In a recent paper, a theory explaining the wake-induced flutter of smooth circular cylinders was developed and vindicated. It was shown that, for moderate spacing of the cylinders (where the aerodynamic coupling between the constituent motions is small), flutter could occur only when the natural frequency, in still air, of vertical oscillations exceeded that of the horizontal motions. The theory, however, took no account of the possibility of mechanical coupling between the constituent motions and, while the results from this theory were corroborated by wind-tunnel tests in which a statically uncoupled mechanical support system was used, no indications were given of the changes in aeroelastic behaviour which might accompany the introduction of simple coupling terms.

In this paper, the class of cases wherein the mechanical support system for the leeward cylinder exhibits static coupling is studied using “undamped flutter theory”. It is demonstrated that the appearance of static coupling terms can lead to quite dramatic changes in the flutter characteristics, and that considerable care must be exercised in the design and operation of wind-tunnel dynamic models if meaningful results are to be obtained.

An Appendix deals with the general problem of mechanical coupling, using the normal coordinates approach, and aspects of the problem which bear on the subconductor oscillation phenomenon experienced on “bundled” overhead power transmission lines are highlighted.

In constructing an X-type hot-wire probe it has been the policy of a number of experimenters and manufacturers to place the two wires forming the X close to each other to assure that they are both measuring in effectively the same plane. A number of important experiments have been made using such a probe design. Recent experiments at the University of British Columbia and McGill University have shown that an X-wire probe which has two wires almost in the same plane is quite sensitive to movements of the velocity vector out of that plane (defined as pitching motion). This has been attributed to the influence on one wire of the hot wake produced by the other. In this note a Disa X-type probe (type 55A32), with the wires 0·15 mm apart, is tested and found to have a static sensitivity to small angles of pitch, which is very significant for low wire Reynolds numbers (< 5) but becomes small for Reynolds number greater than 10. A modified probe, having the wires one wire length apart, is suggested and when tested was found to have no pitch sensitivity.

Using a perturbation method, the formulae for the pitching stability derivatives of sharp wedges at zero incidence in viscous hypersonic flow are obtained in closed form. The results include the effect of unsteady reflected waves from the bow shock and may be applied to any wedge provided that the bow shock is attached. The effect of viscosity of the gas is included in two parts: first, it thickens the wedge, thus introducing the concept of an effective wedge, thicker than the original wedge by a semi-vertex angle equal to the average inclination of the displacement boundary layer; secondly, it makes this effective wedge deformable. Comparison with previous theories is given.

In many flight situations an automatic control is used to minimise the deviation of one or two aircraft motion variables from their trim values. The same effect may be brought about, in certain cases, by pilot action. If the control is very strong it may be imagined that the aircraft motion is kinematically constrained in the sense that the controlled motion variables are actually held irrevocably at their trimmed values throughout a disturbance.

More than fifteen years ago Neumark presented an analytical treatment of startling simplicity which dealt with the motion of an aircraft under strict kinematic constraint. By adopting the concept of kinematic constraint and so eliminating one or more variables the motion of the aircraft is described by a simpler set of equations: yet, clearly, the addition of a control system to the aircraft will, in fact, increase the complexity of the complete set of describing equations. However, there is no doubt that Neumark’s theory does give meaningful and useful results.

The analysis described in this paper shows how the motion of a strongly controlled aircraft can be described in terms of simpler sub-systems and indicates that Neumark’s theory appears as the limiting case of infinite control strength. The role of the pilot in applying strong control is placed in the context of the general theory.

Shear stress, eddy viscosity and mixing length distributions have been obtained from measured boundary-layer developments over porous surfaces with air and carbon dioxide injection at Mach numbers up to M=3·6. It is found that, if the eddy viscosity is non-dimensionalised by dividing by the product of the free-stream velocity and the kinematic displacement thickness then this non-dimensional ratio is almost independent of injection ratio, but decreases slightly with Mach number.

A non-linear mathematical model for the study of the dynamics of an extensible cable subjected to aerodynamic forces generated by a uniform flow field is formulated. Solutions are found considering large displacement caused by suddenly applied loads (i.e., gusts, shock waves, turbulence) for a range of flow speeds and cable lengths. Transition from overdamped to oscillatory motion is observed when flow speed and cable length are increased and decreased respectively. The stability of the system is discussed.

A study is made of the interaction of a combination of free-vortex and source flow with a stationary surface. The laminar boundary layer flow can be expressed in ordinary differential equations by choosing suitable similarity transforms for the Navier-Stokes equations. When simplifying boundary-layer approximations are included, the equations do not yield any unique solution. Solutions to the complete equations are calculated numerically for the special case of equal source and vortex strengths for a limited range of Reynolds number. The results show the presence of “super” velocities and large pressure variations within the viscous layer.