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FMS Alpha Jet 2ch R/C conversion for PSS

Converting the 18" span EPP free flight 'chuckie' for use on the slope.

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Phil Cooke04/03/2017 12:52:27
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A few weeks back PSSA stalwart Matt Jones mentioned to Andy Meade and I that he'd seen online a small scale, free flight 'chuckie' version of the Dassault Alpha Jet - which he thought might lend itself well to potential R/C conversion for slope use.

You might say 'So what, why bother?' (We did!) Well, we believed it might enable us to fly a PSS model in very light slope lift conditions - those frustrating moments at our events when even the lightest of Jet Provosts and Vulcans would be grounded, hence increasing our ability to fly PSS models when traditionally we'd be forced to 'go electric'.

alpha jet box.jpg

A bit more research showed it to be an 18" span, all EPP moulded model designed really for the kids in the park. As scale model fans it was about the right shape - other than the horrible dihedral set into the wings and the flat tailplane - both of which we would have to address!

alpha jet dihedral.jpgNot surprisingly there were other electric power conversions already evident on the web - but all had 'lazy' surface mounted R/C gear with external linkages ruining the scale lines. Could we convert it with all linkages hidden away? We agreed that with a focus on scale - that must be our design intent...

Finally, the model was on sale for less than £10, so what did we have to lose? We convinced ourselves it was a great idea to get them ready for the start of the 2017 season and we committed with the credit cards...

What follows isn't really a build blog, but a pictorial account of the work needed to convert the little foam model from a free flight toy into a scale 2 ch R/C, lightweight PSS model.

Phil Cooke04/03/2017 13:00:37
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The first job was to remove the belly pan/intakes from the single piece fuselage - there were 'blobs' of hot gun glue at the front and the rear which could be tackled with a little careful knife work. Once detached, you can see it's already hollow inside, and this needed very little further work.

aj1.jpg

Phil Cooke04/03/2017 13:04:54
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Next, the single piece wing was cut into three to remove the as manufactured dihedral. The ailerons were cut out (along the pre-moulded simulated hinge lines) and both wing and flying surface were stiffened with carbon rod, glued with medium cyano into small grooves machined on the underside. 1.0mm diameter for the wing and 0.75mm for the aileron.

aj2.jpg

Phil Cooke04/03/2017 13:06:28
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Similar work on the single piece tailplane, shown here prior to being cut in half ready for some scale anhedral.

aj3.jpg

Denis Watkins04/03/2017 13:11:17
4334 forum posts
104 photos

Spectacular work Phil

We are watching

Phil Cooke04/03/2017 13:11:29
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2546 forum posts
1795 photos

Once the anhedral was set (by eye referring to a 3-view drawing of the full size) some tiny torque rods were made up in plastic tubing and fitted very carefully into the tail. The use of Vaseline is essential here to ensure no glue enters the tubes... the small scale and unconventional 'soft' materials made this a little tricky.

aj4.jpg

The elevators were sanded round on their L/E and hinged conventionally using CA hinge strip.

aj5.jpg

Phil Cooke04/03/2017 13:16:33
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The wing was then glued back together - to remove the dihedral. We agreed a compromise to set the wing FLAT on the board as we proved the taper in section gave the illusion of scale anhedral when viewed from the front. So the wing was glued with a smear of gorilla glue and left to cure overnight whilst held flat on the board. The expanding gorilla glue 'ooze' proved easy to remove once set.

aj6.jpg

Phil Cooke04/03/2017 13:19:14
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2546 forum posts
1795 photos

With an unballasted CoG position in mind, we began to think about the positioning of internal R/C gear - here the 2 x 6g servos are shown in their final agreed positions prior to fitting.

aj7.jpg

Phil Cooke04/03/2017 13:24:08
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2546 forum posts
1795 photos

The wing was fitted with torque rods, similar method to the tailpane - taking great care not to glue everything up!! I used the excellent Dubro Micro Rod system (they cost as much as the foam airframe! - but the fittings were excellent and just perfect for this scale of model)

aj8.jpg

Phil Cooke04/03/2017 13:25:53
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2546 forum posts
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Ailerons with sanded L/E, hinged and fitted - servo in.

aj9.jpg

Phil Cooke04/03/2017 13:26:59
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1795 photos

More Dubro micro parts...

aj10.jpg

Phil Cooke04/03/2017 13:29:29
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2546 forum posts
1795 photos

With the servo fitted part of the fuselage had to be removed to allow the wing to be 'dropped on' as opposed to it sliding through...

This section of fuselage also needed significant material removed to allow the servo and pushrods to operate freely without binding - shown here dry fitted.

aj11.jpg

Phil Cooke04/03/2017 13:38:33
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2546 forum posts
1795 photos

Back on the tailplane, this was the trickiest part of the conversion.

I had originally connected a single 'forked' V pushrod to the two torque rods using heat shrink tubing. The anhedral and sweep on the elevator hinge lines meant the torque rod end effectors moved away from each other as they pivoted forward and moved towards each other when they pivoted back. The single pushrod needed too much force to operate and I soon failed the heat shrink connections! A new concept was required.

aj12.jpg

After some thought I made up 2 tiny little horns out of brass crimp tubing and glued them to the torque rods with metal cement. A tiny wire 'coat hangar' pushrod fitting was then made in the vice which allowed the horns to 'pulse' as a working pair along the wire as the pushrod was moved for and aft. This worked well, there was no binding and the required pushrod load was reduced to something more akin to the tiny 6g servo.

aj13.jpg

Phil Cooke04/03/2017 13:43:10
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1795 photos

Considering the installation of the battery, regulator and Rx, the forward fuselage was machined out to allow sufficient storage.

aj14.jpg

A small magnet was surface mounted at the rear of the canopy for retention, with a carbon down arrangement running into a tube at the front.

The removable canopy was also relieved.

aj15.jpg

And to allow the Battery wiring to run to the Rx, a hole was burrowed through from the cockpit chamber into the wing bay.

aj16.jpg

Phil Cooke04/03/2017 13:45:42
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2546 forum posts
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Worth noting the R/C gear chosen - all of this was new to me as this is my first 'micro' model - so the Rx, 1S Lipo battery and 5V regulator all needed procuring! There goes the budget!

aj20rx.jpg

aj20batt.jpg

Both very impressive, lightweight, compact units.

Phil Cooke04/03/2017 13:53:16
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2546 forum posts
1795 photos

With the wing glued up back onto the fuselage, and the fuselage 'plug' reglued below it, that just about completed the conversion! A full radio check was performed to ensure everything was moving freely and she was done apart from the balancing.

The airframe was gently sanded back, any injection mould 'flashing' was removed with a scalpel and I attempted (not very successfully) to remove all of the little EPP mould 'pimples'. The EPP airframe was then 'etched' with some Multiplex EP Primer. Do take care with this stuff it is extremely powerful - it has to be applied with a cloth then immediately wiped off with another - left on too long and the surface of the foam model can quickly 'decay'!

With the etching applied I gave the entire model a dusting coat of matt white Acrylic primer and then began to apply some camo with the airbrush. Again, all matt acrylics.

aj17.jpg

Phil Cooke04/03/2017 13:56:16
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2546 forum posts
1795 photos

I made some paper masks to avoid too much overspray at this scale, took some time but worked well. The cockpit was brush painted. A few panel lines added with a permanent marker.

aj18.jpg

The tape required to mask the blue lines DID remove some of the camo paint, annoyingly, and I had to sand back and repaint some areas, chasing my tail a bit, and adding more weight no doubt, but we got there in the end.

aj19.jpg

Edited By Phil Cooke on 04/03/2017 13:57:18

Phil Cooke04/03/2017 14:00:19
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2546 forum posts
1795 photos

With the paint applied, I focussed again on the final fitment of the R/C gear, and balance. A bit fiddly, but in the end I had the RX in place just on the wing LE and the battery up front as planned, giving the required balance point of 78mm from the wing LE.

aj20.jpg

aj21.jpg


Balanced on two BIC biros...

aj23.jpg

Phil Cooke04/03/2017 14:03:38
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2546 forum posts
1795 photos

A few detail decals applied, cut from solartrim with the bespoke Nigerian Airforce roundels and fin flashes provided by Tim at Modelmarkings.

aj22.jpg

finished alpha 2.jpg

AUW ready to go - 122g - not too shabby, although I know Matts is lighter still (less paint?)

aj24.jpg

Phil Cooke04/03/2017 14:07:33
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2546 forum posts
1795 photos

So that's that! a 2ch RC PSS Alpha Jet - 18" span with a 1/4lb flying weight.

A bit fiddly in places, and it took way more workshop time than I expected, but I think that will all be justified assuming she flies in the light slope conditions we hope she will, and enables PSS flight at that end of the windspeed spectrum!

finished alpha 3.jpg

I'm hoping that Matt and Andy will both post pictures of their models here now too, and I will post again to report on how she flies (if she gets her chance of course) in April.

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