Mills Bomb (oz5941)

About this Plan

Mills Bomb. RC sport model.

Direct submission to Outerzone.

Quote: "G'day Steve, I hope this finds you well, Attached is a copy of a David Boddington plan called the Mills Bomb, it was also known as the Mills Beam in the USA I believe. The plan is pretty self explanatory although for what its worth I've made mine 3 Channel R/C using Hitec HS 55 servo's and have an R/C PAW 1.49 up front. Can I say a special thanks to my mate Fiery in Hervey Bay Qld Australia for getting this scanned for me... Cheers and all the best to you and the few that keep Outerzone running,"

Update 20/02/2021: Added article, thanks to RFJ.

Quote: "A 35 in wing span 'stick' model for two to three function radio and .06 to .09 cu.Ins. Engines, but specifically designed for the Irvine Mills 1.3 cc replica diesel engine. Designed by 'Diesel Freak Boddo'.

When Irvine Engines introduced their replica of the classic Mills .75 cc diesel engine there was great rejoicing in the hearts of 'old-time' modellers. Those enthusiasts sufficiently experienced in years to recall the original Mills engines (.75 cc, 1.3 cc and 2.4 cc versions) remember them with great affection and many of those engines remain in the hands of vintage buffs. Easy starting and handling, smooth performancel, reliability and the ability to swing a wide variety of propellers endeared the Mills engines to modellers and the lack of gloplug or starter batteries made them ideally simple to operate. That the Irvine replica actually out-performed the original Mills .75 cc is a tribute to the craftsmanship of the present manufacturers for the modest power increase was not achieved at the expense of the fine starting qualities inherent in the earlier versions.

In celebration of the return of the Mills .75 (although there had been other copies - some good, some not so good - of the engine) I designed a 'stick and sheet' model called the 'Mills-Beam' and this proved to be quite popular. Quick construction and good flying characteristics were the aims of the design and these were achieved in good measure. It is the ideal type of model to have stowed in the boot of your car ready for the moment that you can spare half hour's flying - and remember, all you need to go along with a diesel powered model is a small can of fuel, a transmitter and a piece of cleaning rag.

Big Brother: When Irvine announced that they intended to produce a replica of the equally famous Mills 1.3 cc engine it seemed only reasonable to mark the occasion with an enlarged version of the 'Mills-beam' design, hence the 'Mills-bomb'. (I will refrain from mentioning the four years before the Irvine Mills became available!) This features the same Jedelsky type sheeted wing structure, the sheet balsa tail surfaces and the ultra simple 'pod and boom' style fuselage structure.

Of course, you don't have to fit a diesel engine in the model but, be warned, if you do you may well become hooked on this form of internal combustion engine. No throttle control is fitted on the Mills 1.3 cc at present and therefore two function radio is adequate in this form. If you install an R/C engine, diesel or glo, then you can fit three servos although you will probably have to go to mini servos.

Building the 'Mills-bomb' will neither cause you to break into your piggy bank, nor will it take you away from your favourite TV programmes for too long. The model can be constructed and finished in two or three days and only requires three sheets of 1/8, four inch wide balsa, a piece of 3/16 x 3 in, a few sticks of spruce, scrap plywood and hardwood and some odd pieces of music wire. So let's get started.

WIng and Tail Surfaces: Cut the 1/8 and 3/16 wing panels to size and profile the leading edge sheets to section. Thoroughly sand the panels and round off the external edges. Fill the grain - I use an excellent water based lightweight filler produced by DeLuxe Materials. It uses a microballoons filler but is super easy to sand and gives a nice creamy finish to the balsawood. This is followed by one coat of a conventional cellulose grain filler (clear dope with talcum powder added) or an epoxy filler if epoxy finishes are to be used. Sand smooth once more. The reason for filling and sanding the panels before glueing together is that, with the ribs attached it is difficult to sand the underside between the rib stations.

Pin down the sheet ribs, note that they extend beyond the wing panels to allow pinning at the ends. Note also that the ribs are the same length throughout even though there is a slight taper on the trailing edge of the wing panel; this automatically builds in a slight wash-out effect in the wing panels and improves stability. Angle the root rib for the dihedral and check the chamfer fit of the joint between the front and rear sheets. Glue the panels to the ribs and pin in position. I tend to use a variety of adhesives, PVA for most of the wood joints, 24 hour epoxy for hardwood butt joints (five minute epoxy does not give time for penetration into the wood fibres) contact adhesive to fit the .4mm ply doubler and cyanocrylates where something is needed in a hurry.

When the panels have dried, remove from the plan, trim the ends of the ribs and block sand the root rib sheeting to the dihedral angle. Prop up the outside ribs by 1 1/4ins. and epoxy the wing panels together, followed by a cloth and resin reinforcement over the centre section top.

Tail surfaces are simple sheet structures, but do select a light even grained balsawood for these items. Slots in the fin and tailplane allow for a cruciform structure and these surfaces are joined after they have been sanded, filled and slots cut for the hinges, but before the control surfaces are added. Also, the fin/tailplane assembly is glued to the fuselage before the rudder is attached. Incidentally, I use mylar hinges for small R/C models, the type used by draughtsmen for tracings. This is quite strong enough and with matt surfaces will hold the adhesives well.

Fuselage: Construction commences by building the Warren girder spruce frame directly over the plan and to this are added the 3/8 sq beech or maple engine bearers, the 1/8 sides and formers F1 and F2. On removal from the board the .4mm ply fuselage outside doublers and underside sheeting are added, plus the servo bearers, undercarriage retaining screw and wing dowels..."

Update 24/2/2024: Added build log thanks to Nick Ward.

Supplementary file notes

Article.
Build log, by Nick Ward.

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