PFCUs Pt 1

Powered Flying Control Units (PFCUs)

Aim:

Objectives:

• Describe the methods used for providing artificial feel

• Explain what is meant by the term ‘fly-by-wire’

• State the basic component parts of a PFCU

To provide an understanding of how Powered Flying Control Units operate

• State the fundamental requirements of a powered flying control system

• State the other forms of input that can assist the pilot

Powered Flying Control Units (PFCUs)

Powered Flying Controls.

When aircraft are flying at high speed, the aerodynamic forces acting on the control surface are such that it is not possible to move the surfaces without some form of assistance.

This assistance can be provided in the form of hydraulic or electric power or a combination of both.

With total hydraulic or electric power moving the control surface, it becomes difficult for the pilot to gauge the amount of control movement required for any manoeuvre. This is because the pilot's control does not have any sensation of the aerodynamic loads on the control surface.

Artificial Feel.

To avoid overstressing the aircraft it is necessary to re-introduce, artificially, this sense of actually moving the control surface, in other words, to provide Artificial Feel.

The fundamental requirements of a powered control system are:

• Sensitivity.

• Rigidity.

• Stability.

The system reaction must be almost instantaneous and the PFCU must be sensitive to the slightest input signal from the pilot. Therefore there must be minimum time lag between input from the pilot and output movements at the control surface.

Obtained by the elimination of flexibility in linkages

Obtained by the elimination of linkage backlash (mechanical wear) and air from the hydraulic system. The system should be fully damped to permit smooth operation of the PFCU whilst offering resistance to any suddenly applied loads. Therefore no input signals must occur other than those required by the pilot.

Sensitivity.

Rigidity.

Stability.

Power Failure Provision.

To prevent the loss of an aircraft due to mechanical failure of the PFCU, most aircraft duplicate the essential flying control systems.

One common method of providing emergency back-up, in case of a hydraulic system failure, is to supply each PFCU with pressure from two independent systems.

This is achieved by the use of a "Tandem Ram" type PFCU ie (Two separate pistons with a common ram).

Power Failure Provision.

There are other ways of attaining power failure provision:

• Manual reversion of controls.

• Duplication of control surfaces.

Output - The movement transmitted by the hydraulic jack to the control surface.

The basic component parts of a Powered Flying Control Unit are:

Hydraulic Jack - converts the input signal into linear movement for the control surface. The jack ram is often "Double Ended" with one end inside the PFCU body and the other bolted to the airframe structure.Servo Valve - controls the movement of the main jack and receives the input from the pilot. System hydraulic pressure is fed through a servo valve to the main jack.

Control input - The movement received by the servo valve from the pilot's control.

The PFCU is located at the control surface and can be either, Moving Ram or Moving Body design. The Moving Body design is a simpler system, but has the disadvantage of requiring valuable space to move into, and any high mass transfer can affect the aircraft balance/trim.

Basic Principle of Operation of a PFCU

Mechanical input to a powered flying control unit is felt at the servo valve. The servo valve can be of a simple spool valve design, which when held in the neutral position, prevents the ram being moved by a hydraulic lock.

Movement of the servo valve by the pilot directs fluid to either side of the piston head and creates movement of the Ram whilst the body is attached to the aircraft structure

Hydraulic pressure is felt at the servo valve awaiting an input from the pilot. Whilst the pilot does not make an input to the servo valve, the PFCU remains in position by means of a hydraulic lock.

Moving Ram PFCU

Input made

As the pilot makes an input by movement of the servo valve, the servo valve moves allowing hydraulic pressure through into the jack body, forcing the piston to move and allowing return fluid to flow back to the reservoir. The jack will stop at the required position by means of a feedback link.

Feedback

To enable the flying control to be moved in relation to the input, the input must be cancelled at the specified point

To achieve this, a feedback link is attached between the jack ram and the servo valve.

As the control moves, the feedback link gradually removes the input, until the servo valve is closed when the selected position is reached

Moving Body PFCU

Hydraulic pressure is felt at the servo valve awaiting an input from the pilot. Whilst the pilot does not make an input to the servo valve, the PFCU remains in position by means of a hydraulic lock.

Movement of the servo valve by the pilot directs fluid to either side of the piston head and creates movement of the PFCU Body whilst the ram is attached to the aircraft structure.

As the pilot makes an input to the servo valve, the servo valve moves allowing hydraulic pressure through the servo into the jack body, forcing the piston to move and the return fluid is permitted to flow back to the reservoir

Input made

Basic spring feel units are normally attached to the control columns for ease of use and adjustment.

Spring Feel

Basic spring feel systems have the disadvantage that the resistance is constant throughout the speed range.

"Q" feel is an artificial force felt at the control column, which increases as the aerodynamic pressure at the control surface increases.

Artificial ‘Q’ Feel Systems

Artificial "Q" feel units have to increase the control column resistance, in proportion to the square of the airspeed. In general, "Q" feel systems can be either mechanically or hydraulically operated.

Thus, a "Q" feel system has to simulate the feel of the actual control surface loading that is lost with the use of powered controls, preventing the pilot damaging the aircraft by pulling excessive "g" loads.

Mechanical ‘Q’ Feel

Thus, a mechanical advantage is attained over the spring, increasing or reducing the effective feel force.

To vary the feel, the effective force provided by the spring cartridge is adjusted for given airspeeds by moving the fulcrum point of the bell crank lever.

Mechanical ‘Q’ Feel

As a pilot demand is made, this lever pivots about the roller and the spring provides a resistance to the movement.

The control rods are connected at one end of the slotted bell crank lever and the spring cartridge at the other.

The fulcrum arm is positioned by means of an electrical linear actuator.

Mechanical ‘Q’ Feel

Relative positions of the fulcrum arm determine the amount of feel felt back at the control column.