by Des Theisz
The Wombat is a three channel model which is meant to be easy to fly, and to be forgiving. The original is powered by a veteran O.S. .20, we have club members younger than that motor. Construction is straight forward, quite suitable for first timers. For anyone who has not built from plans before, Airborne published a series 'Building from Plans' in twelve parts, issues Nos. 126 - 138, which would be of great help, otherwise there are bound to be fellow club members willing and able to help with any problems. The only way to find out if it flies as intended is to build it and see, so on with it.
Select the balsa. That means donıt use anything heavier than necessary. This design is very strong, there is no need for hard timber anywhere except spar, and leading edges for ding resistance.
Adhesive used in the original was mostly PVA. Balsa cement was used for edge gluing balsa, epoxy for joining different materials such as elevator joiners which are metal to wood and some cyano. Start by cutting out all the parts in kit forms.
Leave the 3mm centre ribs in one piece at this stage. Cut them only half way through where the dihedral braces will go. Pin all the ribs together into a single block and sand to shape. File out the slots for the spars and LE while they are in a block. File? A coarse metal file is as good a tool for this job.
With a flat bottom wing each panel can be almost completely built over the plan. For, the port wing pin down the bottom cap strips, long enough to support the spar, wing centre section sheeting from the spar aft, and trailing edge - over the clear plastic you have put down to protect the plan. Add ribs, with centre rib angled using template shown on plan, then top spar and leading edge. Cut away the centre section ribs for the dihedral braces, and glue the braces and the gussets in. The top leading edge sheeting, the top centre section sheeting and top cap strips can now go on.
Keep everything pinned to the building board while doing this and do not force anything. Well fitting parts is the way to a straight wing. Add the tip outline, brace and top sheeting.
Leave overnight then remove from the building board, trim bottom cap strips to length and add bottom leading edge sheeting. Be careful to keep it straight, a warp now will be deadly later.
Repeat the exercise for the other wing. Do remember to leave off, temporarily, enough of the top sheeting to fit the dihedral braces. Pin down again the panel without the dihedral braces. Fit the other one to it and block it up 180mm. (Equals 90mm on each wing). Make sure everything is straight then glue the wing panels together and leave to set properly.
Lastly, glue the ply reinforcing to the trailing edge.
These parts are flat plate sections, which are simply built over the plan. Plane the leading edges of the rudder and elevators to a V.
Add the ply reinforcing for the horns, temporarily fit the horns and hinges, then make up and install the elevator joiner and sand to final shape. Take it apart again and finish the edges where the hinges go, it is necessary to keep oil out and is much easier to do now. I used scraps of iron-on covering.
The parts can now be hinged together permanently. Use a smear of epoxy. Keep a rag soaked in metho handy and wipe the exposed part of the hinges before the epoxy hardens. Keep those hinges free. And pin the hinges. Two 1.5mm holes in each side of each hinge and toothpicks dipped in epoxy will do the trick nicely.
Select all the fuselage parts. Glue the doublers to the fuselage sides - one right and one left. A pinch of sand sprinkled over the gluing surface will stop them sliding around after they have been positioned. Weight them and leave them overnight. Mark the position of F2 on the inside of each side. Measuring from F2 mark the positions of the other bulkheads. Note: the down thrust and side thrust are set by F1. Mark the distance from F2 of each of the four corners. Glue longerons and wing and tail supports to fuselage sides.
Fit the engine to the mount. Assemble the tank and lay the engine, tank and the throttle cable over the plan to mark the locations of the holes in F1. Because of the side thrust, the mount will be off centre at F1 to keep the propellor in the centre. Drill to suit.
Pin down right side of fuselage and glue in F2, F3 and F4. This part of the fuselage is a rectangular box, keep it square. If the sides have curved as the doublers dried, dampen the hollow side only and it will straighten out when pinned. If water doesn't work, try ammonia. Pin down the left side and add the right on top using a square or two, not only at the formers but also at the front, back and wing and tail seats. Care here will make it easier later.
Install the 3mm balsa bottom between F2 and F3 and the servo mount now. They will help to keep things straight while the ends are pulled together. Space the servo mounts to suit your servos. Trim longerons as necessary and clamp the ends. Do this over the plan and at the same time mark the centre of the formers and pull a string line taut along the top of the fuselage. Also, some scrap can be pinned to the sides vertically at the front, centre and rear. By sighting the plan, the string and verticals there is a real good chance of building it straight. Use a slow drying glue and give yourself a chance.
Install the rest of the formers. Fit the tank compartment floor, triangles to F1 and 6.5mm nose triplers. The photos may show a back on the tank compartment. I do this when possible, ever since a tank split and fuel ran everywhere before I woke up that something was wrong.
For the undercart mounting blocks plane a piece of wood, (I used pine), to size and chisel out the slot. Put the end of the wire for the undercart to a grinder and it will be sharp enough and just the right size to gouge the slots. The slot in the vertical is only one wire wide. Glue these in with one slot forward and the other aft then add the triangles.
Finish off the fuselage except for the bottom sheeting and the wing dowels. I epoxy two 30mm lengths of brass tube into F1 rather than try to poke fuel tube through. The tank is then connected one side and the engine the other. Give the inside of the engine compartment a good coat of fuel proofer (epoxy will do nicely).
Make up the undercart including tail wheel assembly. File a flat on the wire where the grub screws on the wheel collars meet it.
This is quite a good time to cover the wing and the tail feathers, except for gluing surfaces (see finishing). Poke the wing dowels through. Pour heaps of silicone sealant (from the local hardware) on the seat, cover the wing centre section with cling wrap and strap it on. The excess sealant cuts off easily and your mates will wonder how you cut the mount so accurately. Glue on the horizontal then vertical tail, all square to the wing and fuselage. This will involve inserting the bottom rudder hinge in the process. Donıt forget to pin this hinge, it supports the tail wheel as well. When set, add reinforcing triangles.
Install the servos then run the nyrods, working through the open bottom of the fuselage. Epoxy the outer tubes at each former. It may be necessary to glue a piece of 3mm balsa across so that the curves are not too tight. Temporarily mount the motor and fit the throttle cable too. Cut clearance holes for access to the needle valve and for fitting the muffler. The hardware can then come out, the bottom sheeted, and the fuselage finished off. Cut the holes for the Rx switch on the opposite side to the exhaust. Soak the balsa around the holes with cyano.
There are no special requirements. I used iron-on film because it does not smell, it is convenient, and is suitable for the purpose. Bright colours, different on top and bottom make it easier to see in the air. Preparation consisted of sanding, fine sanding, filling any gaps (dope and talc or for small areas lightweight filler) then fine sanding again. Plug up fuel lines before sanding. When sanding the flying surfaces, keep the leading edges well rounded. Too sharp here can make the model twitchy. It has been said that ³the finish is only as good as the preparation.² Trim is also iron on film. Of course any of the proprietary trim films can be used.
Assemble all those parts so lovingly made, except for the Rx and battery. Hang the model from the C.G. shown on the plan. Get lateral balance first by pushing nails into the higher wing tip. Place the batteries and Rx on top of the plane also that the stabiliser hangs level. It may be necessary to add some lead to make it balance. If so, do so. Install the batteries and Rx wrapped in foam RUBBER, stand back and admire. Then check again that there are no warps in the flying surfaces.
Saturday was a superb day for flying but Murphy was at Westernport so Sunday it had to be. Five to ten knots of wind, shifting around a bit - we can live with that. Michael Bourne (Club President - no less) did the honours while I carried the camera, that way there was a better chance of photos of a plane rather than a pile of rubbish. Radio Check, controls correct, taxi out to the strip, THE MOMENT. Throttle on slowly, rotate and sheıs climbing away much steeper than expected. Trim it out - no rudder trim needed but a lot of down trim. Some low passes for photos first then testing.
Once trimmed it was time to see if she could take it. Loops, no worries, rolls, barely but that is to be expected, finally a good, flat out spiral dive and pull out with full up elevator. Everything stayed together and it was my turn.
Flying at full throttle the Wombat gains height surprisingly fast. In fact level flight was maintained at only about quarter throttle. At low throttle is is slow, stable and a piece of cake. The stall is a sort of mushy sink. Ten minutes of fun then get ready for landing. A couple of passes to get the feel of it then set id down. Not bad, only a couple of bounces.
The Wombat is easy to fly and fly slowly. A .20 motor provides plenty of power. It is not difficult to land. It requires care on take off - mainly it is a case of not trying to accelerate to quickly. It is a model for making lazy circles in the sky and practising positioning the plane while gaining confidence but it is also capable of quite a few manoeuvres. The Wombat is an ideal beginners model.
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This page was last modified on the 19-May-02