The X-31 captured the imaginations of those who saw it flown at Paris, but does it have a future?

Graham Warwick/PARIS

A spectacular flying-display performance by the Rockwell/Daimler-Benz Aerospace X-31A Enhanced Fighter Manoeuvrability (EFM) demonstrator has rekindled debate over the combat benefits of thrust vectoring. Few watching the Paris air-show routine could doubt the results of 1994's tactical-utility evaluation - that the X-31 is virtually unbeatable in close-in combat - but sceptics continue to question the value of such slow-speed manoeuvrability when the current fighter-design emphasis is on beyond-visual-range combat.

The manoeuvres demonstrated at Paris, were made possible by multi-axis thrust-vectoring, which allowed the X-31 to be flown to post-stall angles of attack exceeding 70°. The same manoeuvres were used by X-31 pilots to defeat a NASA McDonnell Douglas F-18 in 78 out of 94 close-in air-combat engagements during a tactical-utility demonstration, completed in December 1994, which Rockwell maintains "...established that all future close-in fighters will have thrust-vector control".

To demonstrate the wider benefits of multi-axis thrust vectoring, Rockwell is seeking US Navy funds to conduct simulated aircraft-carrier landings with the X-31. An initial series of flights, completed in January, proved that precision approaches could be performed at speeds down to 80-90kt (150-170km/h) using thrust vectoring, Rockwell says.


Simulations indicate that landing speeds of 65-70kt are achievable using thrust vectoring, obviating the need for an arrestor hook, according to Dr Leslie Lackman, general manager of Rockwell's North American Aircraft division. The flight trials were conducted with the X-31 in its "quasi-tailless" mode, with the digital fight-control system reprogrammed to simulate removal of the fin. In this mode, directional control is provided by the aircraft's thrust vectoring paddles.

Simulated carrier approaches were flown down to 100ft (30m), and demonstrated acceptable handling qualities, says Lackman. The flight trials were funded by the US Air Force/Navy Joint Advanced Strike Technology (JAST) programme office and included simulated bombing runs to evaluate the utility of thrust vectoring for air-to-ground missions.

Although flight-testing of the X-31 has been completed, US research into thrust vectoring continues. The USAF will soon begin flight tests of a McDonnell Douglas F-15 equipped with axisymmetric thrust-vectoring nozzles under the Advanced Control Technology for Integrated Vehicles (ACTIVE) programme. The F-15 has been equipped with Pratt & Whitney pitch/yaw balanced-beam nozzles (PYBBNs) for flight tests to evaluate the cruise benefits of thrust-vectoring.

The ACTIVE effort is a follow-on to 1994's Multi-Axis Thrust Vectoring (MATV) programme, which involved a Lockheed Martin F-16 equipped with a General Electric axisymmetric thrust-vectoring nozzle. The MATV flights concentrated on evaluating the manoeuvrability benefits of thrust vectoring, and demonstrated many of the manoeuvres showcased at Paris by the X-31.

The sole NF-16 variable-stability in-flight-simulator aircraft, (VISTA) used for the MATV tests, is now being permanently equipped with a P&W engine and PYBBN. The permanent installation will include dual-redundant nozzle-control systems, allowing thrust vectoring to be used down to low altitude, and Lockheed Martin is hopeful that the VISTA F-16 will available to be displayed publicly in 1996.


Behind the scenes, operator interest in thrust vectoring remains high. Israel is still intent on modifying its GE-powered F-16s, if and when funding can be found. GE, meanwhile, is talking to McDonnell Douglas about a possible thrust-vectoring demonstration on the F404-powered F-18. This would use a scaled-down version of the MATV F-16 nozzle.

Rockwell's North American Aircraft division, which specialises in combat-aircraft upgrades, is talking to potential customers about retrofitting thrust vectoring on the F-15, F-16 and F-18. The company's interest is in performing the system-integration required to interface either the GE or P&W nozzles with the existing digital flight-control systems, Lackman says.

Flight-testing of the X-31 is now complete, and the surviving aircraft - one X-31 was lost in an accident on 19 January - was to be grounded on its return from Paris. Funded flight-tests were actually completed in January, but the two companies, and the US and German Governments, agreed to share equally the cost of displaying the X-31 at Paris.

The JAST office represents the most promising source of funds to keep the X-31 flying, says Lackman, who admits that money will have to be found by the end of the year to avoid the possibility of the aircraft being placed into storage. Other potential funding sources include USAF, Navy and Advanced Research Projects Agency technology programmes, he says.

Rockwell portrays the surviving X-31 as a "valuable asset" for further evaluation of tailless flights, thrust vector control and helmet-mounted displays - all of which have potential application to the JAST effort. In addition to further testing of thrust vectoring for carrier operations, the company believes that the X-31 could be used to demonstrate the utility of multi-axis thrust vectoring for ground attack and short take-off and landing (STOL).


The benefits of two-dimensional, pitch-only, thrust vectoring were evaluated between 1989 and 1991 using the USAF's F-15 STOL/ manoeuvring technology-demonstrator. A derivative of the P&W nozzle used on this aircraft is fitted to the F119 engine under development for the Lockheed Martin/Boeing F-22 air-superiority fighter, but the USAF has no plans yet to introduce multi-axis thrust vectoring on any existing aircraft.

Thrust-vectoring is not included on the "wish list" of upgrades the USAF has drawn up for both the F-15 and F-16 under its Fighter Configuration Plan. This new plan outlines survivability, lethality and supportability upgrades required to keep, existing aircraft in front-line service to 2020.

Lockheed Martin is, meanwhile, looking at offering multi-axis thrust-vectoring on its F-16 "Falcon 2000", an extensively improved aircraft which would be offered to the USAF in the event of delays to the JAST programme, which is intended to produce a replacement for the F-16. Privately funded design of the stretched, delta-winged, Falcon 2000 has been under way for about 18 months, the company says.

It remains to be seen, whether photographs confirming that Russia's latest Sukhoi Su-35 air-superiority fighter, has been fitted with thrust vectoring nozzles, will provide further impetus to Western efforts. Although axisymmetric, the nozzles fitted to the latest pre-production Su-35 appear to vector thrust only in the pitch plane, but Sukhoi has previously indicated that its ultimate design goal is multi-axis thrust vectoring.

Source: Flight International