Control and non-classical (artificial) stability

attack, using flap-like (camber-changing) control surfaces, or by movement of the whole aerofoil, working as a slab surface. Doing so affects the way in which controls feel to the hands and feet of the pilot. Tables 8-2 and 8-3 apply to basic manual controls and give measures for sizing conventional surfaces. With the advent of computerized high-order ‘active’, or ‘fly-by-wire’ control, stability can now be provided artificially

(non-classically) for aircraft which, without it, would be completely unstable. Table 8.2 Further examples including controls

Table 8.3 Control surface area ratios

Ideally, operation of conventional controls should not alter the basic (classic) stability of an aircraft to which they are fitted. This is not always so. For flight at comparatively low airspeeds control surfaces are regarded as rigid. However, as speed increases aero-elasticity intrudes, causing them to deflect under load. Aero-elasticity is always destabilizing. Ailerons are normally located well outboard for economy of effort in roll. However as design airspeeds increase, and forces increase as speed2, ailerons are mounted further inboard to reduce the risk of ailerons twisting the wing structure and thus reversing their effect. Spoilers are used on

many high-performance aircraft to avoid aileron reversal.

Figure 8.19 shows some types of lateral control. As shown in Fig. 8.19(a) the F-105 used conventional ailerons at low speeds, where spoilers are least effective, but relied upon the spoilers at high speeds without recourse to ailerons. The moving wing tip in Fig. 8.19(b) has attractive features, for it has the advantages of a slab surface. It tends to be over-powerful at low speeds and, perhaps because of complexity of gearing, has only appeared very intermittently. The tailerons of the BAC—TSR. 2 shown in Fig. 8.19(c) (see also

Fig. E.4(a)) were slab surfaces that moved either together, as pitch controls, or independently for additional control in roll. It should be noted that modern aeroplanes having large tail surfaces and small wings suffer rolling moments from moving fins. Such cross-coupling between control surfaces makes the problems of stability and control more complex than even before.

8.5.1 Balancing and harmonizing controls

With the exception of spoilers, flap and slab surfaces, in altering local lift circulation, experience opposing moments which are fed back to the pilot either directly or indirectly. The feel of a control system is of great importance, for control forces that are too heavy make an aeroplane tiring to fly, while forces that are too light may result in an aeroplane being broken in flight. The commonest devices for making control surfaces feel right are aerodynamic balances and tabs of various kinds.

Aerodynamic balance

Aerodynamic balance is achieved by hinging a control surface some way aft of its leading edge, so that a portion of the control surface area projects forward of the hinge-line. When the surface is deflected, part of the load in acting forward of the hinge introduces a moment opposing the moment caused by the load acting behind. As the moment caused by the load acting behind the hinge opposes control movement, that acting ahead assists the pilot. Too much area ahead of the hinge leads to control overbalance.

Trim tabs (balance and anti-balance)

Trim tabs are miniature control surfaces set in the trailing edges of control surfaces. Balance tabs move in opposition to the control itself: depression of a tab causes an upload at the control trailing edge, which helps the pilot to deflect the control upwards. An anti-balance tab is used when a control surface can be moved too easily. By moving such a tab in the same sense as the parent surface the moment of the surface about the hinge-line is increased. Tabs are moved by gears, to respond immediately when the parent surface is moved, or they can be operated by independent trimming controls in the cockpit. Trim tabs are used by the pilot in to reduce a control hinge-moment to zero, so that no force has to be tiringly applied by the pilot in steady flight. Mass balancing

Control surfaces, in having mass, are affected by accelerations. If the CG of a control surface lies behind the hinge, then a normal acceleration will deflect the control surface relative to the main aerofoil: the control being apparently depressed by an acceleration upwards, and raised by downwards acceleration of the aeroplane. If such movement is not stopped it is possible to break an aeroplane, or at least to suffer dangerous fluttering of control surfaces.

Surfaces are dynamically balanced by weights, either built into horns, or suspended on arms ahead of the hinges. One such mass balance, in the form of a streamlined weight, is shown in Fig. 8.20.

Fig. 8.20 Forms of control surface balance. 8.5.2 Flying control systems

Mechanical systems for moving the control surfaces are still basically the same as they have always been. Their increasing complexity has arisen from the combined effects of increased speed and size. Figure 8.21 compares two control systems: one simple and straightforward, such as might be found in a sailplane or light aeroplane; the other is more complicated for a moderately high-speed aeroplane. The second is recognizably similar to the first.

Fig. 8.21 Flying control systems, arrows indicate direction of travel for left aileron up, elevators up and left rudder.

A novel feature of the second system (for example the BAe 125) is the rudder-bias strut, which is a pneumatic ram with compressed air tapped from the engine compressor deliveries and fed to either side of the piston. Failure of one engine reduces the compressor delivery on the appropriate side of the piston, causing displacement and rudder deflection to counter the asymmetric yawing-moment from the live engine. The feature represents power-assistance in a most rudimentary form.

If manual controls are to be retained for flight at high speeds it is necessary to use a higher degree of aerodynamic balance as aircraft size is increased. As the degree of balance is increased the net hinge moment becomes more sensitive to manufacturing tolerances. Furthermore, because of the non-static nature of shock waves, compressibility can vary hinge moments very rapidly with slight changes of airspeed and control-surface deflection. At high speeds it becomes impracticable to use manual controls and fully powered controls (or at least power-assisted controls) must be fitted.

Powered controls employ rams, or servo-units, to move the surfaces. The control-column and rudder-pedals become power selectors, moved by the pilot to provide power to one side or other of pistons which, through a system of linkages, in turn deflect the flying controls. Most power controls are hydraulic, but work is being done to improve the reliability of electrical systems for advanced aircraft. With such systems it is now possible to save valuable cockpit space, replacing the traditional stick and rudder-bar with switches on an armrest of the seat. Pilots, however, are conservative by nature and several attempts to replace stick and pedals with smaller levers or switches have been resisted. It should be noted that there is good sense in such conservatism, for some change that might look sensible on a drawing-board could well cost lives in the isolation of a cockpit, with several things going wrong at the same time.

It is worth noting that servo systems of all kinds, designed to aid the pilot, introduce instabilities of their own. These are treated as part of the whole stability problem of an aircraft.