Definitions

stick on

Centre stick vs side-stick

History

Most early aircraft, like the motor cars and more especially the boats, from which they developed, were steered using a wheel. A wheel made best use of the pilot’s muscles, having to cope with large aileron and elevator movements. Eventually powered controls (and in turn fly-by-wire (FBW)) obviated the need for muscle power, but the wheel remained. A two-handed control mechanism was also convenient when mounting positions had to be found for ancillary cockpit controls such as autopilot, rudder trim switch, microphone switches and so on. The wheel was developed into the yoke which is still used by most commercial airliners such as the Boeing 777.

Side-stick control, however, was adopted by The Wright Brothers, and by some of the other aviation pioneers. By the end of World War Two, however, a control column set between the pilot’s legs had become the norm. During the war the side-stick did appear in the B-24 as an autopilot control - a small column or joystick mounted alongside the left leg of the aircraft captain. This enabled him to control the aircraft directly through the autopilot when flying in formation. It was an extension of the system whereby the bomb aimer exercised control of the aircraft in yaw when approaching a target.

During the war years the position of the throttles varied considerably between aircraft, from on the right, on the left and in the centre. German fighter aircraft typically had a left throttle, while British aircraft had the throttle both left and centrally. American aircraft also differed. The historical placement of a left-hand throttle led the side-stick to be generally located to the right.

In the Tu-16 jet bomber and in its civilian version (the TU-104) a side stick autopilot control was mounted on the right arm rest of the captain’s seat. The side-stick idea was revived on and off during the 1950s but even with power assisted controls, there was not enough room to accommodate the mechanical linkages for true side-stick control of elevators and ailerons which was required in case of failure. Only when FBW design dispensed with the need for mechanical reversionary back- up was side-stick fully practicable.

In the 1980s the Airbus 320 was the first production airliner with side-stick primary operating controls operating through FBW, while Boeing from its 707 to 777 stayed with the traditional column wheel and column primary controls.

Modern fighter jets

During the 1980s the Swedish Air Force considered the use of s side stick in the JAS 39 Gripen. This was eventually ruled out for number of reasons. These included the desire by the air force for the pilot to have left hand use of the stick in an emergency, unfavourable comments from Swedish pilots who had flown the F-16, and problems of existing control displacement caused by relocating the stick to the left console.

Although the cockpits of many modern fighter jets, such as the F-35, Rafale and Su-37, feature a side-stick, the choice between side-stick and centre stick is still contentious.

Advantages of side-stick

  1. A main advantage of relocating the control stick to the side is that it no longer obscures, even if only partially, the lower instruments and controls directly in front of the pilot.
  2. If the aircraft is expected to encounter high g-forces and rapid manoeuvring flight, locating the stick on top of a console will enable the pilot to brace his hand and forearm against a hard surface and thus reduce arm flailing.
  3. The smaller size of the side stick compared with the conventional centre stick allows for greater precision of hand movement and less fatigue. It is easier to move under high-g.
  4. Removal of the centre stick may also allow faster access by both hands to the ejection seat grab handle which is now universally located under the seat between the pilots legs.
  5. With the advent of ever bulkier and more sophisticated aircrew clothing, a further minor consideration is that the side-stick will afford easier airflow around the right arm, also aiding free movement and reducing fatigue.
  6. A main advantage of side-stick has frequently been seen as the additional room it affords for tilting the seat further back to combat the effects of g- stress. With no room to tilt the seat back, g-protection must be provided by advances in aircrew clothing, as has happened with Eurofighter Typhoon.

Advantages of centre stick

  1. The main advantage of keeping the control stick at the centre is that design and use commonality with earlier fleet aircraft may be maintained, This can be seen as a major factor in then decision to use a centre stick for Eurofighter Typhoon. The F-16, in contrast, was a fresh FBW design and was thus given a side-stick from the outset.
  2. The centre stick provides more visual feedback of its position compared with the smaller side-stick and may also show more clearly the position of such safety-critical controls as weapons late arm.
  3. A larger stick may mean more room for the incorporation of HOTAS controls, which have become ever more critical in the design of modern fighter cockpits.
  4. Another perceived advantage of centre stick is that, in the event of pilot incapacitation through battle damage, the stick may still be used by the alternate hand. (It may also be argued, however, that if injury was this severe, abandonment of the mission and even of the aircraft might be the only realistic option).
  5. A further perceived advantage of centre stick is one of mechanical protection to the user – pilots may feel safer with a sturdy stick between their legs, even if, in reality, they are not.

Force or Movement control

Even with powered controls, however, pilot kinaesthetic feedback is still important and in many power-assisted systems artificial loads are added to the controls to prevent the aircraft from being overstressed. The pilot must still get the right `feel’ from the movement in the controls. In FBW systems although protection is offered digitally by the processing computer software, feedback forces may still be important.

A side-stick control can either be a displacement type in which the small control column replicates the movement of a joystick, but over much smaller angles. Or it can be virtually fixed and respond to the force being applied by the pilot’s hand. With both methods movement or pressure is detected by sensors that feed data to a flight control system, which in turn controls the power units that move the control surfaces.

There is some debate as to whether the side-stick is better as a force or movement controller. Although the prototype F-16 was equipped with a rigid stick, a very small amount of movement was introduced before the aircraft went into production.

With the advent of more military female aircrew, a consideration of pilot strength characteristics may be more critical. Hodgson (2001) found that one side-stick force gradient could accommodate an extended user population that included females.

Side-by-side conflict

In twin-seat aircraft, with a conventional wheel or yoke control in front of each pilot, the two sets of primary controls are mechanically linked together. In the unlikely event of a disagreement between pilots, the stronger will win. In FBW aircraft, however, large inputs may be ignored by the flight computer, and one or both controls may also include an override switch.

Commercial airline pilots who sit side-by-side have also been asked if they find it difficult or awkward when they change seats, so that they must use their other hand on the stick. Experience during the development of the A320 has shown that when taking off the pilot in the P1 (left) position, with the stick on the left, typically rolled slightly during rotation. This was caused by the natural lie of the forearm and the plane of rotation of the wrist. So the aircraft was re-moded to always maintain a wings-level attitude in takeoff (there was a similar but opposite effect for the pilot in the P2 position).

Since the two side-sticks are not linked mechanically in the Airbus, there was considerable debate over the relative authority that each stick should have. One favourable design which emerged was as follows:

"When one stick is not producing any control demands, the other has full control authority.
When one stick is deflected by less than six degrees demands from the other stick are summed with it.
When one stick is deflected more than six degrees, a demand from the other stick is again summed, but only up to a limit of a further six degrees.”

In this way it was intended that if there were a conflict of input, the consequences would favour the more intelligent pilot not the strongest. The electronic primary flight display attitude indicator for the A320 also includes symbols to show the position of each stick relative to the neutral position.

See also

References

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