THE AMATEUR CONSTRUCTOR,S' GUIDE

by ARTHUR W. l.

Part 5.

Aircraft Metalwork

It is a popular misconception that to work in metal requires great skill, complicated a:rd costly machine tools and a lot of time. This impression is quite erroneous and, with a little experience, the amateur will flnd it no more difficult to manufacture metal fittings and components than to make a wing rib out of wood.

Metal is a medium for construction just as is wood. It does, however, require a different technique in working. As with wood, there is a right way and a wrong way to fabricate aircraft metal parts. The wrong way may seem the simplest and quickest. This is a delusion for it is the right method which saves time, money and embarrassment in the long run.

In addition to goneral workshop tools there are a few special items required, in particular the scriber, hacksaw, centre-punch and cold chisel. The use of these tools has been detailed in an earlier article. A useful accessory which will speed work if a quantity of metal work has to be done is the electric drill. It is well worth going to the expense of equipping oneself w-th one of these for a power drill will come in useful for all branches of aircrait construction and will save much labour and time.

If a lot of sheet metal cutting has to be done, the hand guillotine will facilitate the job. A model which will cut 14 gauge sheet or *" bar costs about f5.

Steel is strong and, when properly worked to make a part or fitting, its strength may be used to the best advantage. However, there are certain factors which can adversely affect its ultimate strength. One of these is structural failure which includes fatigue, excessive wear through improper manufacture and cracking. The other is corrosion. The subject of corro'

G. ORD-HUME

sion has been considered earlier on and it may still further checked by assembly with an approved jointing compound and application of adequate flnish.

Structural failure, when the molecules of metal separate without necessarily being subjected to ultimate load, can, in the main, be avoided during the working of the metal into the fitting.

When bending metal, it is important that the inside line of the bend should have as large a radius as is practicable. In fact, there is a minimum radius to which metal may be bent safely. This is because the metal on the inside of a bend takes a much closer radius than that on the outside of the bend. The inner surface fibres are compressed whilst the outer ones are stretched. The central, internal fibres are the only ones which are subjected to pure bending with neither compression or tension. The tighter the radius of the bend, the more pronounced is this effect. The larger the radius, the less is the effect of this bending and compression.

Certain alloy steels and aluminium alloys may not be bent excessively without heat treatment (annealing) followed by normalising or age hardening. This is due to the inherent brittleness of the internal structure of the metal. The surface fibres, being neither able to stretch or compress, teat under. the load being upon them in attempted bending'

Considering again this stretching a:d stretching of opposite surfaces during bending, it is easy to see that any surface imperfections on the side being subjected to tension will leaC to the metal being torn or cracking. These cracks may be so small that they are not visible to the naked eye, but nevertheless they are there and a highly stressed part thus made wilt crack through and fail in service with possibly serious results.

The fear of this may easily be avoided. Only use aircraft quality steel sheet and do not scratch or mark the surface permanently, for a deep score will most certainly cause a crack to develop if the piece is bent' This also applies to a file mark or a nick with a hacksaw.

In ordinary sheet metal layout work, the shape of the part to be cut out together with any bend lines and so forth, is usually transferred orto the metal by scratching with a sharp scriber. Whilst this is suitable for general engineering, it can be a dangerous practice to adopt on aircraft metalwork as can be appreciated from the foregoing remarks.

There are two alternative methods by which layouts may be drawn accurately onto metal. One is to use a ball-point pen which, on certain metals in particular non-ferrous metals, is quite accurate. A pencil does not produce a: accurate, fine line and is also hard to see.

The second method, however, permits great accuracy and is in use throughout the aircraft industry. The metal is painted with a special dye preparation which will show up light scratch marks without the need for scratching the actual metal surface. This dye, manufactured under the trade name "Spectra-Blue", is applied by brush or swab to the metal after thorough cleaning and de-greasing. It dries almost at once. By lightly drawing the scriber acros the blued surface of the metal, a clear line can be made where the point of the scriber has removed the dye. By using this method, not only is the metal surface protected, but

Popular Flying, MarchlApril, 196O the layout stands out clearly and is easy to follow. The dye is readily removed afterwards by use of a solvent such as thinners.

When marking out sheet metal, it is necessary to allow extra for any bending radii. The minimum bending allowance for most metals is one and a half times the thickness, written as "llT" for short.

It is often possible to simplify the making up of a bent up fltting by "cheating". The metal is bent up f,rst and then marked out carefully (preferably on a surface-table using a scriber block) so that all that remains is to cut the part out-already bent. Whilst this is a sound scheme for the unimaginative approach to simple 90' bends in simple fittings, it is more often than not necessary to tackle the job the long way round. This entails a few simple calculations.

There are two formulae for evolving this. The first is a very accurate method :(0.01743R + 0.0078T)O where R-bend radius in inches

T-metal thickness in inches

O-angle to be bent in degrees.

The second method is a little less involved. Since a radius is a portion of the circumference of a circle expressed in degrees, the length of the circumference of a circle is 2xRx3.14 where R-radius of circle

Therefore for a bend of 90' the length of the bend will be 90

360 or2xRx3.142

4 which is, of course, one quarter of the length of the circumference. If the radius is $", the length of the radius for a 90' bend is 90

2*tx3.142x :0.196 inches

360

This formula holds good for all gauges of metal. As we have seen, however, only the centre of the piece of metal is subjectd to pure bending, so to this formula must be added

SMA.LL ADVERTISEMENTS

Members af the Pooular Flying Associarion may have the use of^tltil iiiu:ii ioi" "aiiri itins ar lii-ti i"wanted, e xchang i or dispo sal. for. o N E iNSiitblON ip to 24 words FREE. Uryq,, than one insertion it must i, ,rii-iitn ii c,lronr"-o-i tie rate of 4d. p"r word.(min' l2-words'l' b"i tlo. ls.6d. extra. Non-Members t'hargeablc at lhe ralc ol 6cl' per word (min. 12 words). Box No. ls.6d. extrc' ""Cniiiii:i iia Fiitii oiirrt shoulel be uossed and sent to P'F'A'

FOR SALE

HALF PLATE AND ENPRINTS of photographs in poputln iit,"rb. *u-nv in .to"t. Also sets of 25 ultra light aircraft' i'.i.A.'h,io6nderry House, 19, Park Lane, London, W'1'

}4-VOLT TWO CHANNEL EKCO V'H'F" RADIO' Box 102, P.F., 19, Park Lane, W.l. GYROCOPTER parts and engines. Shafer A'8 from stock' il"t- pfut"i.- Nyion balls foi release valves' Blades and tiua"-*ut"iiuls.- McCultouch 4138 AX e!8ines and spares' F"rii pi"i"t ticated to order. Scott and Howell (Aviation) at: "R6ild Gardens, London, S.W.7. KNI' 1001"

J.A.P, J. 99 engine rvith log book. .Has^run approximately io^ir^""ii. No'Prop availible. Price [25. Applv^W" S' bl"[, fir"t"rs Edge, Waterloo Road, Crowthorne, Berks'

PROCTOR FOUR Co-ownership eigh,tl-r- shares available'

V.fu.e-., clual, situated Nutts Corner. Ulster Aero Group' 39. Princess Gardens, Hollywood, Co' Down'

TIPSY BELFAIR. Genuine example of the type' Pern.rit t"1"".. tS6t. Brakes and flaps, unbruised. f,850 or near offer. Chris Riddell, Harrogate 3313.

McCullouch engines, type 100' new co-ndition' clw prop ij"ii---gr*to, "and fuei injector. R. J. webb' Vendair, Biegin Hill AirPort, Kent.

TAYLOR MONOPLANE, J'A.P. Fngine' Ful[ permit oentlins.. No reasonable offer refused' All enquiries i"i*.iEa. i. Tavlor, 46, Sunnvside Road. Ilford' Essex'

NEW MOTOR 'TUTOR FUSELAGE and J'A'P' J'99 iir,lCmf. Roberts, 28 Mumbles Road, Blackpill, Swansea'

WANTED

J.A.P. J.99. Crankshaft in good condition-'- Reasor-rable pii;". John Taylor. 45. Sunnyside Road, llford'

Pre-war 600 c.c" Douglas engine complete with -pusher pi"piiio. as fitted to B.A.C. Drone. H. Poulsom, 47, Station Road. Taunton. Somersel

EXCH,A,NGE

WOULD LIKE TO TRADE 35 m'ni. colour slides of present ai" nrit*i"tique aircraft, for similar with anyone interested' R-'B;d lamesb,.t, 1665 N. 4th Street, Columbus I' Ohio, u.s.A.

BUILD AND TLY YOUR OWI{ HIGH PERFORMANCE

SPORT PLANE: BEAT THE HIGH COST oF FLYlilG" folN THE HUNDREoS OF OTHER HOMEBUILDERS "

Simplified <letailed construction prints now available" FOUR proven. economical designs to choose fronl. (l) Single place Playboy. Iow wing, rugged and fast, designed for aerobatics' (2) Two place Playboy. side-by-side, low wing, comfortable cross-cctuntry and aerobatics. (3) Two place Flut-R-Bug" tandem. mid-wing. tri-gear. Very short take off and Ianding" high altitude fielcls. Wings easily removed for torving' (4)

Two place Sky-Coupe, side-by-side, high wing' cabin, tri-gear' All purpose general utility and personal airplane' For cornplete details with descriptive brochures. photographs, 3-views and C.A. regltlations on homebuilt airplanes, mail 1.00 tlollar (packaging and postal expenses) to:

STITS AIRCRAFT, P.O. BOX 3084 B, RIVERSIDE' CATIFORT{IA of the blade with the left hand and draw the file along the edge of the metal parallel to the edge and not across as in the normal manner' As the file marks disappear, lessen the pressure of the stroke. Draw-filine removes metal quite quickly so care must be taken to watch the layout lines to ensure that too much is not removed. Also watch that the file is held at right angles to the fitting so that a true edge is produced ; if the file rocks, the edge will become rounded.

To file a radius on a (netal fitting, or to file a curve, support the metal being filled as close to the vise jaws as possible, frequently repositioning the metal in the vise as the edge is liled. This applies to both inside and outside curves. Never trv to flle metal without properly and securely holding it in the vise' The metal will vibrate, damaging the file teeth, destroying the efficiency of each stroke and' with thin gauge material, will bend the metal.

The final step is to tear a strip of emery cloth a few inches longer than the file blade and about l+" wide. With the emery strip held alonq the file blade, polish the edge in the same manner as draw-filing. Alternatively, use a smooth file having first rubbed chalk into the teeth.

If the fitting has to be bent, now is the time to do it.

When working with aluminium, remember that the metal is very soft and that a hammer blow will cause the metal to expand or stretch at that point. To bend a flange on a light alloy sheet, clamp the flange portion between two stout pieces of wood and, using a block of wood and a mallet. tap the body of the sheet over, leaving the flange straight and true' If, however, it is desired to make up a curved section, such as a cowling stiffener' advantage may be taken of the readiness of the metal to expand by hammering with the mallet directly on to the flange. As the metal expands, the piece will bend uP.

The practical aspects of work in light alloy will be detailed in a later article on cowlings and fuel tanks.

Popular Flyins, MarchlAPrll, 196O

For POP RMTING

PARACHUTE FITTINGS and LIFE.SAVING EQUIPMENT

Aircraft Materials

LIMITED

MIDLAND ROAD, LONDON, N.W.l EUSTON 6151 (7 lines)

MODELLER 60 Pages l/6 Monthly

Famous f or occurate ond highly detailed scale drawings !

Whether it is ultra-lights, the latest jets, or nostalgic old timers, you can rest assured-that they aie covered by "Aeromodeller". Each issue features at least two l/72nd scale drawings always backed by factual information and first class photographs. Current series features "Famous Biplanes" by George Cox, covering well-known aircraft such as the Avro 504, Boeing F484, Fairey Fantome, Fiat CR.42, Albatros DIII, Heyford, Bristol F2B, S.E.Sa, S.P.A.D., etc., with many more to follow. In "Aircraft Described" several ultra-tights such as the Druine Turbi, the Turbulent and the Bebe Jodel have already appeared, in many cases back numbers and reprints of the articles are available.

Send stamped (4\d.) sel.f-addressed envelope for .free specimen copy of the magazine 'togeiher with lotest list of scale drawings to: AERONAUTICAL PRESS LTD.

38 CLARENDON ROAD, WATFORD, HERTS.