Converting the Multiplex Fox to R/C

Here's a very quick guide to converting your Multiplex 'Fox' Chuck Glider to R/C.  Credit to Andy Ellison who wrote an article in RCM&E about this calling it 'Pimping My Chuckie' - here's the way I did it...

(You can click on any of the pictures for a full-size version)

1. Here's what you need for the job plus a couple of options...

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The glider is a Multiplex Fox that you'll have to modify yourself.

I used Cirrus CS101/STD 4g coreless micro servos which, at 15 each, was waaaaay over the top in my opinion but I was in a rush!  Have a look at www.giantcod.co.uk for ones that cost less than 2.00 each! 

The receiver only needs to be two-channel really - the one I used was a GWS type GWR-4N.  The batteries are a single-cell 400mA type that come from a tiny Century twin rotor helicopter.  I didn't need the spruce in the end (it was for the control surface horns). I used 2mm carbon tube and 0.9mm piano wire for the pushrods and 1.5mm x 2.5mm carbon for the wing spar. Normal thin superglue seems to be safe on the Elapor foam used in the Fox.  I also used Graupner hinge tape and a servo extension.

Note:  There is a regulator there that I played with but I didn't need it - more about that at the end. 

Here's what to do...

1.  Build the glider and test-fly it...

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2.  Find the correct Centre of Gravity and mark it on the underside of the wings...

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3. Now mark out and cut a channel for the 1.5 x 2.5 spar...

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4. ...and glue it in with thin superglue.  I glued it in flat rather than edge on - in other words the more springy way.

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5.  Remove the canopy which is 'keyed' onto the fuselage and remove the ball bearing nose weight.  Cut off the 'key' part.

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6.  Now mark out the area that you're going to cut out for the cockpit electronics...

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7.  You'll need enough room for whatever bits you're using.  I hollowed it out using a Dremel with both milling and sanding drum attachments...

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8.  Next cut a channel to accept the aileron servos; you shouldn't end up needing any glue for them...

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9.  Now take the aileron servos out again and cut the hole for the elevator servo...

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10. Use a piece of hot welding wire or similar to make a hole through from the front to where the elevator servo will go.

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11.  I went spectacularly over the top with this one and used a lathe to line everything up.  You really don't need to do this! :-)

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Jon Fox had a much better way to do this.  He very cleverly roughed up the end of a piece of aluminium tube (an old micro helicopter's tail boom) and used it to cut down through the centre of the fuselage - very neat!

12.  Take the tail plane and cut a channel so that LATER you can insert a piece of carbon tube on the underside and glue it in like this...

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13.  Now cut out the elevators...

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14.  And relieve this edge so that there will be space for the elevator to go down...

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15.  Now glue the cross-tube into place and use hinge tape tape to make the hinge...

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16.  You'll need to cut away a little material from the rudder to make space for the elevator cross-tube.  You can just see here where I marked where I was going to cut...

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17.  At the front end, insert a piece of carbon tube to hold the canopy in place...

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18.  The control horns are made from servo horns.  Just chop them off...

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19.  Cut a slot in the control surface in question and glue them in with superglue.  It works a treat!

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20.  The beauty of using that carbon tube is that you can easily make pushrods.   A little superglue is all you need to keep the piano wire in the carbon...

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21.  And this is what you get...

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(The elevator servo needed an extension cable)

22.  Make up the ailerons last.  Once they are finished you can't really get to the aileron servos or the cable going to the elevator servo.  My ailerons were 25mm deep and tapered at the ends.

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23.  Add weight if necessary to get the C of G right (using those lines we drew on the underside of the wings in Step 2) and it's finished...!!!  All up weight is 76g.

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Seems to fly very nicely...enjoy!

Nigel Fraser Ker
August 2008

Final note:  I got the little 5 volt regulator (the thing with three legs) from Maplin (type TS7805CZ or Maplin No. QL31) with the intention of using two lipos to power the model.  However, the receiver and the servos work fine on 3.7 volts (in fact they seem to work all the way down to about 2.6 volts) so I didn't think it worth it.  However, if you want to try it, this what I worked out...

PCB for Jon.jpg (2607086 bytes)

I made up this little PCB with a piece of Veroboard and some 2- and 3-pin connectors.  Look at the bottom right (rather unclear) picture...The centre copper rail (running left to right) is a common ground (i.e. -ve).  It's hard to see but the bottom and next to bottom rails are cut at their 3rd holes from the right-hand side.  Although it's not clear the top right joint isn't connected to the one below it.