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FauxJet scratchbuild


Aussie in Belfast
I started a new build last week, still a work in progress: 32" WS, 28" tip to tail with a KFm4 airfoil (5" chord at wingtip, 2.5" KF steps at wingtip) - based on the hand drawn plan of the FauxJet. Built using a very large sheet of 5mm foam board (no joints, just layers). Fuse will get a few extra hollow layers of enclosure on top for electronics, and a narrow skid layer underneath. Not quite a profile, not quite built up - just enough to house everything nicely and keep the weight down.

Added an 18" spruce spar/support across the wing section to stop the wings flexing up and down as much as they were - channeled out the top section of the wing and glued in the spar using 15min epoxy. Covered up with glass tape to keep things tidy.

Weight so far is 358g (12.6oz) and it'll probably grow another 90 before adding the electronics in the form of more foam for the fuse shaping and the motor mount. AUW is estimated to be around 650-700g.

Planning to try the following electronics:

  • Two Corona CS-929MG Anolog Metal Gear Servos
  • Turnigy NanoTech 2200mAh 3S 25-50C Lipo
  • 25-30A Brushless Motor ESC
  • 4.5x4.5E TGS Sport Propeller
  • A2208/8 20A 2600KV Outrunner Brushless motor (2208/8 Motor RCM)
  • FlySky 2.4Ghz 6 channel receiver FS-R6B (R6B)


Aussie in Belfast
Got the motor mount sorted today - 3mm 3ply securely epoxy'd to the rear of the plane using a long screw into the foam with a lather of epoxy along it's shaft to keep it tight. I recessed the four engine mount nuts nicely into the back of the plywood, secured with epoxy and cyano. Bolts move freely in and out so thankfully no glue got into the threads!

Left a hole big enough for the bullet connectors to run into the back of the fuselage and into the internal cavity of the plane. I'm waiting on some more bullet connectors and extension leads to arrive from ebay, so it will be a few weeks before the maiden.

I put all the electronics into rough place including the battery, RX, ESC, separate BEC (in case of ESC overload, I'll still be able to control the plane, hopefully) and the CofG is roughly 1" back from the front-most leading edge. From other similar builds, it seems about right - but we'll see what happens during the glide test.

The AUW including battery and electronics is now 750g (26.5oz) - a bit heavier than I hoped - but not surprising given the build technique (layering foamboard without removing the paper). As a prototype, it will do the job. I don't think I'll be breaking any speed records with it though!


Aussie in Belfast
Back after a week away for work - spent Sunday mounting the engine, running cables, positioning the RX, ESC and BEC, soldering the cables, connectors, etc and getting it all to fit inside the tiny space in the fuselage... it just fits! I need to add something to grab onto underneath serving both as a launch grip and perhaps also as a landing skid to save the prop - later in the week perhaps.

If the weather is any good on the weekend, it might even get a maiden toss or two to see how it glides (CoG test) - and then it's time to see if it can actually fly!


Aussie in Belfast
Some more pics (click for full size):

Fuselage built up and shaped from layers of foam board - paper still in place, this brand is almost impossible to remove without using alcohol spray (which I don't have handy) - but for a prototype, this will do the trick!

Motor mount is 3ply with nuts embedded in rear (glued in place) and a long screw embedded in the foam, all secured with epoxy - motor mount screws turn freely in and out without binding:

The hatch has just enough room for a 2200mAh nanotech 3S:

Rear view - had to add a small hatch to get the motor wires hooked up to the 25-30A ESC - I suspect I may need to cut a cooling vent hole to prevent the ESC from burning up inside - thankfully I'm running a separate BEC for RX:

Tiny MG servos and battery alarm in place:


Aussie in Belfast
During the week I spent some quality time in the shed with my woodworking tools (such as they are) building a thrust meter (see this article on the benefits of such a device):

Basically, you mount a motor on the top of the right hand side of the vertical boom, facing right and thrusting left (wind blowing to the right). The hinge in the lower right corner lets the horizontal boom press down on the scales with a force equal to that which the motor is generating at the top of the vertical boom. The critical piece is that the booms must be offset at 90 degrees angle and the distance between the motor mount center point and the point which pushes down on the scales must be the same.

The end result is you get to measure the grams of thrust that your motor generates while also measuring the current draw and power consumption through a watt meter - which lets you find out which propeller is the most efficient while providing the maximum thrust for a given motor/ESC combination.

I did a few tests with the motor from the FauxJet - The motor was an A2208/8 20A 2600KV from giantshark(cod) - they don't sell them any more (not surprised, the quality was very variable). Anyway, the results of my tests were:

The 4.5x4.5 prop which I flew with ran at 19.5A (200W) and produced 570g of thrust.
The 6x4E prop I tried on the bench drew 26A (260W) and produced 840g of thrust.

Both tests were done using a 30A hobbywing ESC.

So in theory I could have swung the larger prop and maybe got some vertical performance (rather than none) as the FauxJet was around 800g flying weight, and I might have even run a bit faster than it did down the line, but I'm fairly sure the motor would have burned out eventually as it is only rated at 20A max.

In summary, an interesting exercise and I'm glad I now have my own thrust meter! It was fun to build too :)