L’un sobre l’altre: l’acte nòmade i les llàgrimes verticals del jo testimoni

Fig. 3.4 Active System symmetric

manoeuvre

There are no isolated roll loading cases as such, rolling load cases always being associated with symmetric loading conditions. It is necessary to refer to roll per- formance requirements to establish the capability of the roll motivators, typically the ailerons, to provide some basic design data. This is because the loads arising during rolling are a direct result of the roll motivator capability defined by the required roll performance.

Aircraft loading a n d layout

Roll performance requirements can be divided into two main categories:

Low-speed handling, especially the need to be able to rapidly lift a wing during approach to landing or in the event of a powerplant failure.

High-speed handling. especially for combat aircraft. It should be noted that at high speed there is a significant aeroelastic distortion of a wing which results in a reduction of aileron effectiveness relative to the rigid condition. However, as the conditions relate to rates of roll, the overall loading is not affected by this unless there is a restriction on the motivator application force.

A summary of roll performance requirements is given in Appendix A3.

3.3.2.3 Rolling manoeuvre conditions

A rolling manoeuvre has to be analysed at three or four specific stages. These are illustrated in Fig. 3.5.

(a) Roll initiation due to an instantaneous or rapid roll motivator application, the resulting rolling moment being represented by The result is an initial roll acceleration at effectively zero roll rate.

The steady roll rate state achieved when the roll damping moment of the aircraft. represented by L,,, is numerically equal to. and balances, the applied increment in rolling moment.

The start of roll when the applied rolling moment, is removed so that a roll deceleration is imposed upon the steady roll rate as a consequence

of moment hy

A reverse roll which is similar to case except that the motivators are moved to give an equal rolling moment to the original hut in the opposite sense.

This case only applies to military combat types and results in a roll acceleration which is numerically twice that of the initial value due to algebraic sum of the representative moments and

Combined roll and pitch manoeuvre

Specific loading conditions are prescribed when a rolling manoeuvre is combined with a pitching manoeuvre. It is assumed that the two effects can be analysed separately and the results superimposed in appropriate proportions. The symmetric part is analysed with a prescribed value of normal acceleration, the aircraft being assumed to be in a steady manoeuvring, zero pitching acceleration. condition. The additional effect of an appropriate application of roll motivators is then added.

The requirements usually specify that throughout the manoeuvre the yaw motivators are either held fixed in the position required to trim the aircraft wings level. or deflected to minimize any sideslip angle. When airbrakes are fitted the analysis should include

Flight loading cases 51

Steady level flight

Roll initiation

Steady roll rate

Roll arresting

or

Reverse roll

the open and closed settings. All flight speeds up to and all altitudes must be covered.

The roll motivator deflections to be used are those corresponding to the most critical of:

Fig. 3.5 Phases of a

The deflection, or set of deflections, which results in one-and-a-third times the specified minimum rolling performance at that speed and altitude (see Appendix

Aircraft loading and structural layout

For combat aircraft the deflection, or set of deflections, given by whichever of the following is appropriate to the particular design:

that corresponding to the maximum output permitted by the flight control system by the power unit of a single roll motivator. or of the individual power units for a number of roll motivators used in

where the motivators are driven solely by the deflection of the pilot's stick or wheel, that corresponding to a control force of 267 N (60 for a stick control or 222 N (50 applied to the rim of a wheel of diameter D m or in), resulting in a couple of magnitude N m or (50 D in). In applying these conditions the accuracy with which the actual hinge moments of each individual motivator may be predicted may sometimes he such that arise as to the magnitude of the deflections so defined. assumed deflections are to he increased 30 per cent in such cases:

if the deflection (or any one deflection), so defined, exceeds that for full travel, it is replaced by the maximum available deflection.

For the roll motivator deflections prescribed above, a range of normal accelerations have to be considered:

transport aircraft to

other aircraft - to and in addition motivator deflections of half of those prescribed above are to be combined with a acceleration of with linear variation between and

In some requirements there is a of the need to allow for distortion resulting from aileron application.

In case of the light aircraft requirements, JAR-23, there are somewhat different requirements. Similar conditions as those above are given as one design case.

However, for wing design it is also required that a condition of 100 per cent load on one side of the aircraft should be combined with 75 per cent on the other side (60 per cent on

designs), which may override the more conventional condition.

manoeuvres General considerations

In the case of manned aircraft it is usual to assume that yaw motivator induced loading cases occur when the aircraft is initially, and remains, in steady level flight. The

Flight loading cases

exception of low aspect ratio, highly swept layouts is referred to in Section where the pitch-yaw coupling may result in a departure from steady level flight.

The specified cases relate to the deflection of the rudder, or other yaw motivator, through angles which may be limited in some way by available travel or applied hinge moment. The actual motivator deflection required in a given condition is determined by performance requirements, such as handling i n cross-winds, powerplant failure at low speed or combat manoeuvre at high speed. Frequently the application of the full theoretically available deflection at high speed gives rise to manoeuvres which are more severe than is needed and consequently to unnecessarily high loads. Some form of limitation of the movement is then desirable. This can be done by limiting the available control motivator operating force, by introducing a gearing which is variable with speed or Mach number to reduce the allowed deflection appropriately, or by limiting the control demands in an active control system.

The determination of the necessary motivator deflection at high speed is not always easy. Considerations which may assist in determining rudder deflection limitation are:

(a) It must not be possible to stall the fin dynamically as a consequence of rudder application. The fin dynamic stall angle may be up to 1.5 times the static value.

A dorsal fin assists in delaying fin stall if it is of consequence.

Lateral manoeuvre acceleration is limited by occupant tolerance. A rather arbitrary figure sometimes used is that the maximum lateral acceleration at the head of the pilot should not exceed 2g. This applies only to a high-performance combat aircraft and is a severe condition.

An automatic control system may incorporate some of fin load limiting system. The value of this maximum limited load is a matter of design decision, but suggests that it is unlikely that operational requirements will require a load greater than about where is the maximum normal acceleration factor and is the normal take-off mass.

In a fully active control it is usual to specify appropriate combinations of normal acceleration with lateral and yaw rates and accelerations, as is illustrated in Fig. 3.6.