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An article by DaveT on converting the Multiplex Fox glider for powered RC flight.
Has the Fox overtaken the TB1 as the most popular plane at our field? If not it can't be far off. Favourite model for many as it flies so well even in brisk winds. Twinkle rolls are its forte!Suggested parts; local suppliers available as well.
The Multiplex Fox is a hand-launched glider which costs less than £10. A good conversion is to add a 10g brushless motor in the nose, a 6A ESC, 2S 300mAh lipos, two 3.7g servos and a small receiver.
The model is made from moulded EPP. This is a very resilient type of foam, the same as we use to make Bounce. The best glue is UHU POR. POR is usually used as a contact adhesive in that it is applied to two surfaces and allowed to set for 10minutes before joining. However, it can also be applied to one surface as a 'bead' and joined immediately. This typically takes longer to set.
The fusalage is solid foam so the simplest conversion involves mounting servos on the outside surfaces. However, a neater finish can be obtained by hollowing out the fuz. If you are going to hollow the fuz, first cut the section of fuz away immediately above the wing. This will allow you to mount the servo in the wing, then glue that to the fuz and then glue the 'top part' back on.
You then need to slit the fuz open, probably along the mould line. A long blade is best, the snap-off type ideal. Blades have to be sharp and you will go through many to do the job. EPP cuts best when 'sliced' rather than 'ripped'. Experiment with the blade angle and 'saw' to slice if that helps.
Once the fuz is in two, hollow it out with the blade leaving about 3mm (1/8") wall thickness. How do you do this? You just keep cutting bits away! (you never built log canoes before?). If you cut too much away, simply glue it back with POR. The side thickness should ideally increase to around 6mm (1/4") as it approaches the nose The forward-most nose area should be left solid to provide a more rigid mount for the motor (the part in front of the canopy).
The motor's aluminium mount can be glued directly into the foam, or an easier and more robust approach is a thin ply firewall first. If the latter, cut part of the nose away to glue the ply where it needs to be. No up, down or side thrust is needed. This is determined relative to the stabiliser (not the wing which has some positive incidence).
The plane is likely to be nose heavy. So it is best to mount the elevator servo behind the wing. Shape a small block of EPP to support the servo in the fuz. Glue this block into the fuz with POR (but not the servo yet). Make and trial fit a balsa or carbon pushrod with wire ends. The front end can be a 'Z' and the rear a simple right-angle bend to allow it to be hooked up to the elevator later. Make sure the servo leads will reach in front of the wing for inserting in the Rx later.
Ailerons are cut out of the trailing edge of the wing. 20x170mm each is a good size. The EPP can serve as hinge material if not cut through completely (if you do cut through completely in places it can be repaired with POR + scraps of EPP). Cut a 'V' in the bottom of the wing to allow 'down' movement. The best position for the aileron servo in the fuz is beneath the wing roughly mid-chord. Mount it as close to the wing as possible, recessing it so that the pushrods just clear the foam. This is intended to allow the battery and Rx to be placed beneath the servo. Cut slots in the fuz side to allow the pushrods to reach their horns. 12mm throws both ways will get the juices flowing. Exponential is your friend.
The wing won't collapse without reinforcement but does 'bow' a bit in turns. Embedding some 1 or 1.5mm carbon for half to 2/3rds the span can reduce this. Not essential but is an improvement. Since the wing is simply glued on, it is easy to cut it off and add the spar later if you want it. Advanced aeronautical engineers can also shave the bumps off the wing and any moulding flash.
The elevator is cut from the stab in the same way as the ailerons. 15x190mm is about the right size. Throws should be about 6mm each way. To allow the elevator to clear the fuz, one way is to elongate the slot in the fuz so that the hinge line is at the end of the fuz. The hole in front of the stab can then be filled with some offcuts.
Glue the two halves of the fuz together with POR. Glue the stab in place and then position the elevator servo in the fuz and hook up the elevator. With the radio on, trims neutral and control directions the right way, glue the servo with POR wherever it needs to be for the elevator to be neutral. Glue the wing on once its servo and pushrods have been set up.Mount motor. Velcro beneath wing to secure battery. Job done.
3.7g servos (the link below may not be the best available)
5x4.3 Gunther prop or GWS 5x3
2S 300mAh 20C lipo eg: