Fighter design

by M A Bugnon

Because most military aircraft are designed to a government specification, the engine is seldom selected by the aircraft designer. Sometimes only one engine type offering the desired performance is available at the time the design is contemplated. On other occasions, official dictum may specify that a particular aircraft is to have liquid-cooled or air-cooled engines for operational reasons.

Thus it may be assumed that most plane designers start with a given powerplant, develop a design to accommodate that particular engine, while hoping for development of an improved engine before his plane goes into production.

With a variety of single-engined fighters serving the belligerent powers, aircraftsmen working on US planes should be interested in the extent to which individuality of designers has influenced design. In the main, all of these fighters were designed with similar purposes in view, yet each one is completely different.

In aerodynamic design, most of our modern single-place warplanes are similar because the natural forces permit little originality. The engine must go in the nose of the fuselage which, in turn, must be of the best streamline form. The smallest pair of wings capable of carrying stipulated fuel and armament loads, landing them at a tolerable speed and supporting the plane in climbs and maneuvers are chosen. The undercarriage must retract, and means must be provided for cooling the engine. Except for "freak" designs, there is thus relatively little the aircraft designer can do. In structural design, however, there is a far greater choice of alternatives, and an examination of existing fighter types reveals a variety of "systems". We have picked five of these as being fairly representative, although several more could have been included without duplicating the design.

It might not be out of place to call attention here to a fact which is too often overlooked. The percentage of weight over which the aircraft designer has any control is surprisingly small. He is given an engine of a certain power and weight. Usually the flight duration, and therefore the weight of fuel, is stipulated in the specification. The types and number of guns and the number of rounds of ammunition is also decided in advance.

It will thus be seen that the designer is left with only the main aircraft structure on which to exercise some ingenuity. His ideas are frequently compromised, however. He may wish to adopt certain forms of construction which would be light and strong, but some of the materials may be on critical lists or unavailable because they are slower to make.

Broadly speaking, fuselages are of two main types: the girder type, in which the material is concentrated into struts and ties; and the stressed-skin type, in which the material is disposed almost entirely at the surface of the body, as far away from the neutral axis as possible. Basically, one would assume that the stressed-skin construction should be the lighter. If a fuselage were merely an unbroken shell, that would probably be so, but unfortunately openings have to be cut in the shell, and the loads then have to be taken around the openings in such a way that they are distributed to the surrounding metal. Moreover, there are several concentrated loads which have to be distributed into the skin structure, so that the shell type of construction can rarely be used in its fundamentally light form.

Despite these considerations, however, the stressed-skin aircraft is usually lighter than the girder type. But certain practical considerations enter into the problem to add further difficulties to the designer's choice. For example, the girder type of fuselage affords a better opportunity for installing the multitudinous equipment of modern military aircraft. Workers can reach in between the girder members to get at the equipment being installed, and a considerable number can work on one fuselage at a time.

With the stressed-skin fighter, small in proportion to the equipment it must carry, workers must lie flat on the floor while attaching certain equipment. Attempts have been made to solve this problem by using a "split" form of construction, in which the equipment is installed on a portion of the fuselage only, the shell being completed after the equipment is in by riveting on the remaining portions of the skin.

This would be satisfactory but for inspection, maintenance, and the relative certainty of damage by enemy action which frequently necessitates opening the skin at regular intervals.

Wing Variations

In the case of the wings, there is little scope for alternative arrangements. Because wings of modern high-performance aircraft must be metal-covered, the broad principles of construction are limited to single-spar, two-spar, and multi-spar construction. In the single-spar wing the leading-edge has a thicker covering so as to form, with the spar, a sort of D-section box which is stiff torsionally, while the spar takes most of the bending loads. In the two-spar type of construction the spars share the loads and the skin assists in resisting torsion. Multi-spar wings are rare as they may have to be "cut" for undercarriage wheel wells, armament, or tanks.

There are, of course, many more complicated problems for the designer to solve, but enough has been said to give a fair idea of the extent to which aircraft structural design is a series of compromises between conflicting requirements. Now let us examine a few representative examples.

Basically, the design of the Vickers-Supermarine Spitfire comprises a single-spar wing attached to a stressed-skin fuselage. The accompanying sketch reveals considerable concentration of stress at the point where the single main spar is attached to the fuselage. The lower engine bearer members and the undercarriage struts are attached to the cross-member running across the fuselage. Loads from the upper engine attachment points are distributed to the skin of the fuselage by way of the top longerons which extend to the aft main fuselage former. In the sketch of the Spitfire, as in the sketches of the other four fighters, the lighter intermediate formers have been omitted in order to emphasize the main structure members which, with the stressed skin, form the primary structure. Aft of the rear main former the top longerons trail off to nothing, while the bottom longerons are of lighter construction than those in the forward portions of the fuselage.

The undercarriage folds outward concentrating loads close together, avoiding the high stresses generated when undercarriage legs are attached to the wing spar some distance outboard. The only real fault in this installation is that the wheel track is of minimum width. The armament, in this case two cannon and four machine guns, is of necessity carried outboard of the wheel wells in the wings.

The Spitfire is a good example of the single-spar wing with strong leading-edge covering forming a torsion-resisting "tube". The small auxiliary spar has relatively little work to do.

Girder Construction

The Hawker Hurricane is the only important fighter using girder-type construction. The Hawker firm had, for many years, specialized in this form of fuselage construction, and had, in the biplane days, used rolled and drawn steel strip for the wing spars. Because of this girder know-how, Sidney Camm designed the Hurricane to this pattern to prevent the production delays which would have evolved in a change to shell construction.

The result was the first Hurricane in which even the wings were of girder construction with fabric covering. Later Hurricanes have stressed-skin outer wings but have retained the girder fuselage and wing center section shown in the accompanying diagram. That the type of structure originally designed was amply strong for its work will be realized when it is recalled that originally the Hurricane weighed some 6,000 pounds loaded, and that recent versions have tipped the scales at fifty per cent more than that. This has necessitated some increase in the strength of material used in the structure. but it has caused no major structural alterations.

Both the Spitfire and the Hurricane embrace center-section spars. In the former, the spar is contained within the fuselage and the wings are buttoned in close by the fuselage sides. In the Hurricane, the center section is of fairly large span, and the joint of outer stressed-skin wing to girder center section occurs a considerable distance outboard.

For sheer simplicity of design "motif" it would be difficult to improve on the type of construction employed in the North American Mustang, which combines a single-spar wing with a very simple stressed-skin fuselage. The two are complete units in themselves, attached to one another by four bolts. It should be explained that the diagrams in this article give a slightly false impression in that intermediate fuselage formers are omitted and only the main formers shown. The four longerons run through to the tail, but are tapered off towards their rear ends. At the front they carry the engine bulkhead, to which the fork ends of the very simple engine bearers are attached at four points. These are of box section and are built up of sheet metal channels and plates riveted together. The neatness of these engine bearers must be paid for in the form of some additional weight, since there must be a fairly heavy stress concentration at the point where the three limbs of the "Y" branch out.

It is interesting to compare the Mustang wing with that of the Spitfire. In the latter, the spar is placed fairly far forward in the wing section, but in the Mustang it is located at a point roughly corresponding with the center of pressure, so that when the center of pressure moves aft at high speed, the torsional stresses imposed are not high.

Built as a complete unit composed of the two wing portions, the Mustang wing is bolted together on the centerline. It is offered up to the fuselage as a unit, and attached to it by four bolts.

One result of placing the single spar fairly far aft is that the wing guns, mounted on the spar, do not project through the leading-edge as is the case when they are mounted on the front spar of a two-spar wing, or on a single spar placed far forward, as in the Spitfire. Another advantage of locating the single spar fairly far aft is that it becomes possible to house the retracted wheels in front of the spar, where the greatest possible space is available.

Compactness was obviously the aim of the designer of the Focke-Wulf Fw-190. He started with a radial engine, very closely cowled, and brought it as far into the fuselage as possible. Like the designer of the Mustang, he chose a single-spar wing construction, but placed it very far forward.

In the outer wing portions the spar is very close to the leading-edge. To continue it in that position would have precluded the possibility of housing the wheels ahead of the spar, and to overcome the problem the designer of the Fw-190 cranked the spar as shown in the sketch. This not only enabled him to attach his engine ring at five points by a triangulated structure of tubes, but brought the central portion of the spar back to the region of the center of pressure. The writer of these notes would not care for the task of stressing the wing of the Fw-190!

The Bell Airacobra is unorthodox in every way so far as structural design is concerned, yet in external appearance it does not differ greatly from other single-engined fighters. The form of construction shown was almost inevitable, once it had been decided to place the liquid-cooled engine in the fuselage, aft of the pilot.

One might express the Bell system either as a chassis-built fuselage, with the wing and engine loads taken direct to the chassis members, or perhaps better regard it as a sort of boat hull, using the expression in the nautical sense and not as relating to flying boats. The lower half of the fuselage, in the region of the engine and pilot, is the backbone of the machine and to it are attached the engine and the wing roots. The forward. part has to be torsionally strong in order to resist the torque reaction from the airscrew reduction gear placed in the nose. It must also house the front wheel of the tricycle undercarriage.

The tricycle undercarriage has its two rear wheels farther aft than the wheels of the conventional undercarriage, and in the Airacobra they are housed, when retracted, behind the central main spar. The armament is located partly in the wings, outboard of the undercarriage wheel wells, and partly in the nose of the fuselage.

This article was originally published in the April, 1944, issue of Air Tech magazine, vol 4, no 3, pp 27-29, 76.
The original article includes 2 uncaptioned photos and 5 sketches which include thumbnail photos.
Photos are credited to Air News, Harold Kulick, USAAF, British Combine.