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UNSTABLE AIRCRAFT DESIGN: THE COMPUTER AT THE CONTROLS

Rafale, Gripen, Eurofighter Typhoon or the F-22: the new generation of superfighters have one design feature in common, something that no modern fighter can do without. They are designed unstable. At the end of the 70s the Lockheed Martin F-16, developed back then by General Dynamics, was the first series production fighter to use this technology.

The technological principles for an unstable design, which is intended to increase agility, were known before the F-16. However, there were probably a few reasons for not having this technology implemented into a program earlier. On the one hand computers were not powerful or small enough to be used as regulators on an aircraft, on the other hand redundancy could not be secured.

While there have been regulators installed in aircraft before, mainly in order to improve dampening which has a positive effect on the aircraft's handling, the demands placed on reliability and speed are much higher with unstable designs. Without a regulator the aircraft cannot be mastered, since the human reaction at the controls are too slow.

The main benefits from this new technological includes a much higher aircraft agility and a considerably lower drag. Since agility is important for fighters, unstable designs have so far only been used for military aircraft applications. Furthermore they were only deemed practical for movements along the pitch axis.

In aircraft design, natural stability means that if steered off balance, an aircraft will automatically reassume a balanced position without the need for pilot's intervention. If for instance a gust of wind causes the aircraft to pitch up, its position is automatically corrected. The position of the neutral point in relation to the aircraft's centre of gravity is decisive for this automated feature.

The forces created by the rotating aircraft movement work at the neutral point. To take the pitch-up as an example: If the aircraft lifts its nose, the angle of attack is increased and with it the lift of the wings. This additional lift takes effect at the neutral point. This way, if an aircraft assumes an unbalanced position there is always a counter force, which counteracts the unbalancing force. A bigger unbalance will result in a bigger moment.

A measure for the stability of an aircraft is the spacing between the centre of gravity and the neutral point. The further these two points are apart, the more stable is the aircraft. This explains why the neutral point has to be located aft of the centre of gravity in a stable aircraft. The existence of a correcting moment is also called static stability. These considerations do not only apply for movements around the pitch axis but also around the roll and yaw axis.

Positioned at the back, where the strongest side force is created, the vertical tail generates a stable condition around the yaw axis for most applications. Rolling is a different matter, because there is no correcting momentum. Some stability is reached though through the combination of roll and yaw controls.

If an aircraft is stable, there is always a compromise with the desired control effectiveness. The reverse momentum (as explained above), which is generated through the aircraft movements actually works against the aircraft's intended direction of movement. In principle it makes no difference whether the destabilisation is caused by a gust of wind or by a pilot control input - in both cases the aircraft reacts the same.

High stability and highly effective flight controls are therefore incompatible requirments. In stable aircraft a compromise has to be made. However, no concessions need to be made in unstable aircraft. The demand for natural stability has been foregone completely. The neutral point is not located behind but in front of the centre of gravity. The force, which in conventional designs counteracts the movement, supports and amplifies it in unstable aircraft designs. If an unstable aircraft pitches up, the additional lift works as lifting moment in the neutral point. Therefore, with unstable designs, righ rotary acceleration can be achieved with small control surface deflections. The aircraft is nimble and agile.

Any pilot will find it very hard to control an aircraft like this without assistance. A computer is needed and a regulator will supply the necessary stability. It will not generate the natural stability of conventional aircraft, but will automatically trim the aircraft to the actual flight conditions.

If the pilot moves the aircraft by pulling the control stick, and then lets it go, a stable aircraft will move back into its trimmed initial position. An unstable aircraft, however, remains in the same flight position, which corresponds with the trimmed balance generated by the regulator.

In the flight control computer the pilot's steering commands are being superimposed with the necessary commands for the stabilisation of the aircraft. It is furthermore constantly fed with data of the aircraft's position, i.e. speed, altitude and attitude. The computer also knows the aerodynamic limits of the aircraft, which enables it to calculate the optimum rudder movements.

Fly-by-wire flight control systems seem the ideal technology for these applications. Steering commands are no longer linked mechanically from the cockpit to the rudder, but via electric wiring.

There is another benefit from unstable design: Along with the aircraft's increased agility, there is also a reduction in drag. Arranging the neutral point behind the centre of gravity in stationary flight, i.e. a stable layout, results in reaching the balance of momentum through a downward lift of the elevator. This downward lift has a negative effect on the total aircraft lift. More lift must be generated at the wing, than there is normally needed to counterbalance the weight force.

In an unstable layout the elevator's lift is direct upward to counterbalance the momentum. This way, the aircraft's total lift is increased. The aircraft can therefore be designed smaller and lighter and still give the same performance.

A further less obvious aspect can add to a drag reduction in unstable aircraft. Vortex is caused at the leading edge of swept wings during subsonic flight, which result in the largest air stream losses at the highest angles of attack. In order to reduce the AOA (and the corresponding losses) while maintaining the lift, the lift can be repositioned towards the aft wing area by lowering the trailing edge flaps. However, this flap movement generates a pitch-down force which must be balanced.

Stable designs already have a down-pitching force because of the position of the neutral point aft of the center of gravity. Any balancing force would need to be generated by a larger downward lift of the horizontal tail. This, again, would decrease the maximum possible lift and increase the trim drag.

Unstable designs, on the other hand, allow to kill two birds with one stone. For one, the downward moment, which is generated by lowering the trailing edge flaps, is balanced already by the already existing upward force. Secondly, the lift that the horizontal tail needs to generate is lower, such also reducing the trim drag.

Since agility is a decicive design criteria for fighter aircraft, there will be only few new developments which do not apply the unstable design philosophy. In civil aviation it looks different: While the engineers are doeing almost anything to reduce the drag (and such the fuel consumption), none of the manufacturers has so far seen a justification for the technical effort of an unstable design.

From page 74 of FLUG REVUE 9/99