FLUG REVUE July 2003: X-31A demonstrates ESTOL landing

SUCCESFUL ESTOL LANDINGS WITH THE X-31A

By Karl Schwarz

„It was the high point of my career as a pilot,“ said Rüdiger Knöpfel of the Bundeswehr Aircraft Test Center (WTD 61) with enthusiasm, as he looked back on the first landing with the X-31A in extremely short take-off and landing (ESTOL) mode. In the critical minutes of the flight he had had virtually nothing to do, as it is only thanks to the unbelievably accurate position finding and automatic control functions of the complex software that the German-American experimental aircraft is able to perform extremely short landings.

X-31A ESTOL landig

ESTOL is pure high-tech. The systems on which it builds include the Integrity Beacon Landing System (IBLS) developed by the IntegriNautics, which functions to within an accuracy of two centimetres. It is based on the Global Positioning System (GPS), but is supplemented by two virtual satellites or „pseudolites“ to the left and right of the runway. The X-31A had to overfly them and then to stay below 700 m and within a nine km radius to maintain maximum accuracy.

The IBLS not only evaluates data transmitted by the pseudolites and at least four satellites, but it also uses the phase shifts of the carrier signals for its positional calculations.

Position and flight attitude data constitutes the basis for automatic control. For the ESTOL landings on the runway at Patuxent River Naval Air Station, the X-31A was specifically upgraded with version 124 of the Operational Flight Program (OPF), which takes into account factors such as landing gear extended and ground effect. Over 600 simulator hours, 320 of them with pilots, were required just to review the software developed under the leadership of EADS.

Finally, a short landing requires an approach with a high angle of attack, and this is only feasible with integrated thrust-vector control. For cost reasons the X-31A does not have a vectoring nozzle but three carbon-carbon composite paddles.

An ESTOL landing begins with flying into the area covered by the pseudolites, followed by a manually controlled circuit. The pilot then brings the X-31A into the engagement box and switches over to autopilot. The aircraft now assumes a high angle of attack and follows a curved path through to the landing point.

Only just before touchdown, when the paddles of the thrust-vector control system are only half a metre above the asphalt, does the nose of the X-31A drop back to the normal 12 deg. angle of attack. The aircraft sets down and the pilot takes over control again.

Originally a maximum 40 deg. angle of attack was planned, but for cost reasons a less ambitious goal had to be accepted. During the first ESTOL landing by Knöpfel on 22 April, they made do with 12 deg.. Eleven test flights later Major Cody Allee of the US Marine Corps managed 24 deg. at around 17.30 hours on the last flight of the X-31 on 29 April.

„There are no g forces and you sit leaning somewhat backwards in the ejection seat while the nose is pointing sharply upwards,“ said Allee, describing his experiences. At angle of attacks greater than 15 deg. the pilot cannot see the runway except on the screen on the right-hand side of the instrument panel. „Whereas on a normal landing the landscape flashes by, now everything takes place as if in slow motion.“

„You don't have time to be nervous,“ adds Rüdiger Knöpfel, although the manoeuvre is extremely risky. „If the computer reduces the angle of attack too early, the nose landing gear could buckle under; if it acts too late or too little then the tail could hit the ground.“

Neither occurred, a major success for the small test team, which had to contend with a very tight budget. But what was the point of all this expenditure? On the last test flight the landing speed was 121kt (224km/h), down 31% from the normal 175kt (324km/h). Whereas on a normal conventional landing the X-31 requires some 2,400m of runway to come to a halt, all it requires now is 520m.

Such figures naturally depend on the aircraft type, but have the attraction of making landings on short temporary airstrips feasible. The reduction in landing energy is important for aircraft carrier-based operations. The aircraft structure could be lighter or it would be possible to set down with greater fuel reserves remaining or with unused weapons without overstressing the airframe. Again, the requirements regarding headwind on the deck would be less stringent if planes could land at a lower speed.

As well as the spectacular short landings, also tested on the X-31A was the groundbreaking Flush Air Data System (FADS) developed by EADS. Unlike ordinary pitot tubes and angle of attack transducers, FADS is integrated into the nose of the fuselage. With a dozen miniaturised sensors consisting of tiny ceramic capsules with highly integrated processors, it captures the pressure ratios extremely accurately even at high angles of attack. The speed, height, angle of attack and angle of sideslip can then be calculated from the results.

FADS offers a significantly smaller radar cross-section than conventional sensors. It could be used on future versions of the Eurofighter. Due to the small size of the FADS, there would be no problem fitting FADS in front of a radar antenna.

From FLUG REVUE 7/2003

Last updated 11 June 2003

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