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Issue 7 - Avro Vulcan: Delta Design

Many people, and not only those connected with the aircraft industry, are speculating on the reason for AV Roe and Co Limited designing and building the 707 series of Delta research aircraft, the first of which appeared at the 1949 SBAC [Society of British Aerospace Companies] show. After all, the name of Avro is closely associated with the very much larger aircraft, such as the Lancaster, Lincoln and Shackleton in the military field and the Tudor in the civil field.

The little 707s seem to be a complete departure from this tradition. The reason is partly explained by the fact that the aircraft are research aeroplanes and are intended to find out more about the flying qualities of this sort of an aeroplane which is known as a Delta, because of the close similarity between the wing plan form and the Greek letter Delta. We, like other large aircraft concerns, cannot afford to stagnate and merely produce the same basic type with miscellaneous detailed alterations and improvements as the years go by. The advent of the jet engine has raised the performance levels of all military aircraft and also long-range commercial aircraft, and with the radical change in power plant must come equally radical changes in the airframe to match it. Considering that a military bomber or commercial transport is essentially an aircraft designed to carry pay loads for relatively long distances, the basic aerodynamic problems are rather similar and just as from a military point of view the highest possible cruising speed is necessary, so in the case of a transport aircraft, it has been proved that high cruising speed can lead to overall economy and the lowest overall cost per passenger/mile. From an economical point of view there is a limit to the cruising speed which as is well known, is set by the so called ‘barrier’ of the speed of sound. Some aircraft have flown faster than the speed of sound, but it is not yet an economical proposition to attempt to cruise for long distances at these speeds. However, the nearest one can get to it the better, and it is this consideration which starts the designer thinking on radical lines.

In order to fly economically the aircraft must have the minimum possible drag and in order to keep the drag down at speeds approaching that of the speed of sound it is necessary for technical reasons to sweep the wings back at a very pronounced angle to the fuselage. Also it is necessary to keep the thickness of the wing as low as possible in terms of the chord; that is to say, whatever the wing chord is at any particular point along the span, the thickness should be kept down to a value of 10% of the chord or even less. Furthermore, if you want to fly at a true air speed and go as far as possible with an economical fuel load, it is well known that you must go as high as possible where the air is less dense.

Unfortunately, the speed of sound drops with increasing altitude and, therefore, as you design to fly higher, you must not only take the steps mentioned previously but in addition you must keep the angle between the wing and the flight path (known as the angle of incidence) low or else the drag will rise rapidly. In order to keep the angle of incidence low it is necessary to keep the wing loading low, or in other words, for a given gross weight of aircraft the wing area must be larger than we have become accustomed to in the last 15 years.

Another factor to be borne in mind, is that on a commercial aircraft or long-range bomber, if you want to fly long distances you utilise wings of high aspect ratio; that is the span is largely relative to the chord; the ratio varying from, say, nine up to 14. This is necessary in order to keep down that part of the drag (known as the induced drag) which is the penalty we pay for the wing lift.

Triangular plan form

Now you can get a broad picture of what the designer of a large load-carrying, long-range aircraft is faced with, if he wants to fly at speeds comparable with the speed of sound. He has at one and the same time to sweep the wings back, make them thinner, increase the area and keep up the span. This poses very great structural problems, and in fact to try to keep a really high aspect ratio and do all the other things simultaneously is economically impossible. One solution is to reduce the aspect ratio, in order to keep the structure weight down, and let the induced drag rise with the hope that so much saving can be made on the rest of the drag that the total will still not be too high. If you combine what is structurally desirable with what is necessary aerodynamically you soon arrive at the solution that the best thing to do is to taper the wings very drastically so that in the limit the plan form becomes triangular in shape.

A little thought will show that with such a wing the required sweep back is achieved and a large area can be automatically obtained at the lowest possible structure weight, since the area out at the tip which causes the big bending loads on the wing structure is reduced to a minimum and, therefore, such a wing of large area can be obtained with the minimum possible structural penalty. Furthermore, our aim of keeping the wing at the centre portion nearest the fuselage is quite large in terms of feet and inches.

We now find that as an interesting by-product of the theme we have got a relatively large usable volume in the wing that can be used for packing away the engines, undercarriage, fuel, etc, so that the excrescences hitherto so evident on the wing of an otherwise clean aeroplane have completely disappeared. Furthermore, the thickness of the wing at the centre is sufficiently large as to absorb the fuselage almost entirely so that it is reduced virtually to a streamlined projection ahead of the apex of the triangle.

Another by-product of this type of wing with its low loading is that no special devices such as slots or flaps are necessary to keep the landing speed down. The wing loading is sufficiently low as to enable quite normal take-off and landings to be done on existing aerodromes. Once we have abolished the need for landing flaps, which produce big changes of trim that have to be balanced out by the tail, the very need for the tail itself becomes questionable. The large wing chord of the Delta type of wing enables us to fit elevators at the trailing edge of the wing and these elevators have sufficient power to enable the aircraft to be flown through all normal manoeuvres.

We thus, by a fairly logical process arrive at an aircraft capable of high cruising speeds for long distances with a respectable pay load and consisting of nothing more than a smooth wing, streamline fuselage nose and vertical fin and rudder to look after directional control. If we have done our calculations properly we have now reduced the drag to the absolute minimum possible, and, therefore, have achieved, whether by military or commercial standard, the highest possible cruising efficiently.

Technically, therefore, the case for the Delta on paper is proved provided that in fact it flies in a respectable manner and does not suffer from hidden vices which have been overlooked in thinking only of the performance. Any aircraft company interested in the large type of aircraft cannot afford to ignore the possibilities of the Delta configuration. It is one thing, however, to prove a theoretical case on paper and it is another to sell it to the customer. What more obvious step, therefore, to take than to build a small one and fly it and this the Avro Company has done.

This, however, is only the beginning of the story; to translate this rather hopeful lesson into a large and intricate piece of hardware such as a bomber or a transport aircraft requires an enormous amount of investigation into the engineering details and it is here where the designer’s art is more important than his science, where time is dictated by the speed with which materials can be obtained, fabricated and assembled equipment provisioned and tested, all of which adds up to a process which can run into many years.