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Old Apr 29, 2010, 12:03 PM
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Kuopio, Finland
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WMD's Jackal Twin with a twist: A Water Jackal

I stumbled upon WMD's Jackal Twin ( http://www.rcgroups.com/forums/showthread.php?t=1181441 ) and immediately thought about converting it to a flying boat. I spent some time (over)engineering the design with SketchUp (WMD posted his .skp file in his thread), and I just had to do it. I extended the bottom to make a planing hull with a step, widened the fuselage a bit, made the already present sponsons into real floaty ones, and moved the motors up just a bit.

Today I stopped planning and started to build, and after only a couple of hours I have a wing!

-kepa
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Old Apr 29, 2010, 05:04 PM
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Here I am
Bouncing like a gazelle,
I just have
Made a nacelle!

Sorry about that. I'm not actually jumping up and down, but it's good to see the thing taking shape. And I have made two nacelles. Almost. The epoxy for the firewalls will take overnight to cure, and then I have to set up some electrickery before I can close up the nacelles.

-kepa
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Old Jun 08, 2010, 03:14 PM
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Some (a little) progress at last!

For those who have been checking on this thread: I haven't given up or lost interest. I've had some distractions. First, my sister was back in town for a week and I spent all my free time with her and our parents. Then I managed to arrange a bit of repair work for myself (a couple of crashes). Now most of my fleet is in airworthy condition again and I'm back working on the Water Jackal. I just made the sponsons. The assembly is like a tiny wing. I gave some thought on putting some dihedral in it, but built it straight anyway. How this works out depends on how deep she will float in the water. And because this is a simple conversion of a land plane, there's no secondary step behind the primary one. If she'll float with her tail and lower parts of the rudder in the water, great. I'm not putting on a separate water rudder, and plan on using differential thrust for steering anyway.

<edit>Whoops! I didn't have the big picture in mind when I wrote the above. Of course in this design the rudder can't reach the water in any way. The point about differential thrust still applies, and even more so.</edit>

-kepa
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Last edited by kepa; Jun 09, 2010 at 03:00 PM. Reason: Realized something about the rudder
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Old Jun 09, 2010, 07:59 PM
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The fuselage started

Today I started working on the fuselage. I had to scratch my head for a bit though. It's so easy to draft a perfect structure in SketchUp, and then find out it's not possible to actually build it. Fortunately it wasn't as bad this time. I decided to start working from the bottom up and inside out. I first glued the rear bottom piece on the underside of the sponson, and then added some supporting structure, or sub-formers. I also decided to cut off the pieces that go under the sponson from the fuselage sides. I then glued on the battery tray, fuselage former 4 behind the sponson, and finally the sides. There had been a slight communication breakdown somewhere between my eyes, brain and the hand wielding the knife, and I had to add a filler piece under the sponson in the step area.

Next I'll add some more formers and pull the fuselage sides together.

-kepa
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Old Jun 10, 2010, 07:19 PM
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Got quite a bit done with the fuselage.

I started by adding two more formers under the battery tray, and then glued the fuselage sides to the tray and formers. I decided to put in some strips to more positively anchor the sides to the battery tray. Next I put on the front bottom.

The tail side was next. After adding two formers I pulled the tail ends together, and glued the bottom piece to the sides and formers.

The whole fuselage structure is almost straight. I say almost, because the curves aren't exactly identical between the sides. It's no banana though.

Now that the fuselage had its form it was time to think empennage. I noticed I hadn't previously drawn the vertical fin down to the bottom skin, but that was easily fixed while cutting. The fin, a single layer of 6 mm Depron (actually closer to 5 than 6 mm), seemed quite flimsy. So I decided to add a 3x1 mm CF stip to the hinge line and extending to the bottom of the fuselage, and a 1.2x.8 mm one on the front edge. The horizontal stabilizer received a 3x1 mm strip on its hinge line, too.

I concluded the day's (and part of the night's) session by making a servo tray for the tail servos. I decided to put them inside the main compartment instead of leaving them out in the open air and possibly water.

Next I'll route some flexible pushrods to the tail and put in the dowels for the wing rubber bands.

-kepa
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Old Jun 13, 2010, 06:48 AM
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Pushrod guides in place

Building has been a little slower in the last two days, and will remain so for a couple of days more. I hope I can put a moment or two aside for it though.

I installed the push rod outer tubes. There's nothing special about it. I tried to find a way of least bends, and put in pieces of thin plywood to anchor the tubes into the Depron bulkheads.

-kepa
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Old Jun 15, 2010, 02:40 AM
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I put the dowels in place. Actually I used 4 mm CF tube instead of wooden dowels, partly because of the strength-to weight ratio, and partly just because I had some leftover pieces of the tube. I put in small pieces of plywood on the insides of the fuselage side to distribute the load, and will put similar pieces (20x30x.8 mm) on the outside too. And I used some balsa as cross-fuselage beams to strengthen the fuselage in the wing attachment area. I next glued the tail top skin in place.

Yesterday's free time was spent soldering all the connectors into the motors, ESCs and extension leads, and inserting the leads into the wing. I also temporarily attached the motors to the firewalls. I was then able to check the balance. Because I'll be using a lighter power train than WMD did (in power and weight) I'll have to put the battery quite near the nose and make a water tight hatch on top.

-kepa
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Old Jun 17, 2010, 07:44 AM
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Here's what I have done lately:

I glued the aileron servos on top of the wing behind the front parts of the nacelles, and built the rear parts around the servos.

At first I wasn't going to add the bulge on the tail fin that WMD has on his, but then decided to put lights on the aircraft. So I put in the bulge after all, mounted the tail light LED in it, put the wires on the front edge of the fin and covered them with a thin strip of Depron. It would of course have been easier to hide the wiring from the root of the fin to the radio compartment before gluing on the tail top skin. I put the wires on top of it instead and covered them with 3 mm Depron, most of which will be sanded off later. I also installed nav lights onto the wing tips. I might put landing lights somewhere on the wing later but not before actually weighing and possibly flying the completed model.

I put 1 cm wide Depron strips surrounding the wing saddle area and the opening in the front of the fuselage. I also put in some balsa and plywood to facilitate mounting the removable hatch. There will be a plywood tongue at the front end, and magnets at the back end. I then glued on the short nose top skin piece.

I also put some Velcro on the inner floor to keep the battery in its correct spot. The strips are quite long to allow me to use a wide range of battery sizes and weights.

I'm sorry there isn't complete photo coverage of each step. And I don't usually publish my blurry shots but I'm away from the home base now so can't re-shoot them at the moment.

-kepa
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Old Jun 27, 2010, 05:02 AM
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I haven't done much lately. I did put the hatch attachment hardware in place. I haven't pre-bent the cover and so the rear end tends to snap away from the magnets. We'll see if putting in a permanent bend will cure this, or whether I'll need to put some kind of ribs or formers on the inside.

-kepa
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Old Jul 08, 2010, 02:19 AM
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Got something done again. I rolled a bend into the front cover using a highly sophisticated tool, a rattle can of paint against my belly, and put in some structure to make sure the bend stays on permanently. After that I spent a good amount of time sanding the fuselage smooth, and finally attached small pieces of plywood around the wing hold-down dowels. The fuselage is now ready for a coat of WBPU.

-kepa
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Old Jul 08, 2010, 06:49 AM
If it flies - I want one!
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Werribee, Victoria, Australia
Joined Jul 2008
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Looking good!

Kepa,
She is really coming together nicely!
I look forward to news of the maiden flight.
Cheers,
PeteM
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Old Jul 08, 2010, 09:40 AM
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Quote:
Originally Posted by Petem View Post
Kepa,
She is really coming together nicely!
I look forward to news of the maiden flight.
Cheers,
PeteM
Thanks! The fuselage is ready and I'm working on the wing now. After that there's only the control surfaces left. Oh, and a few (million) little things that won't take long (only about most of the time it takes to build a model)...

-kepa
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Old Jul 08, 2010, 09:50 AM
love rc bombs :)
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awsome plane your building hope to see the maiden when its done.
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Old Jul 10, 2010, 06:45 AM
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Kuopio, Finland
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Quote:
Originally Posted by swedish rc guy View Post
awsome plane your building hope to see the maiden when its done.
Thanks! Full credit belongs to WMD. This is only a slight modification to his Jackal Twin. A little stretch here and there.

I don't think there will be video of the maiden but after a couple of test flights I will definitely put my keychain camera on board. And I'm almost certain my dad will be there shooting stills.

Quote:
Originally Posted by kepa View Post
The fuselage is ready and I'm working on the wing now.
I spoke too soon. I had read, and now learned by experience, that you can get Depron really smooth only after the first coat of WBPU. So there was still some more sanding left to do.

The sanding got done and more WBPU got applied. I then used fiberglass tape to hinge the control surfaces and to protect the leading edges of the stabilizers.

I was first going to set up flaperons and couple the differential thrust with rudder, but I couldn't get both the ailerons and flaps working in the correct sense (I have a basic six channel radio with the reversing switches located under the battery and set up for my other models, and had to use external V-tail mixers). So I left the ailerons as just that and connected the thrust differential to the flap knob. I now have separate rudder and ground (water) steering functions, and I can practise one engine out -scenarios.

Here are some specs for the setup:
Motors: 2 * F1818 2500 kV 15 g ( http://www.r2hobbies.com/eng/proddet...od=rcps81511_1 )
ESCs: 2 * Pentium 10A
Propellers: 2 * Cox Thimble Drome 5x3 3-blade
AUW with 800 mAh 2S: 580 g (~20.5 oz)
AUW with 1700 mAh 2S: 630 g (~22.2 oz)

I've flown my Herr Breezy Day Park Flyer J-3 Cub with the same power setup (single of course) and with floats she weighs 320 g (~11.3 oz) and ROWs just fine so I should be good to go.

So, she's finally ready for flight! It may happen today or tomorrow, or at a later date, depending on the weather.

Again, thank you WMD for the simple yet elegant design!

Edit: I got the pictures uploaded.

-kepa
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Old Jul 12, 2010, 09:32 AM
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She has flown!

And earned her first repair too, but more on that later.

After my previous message I packed my things RC and headed for my water flying base a.k.a. my parents' house. The wind was too strong during the day and so I did some things I didn't have time for at home. I made a spacer block so that the receiver hangs about an inch below the bottom of the wing. This way no matter which side up the aircraft ends up floating, the receiver will stay (mostly) dry. I also Velcroed the ESCs to the front side walls.

In the evening the wind died and I headed out. Unfortunately the bronze medal football game was on at that time so my dad wasn't available for taking pictures.

So I set the bird on the water for the first time. She floated in a level attitude, both laterally and fore-and aft, with the bottoms of the sponsons on the surface or just slightly submerged. I then started to taxi. The thrust differential on a separate knob takes some practise but I soon started to get used to it. Next it was time to test whether she would rise on the step. Yep, she did. Actually she floats so high in the water that she doesn't really rise, let alone climb, on the step. She just starts to glide faster and faster.

So, she planes beautifully. Next it was time to floor the throttle. She accelerated quickly yet gracefully on the step and kept rolling for a while. I'm sure I could have yanked her off the water quite soon but I'm glad I didn't. When she lifted off by herself she started to climb very steeply. I pushed the elevator trim fully forward and still had to maintain pressure on the stick. The flight went rather well, although the controls were quite sensitive. And she didn't really want to gather speed, and rather tended to prop-hang.

After the first flight I checked WMD's pictures in his build thread, and sure enough I had guessed the aileron angle wrong. I had aligned the top sides of the ailerons with the top side of the wing and created flaps. I leveled the bottoms, and added a bit more down thrust. This cured the main problem and there was only some normal minor trimming left to do.

With my power set up she's no speed demon. I hadn't figured out earlier that with a V-tail mixer I'm not going to get full power from both motors at the same time. Only if I turn the knob will one of the motors go full speed, and the other one of course will slow down at the same time.

By the way, there's no problem whatsoever with asymmetric thrust in flight. Some aileron and rudder trim, and she flies straight and level with only one motor! And during water taxiing exercises I noticed that if I keep her moving just at about walking speed, or faster, I can steer her with the rudder and have only one of the motors pulling. That's some rudder authority!

I flew her again yesterday, a bit earlier than the day before, and so this time got some pictures too!

Unfortunately the flights ended prematurely. There was suddenly a "froppfropww"-like sound. I cut the power and made an emergency landing. One of the motors, still attached to the firewall, had become loose and cut the wing and nacelle somewhat, as well as pulled the motor wires out of the wing. I found out the epoxy didn't have full contact with both the firewall and the LE of the wing.

It only takes some minor repair and modification work and she'll fly again. And the improved nacelles will hopefully cope with more powerful motors which I plan to install as well.

-kepa
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Last edited by kepa; Jul 18, 2010 at 03:38 AM.
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