Before commencing the installation of your engine, ensure both the engine compartment and tank compartment are treated with a good fuel proofer. Failure to do this will result in rapid deterioration of the balsa wood and plywood within these compartments. ou can investigate and decide on what type of fuel proofer to use either from the internet, local model shops, club members or other experienced modellers.
Mounting your engine upright is always the best choice if possible. Most commercially designed and kitted trainers are designed for upright installation of the power plant. The kit instructions will normally give step by step directions for this process. ARFs and most kits include an engine mount for you to use.
Mounting the engine upright provides easy access to the fuel adjustment needles and makes it very easy to attach the glow plug ignitor for starting. Upright mounted engines tend to start easier due to the position of the carburetor in relation to the fuel tank location inside the model.
I suggest you attach the engine mount to the firewall first. When this is done sit the engine on the mount and position it so that the back plate of the spinner is in the correct location as indicated in the instructions.
Mark the position of the holes in the engine mounting lugs on the face of the mount with a sharp object (Nylon types) such as a nail or a felt tip ink marker (Aluminium types) . It also helps to scribe the mount on either side of the engine mounting lugs.
The engine should not be forced between the two mounting bars. If this is necessary then the mount is to small for you motor and should be replaced. The only parts of the engine that should be touching the mount are the mounting lugs through which the engine is actually bolted to the mount.
Check that the engine sits nice and flat on the mounting arms. If the arms are uneven they will distort when you tighten the bolts. This could throw off the thrust angle or, even worse, crack the mount.
Having carefully marked where the holes need to be on the mount, remove the mount from the firewall. Do not attempt to drill the holes for the bolts with the mount installed! If you dont have a drill press you can put the mount in a vice and use a standard hand drill. You must be very careful to drill the hole straight down. Drilling the holes at an angle will put stress on the engine mount and mounting bolts.
When you have drilled all four holes, bolt the engine mount to the firewall, and bolt the engine to the mount. Some kits supply self tapping screws to fix the engine to nylon type mounts. I would recommend discarding these screws and using High Tensile Steel bolts and nuts with spring washers behind the nuts or flat washers and “Nyloc” type nuts.
Fuel Tank Installation
Most kits and ARFs will include a fuel tank or will recommend what size the fuel tank should be. You can go slightly larger than the recommended size but dont over do it! A slightly larger fuel tank will enable longer flights before running out of fuel. Dont be tempted to install a much larger tank even if it will fit, you may cause to much of a rearward change in the C of G or balance point as the tank empties. This could cause your plane to become more unstable towards the end of the flight.
A typical fuel tank has three pieces of tubing that fit through a rubber stopper in the front opening. A screw fits through the centre of the stopper and screws into a back plate inside the tank behind the stopper. Tightening up the screw into the back-plate squeezes the stopper to form a seal around the tubing and opening of the tank. One piece of tubing is a vent line, one is a filler or fuel return and the other is the fuel feed line.
The vent pipe is bent upwards towards the top of the tank. A length of fuel tubing is connected from this pipe to the nipple on the exhaust silencer to pressurize the tank helping the engine to pull the fuel from the tank. It is turned upward inside the tank to minimize unwanted air bubbles in the tank.
The other tube, the fuel feed, extends in to the tank a short distance and stops. A length of fuel tube is connected to this pipe. A fuel clunk is attached to the other end of the tubing the weight of which keeps the feed submerged in the fuel no matter how the model is orientated. The tubing length should be such that the clunk is as close to the bottom of the tank as possible and swings freely back and forth without fouling the tank bottom when it is held vertically.
A further length of fuel tubing is connected from the pipe exiting the bung to the carburetor intake nipple. This pipe is usually positioned at the bottom of the bung when viewed from the front. The other pipe is usually bent upward inside the tank and is fitted with a short length of fuel tubing externally for the purpose of filling the tank. This tube must be sealed off with a small stopper to prevent air being drawn into the tank during flight.
When all the fuel lines have been installed you will need to check for leaks. With the filler tube sealed by its stopper, you can pinch one line and blow into the other line while the tank is submerged in water and check for bubbles. This is an important check as any tiny leak will prevent the engine from running correctly.
Wrap the tank in foam before installing it into the airplane to help prevent air bubbles forming in the fuel. You can use rubber bands or tape to hold the foam in place during the fuel tank installation. The tank compartment of the airplane should also be filled with foam.
Connect the fuel lines to the tank. Hold the airplane nose down and with the help of gravity you can thread the three tubes through the hole in the firewall. Gently pull the tubing through from the other end while pushing the fuel tank forward until the tank is in the correct position. Add more foam until the tank is supported firmly in all directions. Glue a piece of hard balsa across the aperture behind the tank.
Finally, connect the fuel feed line to the carburetor nipple and the vent tube to the silencer nipple. Your plumbing is now complete.
Engine Throttle Linkage
The picture above shows a typical connection to the throttle control arm from the throttle servo using a nylon ball joint. What method you use will depend on the layout of the control run within the fuselage from servo to engine. The kit manufacturer will usually have supplied appropriate parts to suit their model. It is important that you ensure there is no resistance to freedom of movement within the cable or push rod assembly.
Watever method of connection used, adjust the connector on the throttle end of the cable so that there is equal adjustment left in both directions, then attach it to the throttle arm (If using a solder type connector,this should not be soldered to the cable at this point). Move the throttle arm on the engine’s carburetor until the hole in the throttle barrel is just barely open.
You will need to have installed the throttle servo to carry out the final connections. If you haven’t done this yet, go to Servo Installation (below) before proceeding further.
Having fitted a suitable connector to the servo arm end of the pushrod, attach this to the servo arm. If the hole in the servo arm is to small for the connector locating pin, carefully open it out using a fine drill bit that is the same size as the location pin. If you use to big a drill bit you will cause slop resulting in lack of accurate control of the throttle.
Position the throttle stick on the transmitter to the fully closed setting with the trim centred. Gently pull the cable tight between the servo arm and carburetor arm and mark the location where the cable meets the throttle connector. Cut the cable at this mark. The final step is to solder the adjustable throttle connector in place and attach to the throttle.
If using a solid pushrod and screw clamp style connector, cut the rod at the throttle end with sufficient spare length to enable adjustment of the position as above.
Your kit manufacturer will have provided instructions on how to install the servos along with the appropriate mounting plate for the servos. A basic glow powered trainer will require either 4 or 5 servos depending on whether the kit manufacturer has arranged for the wing ailerons to be driven by one centrally mounted servo or two servos, one in each wing panel. Whichever the case, each servo is mounted in exactly the same way. Just follow the manufacturers instructions.
In the above photo you have three servos mounted in the fuselage directly behind the fuel tank compartment. The arrangement is as follows:
Servo 1 on the left is the Throttle Servo. the pushrod exits left through the tank compartment to the front of the model where it is connected to the engine throttle control arm.
Servo 2, top right is the Elevator Servo with a single pushrod that travels down the fuselage to the elevator control horn.
Servo 3, lower right is the Rudder/Nose Wheel Steering Servo. Notice the two pushrods, one giong to the Rudder at the rear of the plane (exiting right), whilst the other is connected to the opposite side of the servo arm and exits left through the tank bay and passes through the front bulkhead to an arm on the steerable nose leg.
There are a number of ways to connect pushrods to either servo arms or control surface horns. Kit manufacturers offer their own preferred types so it is impossible to comment on all options. My own preference is for either Metal or Nylon Clevices as shown in the pictures below.
In the picture of the metal types you will see two short lengths of silicone fuel tubing. These are fitted over the end of the pushrod prior to fitting the clevis to the threaded rod end and is slid up over the clevis blades once they are connected to the servo arm or control horn. This acts as a keeper to prevent the blades opening under flight loads and consequent loss of control in flight. I use them on both metal and nylon clevises.
Take the time to install the linkages properly and securely to guarantee control surfaces that work properly for the life of the plane. Make sure that the servo arm travels parallel to the movement of the pushrod so that it won’t cause undue friction in the linkage. Don’t allow the servo arms to contact other servo arms or bind with the clevises at the maximum extent of their movement.
Your kit manufacturer will recommend the maximum amount of movement for each control surface. I suggest that initially you follow their guidelines and set up your throws accordingly.
When installing the servos the brass eyelets install from the bottom of the rubber grommet, and then the screw is tightened to where it just touches the top of the rubber grommet. The purpose of the rubber grommet is to isolate the servo from the vibration in the airframe during flight. this is particularly important in glow engine powered planes. If the screw is over tightened to the point that the rubber grommet is compressed and the isolation characteristics of the grommet are gone. Vibrations will travel directly into the servo, which can damage it.