Experimental methods

In the course of this book there has been frequent comment to the effect that theory has tended to lag behind practice as the means whereby we have acquired aeronautical knowledge. Some critics have said that this claim has been exaggerated, but the author still believes that it is fair comment. It cer­tainly isn’t a new idea for this is an extract from the 14th Annual Report of the Aeronautical Society of Great Britain (now the Royal Aeronautical Society) – ‘Mathematics up to the present day has been quite useless to us in regard to flying’ – the date of that report, 1879!

But whatever may have been the relative importance of theory and experi­ment in the acquisition of knowledge about subsonic flight, even the most theoretically-minded critic will surely agree that we have had to rely almost entirely on experiment in solving the problems of transonic flight, for here we have a mixture of subsonic and supersonic flow, compressibility and incom­pressibility, two completely different theories all mixed up. Curiously enough, in real supersonic flight, theory comes into its own; supersonic theory is simpler and older than that of subsonics or transonics – Newton’s theories used to calculate air resistance of a body at 515 m/s give an answer nearer the truth than if they are used to calculate its resistance at 51.5 m/s.

But one of the fascinations of this subject is that the experimental methods themselves are so interesting, involving as they do their own theories quite apart from the facts that they reveal. So far as transonic and supersonic flight are concerned we have already referred to ingenious methods of photography which give us pictures not only of shock waves, but also of smaller changes of density; and by taking films by these methods we can watch the changes of density and of the shock pattern as speed is increased from subsonic to tran­sonic, then through the transonic to the supersonic region. At subsonic speeds we have devised methods of seeing the flow of air, but the schlieren and other methods show something even more important – what happens as a result of airflow. We have learnt a very great deal by these methods, and we shall look at some more pictures later.

But though it is conceivable that by very elaborate means such photographs could be taken in flight they really require laboratory conditions, which means wind tunnels. Now wind tunnels present quite enough problems at subsonic speeds; but as we approach the speed of sound the very shock waves which we want to investigate and photograph, obstruct the flow through the tunnel (even when it hasn’t got a model in it), and raise a barrier so effective that the tunnel is virtually choked and the high-speed flow cannot get through. Even if the tunnel design can be modified so as to allow the flow, the shock waves on even the smallest of models will cause very severe interference between the model and the walls of the tunnel, and so make the results of the tests value­less. This choking of wind tunnels, which was particularly difficult to overcome between Mach Numbers of 0.85 and 1.1, is the explanation of two rather curious facts in aeronautical history, that a truly supersonic wind tunnel became a practical proposition before a transonic one, and that flight at super­sonic speeds took place before such speeds were reached in wind tunnels.

The problem of the transonic wind tunnel has now been largely overcome (though rather late in the day) by using slotted or perforated walls, and there are many types of supersonic tunnel in use today. Some of these are very similar to subsonic types in general outline; the extra speed has simply been obtained by more power together with suitable profiling of the duct – perhaps simply is not quite the right word, because the increase in power required, and consequent cost, is tremendous. It might be thought that fans of the propeller type would not be suitable for such tunnels; but in fact they can be used because they are situated at a portion of the tunnel where the speed is com­paratively low, the cross-section of the tunnel, and therefore the propeller, being correspondingly large. The great size of the propeller, often larger than any used on aircraft, presents problems of its own, but none the less this con­ventional type of tunnel, which may be straight through, return flow or even open jet, is probably the most satisfactory where a large tunnel is required.

For smaller types, and higher speeds, it is more usual to employ some kind of reservoir of compressed air and, by opening a valve, to allow this to blow through the tunnel to the atmospheric pressure at the exhaust (Fig. 11.10). By arranging for the exhaust to be into a vacuum tank, even higher speeds can be obtained. One great disadvantage of this method is that continuous running for long periods is impossible; in fact constant speed is only achieved for a very

Fig 11.10 High-speed wind tunnel: blow-through type

If return flow type excess air is blown Air injected Can be of return off during return underpressure flow type Air injected under pressure

Fig 11.11 High-speed wind tunnel: induced flow type

short time. It sounds rather primitive, and in some ways it is, but Mach Numbers of 4 or more may be reached by this method, though the cross-sec­tional area of the working section is usually small.

More efficient, at some Mach Numbers, than this straight blow-down type of tunnel is the induction or induced flow type, in which air is blown in or injected just down stream of the working section (Fig. 11.11), thus ‘inducing’ a flow of air from the atmosphere through the mouth of the tunnel. Notice that in this type the compressed air does not flow over the model at all, only the induced air does. The injected flow can be the jet of a turbo-jet engine; in fact the jet engine itself can be in the tunnel. An induction type tunnel may be of the return flow variety, in which case provision must be made for the excess air to be blown off from the return passage.

It is one thing to obtain a high Mach Number in a wind tunnel – it is quite another thing also to obtain a high Reynolds Number (see Appendix 2) and so eliminate scale effect. This brings in the question of high-density tunnels, low – density tunnels, cryogenic (low temperature tunnels) and even the use of gases other than air; this interesting problem will be touched on in Appendix 2, but it is really beyond the scope of this book.

A simple and fascinating way of observing patterns very similar to shock – wave patterns is by using what is sometimes called the hydraulic analogy. Anyone who has watched the bow wave, and other wave patterns caused by a ship making its way through water, and has also seen the shock-wave pat­terns in air flowing at supersonic speed, must have been struck by the similarity of the patterns. If bodies of various shapes are moved at quite mod­erate speeds across the surface of water (not totally immersed), or the water made to flow past the bodies, many shock-wave phenomena can be illustrated and, by a suitable system of lighting, thrown on to a screen. Some people say that such demonstrations are too convincing, because they make one think that it is the same thing – which of course it isn’t. The patterns are similar but the angles and so on of the waves are different.

Finally, we come to full-scale or free-flight testing, and even this may be divided into two types, tests with piloted aircraft and those with pilotless air­craft or missiles. Not only are these the eventual tests, but in the case of transonic flight, in particular, they have also been the pioneer tests and most of them have been made in piloted aircraft. In the early investigations, piloted aircraft certainly had their limitations because we could only approach the speed of sound in a dive; this had rather obvious and rather serious disadvan­tages. It took a long time (and a lot of height lost) to reach the critical speeds and then, if the symptoms were alarming – as they sometimes were – we were unpleasantly near the ground by the time a recovery could be made; moreover the making of such a recovery was not made any easier if the symptoms were severe buffeting, or worse – a further dropping of the nose and steepening of the dive – or worse still – heaviness of the controls so severe that they could not be moved. In such circumstances the point made in an earlier paragraph that the Mach Number might be decreasing as we hurtled towards the ground was hardly sufficient consolation.

As the thrust of jet engines increased – and it was this thrust which made even the approach of the speed of sound possible – the shock stall could be reached in level flight in some types of aircraft, and later still in climbing flight. This was an altogether different proposition from the pilot’s point of view and, as testing at transonic speeds lost its terrors, our knowledge increased corre­spondingly more rapidly.

Test on bullets and shells moving through the air at supersonic speeds have been made on ballistic ranges since before the days of practical flight, and pho­tographs of shock waves were taken more than 60 years ago, but it was only when aircraft themselves began to approach the speed of sound that the sig­nificance of such tests in respect of aircraft design began to be realised; and it was the development of the ramjet and rocket as means of driving missiles, and the parallel development of electronic instruments which could not only guide the missiles but take readings and keep records during the flight – in some cases even transmitting them back to earth – it was these that con­tributed most of all to our knowledge, and to the solution of the problems of transonic and supersonic flight.

So we can sum up the experimental methods that have been used to inves­tigate these problems as coming under the following headings –

(a) Photography of shock waves.

(b) High-speed wind tunnels.

(c) The hydraulic analogy.

(d) Free flight in piloted aircraft.

(e) Rockets and missiles.

Now let us see what all this has taught us.