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By: Robert Lane
I tried to find out how to buy a dummy Radial engine for my C30 Autogyro that had a Wright Radial 7 cyl engine, Wrights were some of the really nice looking engines, but no one seemed to make them now, although Williams did at one time, Flair Products advertise a plastic version but it must be inside a cowl so is not very realistic. So I decided to make my own. First I must say that I tend to collect scrap card and plastic tubes and anything that looks remotely useful, Round toilet brush holders make good radial cowls, and many clear plastic bottles make windscreen canopies, as my Dad used to say, it will come in useful if I never use it. So here goes. Find a photo of the engine you want to copy and using a scale rule make a scale drawing. Bear in mind if you want an electric or glow engine to fit inside. A two stroke is not easy due to the large silencer to fit in, but Four strokes are very easy, but inverted only as the silencer can be pointed down and under the model using a flexible exhaust tube, OS, SC, and Super Tiger engines are best suited but Laser engines due to the carb position need the fuel tank very low down, not easy in most models, also when using a glow engine a five cyl is better, more room to fit the barrel in. Its unlikely that you will be able to make an exact copy unless you are a skilled model maker, I have seen some at exhibitions, but when fitted on the front of a Stearman or similar open engine type no one will not find it appealing. Open rocker types are best but a bit fiddly to make, when I run my Radials the open rockers seem to fasinate viewers, I supose seeing moving parts brings it to life, so here goes.
First find a round opaque plastic bottle, the thicker the better, Sainsburys sterilizing fluid containers I used, of the correct diameter to suit the scale required then fill with dry sand and screw the lid tight to make it solid, this is to make it firm for drilling all the small pilot holes.
Use a paper tape measure to find diameter then divide into the number of cyls .EG. 5, 7 or 9 .EG. if 7 cyls and diameter is say 250 mm divided by 7 = 35.7 mm. Wrap masking tape around bottle and mark the 7 positions with pencil.
Then mark a line in from the front of the bottle were the cap screws to, to the centre of the cyls will be, also were the push rods at the front and plug leads at the rear.
Centre all these points with a pointed tool heated slightly and pilot drill all points. Then empty out the sand and cut off the bottom of the bottle. Get the card tubes from cling film, baking foil, or Solartex of the correct size for cyl liners. Next make a ply disc to fit inside what is now the crankcase to stiffen it up and drill holes with a hole saw in the crankcase to the size of the cyl tubes. Now cut of the spout were cap is screwed on, this will leave a hole around the prop nut for cooling. Next make a ply disc to fit inside what is now the crankcase of engine to stiffen it.
Now drill all the push rod holes at the front to the push rod size and tiny holes at the rear for the spark plug lead. Now paint the crank case black, Humbrol paint is best as car spray paint chips off very easy.
Cut 7 lengths of cyl tube to correct size and mark the fronts with an X, next cut slots at about 1/8th inch spacings but not right through to the left and right of this X, then drill tiny holes at top and bottom of these tubes at the rear, then wind plastic strimmer cord around and fitting into the slots to simulate fins using the holes top and bottom to lock the cord until fixed with Super glue, then seal with dope. Now paint Silver and when dry paint Black then wipe of leaving black grooves and Silver fins Then push the cyl tubes into the holes in the crank case and glue.
Make cyl heads from Liteply as photo by bolting together then after drilling and separating seal with dope then paint black. The holes to bolt together will be hidden under the rocker covers when fitted.
Glue cyl heads on to barrels then make 14 push rods from thin dowel, and rocker covers from Balsa symmetrical leading edge, round of the front and rear and dope to seal. Paint push rods and rocker covers Silver. Use thin Red wire for spark plug leads, now assemble every thing together.
If using electric power fit motor to ply disc and secure disc with small self tap screws to crankcase. If using a Glow motor you will have to omit the bottom or top cyl and leave some holes for cooling at the rear. VERY IMPORTANT. The length of the case from front to rear depends on the engine used but bear in mind that the dummy engine must not have any thing fixed to it as it is only for show, and must be removable after removing the prop. Cooling outlet holes at the rear will be left to the common sense of the maker but should be as large as possible but hidden if possible. My dummy will be fixed on the front of my C30 Autogyro to 1/6th scale. Using the same method an inline or V engine can be made, but not a Rotary as the centripetal forces would destroy it if it was fixed to the prop driver it would fly apart.
Now to make a exhaust ring I used a length of pvc pipe joined by a wood dowel to the correct diameter then place the mock exhaust stubs on it laying down and mark and drill holes for the stubs to fit in, glue them in and paint black.
Happy modelling. Bob Lane