Many months of work by numerous specialists are required to complete a new design
PLANS ARE DRAWN UP by draughtsmen for every part of an aeroplane before construction can be begun. The work is divided up into sections, each section being in the hands of an experienced senior draughtsman. If the aircraft is a large one there may be as many as twenty men working under each section leader. The details of the design are worked out by the draughtsmen themselves.
THE popular conception of an aeroplane designer is that he is a solitary worker, to whom there conies in an intuitive flash an idea which he embodies in a revolutionary invention. Such a picture is far from the truth.
In these days no aeroplane is ever designed by one man single-handed. Every firm of constructors has a Chief Designer, but he is only the head of a large staff of designers. Some members of the staff are mathematicians, others draughtsmen, still others are engineers who work in the wind tunnel. Large contributions to a successful design may come from the Government research laboratories and, on the more practical side, from the factory building the machine. If the aeroplane is being made to the order of a customer such as the Royal Air Force or Imperial Airways, many of the most important features of the design are already specified before even the “designer” hears of it.
The Chief Designer is generally an experienced engineer with a working knowledge of economics and no patience with visionaries. Yet many, if not all, of the most successful aeroplanes are beautiful things to look at, with fair lines and flowing curves. Those lines and curves have not come from some rapturous dream, but because wind tunnel tests, mathematics and experience have proved them to be right. The smooth, glossy surface of a high-speed machine is not designed to please the eye, but because it allows the machine to travel faster and earn more profit for the same expenditure on petrol.
Perhaps the popular but false picture of how an aeroplane is designed is due to the secrecy in which designers have to work. Their products, for obvious reasons, may not generally be discussed until long after they have been designed. Months pass while a machine is being built and tested. It may even be two or three years before the public hears of it. By then it is in the hands of the pilots. The designers may not themselves lead dangerous lives, but they carry a heavy load of responsibility for the lives of other people. By the time their aeroplane has become “news” they are more interested in some new design of which the public has not yet heard. The career of a designer depends little on publicity, but much on a sound reputation built up in the minds of those who sponsor him.
It is rare for a designer to start with a clean sheet of paper. Many of the most successful, if less revolutionary, designers try to base a new design on an older one, the characteristics of which are well known and to which obvious improvements suggest themselves. Thus well-tried features tend to reproduce themselves in every new type from a particular factory.
One famous designer has concentrated on single-seater fighters, and since 1915 has produced a series of highly successful machines, every one an improvement on its immediate forerunner, but all having a strong family resemblance to one another.
Another designer became world-known for his commercial aircraft, ranging in size from small two-seater sporting biplanes through four-seater cabin machines to aircraft seating eight, ten and twelve people.
AN ENGINE AND ITS NACELLE being fitted into place in the wind tunnel of the Royal Aircraft Establishment at Farnborough, Hampshire, to check the effectiveness of the streamlining. The wind is forced past the engine from the front and sucked in behind by the fan, the size of which can be appreciated by comparison with the size of the men. Smoke is used to make visible the course taken by air in its passage at various speeds past the engine, aeroplane or other item that is being tested.
PART OF THE STRUCTURE of the Farnborough wind tunnel. Scale modes of a new design of aircraft are built for test in a wind tunnel. The tests indicate the accuracy of estimated performance figures and the stability of the aircraft. If the new aircraft is to resemble one already in existence, these tests with a model in a wind tunnel may not be necessary.
One type grew out of another and the designer was most successful when he adhered fairly closely to precedent. Only when aerodynamic and engine research had advanced far enough was he able to break away and produce the clean high-performance monoplane. He has now begun a new kind of activity for his company and it is to be expected that a range of successful monoplanes will follow.
When this is the method which the designer can follow he has sound information of existing aircraft on which to base his design. Certain modifications will produce certain effects and the designer makes adjustments in accordance with the new requirements.
Let us follow the procedure of a new design from start to finish. Whether the aircraft is commissioned by the Air Ministry or by one of the big transport companies, the process is roughly the same. A specification is drawn up which clearly defines the outstanding points of the customer’s requirements. This specification will represent the points of view of the pilots, crews and maintenance engineers who will have to handle the machine. The specification will represent also the requirements of the commercial management who will have to produce a profit with it or, if it is a Service aircraft, of the officers who have its particular purpose in mind.
These people have already designed the machine in the rough. They will state whether they want a high-wing or a low-wing monoplane and they will define the type of construction, the kind and number of engines, the performance and capacity. They will give exact particulars of the cabin arrangements, wireless and other electrical equipment, and instruments. If it is a military aeroplane, they will specify the armament as well.
The specification is then sent to a number of selected firms which are known to have experience of and a reputation for the particular type. These firms will then decide if they wish to compete and their respective designers will study the specification in great detail. As each designer reads it a picture of the aeroplane-to-be will form itself in his mind. He is, however, closely tied down by the requirements of the specification and he is not allowed to depart from them without overpowering reasons.
If the type of engine is stated this, with the required range or endurance, defines the amount and weight of petrol to be carried. By adding to that the stated payload and weight of crew and then another 50 per cent to this total for structure weight, the designer will have the gross weight to within narrow limits. The landing speed, which is always given in the specification, defines the wing area needed to carry the weight (see the chapter “Wing Loading Problems”).
The length of fuselage and the size of tail plane, elevators, fins and rudders are all dependent on the span and area of the main plane; when that is known the others follow. The designer thus has his first mental picture of the aeroplane and up to this point there will be little difference between the pictures in the minds of any of the competing staffs.
The designing staff now begins to go into greater detail. This will be the work of several highly skilled men for perhaps eight or ten weeks. They prepare drawings of the main structure, of the accommodation for the crew and, if it is a military machine, of the stowage of bombs, machine guns and so forth. The corresponding work on a civil air transport machine includes the passenger cabins, ventilation and heating, and the like.
Strength calculations are carried through for all the main members of the structure and a close estimate is made of the weight of every item. The company is then able to say how much it would cost to build such an aeroplane and how long it would take. On the appropriate day the company must submit its tender to the customer.
Then ensues a long wait while all the tenders received from the different competing companies are examined. The successful firm is not necessarily the one which submits the lowest price. Many factors have to be taken into consideration — the reputation of each company for that particular kind of machine, the delivery dates, the estimated performances, the type of structure suggested, whether metal-clad or a fabric-covered framework, and so forth.
Eventually the order is placed and the work really begins for the successful designer. A scale model of the machine is made and tested in the wind tunnel. This will confirm the estimated performance as given by the earlier calculations and it will probably be used to test the stability of the aircraft. If the aeroplane is to be a flying boat, a model of the hull is made and tested in the water tank at speeds corresponding to those of the full-sized machine up to the speed at which it leaves the water.
THE FIRST FLIGHT TEST of any new aircraft is an anxious time for the Chief Designer. Sometimes the performance proves better than that expected, but almost invariably there are some small adjustments to make and further flights to do before the aircraft can be handed over to the customer for acceptance trials. This photograph shows a De Haviliand Albatross type aircraft being brought out for flight trials.
Another model which may be required is one which reproduces to scale not only the shape of the aeroplane but also its weight and its centre of gravity. This is tested in the “spinning” tunnel. The wind tunnel does not spin round but the model may. If it does spin round the designer learns what will be the control' qualities of the aeroplane at speeds below that of stalling.
Any or all of these models may suggest alterations to the designing staff. If the aeroplane is to resemble some existing one which has already flown, the models may not be needed; if it is a completely new type a great deal may be learned from them.
Finally a full-sized model or “mockup” of the pilot’s cockpit and cabin is built of wood and brown paper. Into this are built seats, controls, instruments and all the equipment of a modern aeroplane. The customer’s representatives are invited to inspect this and to criticize it. They make full use of the invitation.
Further alterations are made and at last the design can go ahead with all the main problems solved. Several months may have passed before this point is reached, and until it has been successfully passed little can be done in designing the details of the machine.
The work is then divided up into sections and each section is in the hands of a senior and experienced draughtsman. If it is a large aeroplane there may be up to a dozen or twenty draughtsmen under each section leader. One section takes the wings, another the fuselage and tail unit. The equipment, the undercarriage and the engine installation are each the work of further sections.
Hundreds of Drawings
While the “stressmen” recalculate the loads in all the members of the structure, the draughtsmen are employed in making drawings of every part. Technical clerks go through all the drawings as they are made and make lists of the material which will be needed in the construction. If this material is not in stock, valuable time, which would be lost later while it was ordered and delivered, can be saved.
For some parts of the structure the Chief Designer may decide that the mathematics of the stressmen may be unreliable, due to insufficiently exact knowledge of new methods. When this
happens a specimen part, which may be only a short length of strut or may be a complete wing, is built and tested under loads of shot bags until it breaks.
Gradually the drawings, of which there may be many hundreds, are issued to the workshops. Here they are examined in great detail and arranged in the correct order for production. Special tools may be needed and these have to be designed and made.. Eventually small parts begin to collect in the “finished part” stores, awaiting assembly.
Space is cleared and at the right time the assembly of the first machine is begun. This will bring more work to the drawing office as small errors are discovered. It rarely or never happens that every part has been so designed that they all fit together in perfect harmony. Moreover, when the aeroplane begins to take shape, improvements may suggest themselves to the designer. He must decide whether he will incorporate these ideas at once, even though it may mean scrapping some of the parts already made.
At last the first machine is finished. Every part has been made and all the parts been checked by inspectors whose task is to look for divergences from the original drawings, and for faulty workmanship of every kind.
Then comes one of the designer’s biggest checks. The finished aeroplane is put on the scales and weighed, and its centre of gravity is determined. The recorded results ought to come, and often do come, close to the designer’s original estimate. If they do not, then he or some of his staff are at fault.
The biggest and most nerve-racking test has still to come — the first flight. Some Chief Designers insist on going up on the first test; others take a holiday. The aeroplane may do everything that was prophesied of it. Small adjustments, however, and further flights will be made before the machine can be handed over to the customer for acceptance trials.
FULL-SIZE AEROPLANES can be tested in the wind tunnel erected by the French Air Ministry on the outskirts of Paris at Chalais-Meudon. The area of the oval aperture through which the wind is forced by six propellers is 100 square yards. Each propeller is driven by a 1,000 horse-power motor, and the wind may be given a speed as high as 105 miles an hour.