying qualities demonstrations and flutter clearance program, clearing the envelope for the first Navy Preliminary Evaluation, or NPE, and tests of the contractor’s airplane by Navy test pilots.
After that I rapidly shifted my attention to Ship Three, which was the first aircraft with a full avionics suite and mission computer on board. Because of my recent operational experience flying against submarines in the Pacific, I found this a fascinating and exciting experience.
Since we had more airplanes than pilots, I ended up flying a lot of different tests and test aircraft throughout the test program. They included performance, flying qualities, engine development, in-flight refueling, ordnance separation, spin chute deployment, autopilot demonstration and all Navy test pilot and Replacement Air Group instructor training.
About the only flight testing I did not participate in actively was the structural testing which was done by Christiansen and the carrier suitability demonstrations that were done by LTV pilots Hoppy New and Don Wilson at NAS Patuxent River, Maryland.
S-3A Performance Requirements
Most, if not all, of the detailed specification performance requirements for the S-3 were met in flight test.
Descent Rate: Because of the need to transition rapidly from a high altitude search pattern for submarines to the low altitude localization and attack mode, the S-3 was required to be able to descend from 30,000 feet to the surface in two minutes.
This required a large amount of drag, so the prototypes were fitted with dual panel upper wing and single panel lower wing spoilers. When they were both extended they resulted in very powerful speed brakes. The S-3 also featured trailing edge flaps. Both the leading and trailing edge flaps featured a break at the wing fold—which was also unique. When folded, the wings overlapped each other slightly.
The upper spoilers were also used to augment the ailerons for roll control in the powered mode, but their deployment resulted in some ugly positive g-loads in maximum defection rolls at high speed. The powerful roll rate was a result of the slow speed carrier approach roll performance requirement, which translated into a fighter-type roll rate at the higher speed.
Even with mechanical stick stops inserted to restrict lateral stick travel when the flaps were retracted, the roll rate at high speed exceeded most utility class airplane designs.
Pitch-Up With Roll: Because of the short coupled fuselage and large spoiler panels, the airplane exhibited unacceptable nose up pitching moments at high speed full deflection rolls. Between 350 and 400 KIAS, there was a peak delta g of about 1.5g, which was considered excessive.
Eventually, the outer spoiler panels on the upper wing were disabled to eliminate most of the pitch up and the two minute descent requirement was eased. Before we disabled the spoilers, I did the descent demonstration point before one of the NPEs.
The absurdity of the requirement was revealed when we had to start the dive recovery at 10,000 feet to ensure safe pullout without exceeding the airframe acceleration limit of 3.5g. It became apparent that this requirement was excessive as we passed 10,000 feet descending at limit Mach number at over 17,000 feet/minute rate of descent. The spoiler panels that were retained were later also used for Direct Lift Control to solve a high fast slow down problem.
Single Engine Climb: Because the S-3A was a dual engine design, it obviously had to be able to fly safely on a single engine. This requirement was most critical for carrier landings. It required the ability to establish a 500 feet per minute climb rate in configuration PA (full flaps) after a bolter at maximum carrier landing weight.
There was also a single engine climb requirement following a catapult at max takeoff gross weight in configuration TO (takeoff flaps) with either what were classified as A-stores and B-stores, which were the were the mines carried on the wing stations. Not only were they heavy, they had a blunt, high-drag nose. Fortunately, the saving grace was that they could be jettisoned quickly in the event of an engine failure off the catapult.
Low Drag Solution. This single-engine climb requirement turned out to be a combination of drag vs. thrust. Since thrust was essentially a constant we could not change, it drove the airplane toward a very low drag count in configuration PA; primarily the flap drag.
It was common for carrier airplanes like the A-4 to make carrier approaches with the speed brakes out to improve engine response and make a quick transition from high drag approach to a low drag bolter at the flick of the switch to bring in the boards.
The S-3A spoiler-type speed brakes did not lend themselves to be extended in the approach, so the only drag came from the flaps in the landing configuration. The A-6A, the carrier approach model, used wing tip speed brakes for drag; the S-3 aileron configuration did not lend itself to this type of design. Because of the focus on reducing drag in the landing configuration, single engine climb performance requirements were met.
Carrier Approach Hop-Up Maneuver: Navy engineers at Patuxent River’s Carrier Suitability Branch developed what was called the hop-up maneuver as a design-to requirement for all new carrier aircraft. The purpose of the hop-up maneuver was to define a requirement for a quick increment of lift at VPAmin (minimum approach airspeed as defined by an approach angle of attack) that would produce enough lift to allow the aircraft to safely correct for a low in close situation on a carrier approach.
Given the state of the art in wind tunnel technology, it was virtually impossible to demonstrate compliance anywhere except in the airplane. As difficult as it was to simulate, it was equally difficult to fly and measure.
The proof finally came on the instrumented field arresting gear site at NAS Patuxent River during the carrier suitability trials. Despite all the hand wringing and stress over this guarantee, the S-3A successfully met the hop up maneuver. Due to some other changes that resulted in an increase in the approach speed with Direct Lift Control, the S-3 ended up flying its approaches at a lower angle of attack/higher airspeed which improved its hop-up performance even more.
Mission Performance. The performance engineers take massive amounts of performance measurements and come up with mission profile fuel usage. The S-3 requirement was 4.5 hours of endurance at a range of 530 miles.
The wing design of the S-3A was geared towards this kind of mission. It was not efficient at high speed with a pronounced Mach-buffet occurring a few hundredths above Vmo, but it was terribly efficient at ASW loiter speeds in the range of 160 KIAS.
The wing had small ailerons for manual control, but virtually the rest of the wing had an efficient Fowler-type trailing edge flap and full span leading edge flaps. The leading edge of the wing was swept fifteen degrees, with a straight trailing edge.
The wing had a variable thickness ratio, a geometric untwist toward the outer tip and a high aspect ratio. All this was an attempt to extract the maximum lift and minimum drag out of the wing to meet all the performance requirements.
The S-3 met the mission performance this requirement or came so close that it did not require redesign or a contract penalty. The Navy did not have to revise the S-3 design specifications in order to meet the performance capabilities of the airplane.
Minimum Approach Speed (VPAMIN): The slow approach speed of the A-6 had set the bar high and the carrier suitability guys were determined that they wanted no more Vigilantes and Phantoms coming aboard ship at high speeds with the carrier having to make wind over the deck to get them on board. The S-3A made this performance requirement in its basic form—about 102 knots, as I recall. There was also a minimum acceptable roll rate at this speed as well as the hop up maneuver, which also dictated the size of the lateral spoilers.
The S-3A demonstrated that it had very adequate roll rate at VPAMIN, but pilots really needed to use the turn coordinator to avoid a bunch of adverse yaw created by the spoilers drag. This is where having the vertical tail farther aft on a longer fuselage would have been a big advantage.
Stall Characteristics: Here was where one of the conflicting requirements came to a head. The very efficient wing for the long endurance ASW mission created a wing that was almost laminar flow and not prone to an orderly progression of separated flow in the stall.
The engineers anticipated this and had designed stall strips for the leading edge of the inboard wing that was supposed to trigger the boundary layer as the airplane approached stall angle of attack. This stall strip was to provide natural stall buffet on the horizontal tail and keep the ailerons effective into the stall as the twist in the airfoil design kept the airflow attached on the outer wing.
Well, the first time we did a stall test, the installed stall strip solution did not work. In the first approach, we went from wings level and no buffet, to a roll angle in excess of nienty degrees in the blink of an eye. It did not take a rocket scientist to figure out this was not going to meet muster with Navy pilots, so we set about to discover how to solve the problem.
Of course you had to have acceptable stall characteristics in multiple flap and power configurations, so there was a lot of flight time devoted to solving this problem in all configurations, some of which eventually required an extra stall strip on the leading edge flap.
After many tufted wing video recordings and numerous stall strip configurations, one set of locations was finally discovered that gave the optimum stall warning. This configuration resulted in the least amount of stall speed compromise, but still an acceptable roll off bank angle.
The stall strip was a ninety degree device which looked like a piece of angle iron attached on the leading edge of the wing. It was discovered that its exact location on the radius of the wing was extremely critical. In fact, a template was used to precisely locate each one of the tested positions on the wing. To our chagrin, the template that had an almost perfect compromise was lost after the test and we never again found that same sweet spot location on the wing.
This became a classic example of what we used to jokingly refer to “at some point in the design, you have to shoot the engineers and build the airplane”.
The success of the S-3 program belies the serious design challenges that the Lockheed team faced and the timely solutions that they came up with to meet those challenges.
The cockpit featured dual right hand control sticks and dual left hand throttles. Tactical information was displayed on a CRT screen for the pilot and a larger CRT for the copilot. The copilot doubled as the non-acoustic sensor operator for the inverse synthetic aperture radar, forward looking FLIR, electronic support measures, and magnetic anomaly detection. Conventional tape engine instruments were installed in the left side of the center instrument panel.
The two forward facing non-pilot crewmember stations sat aft of the flight station. Both operators had integrated control system trays to interface with the central computer. These were stowed during an ejection, either manually or automatically. The Acoustic Sensor Operator sat on the left and had an Auxiliary Acoustic Display in the upper panel; both this operator and the Tactical Coordinator on the right had a large tactical plot display.
Four-Crew, Sequenced Ejection System, Zero/Zero to 450 KIAS Ejection Capability: The design requirement for the S-3 was to have a command escape system that would extract all four crew members from the airplane by activation of either the pilot or copilot ejection seats.
The heart of this system was the four McDonnell Douglas Escapac 1E-1 ejection seats. Each seat was tailored to its position, so that the trajectory of the seats would avoid colliding with other crew members or other ejection hardware.
The seats were to work from zero airspeed, zero altitude up to 450 KCAS and all crewmembers were to be extracted in less than a second. The front two seats and the rear seats went out in pairs; the rear seats went out first, followed 0.5 second later by the front seats. The rear seats had a wider arc trajectory than the front seats for lateral separation purposes. All seats were designed to fire through the Plexiglas overhead canopy enclosures which is shattered by cutters on the top of the seats.
Development of the escape system took place first in the lab, but was tested with instrumented dummies on a rocket sled at NAS China Lake, California, in about twenty-four different combinations and airspeeds. All ejections at all speeds were successful the first time tested with no redesigns required.
In Service Successes and Failures: Several successful in-envelope ejections occurred from the S-3 over the years of service use. The only glitch was discovered inadvertently, when an ejection from an S-3A landing in Burbank for a factory mod resulted in a fatality.
It was discovered that all the tests had been conducted with both seats occupied, even with the extremes of weight in opposite seats, but no tests had been conducted with one seat empty next to a full seat.
The fatality was caused by a single point failure in the seat-man separation mechanism which prevented the seat to sequence. In addition, it was discovered that the deceased crew member had received severe burns from the rocket in the empty seat. From that point on, either both seats had to be occupied, or ballast was required in the unoccupied seat to avoid the possibility of rocket burns from the empty seat.
Longitudinal Flying Qualities and Flight Control System
The Fundamentals: Designing an airplane for an airspeed range from 90 to 450 KIAS was not that unusual, but Lockheed engineering design had established a requirement for the airplane to have acceptable longitudinal flying qualities without any stability augmentation over this entire envelope.
Because there was also a requirement to have an automatic emergency manual back up to the flight control system, this meant that a conventional rudder, aileron, and elevator were going to be required in the manual mode. This manual mode requirement led to a trimmable horizontal stabilizer/elevator rather that a powered all moving tailplane, which was the common longitudinal flight control on most other carrier jets with this speed range in that era.
Stick Force Mismatch. The longitudinal flight control gains were worked out in the developmental flight simulator. It became obvious that the short length of the airplane established a quick responding short period, which not well damped. The pitch response had to be safe at high speed yet responsive at carrier approach speeds.
Because of the requirement for the ejection seat, a control stick had to be used instead of a yoke. This posed a dilemma, because the utility stick force/g flying qualities requirement for a 3.5g airplane presumed a two-handed yoke control, not a stick.
So one of the first things I discovered when I flew the simulator for the first time as the new pilot on the street was unacceptably high maneuvering stick forces at low speeds. In my opinion, these forces were unacceptable for a stick controlled airplane and had to be changed. This created a problem first for me and eventually for Lockheed.
On the flip side, when you lighten up the maneuvering stick forces on a stick controlled airplane that has a 3.5g airframe, the tendency to overstress the airframe at high speed becomes a real hazard; particularly with an unaugmented flight control system at high speed.
Lockheed had initially designed a longitudinal flight control system that met the letter of the specification, but because of its unique stick control on a utility airplane, this design was not acceptable for the mission.
Force Feel Changes Required: The flight control servos in the S-3 had a cam arrangement that created the artificial stick force gradient, so when this problem was discovered it sent the engineers scrambling to redefine the stick force gradient.
I ended up in the hot seat in the flight simulator with the Lockheed VP of engineering challenging my assessment, so I had to show him how I could not accurately maintain altitude during low level maneuvering without excessive concentration and effort and unacceptable handling quality ratings. This was the very environment which sub chasers spent much of their time, so our airplane had to have excellent flying qualities in this part of the envelope.
Many hours of simulation resulted in a compromise at best with a combination of bob weight, viscous damper, and spring constant that would avoid inadvertent overstress at 450 knots but provide adequate pitch response for the pop up maneuver at VPAMIN. The force feel system was optimized for the 160 knot envelope point where I suspected Viking pilots would spend much of their time.
Hundreds of hours testing the ASW systems confirmed our assumptions, at 160 KIAS the Viking was a dream to fly. Altitude control was precise and crew fatigue factor very low.
The High/Fast Slow-Down Problem: Another problem arose during Navy Board of Inspection and Survey trials. Lt. George Webb from the Carrier Suitability branch determined that he could not safely get the airplane down from a high fast start on the carrier approach.
The big fan engines had too much residual thrust and the airplane just did not have enough drag to slow it down to make a timely correction. Many alternatives, such as fuselage speed brakes and split ailerons, were explored but none of them made sense.
Based on my experience flying gliders, I finally suggested that we consider a bam-bam direct lift control system. I suggested using the existing spoilers that the pilot could extend by pressing the nose wheel steering button on the stick.
We flight tested the drag with various spoiler configurations and found the optimum deflection of spoilers that provided enough drag to get George down from his high fast condition and back on glide slope.
So the Direct Lift Control, or DLC, system was designed to extend a fifteen degree increment of spoilers as long as the button was depressed and fully retracted them as soon as the nose wheel steering switch was released. What seemed like a simple fix turned out to be a lot more complicated because the measured AOA was impacted by the spoiler extension and retraction.
The Challenge of Spoiler DLC: The DLC system solved the approach drag solution, but there were other problems associated with extending the spoilers in this configuration. There was an adverse nose up pitch change with spoiler extension. Also, the angle of attack increased when they were extended which drove the autothrottle crazy.
To compensate for both of these problems, we picked an optimum approach airspeed design point to work from, knowing that approach speed would vary with the gross weight of the airplane. We fixed the pitch up problem by introducing a pitch bias in the elevator servo when the DLC button was pressed and solved the autothrottle problem by introducing a corresponding bias into the angle of attack at the same time.
The DLC system actually worked pretty well for what it was designed to do; although I never liked the overall impact to the flying qualities and approach speed as a test pilot. I wasn’t the guy that had to make the high fast correction to get aboard ship, so for him we compromised our simple system.
The production line was shut down at Ship 33, and the DLC system retrofitted and forward fit into all S-3A airframes and the Navy got a better system as a result. Lockheed worked night and day on this change yet did not miss a single contract delivery date and after paying for the change, still made a profit on both contracts.
The High Speed Pilot-Induced Oscillation Solution. Solving a high speed PIO problem involved another innovative approach. Each of the flight control servos had a trim pot capability that was used to bias the no-force position of the mechanical force feel cam. In the rudder and aileron, this neutral spot was relocated by a dual electric trim motor on the servo, but this function was unused on the pitch servo.
Bob Loschke, our brilliant young flight control design engineer, and I came up with the idea of using this unused trim pot on the elevator servo to add or subtract from the pilot stick input through a cam mechanism tied to the flap position.
Our idea was to subtract from and reduce the pitch response to stick input with the flaps up, but add to and increase elevator response with the flaps down. This would have a high rate of response and would act in series with the pilot longitudinal input. We optimized the design in the simulator then tested it in an airplane; to everyone’s amazement, this simple system coupled with the DLC pitch damper made a dramatic improvement in the pitch response in configuration PA, completely eliminated the PIO tendency at high speed and did not degrade the excellent flying qualities we had in the low speed 160 knot search configuration.
We tossed in an APU-powered, electric hydraulic pump in an Engineering Change Proposal, or ECP, so we could fold the wings and check the flight controls without cranking an engine using just the APU generator. This also provided an in-flight, engine-driven utility hydraulic pump back-up.
This airframe change was incorporated into all S-3s as the Flying Qualities Improvement Program (FQIP for short) and resulted in a big improvement in both low speed and high speed flying qualities, as well as an effective ground support hydraulic system.
YTF-34 Engine Development
New Engine; New Challenges: GE had initiated the development of the YTF-34 engine as a candidate for the both of the VSX candidates. The engine had a bypass ratio of 6.5:1 and put out more than 9,000 pounds of thrust on a standard day. It was a derivative of the GE T64 turboshaft helicopter engine which was a state of the art design at the time.
It still had a mechanical fuel control, but incorporated variable inlet guide vanes to produce good acceleration efficiency as long as the airframe was not extracting much bleed air. It had excellent fuel specifics at low altitude, but still produced enough thrust at altitude to get the S-3 up to its certified ceiling of 35,000 feet.
The first instrumented prototype was tested at the GE facility in Lynn, Massachusetts, and then hung on a B-47 flying test bed at Edwards AFB, California. GE test pilot Chuck Anderson flew the B-47 and GE test pilot George Meyers manipulated the throttle to put the engine through its paces. I got to accompany George on one of these flights to learn how to do airstarts and get used to the acceleration response at various airspeeds.
Compressor Stall Discovery: Unfortunately, George uncovered a quirk in the fuel control that caused the engine to roll back. The engine could be driven into a sub-idle stalled condition if the right combination of throttle bogies were introduced.
The worst condition occurred after a snap throttle reduction followed by a call for acceleration. This resulted in the engine fuel control shifting from its deceleration schedule over to the acceleration schedule, but the fuel control could not make the transition without going into a stalled condition.
The good news was that the engine did not flame out (a function of the deceleration schedule), but the bad news was it went into a compressor stall and stagnated instead of acceleration. Since this was considered a throttle movement that could be expected at critical times such as where the commanded power is rapidly reversed from a decrease thrust to an increase thrust command (called a bodie), it was considered a safety risk and needed to be fixed.
In order to keep the engines from this low idle speed in the air for the first flight, a mechanical stop was installed on the throttle quadrant of Viking One before we could go fly. This was fabricated and installed to use for first flight, while GE went to work to solve the problem of the sub-idle compressor stall on the engine.
Eventually, the solution involved a dual idle setting; a higher Flight Idle setting for in-flight and a lower Ground Idle setting that was automatically activated by the weight-on-wheels switch. This protected the engine from spooling down to a low idle condition from which a bodie would cause the engine to roll back in the air. Once on the ground, the lower idle setting would be necessary to kill the residual thrust.
Too Much Bleed Air: Another engine problem surfaced when we started testing the deice system. The S-3 used engine bleed air to run the air cycle pack, pressurize the airplane, deice the engine inlet, anti-ice the first stage guide vanes of the engine as well as the deicing leading edges of the wings and tail.
As long as both engines were operating, everything worked fine and there was ample bleed air to run everything. However, when operating on a single engine, the total bleed loads all shifted to the remaining engine and were more than the engine had been designed to support.
This problem became apparent when we tested accelerating the single engine from idle with maximum bleed extraction. The engine response times were just not unacceptable for a carrier-based airplane. In flight test, we were seeing acceleration times from five-to-seven seconds from idle to maximum power (try that on a waveoff from a low slow condition on a dark night to a pitching deck).
GE was informed that we could not avoid the possibility of a shipboard recovery in icing conditions with an engine out, so they had to come up with a way to solve this problem; not a show stopper for the test program but a definite must before delivery.
Their solution was quite ingenious and was accomplished within the fuel control by changing the acceleration schedule based on the bleed load on the engine. This new fuel control was referred to as the “bleed bias” fuel control which changed the engine designation to a dash-400. The bleed biased fuel control ended up giving us better acceleration times in all modes, with well under three seconds from idle to max; numbers in the same ball park as the J79, which was the fastest accelerating engine in USN inventory at the time; problem solved.
Burner Can Changes Create the Hoover: GE uncovered another problem in the burner can of the YTF-34 in their long term accelerated service life testing in the test cell. They discovered that hot spots were developing in the burner cans that would lead to early hot section removals. This was an obvious design problem that they had to fix, which they did early in the program installing a modified burner can into our flight test engines.
If you’ve ever been around the Viking, you know its nickname of Hoover. This nickname came about because of the weird bellowing moose sound that is emitted from the engine as it accelerates out of idle. This sound was not present on the early prototype engines, but when they redesigned the airflow patterns in the burner cans to eliminate the hot spots, the moose arrived.
Auto-Bolter Trim. Another engine related phenomena was discovered in flight test that was not unpredicted, but turned out to be a bigger problem than anticipated and had to be fixed.
When the final design configuration of the airplane was frozen, it was known that the pod-mounted engines under the wing would have a thrust line that was going to be acting below the center of gravity of the airplane.
What this meant was that an increase in thrust would result in a pitch up and a reduction in thrust would result in a pitch down. What we discovered in flight test was a much larger trim change when we went from an in-trim idle power condition to full power, particularly at low speed.
This unpredicted impact was the result of the huge amount of accelerated air coming out of the fan section of the engine and affecting the flow field around the empennage; the slower the speed, the greater the offset.
The worst conditions for thrust trim change were on a cold day, forward CG and light weight with the stabilizer trimmed near idle power. This point required the most nose up trim for any point in the envelope, but by spec we were required to demonstrate that the aircraft could be trimmed to zero stick force at this point. We discovered that for a full throttle bolter under these conditions, there was inadequate nose down elevator to counter the nose up pitching moment without re-trimming nose down.
A Successful Program By Any Metric: Considering the rapid development schedule and concurrent development and production schedules, by all measurements in the military aircraft development and procurement process, the S-3 Viking was an extremely successful program from both a technical and programmatic standpoint.
The autopilot, autothrottle and ACLS systems all completed BIS trials and fleet deliveries without any gain changes or modifications. Virtually all of the flight control system, autothrottle gains and autopilot development gains were developed and finalized in the developmental flight simulator before they were flown in the airplane.
Good to Go at Birth: The other two new Navy carrier airplanes from this same era, the A-7E and F-14A both spent months in development at Patuxent River to optimize the autothrottle and ACLS systems. Both the technical design and program milestones of the S-3 procurement were met on schedule. There were a number of changes that came out of the test program, but these were addressed and incorporated into the fleet aircraft in an orderly manner.
Viking Fleet Service: The initial intent to deploy the Viking with two pilots eventually evolved into a crew with a single pilot, two NFOs, and a sensor operator. Although there were dual controls that could be operated from either seat, it was designed from the outset to be safely flown by a single pilot, and much of the test work I conducted was done single pilot.
One of the unique features was a left hand throttle and right hand stick for both pilots, making it seat transparent in terms of flying the airplane. The airplane had a good safety record and had several successful in-envelope ejection saves. The S-3 has finished its service life and has been phased out of Navy service—with a couple still at the NASA Glenn Research Center in Cleveland, Ohio—with no fixed wing aircraft forecast to take over its aircraft carrier wing duties of ASW.
The Viking Tanker: In its later years of Navy service, the S-3B configured with a refueling store on one wing and a drop tank on the other, became the only indigenous tanker asset in the carrier air wing. In my opinion, politics killed the prototype KS-3 Tanker and US-3 Carrier On-board Delivery, or COD, programs before they went into production, denying the war fighters badly needed assets.
Womb-To-Tomb Participation: I was blessed to have flown the first flight of the first airplane, as well as many first flights of engineering and production airplanes that followed. I participated in virtually all phases of the Viking development program except for structural tests.
I trained virtually all the Navy pilots who flew the airplane in development as well as the first cadre of instructor pilots and the VX-1 pilots. In so doing, I was the first pilot to reach 1,000 hours in the S-3 while training VX-1 pilots at NAS Patuxent River. Other highlights: flying with the both east coast and west coast Viking Squadron pilots after they received their new airplanes; delivering the last new S-3A to NAS North Island; and making the first flight and development flights on the US-3A COD and the upgraded S-3B
There are very few test pilots who have had the unique opportunity and privilege of flight testing a brand new model airplane in virtually all aspects of the development program over its entire career.
In addition to his work with the S-3, Lyle Schaefer, an Associate Fellow in the Society of Experimental Test Pilots, was pilot in command on the first flight of the C-130J Super Hercules in 1996. Now retired, this is his first article for Code One.