THE AMATEUR AIRCRAFT CONSTRUCTORS GUIDE

ByArthurW'. J. G. Ord-Hume - Pafi2

How Much Will it Cost ?

The series of articles, of which this is the second, is being written for the amateur constructor by an engineer of considerable experience with ultralight aircraft. we think that he has a number of very important things to say, and we think he is saying them well. Even the most experienced of builders, the most confident of constructors, will do well to remind himself of them.

The cost of building an ultra-light aircraft will depend on many things. Initially, it will be governed by the type of machine chosen for construction. Because of the fact that steel tubing is rather costly, a parasol monoplane single-seat aircraft might well cost more to build than a single-seat monoplane of low wing configuration'

The most expensive item will be the engine' Whilst an Aeronca-JAP or Bristol Cherub might be obtained for less than one hundred pounds, a new aircraft eqgine with propeller may cost over two hundred po-qnds.

It is therefof#xtremely hard to estimate the cost of the power unit. If the constructor is lucky, he may be able to obtain a suitable engine very cheaply. With regard to new aircraft engines, the ARDEM 4CO2 as fltted in the Druine Turbulent is approximately f,160. in this country for the basic automobile engine. This has to be convert-' ed before flight. The work involved includes the removal of the air-cooling ducts, flywheel and so forth. The cylinder heads have to be drilled to take the extra sparking plug (two per cylinder) and a duplex magneto fitted in place of the existing coil ignition. Conversion is completed by the attachment of the propeller hub to the brankshaft'

The Aeronca-JAP en$ine, of which the Popular Flying Association holds the remaining stock, may be obtained for something likef'100 new, but this engine is not suitable for the Druine Turbi; investigation is in hand to assess this engine for the Turbulent.

The Walter Mikron engine, of which there are a few available, develops 62 h.p. and is suitable for the Turbi. The cost of these is up to f'200'

The new Coventry Victor Flying Neptune engine, due to fly very shortly in the Popular Flying Association's Turbi, at present costs well over f200.

If the constructor is prepared to allow f'200 for the power unit, this should be sufficient' As stated, he may well be lucky enough to obtain a suitable unit for less.

The airframe cost, whilst still not easy to estimate precisely, presents a more positive case'

Although materials vary in cost from place to place, spruce itself is little dearer to buy than ordinary commercial timber. The constructor should, where possible, request that the timber be cut and planed to size for him' This way, all unsuitable material will be discarded before he gets it. Furthermore, it is a great saving in time to obtain timber prepared in this manner'

Plywood costs between one shilling and sixpence and two shillings per square foot' Sheet steel is usually sold by weight and the price is fairly constant. These factors enable the costing of a particular airlrame to be carried out with reasonable accuracy.

If the constructor has to employ the services of somebody else to manufacture parts or to execute welding, the total cost will, naturally, be more'

As a rough basis upon which to work, the costs relating to a typical single-seat all wood low wing monoPlane are as follows:-

Spruce for fuselage longerons, bracing, etc' Planed to size, f,8; Plywood for fuselage, f'10; t:'-

Spruce for wing and tailplane spars, ribs, etc.

Planed to size, f,10; Plywood for wing leading edge, etc., f.7; Mild steel sheet for flttings, f,3; Turnbuckles, nuts, bolts and other A.G.S., f,7; Control cable, f,l; Steel tubing for undercarriage, etc., f,10; Synthetic resin glue, f,l; Brass brads, screws-and misc. hardware, €1; Fabric, serrated tape, thread, etu; f,S;, Dope (approximately six gallons), [9; Whee]q, f,8; Metal for cowlings, fuel tank, etc., [3; Instruments, f,11; Inspection fees, regi straticiti J eert ifi cat iop, f,3..

These flgures arb intended purely as a rough estimate and cannot be used as a precise guide to expenditure.

The cost of operating the completed atcraft will once again depend on the type of machine, the engine fltted and the overheads to be covered.

Aircraft fuel costs only a few pence more per gallon than ordinary motor car petrol. The largest engine which the amateur may use will only consume approximately four gallons of petrol per hour plus perhaps half a pint of oil. Supposing that our aeroplane cruises at eighty-five miles eorrect way to use these tools and materials to produce an airworthy aeroplane. per hour, that gives us a fuel consumption of The following list details ferrous metals and something better than twenty air miles to the their uses:- sallon. r-e-4 l\l/,, Third party insurance, which is essential before 4 \Dn. aircraft is allowed to fly, costs approximately I 0 f10 per year.

The principle raw materials to be used are wood and metal.

To begin with, we will deal with the metals. These will be both ferrous and non-ferrous in sheet, tube and wire. A ferrous metal contains iron in one form or another. A non-ferrous metal is one which contains no iron. This embraces light alloys such as aluminium and dural, and heavier metals including brass, copper and tin.

Ordinary iron is in the main unsuitable for aircraft use owing to its coarse structure and uneven strength. From iron, however, by addition of certain other minerals, steel is produced. Steel is a high quality, fine textured metal of high strength. By adding again to steel certain precise quantities of carbon, grades of steel can be produced varying in stiffness and brittleness. The higher the carbon content, the harder the steel but, without heat treatment, the more brittle it becomes.

The Popular Flying Association can insure the aircraft against all risks for an annual premium of 9\ of the declared value, should this be required.' The annual premium, therefore, on an aircraft valued at f,500 would be L45. This special comprehensive policy includes third party coverage. Attention should be drawn to the fact that it is only necessary for an aircraft to carry third party insurance.

It willbe seen that the amateur-built aeroplane costs little more than a second-hand car. Its running costs are also comparable to those of a small car.

Materials for Aircraft Construction

Before commencing construction of an aircraft, we must know something about the materials with which we will be working, the tools re' quired to work with on these materials and the cASr rRoN. Brittle and weak. Fairly soft and is readily cast and resists compressive loads. Used for machinery, surface plates and, when processed by 'chilling', for piston rings. Unaffected structurally to any extent by heat. Due to its graphite content, it is slightly self-lubricant. wRoucHT rRoN. Soft, malleable and ductile. Is strong and may be worked by all methods except casting. Is readily welded and magnetised. Used for cores of dynamos, chains, etc. Cannot be hardened by heat treatment. srEEL (row c,LnnoN) MILD. Ductile, less malleable, stronger and harder than wrought iron. May be worked by all methods including casting. Is easily forged, welded, stamped or machined. Used for bolts, rivets, tubes, fittings and all uses where very great strength and hardness is not required. Mild steel cannot be hardened by quenching, but may be case-hardened. srEEL (rvrno. ce.nnoN). Stronger and harder but less ductile and malleable than mild steel. May be worked by all methods but is not so easy to work as mild steel. Used for bearing shafts, high tensile bolts, tubes and stressed parts where great strength is required. srEEL (urcH c.nnnoN). Strong and less ductile. Its strength and hardness depend on the heat treatment employed. It may be forged or cut and rolled from billets. May be made very hard without undue brittleness and is used for cutting tools of all descriptions such as drills, chisels and so forth.

. Steel, when heated, undergoes changes in its internal structure. These changes affect its properties of strength and hardness. If steel is worked continually, it will oage' and become brittle. This will also occur if steel is exposed to prolonged heating. It can therefore be appreciated how important is heat treatment which enables a piece of metal to develop a certain characteristic or state thus ensuring that it gives the best possible service.

The following processes apply to ordinary carbon steels and not to alloy steels.

NoRMALISINc. When a piece of metal is bent or forged, internal stresses are set up which may have a deleterious effect on the ultimate strength of the piece. By normalising, these concentrations of strength are smoothed out thus producing an even internal structure. To normalise steel, it is heated to a cherry red and then allowed to cool in air.

ANNEALTNG. If it is desired to form a piece of steel by pressing, hammering or bending, it is often desirable to render the metal to a soft state when cold. This not only makes it easier to form, but it reduces the likelihood of cracks developing in the metal. To anneal steel, it is heated in a fire to cherry red and then allowed to cool very slowly with the flre or in hot ashes.

HARDENING. To produce maximum hardness in medium or high carbon steel, it is heated to cherry red and then quenched very rapidly in either water or oil.

TEMrERINc. When a tool or a piece of metal is hardened as above, it will become brittle. In the case of a cutting tool, it is desirable that this brittleness should be removed without affecting the hardness to any great extent. This is achieved by tempering which is a 'low heat' treatment. The temperatures involved are insufficient to cause the piece to glow. When a piece of bright steel is heated slowly, it will be seen to change colour. When the source of heat is removed and the steel cooled, the colours will remain, ranging from deep purple near the point of application of the heat, to a pale straw or yellow at the outer edge of the coloured area. These are the temper colours and each one corresponds to a precise temperature. By reproducing a certain colour on the cutting edge of a tool, we can achieve a known degree of hardness. It should be remembered, though, that the edge or section to be tempered must be cleaned to a bright surface, otherwise the temper colours will not show.

As a guide, the following colours and their temperatures are recommended for the tempering of tools:-

Pale yellow-Scribers and scrapers, 220" C.; Straw yellow-Punches, taps and dies, 240' C.; Brown yellow-Drills, saws, 260" C.; Dark purple-Cold chisels and screwdrivers, 290' C.; Blue-Springs, 320" C.

Since the above temper heats are achieved only momentarily as the temperature slowly rises, the right colour must be watched for closely. When it appears, quench the part fully and quickly in either water or oil.

Concerning heat which is visible by the glowing of steel, the following table lists colours and their temperatures'-

Faint red, visible in darkness, 490"-510' C.; Dull red, 700" C.; Brilliant red or blood red, 800' C.; Cherry red, 900'-1,000" C.; Orange, 1,100' C.; White, 1,300o C.; Bright white, 1,400' C.;Dazzling white or welding heat, 1,550' C. CASE HARDENING. The purpose of case-hardening is to combine the hardness of high carbon steel with the toughness of low carbon steel. This may be achieved by the introduction of extra carbon into the surface of the low carbon steel. In practice, the steel is heated for a period of time in a furnace with other materials rich in carbon. The duration of the process depends on the thickness of the 'case' desired. Usually the case is only about .003 inch thick.

Alloy steels are produced for a variety of special purposes by the addition of certain other minerals. Due to the critical nature of their properties, it is inadvisable for the unskilled to attempt heat treatment which is usually carried out under the finely controlled conditions of an electric furnace. Among these alloys are tungsten, chromium, cobalt, nickel, vanadium and molybdenum.

It is not generally appreciated that pure iron is soluble in water. The action of dissolving produces rust. When iron is combined with carbon to make steel, it not only rusts like pure iron, but the rate of corrosion is more rapid due to the galvanic action between the iron itself and the carbides.of iron (the combination of iron and carbon). If steel is alloyed with a percentage of chromium, stainless steel is produced which will resist corrosion. However, the mild steel used in aircraft construction will rust and steps must be taken to prevent this.

The ideal method of protection is deposit via electrolysis a coat of another metal which does not rust on to the mild steel. Such metals include tin, copper, nickel, chromium and cadmium. It is usual for cadmium to be used for protective plating in the aircraft industry. The process may be carried out at moderate charge by any firm which specialises in this type of work.

A further method of corrosion protection, although not so effective, is painting. For this, a suitable primer for metal is used. When painting an assembly of welded tube (such as a control column, undercarriage leg or similar part) it is advisable to paint the inside of the tube as well. Remember that the inside is just as likely to rust as the outside. The best way of painting the inside of the tube is to fill the tube completely with paint taking care to avoid trapping any air, and then let the tube drain thoroughly.

The following list details non-ferrous metals and their uses'-

ALUMTNTUM AND DURALUMTN. Soft, weight/ strength ratio good (weighs only $ of steel). Resists corrosion when in pure state. Used for rivets, sheets and castings. May be cast, forged or rolled. To increase its strength, it may be al- loyed with copper, tin, nickel, magnesium or zinc. Duralumin is less ductile than aluminium and not as soft. It corrodes more readily. Properties improved by heat treatment. This is carried out by the use of a bath of molten salts and should not be attempted by the amateur. coppER. Soft, easily worked, especially if annealed flrst. Used mainly for manufacture of tubing for fuel and oil lines, electrical wires and parts. Rather heavy. Will become hard and brittle with age. rrN. A very soft, ductile metal. Will not corrode. Readily formed and solders well. Often plated on to steel. Used for fuel tanks.

TUNGUM. An alloy of brass and copper. Used in preference to copper for fuel lines as it does not age appreciably in service and is less likely to fatigue with vibration.

BRoNZE. An alloy of copper and tin. IJses confined mainly to bearings for wheels and in engines.

Aluminium alloy should be protected from corrosion. Preferably, the metal should be anodised which consists of immersing the parts in a solution of chromic acid or sulphuric acid and potassium di-chromate and passing a current of electricity through them. After treatment, the parts should be washed clean and painted with primer. Anodising flttings or parts is carried out by the same firms who specialise in cadmiumplating ferrous metal parts.

In aircraft construction there are three other applications of metal.

First of all there are A.G.S. parts. Aircraft General Stores consist of nuts, bolts, washers, turnbuckles and so forth. These parts are readily obtainable from approved stockists.

Secondly there is control cable. This is made from strands of high-tensile steel wire twisted to form a flexible cable. The standard aircraft cable used is made of seven strands, each strand consisting of many more fine wires twisted together. The centre strand, or heart strand, is straight and the other six are preformed around this. These strands are preformed to prevent the cable from kinking easily and allows it to be very flexible. Cables are measured by the safe limit which they will take measured in hundred-weight. The cable used in gliders and light aircraft is mostly l0 cwt.

The final application is hard steel wire, better known as piano wire. This is often used for bracing inside wings and is measured in Standard Wire Gauge. Wire, of a larger diameter and usually of streamlined section, is also often used for external bracing, as on the Tiger Moth. These wires are of high tensile steel rod and are mea_ sured by the diameter of the ends (which are threaded) and the safe load in pounds, e.g. $ in. B.S.F., 35,000Ib.

The methods and processes involved in working metal will be dealt with in later articles.