On the afternoon of 14th April 1987, the Society held a half-day symposium on wing-tip flows and devices. Papers were presented by J. J. Spillman of Cranfield Institute of Technology on wing-tip sails, by A. C. Willmer of British Aerospace (in association with R. V. Barrett and J. D. Coleman of Bristol University) on the tip flow of part-span flaps, and by H. P. Horton of Queen Mary College on measurements in the viscous flow regions of streamlined tips. Written versions of two of these talks and a synopsis of the third* have been prepared and are being printed in the Journal following these introductory remarks. The contribution by J. S. Smith of RAE to the discussion session is appearing elsewhere.

Studies begun over ten years ago have shown that the swirling air flow about the tips of a lifting wing can be redirected using small aerofoil surfaces, called sails, to reduce the overall lift-dependent drag of the wing. Tests on Paris, Jetstream and other windtunnel models, confirmed by flight tests on Paris and Centurion aircraft have shown marked reductions in drag and fuel consumption. Handling tests show improvements in roll control and stalling speeds.

Wing tip sails have the effect of reducing the peak adverse effects on other aircraft flying through the wake of a Paris aircraft but reduce the decay rate so the far field effects are worsened. Sails fitted to a crop-spraying aircraft have significantly reduced the amount of spray lifted above the height of the aircraft by the tip vortices and subsequently blown off-target by a cross-wind.

Windtunnel tests on a model with variable incidence sails show that differential sail incidence can be used effectively to provide roll power, reducing the size of the ailerons required, so allowing a greater flap span. The maximum root bending moments of the wings can be reduced by at least 10% by using negative sail movements in positive manoeuvres or gusts.

Sails can reduce the drag of an aircraft at all speeds leading to fuel savings estimated at US$225 per annum per 1000lb take-off weight per 1000 hours annual utilisation. For an airliner of 300 000lb weight and operating for 3000 hours this means a fuel saving of over a fifth of a million dollars per year!

The work to be described forms part of an experimental study into the overall influence that the end region of a flap has on the flow of a high lift wing system. Here, our attention will be restricted mainly, but not entirely, to the tip flow of the flap itself.

The study has been a collaborative venture over the past three years between British Aerospace-Civil Aircraft Division and the Aerospace Engineering Department of Bristol University, the latter funded by an SER.C research grant. BAe's major contribution was the undertaking of the main test programme, with University participation, using a large swept wing, high lift half model in the Filton low speed wind tunnel. The model was loaned by RAE for the study.

Wing-tip vortices are of considerable practical importance, primarily because of the associated induced drag. A number of investigations have been made in the past into the development of vortices trailing behind wings, but little attention has been given to the mechanism of generation of such vortices. In the region of a wing-tip the boundary layers, which are usually turbulent at full-scale, separate from the wing surface in a rather complex manner to form a vortex sheet which rolls up into the tip vortex. Although the gross effects of the tip vortex can be predicted by inviscid theory, a detailed understanding of the behaviour of the viscous layers around wing-tips could lead to better optimisation of their design.

A simple closed-form formula for the coefficient of surface pressure, Cp is given in this paper. The formula is based on first and second approximations of the full problem and can be used for the Newtonian flow past two-dimensional thin airfoils at small angles of attack and with attached shock waves. It thus extends Cole's zero order theory to the effects of non-zero (γ – 1) and finite values of Mx where γ is the ratio of the specific heats of the gas and Mx is the free stream Mach number. The analysis presented relies on a previous recent formulation of the hypersonic small disturbance theory which has proved to be more advantageous than the hypersonic small disturbance theory. The results are compared with other approximate methods and the agreement is found to be generally good.

An inviscid model for a steady two-dimensional aerofoil-spoiler at low speeds is applied to an aerofoil-spoiler-plain flap configuration. The model is extended to an aerofoil-spoiler-slotted flap configuration. The flow through a slotted flap can result in either attached flow or separated flow about the flap. The location of the separation point on the flap has to be assumed, it is taken empirically to fit experimental data. The inviscid model is extended to the aerofoil-spoiler-slotted flap configuration with the spoiler oscillating in small amplitude simple harmonic motion about a mean spoiler angle. Although both the steady and unsteady models for the aerofoil-spoiler-flap configuration are crude, the results look encouraging.

A method based on the shedding and convecting of discrete vortices has been developed to calculate the transient loading on an aerofoil-spoiler configuration when the spoiler is suddenly deployed to a particular angle. An innovation in the method is the introduction of Kutta conditions at the separation points.

Although the variation of CL with time is highly irregular, qualitative trends can be deduced. For a range of aerofoil angle of incidence and spoiler angle deployment adverse lift effects (ie, when the lift initially increases) have been obtained, similar adverse lift effects have been measured in experiments.