Construction Guide

Part II

In the second article we shall be concentrating on the installation and alignment of the engine and firewall.

The engine installation at first may appear simple but is very difficult to get the accuracy that is needed.  A constant gap of 1.5mm between the back plate of the spinner and the nose is necessary for a neat finish.  The fuselage has the correct thrust angles built in so it is vitally important to get the back plate square with the nose.  It takes is a little thought and preparation for the perfect gap to be achieved, not only will it look good but the model will track straight.  Preparation is all important here, the items we will be using are the engine, a YS140 DZ, the Hyde engine mount, guaranteed for three thousand flights, so is literally fit and forget!  The firewall, this comes ready shaped from Probuild and is constructed using end grain balsa laminated with carbon fibre.  No quarter ply is used here as it is far too heavy.

Although the formers came ready from Probuild it is fairly simple to construct your own, from lightweight balsa, 95g carbon cloth (available from Fibretech or Cherbourg) and epoxy laminating resin.  For a firewall installation the balsa should be 6mm end grain.  Once the balsa has been prepared with a light coat of dope to save weight stopping the resin soaking into the balsa the carbon cloth is cut to the correct size.  The gluing process we use differs from Probuild as they use a vac bagging system which can press thousands of pounds on each surface ensuring the minimal amount of glue can be used so a very good bond is achieved.  Our personal method is to sandwich the balsa and carbon between two sheets of glass that have been primed with release agent.  Weight can then be added to compress it all together.  A couple of car batteries usually do the trick.  Left over night the resin will cure leaving you with a very nice looking, light weight firewall.  This method of construction can be used in any model large or small and can save a lot of weight over the conventional marine ply.

Before we move onto the installation a few notes about the engine, a YS 140DZ (Dingo).  Three years in development and test, YS introduced the Dingo just before the 2001 World Championships in Ireland.  It departs from the usual high pressure fuel system on many YS engines using a crankcase pressure driven fuel pump and direct injection which gives a user friendly engine that will run on up to 30% nitro.  The engine will turn a 15.5x12 wide APC propeller in the mid to high 8000 rpm range giving ample power for the Synergy.  With the introduction of the YS160DZ promising smoother running and easier setup one would think YS have the market cornered, or will Hacker electric motors steal YS’s crown as the pattern market leader.

The first step in the process is to bolt the engine to the Hyde mount.  The mount needs drilling and blind nuts adding.  The blind nuts are home made from titanium or Dural, again to save weight and each one is glued onto the bottom of each beam of the mount using 24 hour epoxy.  The bottom of the fuselage is then opened up to allow room for the engine, this is easily done with a dremel, and a 6-8mm ledge is left all around to retain fuselage strength.

The firewall is sanded so it fits approximately in the right place, care has to be taken here to make sure the firewall is not too tight as it will form bulges on the outside of the fuselage which will always be visible even after painting.  A small gap in places is not a problem as it will allow the resin between the fuselage and the firewall to form a stronger bond.  It is also a good idea to lightly sand the inside of the fuselage to help the adhesion of the resin.  The firewall is then pushed into place and the engine mounted to the mount inside the fuselage.  The spinner and propeller are added to help with the lining up process.  A ply spacer is a good idea here, made the same size as the spinner back plate and around 1 – 1.5 mm in thickness.  Two people are needed for the next process which involves tacking the firewall into place.  The fuselage is held vertical with the nose in the air, the engine and spinner are lined up with the nose with the spacer in place, the firewall is then manipulated up to the mount, ensuring it is flat to the back of the mount, a quick check the engine is the right place and the firewall is not causing the fuselage to bulge, if all is ok the firewall is tacked into place using a few drops of cyno.  The firewall is now tacked into place giving the correct amount of side thrust and a nice even gap between the back plate and the nose.

The engine mount now needs mounting to the firewall.  This is done using four 4BA bolts.  The hard part is drilling the firewall now it is inside the model, I have used numerous methods to do this but the following one has proved very successful.  Double sided stick tape is the answer, a few strips of tape are added to the back of the mount.  With the model still in the vertical position the spinner and nose are once again lined up, the spacer should not be necessary here but can be useful to prevent sticking the mount in the wrong place.  The angle of the engine as well as the alignment with the nose is important here, the engine needs to be set about 20-25° clockwise viewed from the front, this is to stop the exhaust manifold from protruding outside the cowl.  Once the angle and alignment are correct the mount is lowered on to the firewall, don’t press yet, carefully check the alignment and angle, if all is satisfactory then press the engine and firewall together.  If readjustment is needed you should be able to carefully lift the mount from the firewall and realign.   The mount should be temporarily stuck to the firewall now; the engine is then carefully removed.  Once the engine has been removed this will give you access to drill the four holes in the firewall through the mount.  A long drill is needed here, roughly about 12” long.  Our home made one works great, some piano wire is simply brazed on to the end of a normal 4BA clearance drill.  Care needs to be taken to make sure the firewall doesn’t move, so it helps to hold all parts in position.  When the holes have been drilled the mount can simply be pulled from the firewall and the tape removed.  The engine mount can then be bolted in to check alignment and angles.  If all is spot on then the hard part is complete.

When the mount is tightened up to the firewall the end grain carbon laminate can be crushed by the nuts, it is therefore necessary to use large washers or a ply ring that spread the load.  My personal preference is a tapped aluminium ring that spreads the load evenly.

When all checks for alignment are complete the firewall can be permanently stuck into place, this is done using the epoxy laminating resin mixed with cotton fibres.  The cotton fibres not only lighten the mixture but add strength as the tiny fibres tangle together forming a very strong and lightweight bond.  This is then added to both sides of the firewall and a small 3mm fillet is formed all the way around.  With some relief the firewall installation is now finished.

The last part of the engine installation is to secure the nose ring into place.  The nose ring is used to hold the engine on the front housing just behind the drive washer.  The nose ring is a large rubber ‘O’ ring in an aluminium housing, this allows the engine to rock around its centre point but not shake violently.  The nose ring is first fixed to the nose ring former using two 4BA bolts, two blind nuts need to be set into the light ply former.  We manufacture our own blind nuts as the bought ones with spikes are prone to splitting the wood and weakening it.  We produce them from aluminium, machined into a top hat shape and epoxied into place.  Once the nose ring is fixed to the former it can be fitted into the fuselage.  This is again best done with the fuselage in the vertical position as the weight of the engine can alter the thrust line.  With the nose ring situated in the fuselage and the engine installed, the alignment of the spinner back plate and nose is again checked and minute changes can be made using the nose ring.  When all has been checked the former can be tacked into place using cyno or spots of five minute epoxy.  The engine is then removed and the former secured in the same way as the firewall with laminating epoxy and cotton fibres.  When all is dry the engine installation is complete.

Click here for Part III

The Original Series of Articles was published in RC Model Flyer

It is published here with the kind permission of Ken Sheppard. Editor - Model Flyer