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Force Feedback in Flight Systems

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Force feedback in flight systems

As an aircraft flies, the slipstream over the control surfaces causes the load on these surfaces to increase on the primary flight control surfaces such as the elevator, aileron and rudder. The force per unit area, although not linear increases with airspeed and affects the handling of the aircraft. Such effects can be simulated in a flight simulator and this is mainly done by the control loading or force feedback system. Force feedback in a simulator is achieved by attaching actuators to the control columns of the aircraft to give a resistive force to the motion, typically varying with the airspeed.

Prior to 2000, most simulators used hydraulic actuators which typically involved a hollow cylinder with a piston inserted in it (Allerton, 2009). The two sides of the piston are alternately pressurised or depressurised to achieve controlled precise linear displacement of the piston and in turn the entity connected to the piston. Because the aircraft controls are mainly rotary motion, a combination of gears are used together with the liner motion actuators to achieve the force feedback required. Since then, advances in electrical motor drives have enabled control loading to be implemented with a single motor drive per channel. However there have been concerns in the simulation industry that residual torques can be detected around the centre position at very low values of torque where electrical systems lack response (Lee, 2006). With both electric and hydraulic force feedback systems, this is corrected by simply offsetting the null datum for the zero position. To provide the force feedback or feel for a control, the inceptor displacement is sensed using a Linear Variable Differential Transformer (LVDT) or the stick force is measured using a strain gauge.

My research will then go on to investigate the different types of aircraft controls, the loading systems currently being utilised and the important components that make up these systems.

Aircraft Control

Aircraft controls are divided into 3 major parts which are manual, semi-automated and fully automated.

Manual Control

Manual controls are mainly responsible in the control of altitude, airspeed and the rate of climb or descent of the aircraft. These controls are manually controlled by the pilot through the manipulation of the primary and secondary controls. Primary controls include the yoke for controlling the aircraft pitch and roll, the rudder which controls the aircraft's yaw which is the movement of the aircraft's nose. Secondary controls include the flaps and related controls that manipulate the wing movements and braking systems for ground control or operation.

Manual controls are further subdivided into two parts namely the closed loop and open loop controls. Closed loop control occurs when a pilot has to maintain continuous input control in response to continuous feedback such as a pilot attempting to hold a constant heading. This continuous control adjustment in response to dynamic changes in information forms a closed loop. Open loop controls facilitates the input of control values without the need for continuous feedback for example setting the takeoff trim and takeoff thrust values. Most of the complexity in simulating the handling characteristics of aircraft controls is devoted to closed loop control systems where the need for accurate simulation of control forces is imperative.

Semi Automated Control

With the introduction of advanced computer systems, automation of all or most of the pilot manual control tasks are now a reality. Semi automated control systems are usually limited to maintaining airspeed, altitude and heading by providing the primary control inputs that are normally a part of the pilot's duties. Although the automated controls take over some of the primary controls of the aircraft, much of the more complex flight controls still require the intervention of the pilot for them to function as required such as navigation, landing and takeoff.

Fully Automated Control

Fully automated systems have since been developed and introduced into the market for both commercial and military aircraft. Through the use of Flight Management Systems (FMS), pilot navigation tasks have been greatly reduced or eliminated. The advantage of automated controls is that they have the potential to cause a significant increase in the efficiency of aircraft operations. However the disadvantage is that automated controls lead to the degrading of the pilots flying skills as the pilot tends to rely on the aircraft to fly itself. On the other hand, flight simulators play an increasingly important role in maintaining pilot manual skills when regular re-fresher courses on the simulator are scheduled regularly.

Simulating Flight Control

The application of manual control skills in flying a simulator requires that the simulator responds to the pilot's control inputs in a manner closely comparable to flying the real aircraft. Figure 2 below shows how the flight simulator recreates the control feedback necessary for the simulation of primary flight controls such as the yoke or rudder.

Figure 2: Simple Control loading (Allerton, 2009)

When a pilot initiates a control input in a real aircraft, the aircraft control structures provide a counter force as the control mechanisms carry out the input. Usually the input is measured, fed back and compared to the pilot's input and the difference between these forces is what drives the control surface. In small aircraft the pilot input onto the aircraft act on a series of cables, pulleys and levers to move the control surfaces on the wing and tail sections. In small aircraft moving at low speeds or parked, the forces are close to negligible hence the loading on the pilot's controls are said to be static. However this is not entirely true when referring to newer, larger and military aircraft with the opposite being true. This is because with larger aircraft there are larger surfaces to manipulate, hence gearing is used to facilitate the problems arising from the difference in size between the control lever and the wings for instance.

In the case of a flight yoke, the roll and pitch correspond respectively to forces on the aircraft's aileron and elevator control surfaces. As the two controls act on different surfaces, two control loading systems are used to control them which are independent of the other. These are different in that the forces needed to simulate movement for each of the respective



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