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Old Sep 07, 2012, 01:32 PM
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Build Log
Correx Funjet and Fastjet for 2212-06 2200kv motor WITH PLANS

For some time now I have been wondering if the basic Mugi correx flying wing platform could be modified to clone some of the foam funjet designs that are out there. In addition to liking the funjet shape, I wanted to design something that was a little more stable and forgiving than the Mugi - I have two young nephews (12 and 10 years old) who will no doubt outgrow their nutball trainers quite soon. The cherry on the top would be to create something even more crash resistant than the Mugi.

The design came together one afternoon when I was scratching around in my garage and came across some old polyethylene hot water pipe insulation - this solved the last remaining problem of an indestructable fuselage....and the Correx Funjet was born!

The plane is specifically designed to use the 2212-06 2200kv - 1300mAh motor/battery combo that seems to have become the standard for foam parkjets, with a 6x4 prop. There seem to be several versions of this motor on the market for very reasonable prices (Hobbyking's version (the Turnigy D2826/6) comes in at under US$10).

The first prototype weighed in at ~600g (~20oz), and performed way better than expectations - stable in pitch and roll, but crank up the throws and it can roll several times per second plus go from full speed horizontal flight to vertical with a very near to 90 degree change of direction. The foam fuselage has so far protected the plane from the knocks of several test flights gone wrong without any damage.

My eldest nephew started the construction of the second prototype today. I've made some small design changes to get the target weight down to 550g and he is testing that the attached plans are correct.

Construction is almost all 2mm correx, glued using contact adhesive. In South Africa we struggle to get affordable depron, but correx is readily available and cheap. The fuselage is made from a product called Thermaflex pipe lagging. Its a polyethylene hot water pipe insulation material that is readily available in 2m lengths from our local Builder's Warehouse (now owned by Wallmart, so I suspect that the product will be available in other countries as well). Having a fuselage protects the electronic components and makes launching much easier

Construction of the basic aerostructure is very simple and should be possible in a longish evening;add a few more hours to install the motor and radio.

The attached plans were drawn in google SketchUp - this is a free download and permits zooming in (the pdf version is quite difficult to read).

Build photos and guidance to follow...

UPDATE: 17/10/12:

This is turning out to be a productive design concept, with many different variants possible. The key design features are:
1) Pusher prop flying wing
2) Kinked wing leading edge
3) Polyethylene foam fuselage
4) Folded wing construction (I use correx, but Bluecore foam should also work)


So far I have the following designs:
Correx Funjet: Intermediate design with a nice turn of speed and very agile (AUW ~550g).
Correx Funjet Light:
Park flyer and potential second plane - ideal platform for the E-Max CF 2822 or CF 2812 motors (AUW~450g).
Correx Fastjet: Designed for speed - definitely not for beginners. (AUW 450g)

In addition I have applied the same concept to build a semi scale Avro Vulcan bomber - 110cm wingspan ~1000g AUW

What's next? Think a forward swept wing and canard Funjet could be interesting, while on the scale side a DeHavilland Vampire could be a challenge (OK, so its not strictly a flying wing, but the wing and fuse shape lend themselves to this contruction technique).

23/10/12: FastJet plans added

02/01/13: Couple of updates:
1) FastJet CoG is 25cm from the front of the wing. This should be just behind the point where the wing kinks.
2) Keep the foam nose overhang to <100mm for the FastJet and <150mm for the FunJet...if the nose is too long it acts as a weathervane and the plane will be unstable in yaw
3) Added Funjet Light plans to this post. I've been doing most of my testing on the Light version and prefer it to the original design. Apart from looking better, it is more flexible in terms of motor-Lipo combo...mine has flown with many motor sizes ranging from a small E-max 2212 with a 800mAh lipo to a 2212/06 parkjet motor with a 2200mAh Lipo. It has flown well with AUWs ranging from <450g to ~650g. I've also done some trials with two axis thrust vectoring.
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Old Sep 08, 2012, 10:03 AM
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STEP 1: Joining the two wing halves

The wing uses a lamininated 2mm correx spar in place of the more conventional wood spar or carbon tube. This is possible without losing strength because the correx wing skins provide the necessary strength in the same way as the flanges of an I-beam provide the bulk of the I-beam's strength. However, for this to work, two things are critical:
1) There is some mechamism to keep the two wing surfaces apart and stop them buckling (this is where the correx spar comes in), and
2) The flutes run in the correct directions, so that the top skin is orientated in the same direction as the correx's best compressive strength.

Anyone who has built a Mugi will be familiar with the creases on the top skin where the wing compresses when too many 'g's are pulled - the Mugi has the top wing skin flutes running chord-wise and correx does not have a very high compressive strength in this direction. The Funjet has the flutes for the top wing skin running spanwise so that it is better able to resist the compression that occurs when the wing tries to bend. The spar is simply used to keep the two skins the right distance apart in the same way as the web of an I-beam.

Start by cutting out the two wing halves. The easiest way to do this is to make a cardboard template of the top wing (which includes the elevon). Trace the outline (remember to orienate the flutes correctly), then flip it over on the wing leading edge crease line and trace out the bottom skin, only this time leave out the elevon.

Once both sides are ready, cut out the bottom doubler. Then glue the two wings together onto the bottom doubler. Make sure that the joint aligns with the centre line of the bottom doubler. I use contact adhesive for all joints - just remember to sand all surfaces to be joined first.

Finally, cut the elevon hinges by slitting one side of a single flute and then removing the excess plastic (I think there is a post on how to do this somewhere in the SPAD's forum).
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Old Sep 08, 2012, 10:17 AM
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Adding the spar

The Funjet's spar is made from laminating two pieces of 2mm correx together. The main spar has the flutes running vertically and has a 5mm edge top and bottom that is folded over to provide a surface to glue onto the top and bottom wing skins. The spar doubler has the flutes running horizontally and does not have a border.

Start by cutting out the main spar and the spar doubler. Next, cut through one side of the correx on the main spar to enable the edges to be folded over (be careful not to cut right through). Now fold the edges 90 degrees as shown in the picture.

Sand and glue the spar doubler to the main spar.

The spar can now be glued onto the bottom wing skin. Mark out the spar position, sand and apply contact adhesive to both the wing bottom and the bottom spar flange. Note the bend in the centre of the spar - cut the spar flange and score the spar itself if needed to enable it to bend. Also note that the flat side of the spar is the bottom.
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Old Sep 08, 2012, 04:44 PM
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Impresssive! flight video? thanks
spud
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Old Sep 10, 2012, 05:30 AM
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NICE BUILD!! Definitely going to try this...

I have built two Mugi's, and still yet to get one flying

I saw there was a prototype called a Screamer, built bu the Mugi site, but seems it was never finished and I have been looking for something similar...

You have just answered my searches

Correx Building is a bug that has bitten...
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Old Sep 10, 2012, 06:48 AM
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Spudandcat: Video will have to wait a while - Ran my Tx battery flat and the replacement is stuck in customs.

Papsnoek: Mugis seem to be quite temperamental. On some days my current mugi floats out of my hand; on others it refuses to do anything but hit the ground without any hint of flying. I think trim and thrust angle are very important and I have also added a grip on the underside for launching, which helps. I'll post a mod that I have done on the Funjet to deal with the thrust angle issue. High KV - small prop combinations don't help launching either.

I also saw the Screamer, but the fuse looked like it would crumple on first contact with the ground...which was when I came across the Thermaflex idea. Builders Warehouse also sells foam 'pool noodles' for kids...they have a slightly larger diameter than Thermaflex, but also hold potential for use as a SPAD fuselage material. I am busy designing/building a correx AVRO Vulcan as a test case - looking good so far
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Old Sep 10, 2012, 07:20 AM
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Step 3: Completing the wing

Before closing the wing, there are a few things to do:
1) I add a small foam wedge to provide a solid base on which to mount the motor. I found that without it the correx just deforms as I tighten the motor mount bolts, with the result that the motor thrust angle changes...with obvious consequences for flying characteristics. I use a piece of polystyrene, approx 80mm long and 15mm at the thick end, and hot glue it onto the bottom wing skin. Any foam or balsa would work as well - but keep it light. See photo.
2) If you want to hide the servos inside the wing as is done on the mugi, install them now. I'd suggest attaching them to the underside of the top wing skin using a cable tie and installing the push rods before closing the wing. I have mounted my servos on the outside - it adds drag, but makes it easier to adjust throws. Also my nephew needs to be able to move servo's between planes, so cannot leave them buried inside the wing.

Now score the inside of the wing fold line with a small screwdriver. Fold the wing over, being carefull to get a straight bend. If needed, place upside down over a table top edge and 'push' the fold using a plank or large ruler. If the bend is not looking straight, rather redo the scoring....otherwise the leading edge ends up being 'wavy'. If you have not done this before, test it on some scrap (but use a long piece - the 'waviness' only becomes apparent on bends longer than ~30cm)

When ready, sand all edges of the wing top and bottom surfaces and smear contact adhesive. I do one side of the wing at a time. Also add glue to the top of the spar - I use Bostik Clear adhesive here as it has a longer drying time - I don't want the contact adhesive to cause the top wing skin to stick to the spar in the wrong place.

When the glue is ready, carefully fold the top skin over and press the edges onto the bottom skin. Three things help:
1) Keep the bottom wing skin pressed onto the table top, with the trailing edge near the edge of the table.
2) Make sure that the trailing edges of the top and bottom wing skins line up as close to exactly as you can - this ensures that the aerofoil is symmetrical and that both wing halves have the same shape. It gets easier with practice.
3) Very important: As you fold, allow the nose to rise, but keep the tips pressed flat on the table. This ensures that the wing has a suitable degree of washout, which helps prevent tip stalling on launch.
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Old Sep 10, 2012, 07:46 AM
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Where do you fly?

See the video of my first flight(Crash) with my Mugi... Any Tips?

Maiden Flight (Crash) With First Mugi (1 min 19 sec)



Im seriously battling to figure out how to attached the outrunner to the Mugi... Im very keen to see how you have done this with the Funjet
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Old Sep 10, 2012, 10:10 AM
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Mugi troubles....

The last flight looked like it might almost work!

I supect trims may not be right, and its also possible that your CoG is slightly too far back - try some nose weight and a bit more right trim.

I usually launch with about 70% power and hold full up elevator until the plane has reached flying speed - I ease back on the elevator and throttle to full as the plane starts to climb. At full throttle its out of sight within seconds....

That said, I've never tried the discus launch method - it looks way too complicated. Instead I've attached a handle to the underside and launch in the traditional way. The attached pics I did for a mate who was having similar troubles may help - they show the mods I have made.

I make the motor mount from some 20x20 aluminium U-tube. Its simply bolted onto the tail...the foam wedge alows me to tighten the screws properly, which is a problem in the normal mugi design (since it used an in-runner). Pics may help. Thrust angle must be horizontal and straight foreward.

None of this guarantees that launches will be easy...mugis remain temperamental until properly trimmed out. Sometimes I think the plane decides what to do and my job is just to complete each manoeuvre!

What motor (size and Kv) and prop are you using? I use a 6x4 or 7x6 prop - the 7x6 is better as there is more low speed thrust to get the plane to flying speed.

I usually fly from a park in Bryanston - if the above tips don't work, bring the plane round sometime -looks like you were flying just north of Fourways
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Old Sep 11, 2012, 03:09 AM
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wow! you have some awesome ideas, they so simple I find myself saying... "Why didn't I think of that?"

That flight Video was with a 3000kv 6.5x5 Prop (don't laugh ) Hence why it took off like a rocket at only half power Last Sunday I tried with a 2200kv 5.25x6.5 Prop, and crashed it on first launch and motor fell off... Hence why Im looking for a better way to mount the motor.

You were correct with your location, I guess the huge sewer pipes gave it away I fly at the JOMAC flying field, Im a member there... I require ALOT of space as I lack the flying skills a bit I live just down the road from Bryanston, So would really appreciate the help if you are volunteering
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Old Sep 12, 2012, 02:22 AM
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Step 4: Adding the top wing doubler

After closing the wing, cut out the wing top doubler, noting the flute direction.

Next, score the two diagonal fold lines at the front and test fold. Test fit the top doubler to the wing and trim the bottom doubler if needed. The top doubler should fit snugly, with the two flaps folding around the bottom doubler (the pictures may be worth more than the words here...). Make sure that the centre line of the doubler aligns with the centre line of the wing.

When ready, sand all surfaces to be glued and apply contact adhesive to the top of the wing (it helps to mark out the shape of the top doubler to ensure you don't add glue to the wrong areas and to make the next step easier) and to the inside of the top doubler. Do not glue the two flaps yet.

Carefully fit the top doubler. I suggest folding the flaps closed and inserting the doubler from the front and getting the fit right before allowing the glued surfaces to make contact - once the glued surfaces touch, you will struggle to make any adjustments to the fit. This will be easier if you marked out the shape of the top doubler on the wing.

Now turn the wing over, trim the flaps if need be so that they do not overlap. Then sand and apply glue. Finally, press the two flaps into position.
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Old Sep 13, 2012, 06:04 AM
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Step 5: Adding the motor mount and foam fuselage

Everyone has their own methods for mounting motors; for the funjet I have used a small piece of aluminium angle and some 4mm bolts. The picture should speak for itself. The only thing to remember is to add a backing plate (aluminium or thin ply) on the underside to stop the bolts pulling through the correx. Set the thrust angle using some small wood shims (or just bend the alumimium). The motor thrust line should line up with the plane's centre line.

Next trim the plastic joint off the thermaflex. I use a sharp knife for this. Cut a length that is as long as you want the fuselage to be - mine overhangs the front of the wing by about 120mm. Make the fuselage bottom first. Cut off a wedge from the tail and nose to create a gentle curve at the tail and a slighly sharper curve at the front. See the attached photo. Also note the small step cut to match the thickness of the correx at the wing LE.

Glue the foam onto the underside of the wing with contact adhesive. Apply liberal amounts to the foam - it tends to absorb the glue.

Next, work out where your battery needs to sit in order to get the balance right and cut a slot in the top skin of the wing for the battery to fit in. Make it a tightish fit - you will slide the battery in and it helps if fricton keeps it in place. If needed, cut through the underside of the correx wing as well (but do not cut through the foam). I find that for a 1300mAh LiPo I need to cut a small bit of the wing lower skin, but I do not need to cut the bottom wing doubler. About 15mm - 20mm of the battery should protrude above the wing skin, but that is OK as the foam has a hollow centre. Cutting too much of the wing skin will weaken the wing, so cut carefully.

Cut a hatch behind the battery slot in the top doubler and cut away the top wing skin under the hatch area to provide access for the receiver and servos. If you cut 3 sides of the hatch, the 4th side acts as a hinge - see photo. Mark the extent of the hatch as you will need to cut the foam to match later.

Now cut a shape the foam for the top of the fuselage in the same way as was done for the bottom. Only trim the tail enough to get a smooth fit with the motor mount. Hold the foam in place over the battery slot and check that you can slide the battery into its slot through the hatch.

Now glue the top fuselage onto the wing skin using contact adhesive. You will find that pinching the nose section together creates a simple and neat nose profile.

Finally, cut two almost vertical cuts in the foam in line with the battery hatch. If the cuts angle slightly apart as you cut down you can create a slight wedge in the foam that will hold the hatch shut without the need for tape or magnets
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Old Sep 15, 2012, 12:01 PM
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Completing the build

OK, so now there are just a few last items before the funjet is ready to fly:

1) Add the fins. These are simply attached using toothpicks pushed into the foam. For safety add a few drops of wood glue to keep the toothpicks stuck in the foam and in the correx. I had both fins fall out due to an imbalanced prop on a test flight and the plane definitely does not fly without them! (fortunately on that occasion it simply spun to the ground from ~50m up and bounced on its foam nose, so there was no damage)

2) Add the wing tips. These are best glued using hot glue - make sure the front edges are well stuck as the tips tend to hit the ground on landing

3) Add control horns, pushrods and servos. I have used old credit cards for the control horns and just screw the servos into the correx. The correx is a double layer at the point the servos go, so the standard screws supplied seem to hold. Just make sure the servo holes are cut slightly small so as to get a tight fit.

The ESC fits in the foam tunnel between the motor and the access hatch. I threaded some string through from the back, tied it to the ESC cables and pulled them through, and then connected them to the motor. Consider melting some holes in the foam to allow some airflow if you expect to regularly fly at WOT.

I find that 15-20mm throws in each direction (i.e. 30 - 40mm total movement) gives pretty good performance - dial down slightly if you are a beginner, dial up if you want the funjet to spin like a drill bit and pull 90 degree turns.

Prototype 2 weighed in at 535g, a full 100g lighter than the first prototype.

Have fun building and flying - and please post pictures!
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Old Sep 16, 2012, 11:37 AM
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Successful maiden

The second prototype had a very successful first flight yesterday. Flew off straight and level without any need for trim.

The attached video captures the flight. Its not the best quality as it was taken using a cellphone, but it should give a sense of how the plane flies (and its not a bad effort considering the cameraman is only 10 years old..). Note the gentle low speed flying and vertical acceleration at the end.

Hopefully I can get another video at some point with the throws cranked up.

Video here:
Funjet maiden 1 (2 min 32 sec)
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Old Sep 18, 2012, 04:23 AM
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Attached are a few of my build pics so far....

Must admit its a rather easy plane to build and really looks great!

Just had one or two flaws that aren't in the instructions or I misunderstood.

1. Very difficult to cut the plans straight on the correx, you need to make templates as there is no why to work out the angles on the folds.
2. If you look at the pics, I glued the Bottom Doubler incorrectly... I used the build pics as reference, and if you look in the pics, you have a little "Tail" section that overlaps, but thats wrong! the doubler should fit flush with the bottom of the wing, as now the top of the doubler was too short. It still feels fine, but I have a feeling that a nose crash and it will just crumble.

Cant wait to finish this as compared to the Mugi, it feels way more stable and balanced
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