The average orthodox mathematician who writes or reads books about hydrodynamics is so keen about getting a peg on which he can hang what he calls examples that he quite forgets to apply his formulae to compressible fluids. It appeared to me on reading Colonel De Villamil's letter that a useful purpose would be served by writing the present note; it contains nothing but what anyone could write out for himself, but it was only recently that I made the attempt myself and I should be surprised if many readers have seen the same thing before. We know that in the irrotational motion of an incompressible fluid the velocity potential satisfies Laplace's equation, the present object is to find the corresponding equation for a compressible fluid, to grind it out at full length, and (what is more important than grinding it out) to interpret the result.

The theory of thin aerofoils at small incidence and flap deflection is very well established. For certain purposes it is desirable to know how an aerofoil will behave in potential flow when the flap is deflected through large angles.

To this end, an attempt has been made to develop a theory of thin aerofoils in which the flap deflection angle may be large.

The results obtained are compared with those derived by an alternative method of direct conformal transformation by F. Keune.

The structural problems introduced by the kinetic heating of high speed aircraft require a radical extension of what has hitherto been regarded as the normal domain of structural theory. Aerodynamics, for example, cannot be regarded as a self-contained subject which merely provides basic data for the structural worker. Certain aspects of aerodynamic theory, particularly the mode of heat generation, have to be considered simultaneously with the structural problem. Thermodynamics and metallurgy are still more closely integrated with structural theory. The laws of heat transfer, and the variation of material properties with temperature, are both essential factors in the structural analysis.

It was suggested by Lanchester some years ago that an aerofoil immersed in a moving fluid gave rise to two trailing vortices of opposite hand, one situated near each of the extremities of span. The existence of the vortex pair has since been demonstrated in a variety of ways, of which perhaps the most striking is that due to Caldwell and Fales, who used the condensation of water vapour in a high-speed wind channel to obtain photographs. The vortex pair constitutes so important a feature in the stream behind most aerofoils that measurements capable of numerical analysis seemed desirable, and an account is given in the present Paper of some work to this end which appears capable of straightforward interpretation.

It may be that the residual motion sufficiently far behind any aerofoil is a vortex pair. Similar experiments to those about to be described, however, which deal with the motion only for a few chords back, have been made with several other aerofoils of a variety of shapes having long, graded wing tips, and with some of these no distinct vortex was found. With others apparently more than one pair could exist in certain circumstances, but opportunity has not occurred to probe deeply into these interesting cases. Attention is confined to two aerofoils representative of considerable variation in a class having uniform section and angle of incidence with blunt, or square-ended, wing tips. The general nature of the results is common even with aerofoils outside this class, but the foregoing remarks will serve to indicate that exceptions are known to exist.

This paper examines the performance of enriching the shape functions of interface finite elements in the prediction of mixed-mode delamination. Enriching second-order interface and solid elements with the analytical solution of a beam on elastic foundation problem yields the correct displacement field ahead of the crack tip. Despite the enrichment being fixed at elements nodes, resulting in non-traceability of the crack tip location, the strategy is shown to perform consistently well, increasing the minimum element size from the typical 0·5mm to 5mm, for a range of classical mixed-mode bending (MMB) specimens.

The structural use of fibre composites in helicopters, both for primary and secondary structures, has been recognised for some time. In 1964, Westlands developed a glass fibre composite tail boom and tailplane for the Wasp helicopter (Fig. 1) and the results were encouraging. The non-corrodable structure of the glass composite is considered a great advantage, especially for Naval helicopters. Strength testing of the tail boom was successfully completed with a weight of the component close to the existing metal tail boom, but resonance testing showed that a small additional weight was necessary to meet the stiffness requirements. The tailplane in glass fibre composite showed an appreciable saving in weight and after satisfactorily completing static and fatigue testing, one is now flying on a Service aircraft.

The aeroelastic phenomenon of limit-cycle oscillations (LCOs) is analysed using a projection-based reduced-order model (PROM) and Navier–Stokes computational fluid dynamics (CFD) in the time domain. The proposed approach employs incompressible Navier–Stokes CFD to construct the full-order model flow field. A proper orthogonal decomposition (POD) of the snapshot matrix is conducted to extract the POD modes and corresponding temporal coefficients. The POD modes are directly projected to the incompressible Navier–Stokes equation to reconstruct the flow field efficiently. The methodology is applied to a plunging cylinder and an aerofoil undergoing LCOs. This scheme decreases the computational time while preserving the capability to predict the flow field accurately. The ROM is capable of reducing the computational time by at least 70% while maintaining the discrepancy within 0.1%. The causes of LCOs are also investigated. The scheme can be used to analyse non-linear aeroelastic phenomena in the time domain with reduced computational time.

The well-known ant colony optimisation (ACO) meta-heuristic is applied to optimise the parameters of a new fuzzy command to line-of-sight (CLOS) guidance law. The new guidance scheme includes two phases, a midcourse and a terminal phase. In the first phase, a lead strategy is utilised which reduces the acceleration demands. A proportional derivative (PD) fuzzy sliding mode controller is used as the main tracking controller of the first phase. Moreover, a supervisory controller is coupled with the main tracking controller to guarantee the missile flight within the beam. In the terminal phase, a pure CLOS guidance law without lead angle is utilised. For this phase, a new hybrid fuzzy proportional-integral-derivative (PID) fuzzy sliding mode controller is proposed as a high precision tracking controller. The parameters of the proposed controllers for the first and the second phases are optimised using ACO. In this regard, the recently developed continuous ant colony system (CACS) algorithm is extended to multi-objective optimisation problems and utilised to optimise the parameters of the pre-constructed fuzzy controllers. The performance of the resulting guidance law is evaluated at different engagement scenarios and compared with the well-known feedback linearisation method. The comparison is also made in the presence of measurement noise.

A system of external forces on the structure of a rigid airship can be analysed into two or more of the component systems due to

1. W. The Weights of the members of the structure and the weights of the masses carried by it;

2. L. The Lift and pressure of the gas;

3. A. The Aerodynamic forces consequent on the relative motion of the airship and the air;

4. T. The Thrusts of the airscrews;

5. R. The Reactions due to either the mooring arrangements, the hauling-in guys, the supports used in the shed, or handling on the ground.

As these component systems, though restricted by their interdependence, are individually variable, a very large number of complete systems is represented by the possible combinations and variations that can occur. It becomes necessary, therefore, for the purposes of design, to attempt to limit the number of combinations to be considered, and to restrict the variations of the components, but in such a way that the structure designed to carry the resulting systems with adequate factors of safety, shall be able to carry any other possible system with an appropriate factor of safety.

This paper presents an automated optimisation procedure for the feature selection stage of a previously proposed structural health monitoring methodology using a genetic algorithm. The same diagnostic is used in the attempt to progress up the levels of damage detection to location and severity. It was validated experimentally on a Gnat aircraft wing. An artificial neural network is used as a classifier and the work is compared with the previous selection strategy based on engineering judgement.

Aircraft handling qualities in autorotation are critical in determining the level of safety of rotorcraft. For helicopters suffering from an engine failure, transcending from powered to autorotative flight occurs rapidly and requires immediate and accurate pilot reaction. Although it is important for the handling qualities in this flight state to be predicted correctly, obvious difficulties will exist in using flight tests as a means of validation when autorotation constitutes an abnormal mode of operation. In the research work presented in this paper an alternative approach is applied, of configuring a generic rotorcraft model as a gyroplane, a type of vehicle for which its main rotor is constantly in autorotation. Flight tests are used for the validation purposes both for steady state and dynamic response cases. Results are produced to complement those already existing for a dissimilar gyroplane type thus increasing the level of confidence obtained. It is concluded that important handling qualities indicators such as the steady state trends are correctly predicted although limitations are imposed due to rotor speed discrepancy.

In systems for transmitting control motion there may be two shafts set in any relative position, each carrying a lever arm, and the ends of the lever arms may be connected by a link having ball ends. In general, the range of working movement for such a system is somewhat limited and the relationship between the angular movements of the two shafts is non-linear.

Although for some purposes it may suffice to mock-up the system and determine the motion experimentally, for other purposes analytical treatment may be required. The general solution given below is readily applied to particular cases, which may be simpler.

Recently published statistics give the impression that an excessively high percentage of non-carrier aircraft accidents investigated during 1948 were primarily due to human error. Further, an examination of available data covering the past decade gives the same impression in respect of all aircraft accidents occurring during that period. The figures quoted convey in fact the unpalatable implication that, although in nearly a half century of aviation great strides have apparently been made in increasing the reliability of the aeroplane, very little has been accomplished in reducing the element of human error in aircraft accidents. Why is this?

It has been claimed that the possibility of accidents in flying is inevitable by reason of the inherent conditions of flying which necessitate, besides reliance on the technical factor, considerable reliance upon the skill, judgment, memory, and physical and psychological conditions of the human being.

ACT embodying FBW and digital computing is now implemented routinely in almost all classes of aircraft, and is often essential in achieving performance requirements. However, careful design, thorough understanding, and attention to detail are necessary if the benefits of ACT are to be realised and attention must be focused appropriately among the issues to be resolved to ensure that development proceeds efficiently and overall costs are minimised. This paper introduces structural coupling as one of the important “details” of FCS design, and shows how the “state-of-the-art” has developed at BAe Warton in order to help achieve the performance specification for a modern weapons system, while maintaining a balanced and efficient FCS design effort.

Experiments to study the behaviour of a compressible free jet issuing from an axisymmetric convergent nozzle are reported. Detailed measurements of the mean flow field for subsonic, sonic and underexpanded sonic jets was made within the pressure ratio range of 1.13 to 4. The results show that the spread and decay rates of the jet vary significantly with pressure ratio. The results further indicate the radial velocity/total pressure collapse within the two-dimensional shear layer data. The underexpanded jet propagates to a greater distance downstream than the low speed jets; this leads to enhancement of the far field mixing.

A computer assisted conceptual aircraft design methodology (CACAD) has been developed to size turbofan powered transport aircraft. New modules for predicting the maintenance costs of each airframe system and subsection of structure, were developed and incorporated into CACAD.

Many aspects of variable camber wing technology (VCW) were modelled. These included different types of drag saving due to chordwise, as well as spanwise camber variation. Models were also derived for mass, maintenance cost, and extra development cost increments for wing trailing edge devices, flight control, and hydraulic systems. These were incorporated into CACAD, and then a multidisciplinary trade-off study resulted in predicted savings of up to 3·5% in direct operating cost (DOC). The technology was evaluated for DOC improvement against a number of existing, future, and derivative aircraft, under different sensitivity conditions. Reliability, maintainability, and development (R, M&D) predictions were shown to be decisive in determining the feasibility of VCW technology. The study showed that long range, low to medium capacity, derivative aircraft are the most suitable applications for VCW technology.

The influence of flap-end geometry modifications on the near-field wake behind a generic high-lift wing/flap configuration is investigated experimentally with a single-tube yawmeter. It is found that flap-end additions tend to split the concentrated area of streamwise vorticity shed by the flap into two less intense regions, associated with the tips of the flap and the addition. For shorter additions, these regions tend to roll up together, resulting in a slightly more diffuse flap vortex of almost unchanged circulation. For larger additions, the outer region is captured by the tip vortex, which correspondingly gains in circulation at the expense of the flap vortex. In all cases, the vorticity centroid of the rolled-up wake is shifted outboard, and the overall circulation slightly increased. However, the associated lift increase implies a decrease in overall circulation at a given lift coefficient.

Aircraft engineering has been described as “ordinary engineering made more difficult.” Probably most of you will agree that our difficulties are often greater than those that confront other branches of the engineering profession, and that the chief source of them is our inability to forecast with entire confidence what air will do under any given circumstances. Our position in this matter is much better than it was, for instance, at the end of the war. During the last thirteen years we have seen the theories associated with the name of Professor Prandtl receive general acceptance, with a profound effect upon our attitude towards some of the most important practical problems of air-flow with which we have to deal, namely, those associated with the production of lift. The boundary layer, then little more than a scientific curiosity, has become a matter of common concern. It achieved a striking practical success in the rationalisation of the international tests of airship models, which led to a marked advance in our attitude towards the whole problem of scale effect. But in many respects we are still greatly hampered. In so far as there has been any advance in the pure hydrodynamical theory of the flow of viscous fluids, it is hardly too much to say that its practical effect has been negligible. What then are we to do?

Our previous speaker, Mr. Blyth, in his paper on the Design of Incentive Contracts, has given us a lucid explanation of the basic principles of incentives and examples of cost, performance and delivery conditions which could be used as incentives and the objectives to which they could be directed.

Following on this paper, one could initiate a discussion on the basic philosophy behind the use of incentives, whether the incentives as stated are likely to be achieved, and propose various alternative methods of achieving these objectives without using contractual conditions. I have no doubt that many points such as these will be raised later during the discussion.