This is the first meeting at which it has been possible to talk openly about transonic tunnels with ventilated working sections. Many already knew that a large number of tunnels, particularly in the U.S.A. and this country are being built or modified with slotted or perforated working sections. Up to quite recently, however, security considerations have shrouded this work but it is now possible to draw the curtain.

In this short paper on the transonic jet-driven tunnel, I will place the emphasis on the word transonic and deal only with the jet drive as it affects the design or operation of the transonic working section. I also would like to say that, although a fair amount of the information discussed afterwards will refer to the D.H. transonic tunnel, the problems are more or less general to the jet–driven tunnel and some will even be applicable to electrically–driven transonic tunnels using diffuser suction.

The Aircraft Research Association's tunnel was designed from the start as a transonic tunnel. Two important decisions were taken at the beginning of the design of this tunnel. First, that a separate auxiliary suction plant would be used to suck air out through ventilated walls of the tunnel instead of using diffuser suction for this purpose. This decision was taken mainly because the total power required with auxiliary suction is appreciably less than for diffuser suction. The plant is also more flexible and provides, for example, the possibility of running a smaller supersonic tunnel from the auxiliary suction plant.

Early in 1950 it was decided to modify experimentally the original jet–driven tunnel of the English Electric Company—Aircraft Division, from a subsonic to a transonic tunnel. This was inspired by the work of Ward and Wright at Langley Field. As little was known then of the pressure ratio to operate a transonic working section, the working section area was reduced from a 3 8 square foot subsonic section which could be choked, to one square foot. The working section and diffuser for this modification were quickly manufactured from wood. In addition to the new working section and diffuser, a steel fairing was added to reduce the cross sectional area of the mixing length and so match the smaller induced mass flow.

The development of wind tunnel sections with partly open walls has made it possible to operate at continuously variable speeds through the transonic range up to low supersonic speeds. Some consideration must be given, however, to the general reliability of the measurements in these tunnels of the aerodynamic characteristics of air vehicles or missiles. A comparative assessment of the possible overall errors in force and pressure measurements in relation to desirable standards of accuracy is, in fact, required. A number of points connected with such an assessment are briefly discussed (mainly on the basis of experience in the 3 ft. tunnel at R.A.E., Bedford) in this paper, which touches on flow uniformity, accuracy of model manufacture, and interference effects, including a comparison between tunnel and free flight model results; illustrative examples of results for an aircraft model, with some full scale flight comparisons, are discussed briefly and a few comments are made on the problems associated with the achievement of representative flow conditions on sting–mounted models at transonic speeds.

At the start of the discussions, the Chairman (Major G. P. Bulman) gave a warning that parts of the subject of transonic wind tunnel testing were still classified and therefore he asked speakers to be careful in what they said. In spite of this, it was possible to discuss a wide range of topics without transgressing security restrictions. Those members of the audience who were familiar with the subject were able to put many questions to the lecturers and to raise several new points, while the large proportion of the audience to whom the subject was relatively new were able to gain a useful insight into the problems involved in the operation and design of transonic tunnels.

The possibility of explosions in a duct or chambers, which may contain a mixture of an explosive vapour and air, is a hazard commonly occurring in industrial plant, often necessitating a very strong and costly form of construction which would otherwise not be required. Where the structure is not strong enough to withstand explosion pressures serious damage results, frequently with injury to personnel.

Appreciable forces can be obtained for steering a vehicle by the use of aerofoils whenever the relative air velocity is high and the air density is not too small. In practice this means velocities greater than 100 ft./sec. and altitudes below about 50,000 ft. Even when they are not in use for steering, aerofoils necessarily add to the drag of the vehicle.

A simple method is given for the calculation of the heat transfer from a laminar flow surface. Computation is by a quadrature. The method is essentially a simplification and extension of the Eckert(1) method, and is applicable both to two–dimensional and to axisymmetric flows.

It is not difficult to establish equations by means of Castigliano's First Theorem intended to give the deflections and rotations at any point in a cantilever curved in plan. However, when the bending modulus, EI, and the torsional modulus, GJ, are unequal in magnitude and when the plan form is not of simple shape, these equations become difficult to handle, if not unmanageable. For the case in which EI ≃ GJ (a solid circular section or annulus) it is possible to obtain simple, approximate, and explicit expressions for the deflections under a concentrated end load of W1 and end couples M0, T0 for any form of beam. These expressions are now extended in Section 1 to take account of the end loading shown in Fig. 1.