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TYRES get very little attention of the inquisitive sort from those whose business it is to incorporate them in the aircraft. The only information that is usually required (and given) is the load‐deflection curve (obtained experimentally) and the overall dimensions, and that is all that is necessary for most purposes. Yet a theoretical investigation of tyre behaviour should be useful from the following aspects:

OWING to the growing multiplicity of services and the increasing size of aircraft the problem of remote control is assuming considerable importance.

THE existence of compass errors on ships resulting from the presence of masses of iron in the vicinity of the compass and the method of compensating such errors have been known for over a century. In aircraft the problem is essentially similar, but an additional complication arises where there are moving masses in close proximity to the compass, as the effect of such masses in distorting the Earth's magnetic field depends on their position and therefore cannot be adequately compensated by the usual methods.

THIS article has been suggested by an actual problem in undercarriage work, and this problem is used as an illustration.

THE aim of this paper is to provide a survey of some of the principal theoretical considerations affecting the design of aircraft undercarriages.

PROMINENT among hydraulic servos today are those being developed in the aeronautical field, both for conventional aircraft and for guided missiles. In general these applications demand the following characteristics of the servos:

IN a previous article (Ref. 1) the present author outlined some fundamental considerations of general practice. In the present paper the intention is, in effect, to take some pages from a designer's notebook and explain in detail the procedure by which the dynamic characteristics of a leg are determined. A description of one of the author's own designs will provide a suitable area of reference, and although certain of the geometrical calculations will apply rather specially to this, the remainder of the working will be quite general in its application. In particular, the orifice calculations—a vitally useful aspect which has not previously been treated in a practical fashion—have been compared with drop‐test results over a range of differing designs and sizes of shock‐absorber and show very good agreement with them; certainly good enough for a close initial approximation, and in the style of leg described here and in other simple fixed orifice designs, sufficiently accurate to allow of dispensing with dynamic tests provided that tyre and oleo relationships do not diverge too far from the conventional.

The calculation of undercarriage landing reactions and dynamic stresses is discussed, and the merging of the two calculations under certain circumstances is proposed. ? formulation of the combined problem is outlined, and numerical examination of a simple case shows that in some cases the reaction forces are appreciably reduced by the adoption of the combined approach. Finally, a criterion is derived which could be used in the early stages of undercarriage design to predict the elicci of aircraft flexibility.

THE policy we announced at the outbreak of the war of making a speciality of the publication of translations of useful articles appearing in foreign—and more particularly German—technical publications has been amply justified by the endorsement it has received from numbers of readers; and we were particularly interested to hear it mentioned in an address by one of the leading scientists of this country as being probably the most useful service that a technical journal could contribute to the British cause at the present time. Our files show that since last September at least one (and in several months more than one) article from a foreign source has been a feature of each issue of AIRCRAFT ENGINEERING. At a time when it is manifestly difficult to publish up‐to‐date or new information regarding the products or manufacturing methods of the industry in England, which for obvious reasons it is not at present desirable to broadcast, it is clearly of interest to disseminate information regarding developments in other countries. This is, of course, merely an extension of the breadth of outlook on which we of AIRCRAFT ENGINEERING have always prided ourselves. We have never taken the narrow view that it was our metier merely to be a propagandist organ to publicise the quality of British products. We have always welcomed contributions, if of a sufficiently high standard, without regard to the country from which they emanated; believing that in so doing we were best serving the purpose for which we were established—to further the development of aeronautics by spreading the latest knowledge, as it becomes available, among all those working in the common cause. As in time of war individual workers in a belligerent country are of necessity cut off from communication not only with enemy but to a considerable extent with neutral or even allied countries, where there are fortunate ones, like ourselves, whose avocation puts them in an unusually favourable position to obtain information from abroad, it is manifestly their duty to seek out this information and disseminate it to the industry at large.

The pressure control valve is a possible means of overcoming the effects of compliance between a hydraulic servo and its load. The valve consists of two stages; an open‐centre flow control valve schematically analogous to a resistance bridge, and a split‐slider valve. In order to study the dynamics of the system it is convenient to set up an electrical analogue for the mechanical system. This will give the non‐linear equations of the system. Several forms of the various impedance elements making up the system require consideration. The resistance of a length of hydraulic line can be obtained from the friction factor and will depend on the flow regime which prevails. In developing an expression for the inductance of a length of line the assumption is made that the wavelength of the oscillatory motion is long compared with the line length. In considering the resistance of orifices it is necessary to take into account the three regimes for flow through an orifice, laminar, transition, and turbulent flow. The inductive effect of an orifice arises from the acceleration of the fluid as the streamlines converge through the orifice; an expression can be derived by applying Newton's second law of motion to the column of oil passing through the orifice. The effective capacitance of a volume can be derived by considering wave motion in one dimension. An expression for the radiation resistance of a piston is derived, and also one for the incident and reflected waves. In constructing the electrical analogue for the slider valve the mass and viscous damping of the slider as well as Bernoulli forces must be represented.