Comper CLA7 Swift

In 1991 Aviation Metalcraft, well known in the UK and Europe as manufacturers of metalwork kits for plans-built aircraft, were commissioned to produce a kit of metalwork for the second Comper Swift replica to be built in the UK since the demise of Comper Aircraft Ltd. in August 1934. Since then we have received a further order from a builder in the UK.

Apart from Swift aficionados, few are aware that copies of the original factory drawings are still available, from which it is possible to build an exact replica. The term replica conjures up an image of something akin to a reproduction, imitation or model in its widest sense, it is perhaps the wrong word to describe an exact full size aircraft made from the original drawings - without modification and using the original British specification steels throughout. Late Production Model is perhaps a more accurate description. There has been one replica Swift built since 1934, that built by John Greenland. His Swift is a masterpiece of craftsmanship and has been documented in the magazine Popular Flying and by Richard Riding of the magazine Aeroplane Monthly.

The majority of original Comper Aircraft factory drawings still exist and are available to the builder. The set comprises some 250 drawings which represents 95% plus of the drawings. Fortunately it has been possible to gain access to one of the few remaining original Swifts G-ACTF 'Scarlet Angel', now owned by the Shuttleworth Collection to develop some of the missing drawings. It is hoped that further drawings can be derived when VH-UVC owned by Roy Fox is restored. This should then complete the set.

The start of any project is to become acquainted with the drawings and to establish manufacturing processes and procedures. The drawings are exceptionally well drawn and accurate.

Every component both wood and metal is well detailed. Some drawings are of course difficult to read having suffered the ravages of time, but the majority are easily interpreted. Dimensions are shown in fractions of an inch. The metalwork drawings in general have the bending allowance included in the developed profile - something which is rarely found on drawings of homebuilt designs, it usually being necessary to derive the correct bending allowances.

Why then when drawings exist, and suitable engines can still be found that so few replicas of this outstanding racing/record breaking aircraft not been built? The answer is fairly simple. Until Aviation Metalcraft produced a metalwork kit for the aircraft, it was necessary to build everything oneself. There is of course nothing unusual in this, until one realises that there are approximately 800 metal components to be made. The design of a good number of the parts is such that they are beyond the capabilities of all but a few highly skilled/well equipped builders.

The Comper Swift is different from other light aircraft of the era in several respects. The following highlights the constructional areas of the aircraft which may be of interest to those considering building an exact Late Production Model Comper Swift.

It is not apparent when viewed from the outside, that the fuselage is in fact made in three sections. The front of the fuselage, ahead of the leading edge of the wing provides a mount for the engine. One reason for the range of engines which were fitted was undoubtedly due to the ease with which the front section could be changed. Of the 41 Swifts built between 1930 and 1934, 7 different engines were used.

ABC Scorpion 40 hp G-AARX
Salmson AD9 54 hp
Pobjoy P 50 hp
Pobjoy R 75 hp
Pobjoy Niagara 90 hp
DH Gipsy III 120 hp
DH Gipsy Major Special 146 hp

At a time when engine development was proceeding at a pace, it was a simple matter to take advantage of the latest technology. The centre section of the fuselage - from the front of the wing to the rear of the pilots seat accommodated the wing support structure, fuel tank/s and cockpit.
A minor point of interest here, is that the seat is supported on a cross tube which is carried at each end by bearing plates. It is possible to rotate the seat to adjust the angle for maximum pilot comfort. The seat finally being secured at the desired angle by means of brace wires tensioned by turnbuckles. The seat itself is formed from several laminations of thin plywood curved to form an extremely elegant wrap-around 'bucket' style seat.

The tail section of the fuselage, incorporates the turtle deck and a useful luggage locker. Additional luggage space was available in the front of the fuselage where an access door on the left hand side revealed a small compartment. When originally sold the Swift was supplied with a fitted suitcase! In addition, much emphasis seems to have been placed on the ability to carry a set of golf clubs in the turtle deck locker. Whether golf course owners would look kindly on having a Swift alight on the fairway today is somewhat debatable!

The woodwork in the Swift is conventional and would not present difficulties to the homebuilder. Spars are solid spruce routed for lightness, the ribs are of the standard truss type, reinforced with semicircular ply gussets. The fuselage members are square section and unusually for an aircraft of this size routed with a semicircular groove on each face, saving a few ounces. A case of over zealous draughtsmanship perhaps, but the result is very smart.

What is it about the metalwork which makes it unusual - apart from the quantity of parts required to build such a seemingly simple and small aircraft? One could justifiably state that the Comper Swift represented the 'state of the art' in terms of design in the 1930's, with no compromise being made when designing parts for minimum weight. There is always a trade off when designing for minimum weight, whether for instance to use an aluminium alloy - probably of heavier section, or to use very thin steel sheet, flanged to increase bending stiffness and drilled with numerous lightening holes. Nicholas Comper chose the latter technique. The design of almost every piece has several lightening holes - some as small as a ¼" diameter. Comper Aircraft Ltd. was a small company and it is known from existing photographic records that the woodwork and assembly was carried out by the company. What is not certain is whether they also produced the myriad metal components. It is possible that given the quantity involved they may well have sub-contracted the work to engineering companies on the same site - one other being the neighbouring company Pobjoy Airmotors. The cost of producing the metal fittings may have been less of a factor when there existed an abundance of skilled labour available in the vicinity. The situation today has of course changed, skilled labour is exceedingly expensive and the only economical way for Aviation Metalcraft to provide this kit has been to make extensive use of CNC technology. Apart from acting as an 'enabling' technology, the results are an order of magnitude improvement in accuracy over the original techniques available in the 1930's.
Not only have modern CNC machine tools been used but we have used the Tig welding process throughout which was not available in the 1930's, when the only suitable means available would have been the oxyacetylene process. The result is a much neater weld with less distortion of the fabrication.

A brief look at the drawings reveals another interesting fact. Because of the very short length of most flanges - typically 5/16" or less and the small width of many of the parts a conventional folder/press brake could not be used to form the flanges. There is only one way of producing parts of this nature, that is with a dedicated press or form tool for each component. Some parts requiring a number of tools to achieve the desired result.

Extensive use was made of the Guerin process. This reduced the toolmaking workload somewhat as it was only necessary to make the male part of the tool. To maintain the accuracy of the part, the form tooling was also produced on CNC machines guaranteeing that tool and profiled blank were of equal accuracy.

Even though initially only one kit of parts was being made, it was decided at the outset that all form tooling would have to be made from steel if the necessary accuracy was to be achieved. This would enable interchangeable parts to be supplied at a later date, should an aircraft suffer a mishap or simply require spare 'service' parts. This feature of interchangeability is common to all metalwork kits manufactured by Aviation Metalcraft.

The fact that the fuselage was made in three pieces meant that for every joint in the fuselage there are least 8 joint plates per longeron - representing a minimum of 32 plates plus the brackets which stiffen the structure in the transverse direction. This accounts for a large number of the parts in the fuselage.

The legendary racing achievements of the Swift have been extensively documented, performance figures are astonishing and approached those of contemporary fighter aircraft. The Swift rapidly established a reputation as a fast aircraft establishing many world records. What actually created this situation is not entirely obvious. If one takes a look at the Swift from the front, the frontal area is reduced to a minimum by the waisting of the wing support structure which at the same time enhances the pilots view forward. The undercarriage is also unusual. All the shock absorbing components which are generally the high drag items have been placed inside the fuselage. Considering the wheels, the frontal area was kept to a minimum by using the narrow profile Palmer wheels and tyres.
Later models used smaller diameter conventional aircraft wheels and tyres, some with elegant hand formed aluminium spats.

The lift struts bracing the wing are made from round section tubing which is faired with balsa, wrapped in fabric and doped to give a low drag streamline section. For those building an exact Swift replica this neatly overcomes the problem of sourcing some long since obsolete streamline section tube. Frequently one is faced with using a near equivalent section which usually works out heavier than the original.

Considering the tail end, The fin and rudder are built of tubular steel with pressed steel ribs. The tailplane and elevators are conventional and are made of spruce. The tailplane is braced on the underside with small diameter round tubes, the angle of incidence being ground adjustable by means of a vernier plate device.

Frequent mention is made of the Swift's wing folding mechanism. When folded the aircraft is less than 8' wide. When originally advertised it was emphasised that with the tailskid attached to a car tow hook, it was possible to move the aircraft about on its own wheels.

The wings hinge backwards about the rear spar joint. The wing is locked in place by means of a 5/16" mild steel pin passing through the two parts of the front spar joint assembly. Because the lift struts are attached to a single point on the fuselage, when hinged backwards, the wing is self supporting requiring no jury struts. Special clips engaging the lift struts are attached to each side of the fuselage retaining the wing in the locked position. The wing locking pins are retained in place by elegant chromium plated sprung plunger assemblies - consisting of 14 metal components - a leather strap as used on the DH60 would have been even lighter and simpler! The rear spar hinge incorporates an articulating mechanism of considerable complexity which enables the wing to be folded without disconnecting the aileron control circuit.

The control system uses a combination of wires and push/pull tubes. Actuation of each control surface is by means of push/pull rods attached to a horn on the control surface. Internally, the bellcrank operating the push/pull rod is in turn actuated by pairs of cables running to the control column in the cockpit. The advantage of this arrangement is that the only parts projecting into the slip stream are the small single sided control horns and the push/pull tubes. This has added considerably to the complexity - particularly in the wing which incorporates two sets of aileron actuation bellcrank mechanisms per wing. These items are interesting, for although complex, they are extremely light weight, the larger outer units weighing just 15 oz. each.

The aileron actuation bellcrank mechanisms incorporate an unusual feature which is used throughout the control system. Wherever it was necessary to establish the alignment of two lever/bellcrank type components with a bearing/spacer tube a neat design feature was used. To put it crudely, it is a simple 2 lug 'spline'. The beauty of the design is that it is self jigging for welding, a high degree of accuracy is thus guaranteed.

There is a down side to this arrangement though, and that is the difficulty of producing the parts. The tubular component must have an accurate 'castellated' feature machined on one or both ends - in the correct angular relationship and the lever or bellcrank must include the 'keyhole' into which the tube fits. The latter is most easily achieved with a special punch and die - the method probably used on production Swift's. The alternative today is to produce the hole by hand using files or perhaps to use wire EDM machining. A secondary benefit of locking the parts together in this manner is that not only can a plate be attached to a tube, but two coaxial tubes can be locked together maintaining the angular relationship between a lever/bellcrank at each end of the spacer tube. The outer aileron actuation mechanism is an example of this, three tubes and three sheet metal parts are locked together in perfect angular relationship. In the unlikely event of the fastening coming loose there is a fair chance that the assembly will hold together.

Finally there is a rather unusual device buried within the structure of the Swift which perhaps deserves some explanation - the funk wire assembly. The funk wire is essentially a cable which runs from the top of one lift strut down through the strut, then wraps around the lift strut hinge on the fuselage, passes through the fuselage and similarly up through the lift strut of the other wing. The device is tensioned by an over-centre toggle mechanism which is safety wired in place once the correct tension has been established. What on earth does it do? What does funk mean?

'Funk' ~ n., A state of nervousness, fear or depression.

The funk wire is in effect a belt and braces safety mechanism which would hopefully give the pilot sufficient time to effect a safe forced landing in the event of one of the wing attachment fittings failing. Hence the term funk wire!

Now that metalwork kits are available for the Comper Swift from Aviation Metalcraft it is hoped that more builders/collectors of vintage aircraft will rise to the challenge of building their own pocket sized racing machine of breathtaking performance and excellent pedigree.