63.2" Aichi M6A Seiran (scaled-up 3DLabPrint design)

Joe M · Jun 28, 2018

The local club's annual float-fly event is just around the corner (August 18th) and I'd like to bring something a little larger and more interesting than what I've brought in past years. One of our club's most talented builders is planning on finishing and flying a 1:7 scale Nakajima A6M2-N that he's been working on since 2012. Here's a link to his build thread on RCU for those interested, it's a genuine work of art: Royal Zero Kit Build into Rufe

So when the 3DLabPrint Seiran briefly went on sale for $16 last month I figured it was a good opportunity to try getting my own Japanese WWII-era seaplane ready for this year's event. At the very least it'll be a fun way to use ALL of the build platform on the ol' 3D printer. Also I imagine it will really make Gerry's Rufe look great in comparison. I'm scaling the model up by 50%, which makes it 1:7.64 scale with a 63.2" (1605 mm) wingspan. Not quite 1:7 scale, but close enough for me. The increased size also makes the wing slightly less of an issue, since 3D printed planes tend to run heavy. The recommended cubic wing loading factor of the 3DLabPrint Seiran is 14! That's a lot! At 63" I would have to break 180 oz (5.1 kg) to match that. As estimated I'm guessing my final WCL will be around 11-12, which is still high, but manageable. I'll also be covering the 3D printed parts in kraft paper and polycrylic to add strength and ease the painting process.

Here's a whole boatload of specs and weight/cost budgets:

Scale:	1:7.64
Length:	53.7" (1363.98 mm)
Wingspan:	63.2" (1605.28 mm)
Wing Area:	794 sq. in (51.2 dm^2)
CG:	4.3" (109 mm) from LE
Airfoil:	LHK508
Motor:	SK3 5045-660KV
Battery:	4S 2200 x2
Prop:	Xoar WWII 15x10
est. power:	~1100 W

Weight (est.):
	Printed Parts:		2500 g
		Wing	856 g
		Floats	800 g (est.)
		Fuselage	770 g (est.)
		Battery Holder	80 g (est.)
	Cover/Paint:		300 g (est.)

Power:				911 g
	2200mAh 4S 45C	273 g	x2	546 g	$64.28
	SK3 - 5045-660KV	280 g	x1	280 g	$44.64
	CC Talon 90	85 g	x1	85 g	$65.32
Electronics:				235 g
	Corona DS939HV	12.5 g	x2	25 g	$13.42
	Corona DS339HV	32 g	x4	128 g	$36.92
	OrangeRx R920X V2	16 g	x1	16 g	$20.58
	Rx Voltage/Temp Module	6 g	x1	6 g	$1.11
	Servo Extensions	15 g	x4	60 g	$9.99
Total:				3946 g (139 oz)	$256.26

Wing Loading:	25.2 oz/sq.in (77 g/dm^2)
WCL:	10.7
W/kg:	278
W/lb:	127
Est. Flight Time:	5.3 minutes
Est. Current at Max	80 A
Est. Static Thrust:	177.4 oz (5028 g)
Est. Thrust/Weight Ratio:	1.26:1
Est. Max speed in level flight:	61 mph (99 km/h)
Est. Stall Speed:	25 mph (41 km/h)

So far I've printed and roughly assembled the wing:

Huge, isn't it? That's 856 grams of filament including the ailerons. I still have to do some solvent welding between the panels due to some curling as the result of an inadequately heated enclosure. I don't usually have to worry about PLA parts curling, but these were just too huge not to (the wing chord is >15.5" at the root!) The extra holes and servo extensions are for water rudders, which will be installed in the floats. The single large water rudder on the original design looks super flimsy, and dual water rudders just make more sense at this scale.

Currently I'm printing the floats, which have required slightly more print process modifications than the wings, though it looks like they should come out strong at about 400 g/each. That would give me 844 grams of weight allowance for the fuselage and battery holder, which should be adequate. I'm expecting to go over the 300g weight budget on the covering and paint, though given the 5+ kg of thrust as long as I can keep the WCL under 12 it shouldn't have a problem rotating off water... I hope.

Now to start covering the wing!

Joe M · Jun 29, 2018

The first massive float has finished printing (banana for scale). Final printed weight is 415g, so 15g overweight. I'm very pleased with the strength of the printed parts, though, so I'll use the same processes for the other float. Wing covering is about 75% finished. I'll post some photos once that's done, then it's on to smoothing and priming for paint.

Joe M · Aug 3, 2018

Phew, scaling this up required a lot more work than I had anticipated.

Aside from dialing in the printing processes a new motor mount had to be designed in CAD since none of the included files was going to work with the motor I had planned to use for this build. I printed the motor mount in PETG since I like its dampening characteristics and layer adhesion compared to ABS and PLA would have been susceptible to cracking and failing due to motor vibrations. I imported the STLs for the floats into CAD and designed some scale functional water rudders that I hope will be sufficient for maintaining tracking. I designed them to be installed directly onto one of the seam lines on the printed floats. I just cut the original mock rudders off with an x-acto knife tied to a soldering iron and affixed the replacements with solvent weld. I felt pretty good about keeping them scale during the design process, but now that I'm looking at them installed on the aircraft I'm wondering if I should have made them larger. Oh well. With blind nuts epoxied in place the whole thing assembles nicely with nylon M6 screws. The spinner had to be drilled out to accept the larger prop shaft, but otherwise looks and works great, and there seems to be ample room in the plane for all of the power and electronics. If it's not too heavy I might even throw a sound generator in there after the maiden flight. I really hope those floats are large enough to support a 9-pound airplane.

There's still a bunch of covering left to do and then I'll have to design and print some new servo trays for all of the control surfaces, but this should be primed and ready to fly by the 18th. If I'm very quick and very lucky it might even be painted by then.