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Chapter 4 - Dynamic Stability and Control.

401. Directional Stability

The vehicle design shall be such that within the design operating envelope, moments due to aerodynamic yaw are kept to a minimum and shall, as far as practical, be stabilizing. Changes of yaw moment should be smooth and continuous.

402. Cushion Stability and Lift System Performance. 

Design of the cushion shall be such that any disturbance in roll or pitch when cushion-borne shall generate a restoring moment.

403. Design calculations of cushion-borne stability in pitch and roll shall be supplemented if required by representative model tests, and in any case by full scale experiment. (See Division 4 Part A)

404. In designs where air delivered for the cushion may also be used for other purposes, the lift system performance shall be evaluated under all conditions of air usage.

405. In designing the lift system as a whole, the designer shall fully consider the relationship between total air requirements and the fan characteristics, and establish that under normal operational conditions, fan operation will be stable. Such consideration shall also include an assessment of flexible structure instability and cushion system heave instability, and due provision shall be made to ensure that unstable operation will not occur within the vehicle design operating envelope.

406. Flexible Structures. 

Designs of all flexible structure, including any inflated structure, shall be such that any deformation due to dynamic contact with the surface does not contribute a destabilizing moment to the vehicle. In particular, the designer should consider the moments generated by scooping or tuck-under of any part of the flexible structure when operating anywhere within the design operational envelope and at extreme angles of hydrodynamic yaw.

407. Design Features Influencing Capsizing. 

Within the very broad spectrum of amphibious vehicle and cushion system designs and the range of environmental conditions in which they operate, it has been established that a large number of design parameters, some of which are inter-dependent, have an influence on capsizing. In order to assist and guide designers, a list of these which may not be exhaustive, is provided in the Appendix to this Chapter, including ranges of values which have been found to be in normal use.

408. Hydrodynamic Design. 

The hull design shall be such that underside or superstructure contact with the surface when operating anywhere within the design operational envelope and at extreme angles of hydrodynamic yaw will not contribute a destabilizing moment to the vehicle in roll or pitch.

409. Underside surfaces of the hull periphery should present a stabilizing planing surface to the water; in some designs, an equivalent effect may be provided by appropriate peripheral skirt and support structure design.

410. Peripheral superstructure immediately above peripheral flexible structure shall be so shaped as to :-

1.  minimize the possibility of trapping water, and
2. generate a restoring moment upon contact with the water at any expected angle of roll or pitch.

411. Any external structure which is subject to water accumulation shall be provided with adequate drainage, which may require to include non-return provisions, effective under all conditions of trim up to 8° in any direction.

450. Stabilizing Systems and Directional Control. 

Definition:- A stabilization system is a system which, when active, is designed to supplement or augment the stability of vehicle motions in roll, pitch, yaw or heave, or any combination of such motions.

451. Where such a system is installed, it shall be selectable at the discretion of the crew member operating the vehicle, and controls shall be provided at the vehicle control station.

452. The system shall be designed so that, when not selected, or under fault conditions affecting the functional operation of the system, directional control and stability devices which it commands revert to and remain in neutral positions without affecting any other input commands to which the devices may be subject by the crew member at the vehicle controls.

453. Where stabilization systems are automatic in operation, provision shall be made at the control position for the system to be over-ridden, and for cancellation of the over-ride. Such systems shall also include provisions for automatic disengagement should their operation cause the vehicle to exceed prescribed safe operating limits, and such disengagement shall be indicated.

454. The parameters subject to the commands of any stabilization system, and the degree of command provided, shall be approved.

455. The dynamic effects of the stabilization system and of any modal failures, shall be demonstrated and safe operating limits established.

456. Directional Control. 

The vehicle shall be provided with directional control surfaces or devices to enable safe and effective manoeuvring under all conditions within the design operating envelope.

457. As appropriate to the particular directional control device and it's input system, it shall be provided with either:-

1. Two independent means of controlling deflection, or
   
   
2. Means to centralize the device or neutralize it's effect, or
   
   
3. Design features such that failure of power input causes it to revert to and remain in a neutral position.

458. The design and installation of directional control devices shall ensure that deflection will not generate any adverse or destabilizing moment in roll or pitch at any point within the vehicle design operating envelope.

459. In designs incorporating multiple directional control surfaces or devices, consideration shall be given to the effectiveness of control with one surface or device failed or inoperative.

460. In complying with the requirements of 458 and 459, restrictions or limitations which may be established as the result of demonstration are to be approved and contained in the vehicle Operating Manual.

461. Where provision is made for directional control to be effected from positions other than the compartment from which the vehicle is maneuvered, such positions shall be provided with 2-way communication with that station, and with means of determining the control device deflection.

462. Where directional control devices depend for their normal operation upon power, any failure of that power shall be indicated at the crew station, from which the vehicle is normally maneuvered.

463. In vehicle designs incorporating only one directional control device (or group of devices which are controlled and operate as one), it shall be demonstrated that the vehicle may be safely handled and maneuvered in the event of failure of the input control to the device or devices. Any precautionary actions, or restrictions or limitations to be observed shall be contained in the vehicle Operating Manual.

464. Directional control systems are to so arranged that failure of any one device or of it's input system will not render any other directional control devices inoperative.

465. Directional control devices depending for their effectiveness upon the generation of aerodynamic or hydrodynamic forces shall be of adequate strength, and be attached to the vehicle adequately such that these forces will be reacted without distortion of the device or of it's attachment.

466. Where directional control or vehicle stabilization is dependent upon one device which is itself dependent for it's efficient operation upon power, such power shall be provided from two independent circuits, in the event of electrical power being used, one circuit may be provided from the emergency power source, provided with short circuit protection and an overload alarm.

Appendix to Chapter 4 ^

Designs features influencing capsizing 

This Appendix is based upon information contained in C.A.A. Paper 75017, and is included by kind permission of C.A.A.

Definitions

'Plough-in' - A rapid divergent pitching motion involving an increase in drag and reduction in pitch attitude.

'Tuck-under' - Deformation of the skirt, by local hydrodynamic drag forces pulling the skirt under the craft structure.

The combination of adverse design features, together with poor operating techniques and/or severe environmental conditions, is generally necessary for capsize. From extensive model research and studies of full-scale capsizes, it has been established that a capsize is usually associated with a plough-in and tuck-under during operating with an appreciable angle of hydrodynamic yaw. The vehicle speed in the direction of travel at which a capsize occurs is relatively low.

The investigations included extensive consideration of the contributions which vehicle design features make to vehicle motions in conditions favourable to a capsize. General guidelines, applicable to vehicles with peripheral skirt systems, have resulted for minimizing these contributions.

Designs features can be broadly classified into 3 groups:-

1. Independent structural and skirt geometric variables which can be selected by the designer with the objective of minimizing tuck-under;
   
   
2. Vehicle structure features which can be selected by the designer with the objective of resisting motions leading to a capsize, and
   
   
3. Vehicle stability characteristics, which are partially influenced by (a) and (b).

The principal parameters, and the range of values in general use are given for (a) and (b) in the tables overleaf. In respect of (c), the designer should establish that aerodynamic yaw moment derivatives with respect to both aerodynamic and hydrodynamic yaw angles exhibit no dangerous discontinuities and are stabilizing. He should also establish that yaw control is always sufficient to overcome yawing moments.

The water surface immediately beneath the skirt of a vehicle near the point of capsize has an appreciable slope down into the cushion - in the order of 8 to 10 degrees. This, in conjunction with the roll angle associated with a near-capsize, produces a powerful destabilizing force should the lower structure impact the water. It is therefore essential that an effective hydrodynamic planning surface be provided around the lower periphery - chine angles of 25° or more are recommended. In some designs, there is no structure in this area, in which case space and skirt geometry should be such as to permit the skirt to deform to provide a planning surface.

The following tables are reproduced from the Civil Aviation Authority Paper 75017, and are primarily concerned with "bag and finger" skirt designs; the tables are a summary of the relevant text which includes discussion also of "Loop and segment" skirts; meaningful extrapolation out of context could be invalid, and designers requiring further data should contact the Civil Aviation Authority, whose permission to reproduce the extract is acknowledged. The following notation is used:-

 Design Factors Affecting Leading Sideskirt Tuck-under Boundary 

NOTE: the above statements and numerical ranges, which reflect design practice for several current craft, are provided for general guidance and not as design rules or limiting values. An overall configuration involves a compromise choice of all the factors, and may be satisfactory even if one or more factors are at the least favourable end of the range.

Design Factors Affecting Craft’s Reverse against Capsizing 
(up to Tuck-under Point)

 NOTE: The above statements and numerical ranges, which reflect design practice for several current craft are provided for general guidance and not as design rules or limiting values. An overall configuration involves a compromise choice of all the factors and may be satisfactory even if one or more factors are at the least favourable end of the range.

* CD = w/e h^g(S_c)^3/2;eh is mass density of water; S_c is cushion area

h_G is the height of vehicle c.g. above mean water surface beneath the craft.

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