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More lives for the Navy's'Cat'
By Rick DeMeis -- Design News, September 7, 1998
When the Navy's Grumman F-14 Tomcat first entered fleet service 25 years ago, it was the cat's meow in fighter aircraft--a leap in performance, maneuverability, and weapons capability, along with facility in getting back aboard the "boat" following a mission. It outperformed the F-4 Phantom then in service and was miles ahead of the cancelled F-111B it was to replace. And the 125-mile reach of its Phoenix air defense missiles is still unrivaled.
But after 25 years of service, the design might be considered somewhat long in the tooth--especially in light of newer, fly-by-wire airplanes such as the F-18, which have their flight control responses tailored electronically to pilot inputs. Such digital versatility allows programming the controls for many flight conditions, while enabling great combat nimbleness and controllability in coming aboard ship.
As with many aircraft, throughout its operational life, technology upgrades have endowed the F-14, last-of-the-line of Grumman's navy airplanes named for felines, with new capabilities (see figure). But the most recent improvements are challenging younger aircraft designs, while at the same time driving way down both the cost of changes and their "time-to-market."
So what are these new-lease-on-life technologies?
- Commercial software packages, which enable lean project teams to handle tasks from engineering design and wiring layout to testing, data gathering, and report generation.
- Digital electronics, whose processing utility makes it possible to "plug" new capability "wine" into old airframe "bottles" with minimal impact on existing components. The Navy selected a digital flight-control system originally developed for next century's joint European fighter. And the F-14 will be operational with it first.
- Previous advances in propulsion, materials for structures and stealth, and improved cockpit displays and controls.
Here's the lowdown on the latest life enhancements:
Keeping control. Try this at home tonight: go into your bedroom and open the closet door. Turn out the lights and run into the closet while grabbing the doorknob shut--all without hitting the back wall. Sound challenging? That's what it's like to land a jet fighter on an aircraft carrier at night. Now, add some bad weather and a pitching deck to the equation, and you can understand the difficulty.
With its original late-1960s limited-authority (deflection) analog flight-control system, which includes mechanical linkages, "The F-14 often feels like a family sedan with wings, while [other] fighters feel like sports cars," says Navy flier Eric Wahlstedt, of Test and Evaluation Squadron Nine (VX-9). The later-designed F-16 and F-18 have digital control systems whose computers can command up to full deflections of the airplane control surfaces. And the F-15, developed nearly concurrently with the F-14, is a quarter lighter (not having to be strong enough for carrier operations), so it has to overcome less inertia, and also has higher-authority actuators.
Because of its analog controls and configuration aerodynamics, the original Tomcat is tougher to bring aboard ship than other aircraft today. When making a "power approach" to the ship, its shortcomings include difficulty in achieving small bank angles, and poor damping of coupling motion between the yaw and roll axes (Dutch roll). This lightly damped mode means several overshoots of a desired heading change compared to the higher damped F-18, which usually experiences none. Such characteristics kept the F-14 accident rate higher than that for newer designs.
Why not have an automated system for carrier landing? In fact, an automatic carrier landing system is available. This radar-based system on the boat locks onto an airplane eight miles out. It will command the auto throttle and pitch and roll inputs for hands-off landing. But, according to Gary Kessler, a Naval Air Systems Command (NAVAIR) manager for F-14 technology programs, "To stay proficient, pilots tend not to use it." And infrequently, rain may scatter the signal, so lock-on is closer in with less time to establish a stabilized approach. Thus, when most useful, it may not be available anyway.
And there are other flight-control issues. Initial flight testing quickly found the F-14 is able to achieve angles of attack up to 90 degrees (i.e. roughly nose perpendicular to the airstream)--considerably higher than designed. But such maneuverability has an unintended consequence--the prospect of entering spins. "The F-14 is a very 'honest' aircraft," notes Bill Stiles, a stability and flight-control development consultant to the Navy. "Pulling full aft stick won't result in departure into a spin like other aircraft. But in an asymmetric pull up [such as having roll input] the airplane can enter a spin. The difference now being the F-14 takes significantly longer to recover than an F-15."
High angle-of-attack spin-entry prevention was actually developed by Grumman and NASA in separate test programs starting in the late '70s. But concerns over redundancy using single-channel hardware to validate sensed angle-of-attack values scotched implementation of the analog system. Spin prevention was left to the pilots.
Help from abroad. Circumstances and technology have come together to produce a digital-electronics remedy. And Stiles notes other incentives to convert the analog flight controls to digital. The original system was experiencing increasing parts obsolescence and maintenance, and reaching its limit for improvements. A new digital system would offer reprogrammability ease. To address these issues, the Navy surveyed industry to see what might be provided. Based on the responses received, the service selected GEC-Marconi Avionics (Rochester, England) for its Digital Flight Control System (DFCS).
The British company developed the system for the Eurofighter, a multinational aircraft now under test, that won't become operational until after the turn of the century. Thanks to digital electronics' versatility and the company's computer architecture adaptability, a derivative of the original DFCS will see operational use in the F-14 first.
The timely selection and adaptation of the GEC-Marconi system to the Tomcat was the result of engineering management resourcefulness at NAVAIR, according to Stiles. Back in 1991, without a specific development budget, Navy Commander Bob Baker found money to integrate and test the off-the-shelf-computer DFCS under the Foreign Comparative Test (FCT) provisions of the Nunn-Quayle defense reform legislation. The Navy flight-tested the system under the FCT program from July to December 1995.
Good thing. Since F-14 production had been halted in 1994, no replacement airplanes for those lost in any accidents were coming off an assembly line. The success of the tests, and a series of accidents in early 1996, prompted Congress to fund an $80-million production program, less than the cost of two airplanes, to outfit nearly 200 Tomcats with the "plug-in" DFCS.
Teamwork. With low overhead, which helps keep costs down, the Navy is acting as the program's traditional performance and system-integration prime contractor and flight tester. GEC-Marconi (repackaged hardware) and the now Northrop Grumman (aircraft design and structures, flight-dynamics simulation, systems safety analysis, and flight test) function as subcontractors. The 25-member integrated product team (IPT) from all involved organizations is now co-located at the Naval Air Warfare Center-Aircraft Div. at Patuxent River, MD (DN 5/18/98, p. S23).
Concurrent engineering has been vital since the start of the project. To this end, a linked, independent computer station at GEC-Marconi in England was established for quick local download and review of any specification changes developed by the F-14 IPT, according to Bob MacKrell, NAVAIR lead engineer for DFCS. He notes that if the program were started today, with current firewall security technology, Internet transfer of such information would be used.
Another boost to concurrency is having the test-aircraft hanger "out back" at Patuxent River, facilitating quick-turnaround integration and testing for everyone. MacKrell adds that in weekly team meetings, each team member speaks to a specific technical area of responsibility and not for the organization or company that person came from. This small project team formed by the service and subcontractors is "very open, without any egos, with lots of give and take," according to Stiles. The arrangement is working so well, the Secretary of Defense gave the DFCS team the first FCT Project of the Year Award, citing timely completion of DFCS evaluation and overall management excellence.
Adaptable architecture. The Navy started installing DFCS into fleet F-14s in May of this year, along with other improvements, including night-attack systems, satellite navigation, and countermeasures systems. Northrop Grumman is outfitting three to four airplanes each month at Oceana Naval Air Station, VA, with the rate increasing to six to eight this month. By mid 2000, 192 aircraft (75 F-14As, 67 Bs, and 50 Ds) will have the system. Additional operational savings will then be accruing because the mean-time-between-failure for DFCS is more than the required 800 hr, compared to less than 100 for the analog system.
The three F-14 DFCS computers are direct form, fit, and functional replacements for the three original analog computers, and are loaded with the control laws developed by the Navy, NASA, and Northrop Grumman. Rewiring is minimal, involving just the needed inputs and independent power wiring to the new computers and a cockpit panel. These digital computers direct rudder and differential horizontal stabilizer deflections to prevent spins. In the power approach configuration, turn coordination is better, Dutch-roll damping made equal to that of the F-18, and roll control is more sensitive, thanks to elimination of a deadband before spoiler extension. Pilot reaction? Long time F-14 driver and fighter wing commander Dale Snodgrass says, "You have finally given me the F-14 I always wanted--big motors and it goes where you point it" after his first flight in a DFCS equipped F-14D.
Each dual-processor computer is responsible for one control axis (roll, pitch, or yaw). The unique GEC monitoring and voting architecture gives the required triple redundancy. Both redundant sensor input data and output commands are monitored, cross compared, and voted on. If one computer's processor fails, the cross-channel data links between computers allow functions to be picked up by the others. Stiles adds any degradation is graceful in that lower functions and some redundancy are lost first.
Off the shelf. Commercial engineering software was key to keeping costs down and the project moving. NAVAIR's MacKrell mentions the use of The MathWorks (Natick, MA) MATLAB(TM) for the control system design, coupled with its SIMULINK(TM) graphical user interface. Several software tools developed by Navy and Northrop Grumman engineers over the years also have been redesigned and implemented as MATLAB toolboxes.
The Navy used its manned-flight simulation laboratory for DFCS integration and testing along with Applied Dynamics Int'l (Ann Arbor, MI) virtual-machine environment (VME)-based Real-Time Station (RTS) hardware and software. MacKrell says RTS flexibility "allowed engineers to effectively automate DFCS testing. It provided a user-configurable graphical user interface (GUI) for ease of use by many personnel for concurrent software coding and writing test procedures."
Engineers analyzed flight-test data with BBN's (Cambridge, MA) Probe package. For ground testing, they used an On-Aircraft Test Set (OATS). They configured a laptop PC with a 1553-bus board from SBS (Albuquerque, NM) to use on the airplane. MacKrell says the alternative to this low-cost, off-the-shelf solution was "a full blown" VME minicomputer. The OATS, featuring National Instruments (Austin, TX) LabVIEW(TM)-software's customizing GUI, permits an engineer to enter inputs into the DFCS flight control computers as well as acquire data.
To achieve the "plug-and-play" computer installation, several important criteria were met, again within the confines of no or minimal changes to the airplane. Perhaps the most critical of these is air-data (for measuring angle of attack) sensor redundancy. The DFCS had to use the existing sensors on the F-14, posing a problem for the stipulated triple redundancy, according to Kurt Grobert, Northrop Grumman F-14 manager of vehicle integration. These sensors consist of a pneumatic-pressure angle-of-attack sensor at the tip of the radome, backed up by a mechanical rotating cone Airstream Direction Detector just ahead of the cockpit on the left side. Although adequate for normal flight with the analog system, they are not sufficient for reliable, high angle-of-attack spin prevention.
Grumman had a solution. Further aft, below the canopy on each side of the fuselage, are pneumatic probes that gather data used to control the engine inlet ramp positions. Why not use these to furnish a third source of aircraft angle of attack? While these probes are subject to airflow masking during sideslip (airflow at angles off to the side) and are sometimes affected by the presence of stores hung on the airplane, the complex compensation routines and redundancy management could be accomplished with a digital system. The sideslip estimation routine uses a new rudder-pedal position sensor along with the existing differential (left versus right) horizontal-stabilizer angle measurements and lateral accelerometer. In further developments, Stiles sees such digital-control-system utility finding use in processing data needed for intelligent-highway vehicle guidance.
Eyes in the dark. Another timely technology upgrade has given the Tomcat night vision for navigation, targeting, and attack. In another adroit IPT effort, fleet F-14s were given the Lockheed Martin LANTIRN (Low Altitude Navigation and Targeting Infrared for Night) system (see sidebar). The airplane can now place laser-guided weapons on targets day or night, thus taking the place of the retired A-6 Intruder long-range attack plane. Half of the operational F-14s have LANTIRN capability, with the entire force coming up to speed by the end of 2000.
The podded LANTIRN system is so self-contained that no equipment or software on the aircraft had to be changed, says Al Eusini, F-14 sensors and weapons upgrade director for Northrop Grumman. "Any changes were just additions, basically just a bundle of wiring," he adds. While validation of aerodynamic and structural loads imposed by the pod, along with laser-guided-store trajectory clearances from the aircraft, had to be done via software previously developed by the company, the installation's overall simplicity allowed a fast-paced program.
Engineers designed the aircraft wiring runs using AutoCAD(reg) from Autodesk (San Rafael, CA). The wiring connects the joystick control panel used by the back-seat crewman or RIO (Radar Intercept Officer) to the pod along with powering the pod. In use, LANTIRN infrared video is shown on the existing RIO and pilot displays. The RIO designates the target with a laser and the system cues the pilot for weapon release.
Such lean engineering efforts give older aircraft like the F-14 new capabilities and usefulness for many more years. By upgrading existing assets, defense planners have wider options in deciding how to give US forces more capability in lean budget times.
Associate Editor Rick DeMeis once worked as a flight-test engineer on the F-14 program at Grumman.
Design challenge
Integrate up-to-date systems and capabilities into existing aircraft Technology solutions:
- CAD software fits and aligns devices into available volumes
- Thermal analysis provides adequate heat dissipation
- Finite element modeling determines structural stresses due to changed loadings
- Layout programs allow interference-free wiring runs
- Device specifications ensure electromagnetic compatibility
Cat tales: the roller-coaster lives of the Tomcat
Technology improvements fuel new capabilities
10 July 1968: The Department of Defense stops work on the Navy version of the F-111 because of overweight and lack of performance. The already-developed long-range Phoenix missile and TF-30 engine are to be used on a replacement aircraft.
14 January 1969: Navy awards fighter contract to Grumman for what will become the F-14.
21 December 1970:F-14 makes its first flight.
1973: Air Force general officers recommend procuring F-14s as interceptors. Eventually the F-15 is chosen in this role despite fewer intercepts-per-aircraft capability.
1971-1973: The remaining 11 prototypes come on line. A comprehensive flight-test effort, keyed by telemetering and recording test data, as well as in-flight refueling, brings the F-14 into Navy service.
30 December 1970: On its second flight, the first F-14 crashes due to hydraulic system failure caused by vibration and resonance in the lines.
1 July 1973: First Tomcat fleet squadron, VF-1 "Wolfpack," receives its aircraft.
1974: F-14 crews fly their first combat missions providing air cover during the evacuation of Saigon.
July 1975: The Pentagon decides the Marine Corps will not be equipped with F-14s. These aircraft are used by additional Navy squadrons instead.
June 1974: Iran orders F-14s. The Nixon Administration approves the sale to the Shah-led government in order to cut program costs.
1981: Initial deployment of the F-14 with the Tactical Air Reconnaissance Pod System (TARPS). Today's versions furnish near real-time digital images to commanders via data link.
19 August 1981: After coming under missile attack, two F-14s down two Libyan Su-22s over the Gulf of Sidra.
October 1985: In a plan devised by Lt. Col. Oliver North, Tomcat-led forces intercept and escort the airliner carrying the Achille Lauro highjackers to Sicily for capture.
4 January 1989: Two Libyan MiG-23s threatening U.S. forces in the Mediterranean fall to a pair of Tomcats.
16 November 1987: Maiden flight of the first production F-14A+ (later redesignated F-14B) which is powered by F110 engines. This powerplant solves the prolonged troubles (compressor stalls, blade failures) of using the originally installed but "interim" TF-30 engine in the stressful F-14 flight regime
January 21, 1991: The Gulf War begins. F-14s serve as effective "MiG repellant," escorting Navy strikes from carriers in the Persian Gulf and the Red Sea..
July 1992: F-14D with advanced avionics, sensors, and systems, as well as the F110 engine, enters fleet service but only 55 are built.
May 1998: Navy begins installation of low-cost "plug-in" Digital Flight Control Systems (DFCS), improving F-14 handling and control.
14 June 1996: In just 223 days, Lockheed Martin delivers the first LANTIRN (Low Altitude Navigation and Targeting Infrared for Night) pod to Navy squadrons.
1996: The Navy's Intruder attack aircraft fly their last missions. More F-14s are given ground-attack capability to make up for the loss of these long-range strike aircraft.
The cat with two tails
Form follows function, or why the F-14 looks the way it does
- Twin vertical tails--these were dictated by having enough rudder force available to handle the large yawing moment generated if an afterburning engine fails at low speed during takeoff. A bonus was increased high-speed stability, maneuverability, a lighter airframe, and more compatibility with aircraft carrier space limits.
- Swing wing--resulted in a lighter structure than if a fixed wing and complex flap arrangement were used in obtaining the low-speed lift and handling needed for carrier landing a high-speed fighter.
- Widely spaced engines--offer minimal exhaust flow interference with each other for greater total thrust. This resulted in the "tunnel" area between the engines for low-drag weapons carriage.
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