System components

The constituent parts of most AFCS can be grouped into four classes of components, namely:

• Inceptors. Devices that the pilot uses to communicate with the AFCS and to make control inputs.

• Sensors. These are used to measure the relevant reference parameter and transmit the necessary information to the computer.

• Computers. These convert the sensor information and pilot demands into signals to drive the output devices.

• Output devices. These convert the computer signals into a form that will result in the required helicopter movement. The output may be direct, in the form of actuators which move the pitch change links, independently or via the cockpit controls, or indirect in the form of a cockpit display, a ‘flight director’, which will direct the pilot to make the necessary control movements himself. Inceptors

Inceptors refer to the devices used by the pilot to make inputs to the FCS. The simplest and most conventional are the traditional cyclic stick, collective lever and pedals which the pilot uses to command changes to the aircraft attitude and power setting. The selection and adjustment of holds, such as heading, height or airspeed are often made by means of switches and knobs, either located on the primary inceptors or close to the associated flight instrument. It is worth noting that the use of electronic flight information systems (EFIS) have allowed some consolidation of these controls.

The absence of a mechanical link between the cockpit control and the rotor head, which has been made possible with the advent of FBW, has given the cockpit designer greater freedom. Although mechanically coupled side-stick arrangements have be used in some non-FBW attack helicopters, the mechanical tailoring has been necessarily limited. Consequently there has been a great deal of research into the best style and location for active versions of these inceptors. It has now been generally agreed that a small stick, located centrally or to the right, and some form of collective lever, or second side-stick controller, will be used. Such an installation can improve pilot comfort by allowing the optimization of inceptor position, FCMC and adjustment, and improve cockpit layout by allowing more appropriate positioning of displays, instruments and switches, due to the smaller envelope occupied by the side-stick controllers (SSCs) and the possible removal of the yaw pedals. At present little agreement exists concerning the controls that will be assigned to these novel inceptors. Three configurations have been evaluated, these are:

• A two-axis SSC located to the right (2R), a collective (1C) and pedals (1P) or (2R + 1C + 1P).

• A three-axis SSC located to the right and a collective or (3R + 1C).

• A four-axis SSC located to the right or (4R).

Research conducted in Europe as part of the EuroACT programme concentrated on a passive (3R + 1C) system and an active (2R + 1C + 1P) system. The passive system provided the pilot with a fixed set of spring feel characteristics, whereas the active system allow the tailoring of the feel characteristic to suit the current handling task. The more conventional configuration was selected for the active system in order to simplify the basic stick design so that incorporation of actively variable FCMC was possible. Recent research has highlighted the following:

(1) SSC design characteristics, such as grip shape, arm support and the length of the pivot arm, can have a profound effect on the handling qualities due to anatomically induced cross-coupling.

(2) Isometric (or rigid) and low compliance (or movement) SSCs are undesirable because of their lack of control magnitude and control position cues.

(3) Four-axis SSCs have the advantage of freeing a hand for other tasks. But significant handling deficiencies are experienced due to anatomical coupling. It is very difficult for the pilot to make single axis inputs, particularly in heave, thereby complicating certain piloting tasks, such as slope landings. Dynamic multi-axis tasks such as quickstops are also difficult to perform with this type of inceptor.

(4) Surprisingly, three-axis SSCs with twist for yaw control are well accepted by pilots provided it is coupled with an accurate heading hold incorporating an integral trim that obviates the need to hold-off a force during out-of-wind hovering for example.

Control of autopilot modes is generally by means of appropriate knobs and switches. These may be duplicated so that a range of the more important functions can be selected or deselected from the primary flying inceptors. The selection and control of certain modes, such as airspeed hold and vertical speed hold may be via controls incorporated into the appropriate flight instruments. Alternatively, a repeater screen may be fitted that displays the current hold setting. The displayed data changes depending on the hold selected and the datum is adjusted by means of switches on the primary flight inceptors.