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42" Savoia/Macchi "P.R. S.21"

So I harbor a love of seaplanes, particularly the Italian racing seaplanes that frequented the Schneider Trophy races. I also enjoy fictional planes in film and animation (ex. the Sea Duck, I wanted this plane as a kid!). Long before getting into RC in a meaningful way, I wanted to at least build a model of the Savoia featured in Studio Ghibli's Porco Rosso. The plane doesn't exist in real life, and though it's called an S.21 in the movie, it doesn't resemble the actual S.21 at all, and instead is closer to a Macchi design. It's described as not being particularly well flying at low speeds, requiring fast takeoff and landing speeds but being fairly maneuverable, matching and exceeding nearly all other planes in the film.
There are a few videos of working RC examples, some flying and floating excellently. I've found some plans here and there, most aren't too detailed or super useful - but one set of plans in particular has caught my eye for a foam board adaptation. I forget how I found them originally, the writing is in Czech (I think), and authored by Marek Bohac - it details a 42.5" wingspan balsa model of the plane with former drawings as well as airfoils and a complete wing drawing, foam construction of engine cowling, and other small details. I've had the PDF since 2013, and just tonight started adapting it to vector drawings and built the wing with WRFB just to mock it up and see how it might work!


Is there much precedence for this particular fictional plane? I'm excited to figure out the hull and step construction, keeping things as light as possible - it looks like it will balance easily and support okay characteristics with C-pack level performance. Let me know what you think, I'll continue working on the mock-up model and hope to try and actually fly it soon! Of course, more iterations will probably follow depending on how things go.
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Roughing in the fuselage!

I really want to get the hull correct, which is why I've got a center board to help get the bow profile to transition from convex at the bow to semi-concave nearer to the step (without using formers al-la balsa building). I'm not knowledgeable in hydrodynamics, so I don't know if it will actually be beneficial in the water to have the outside edges deeper than the center at the step, but that's how it looks in the movie! (seems like it could work okay, or maybe just cause problems if one side steps up before the other, i.e. with a slight roll on takeoff) I don't mind fudging some things around if it makes for a better flying model, but for the most part I'm getting the rough shape down and freehanding the rest anyway.


Skill Collector
Very cool project, and it looks like you're well on the way to a succesfull model!

I don't have too much experience with flying off the water, but one consistent thing I hear is to keep the edge of the step a sharp angle - not rounded off. If it's too 'hydrodynamic' it will create a low pressure spot behind the step providing downforce, which is not what you need to get airborne :)

If you look into some of Localfiend's builds here on the forums - especially his DTFB floats - he has some great performing designs and lots of discussion of what works well for foam board on the water.


Build cheap, crash cheap
Very nice! You beat me to it! This was on my list of plans to make, and I had started the wings and fuselage but they were an aluminum spar speed wing and a former based fuselage. It has been on the back burner for a couple of months while I work on an odd scale project.

C-pack is what I initially was planning on, but since it wasn't going to be swappable with my other planes anyway I was thinking of using a higher KV motor. Fitting a 10" prop didn't look like it was going to work out at the scale I had chosen (40" wingspan) and I wanted more static thrust than the C pack motor makes with an 8. I'm interested to see how you'll do the engine pod; I was going to tackle that last because it looks like a difficult piece to make from foamboard. The hull on the plane in the animation is inconsistent, depending on the shot the various things change throughout the movie. I was going for a V hull with hard chines (sharp angles, no rounded transitions) in the front transitioning to flat at the step. What you're going for looks like V to inverted V in the rear. That involves some curves and just makes things harder in general, but it should be stable on plane, though you won't spend much time there with a little RC plane unlike a boat.

Looks like you're off to a great start. I hope to see some plans in the future, I'd like to build your simpler version of this plane because I'm not going to finish my plans or tackle a more complex build like that any time soon.
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Thanks, both of you!

I definitely wanted to do it real simple - I've got some very spartan plans, pretty much just enough to get a rough outline going!
Here're some pictures of last night's stopping point:

I'm pretty excited to get past the prow, it's definitely the crux of the build for me. I might be using some very thin and flexible balsa sheeting, I'm not totally sure yet.
Minimal progress this week..
I decided to mount the wing using some 3mm carbon fiber hollow tubing I got from HeadsUp RC. It's a bit higher than originally planned - but it should allow me to use a 10" prop without putting the motor too awkwardly high above the wing. It might break from scale, but I'm mostly looking forward to a flying approximation! The plane feels pretty strong, so far it weighs in at 172g. I just need to build a front fuselage cover that helps reinforce the tubing against the sides, and cover the bottom of the hull still, and then I'll be fitting servos!
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This is coming out really nice. The only thing that jumps out to me is that the combination of high vertical thrust line and the hull step being so far aft of the CG might make it difficult to rotate nose-up on takeoff. Could be fine, too, though. Seaplane design is complicated.

I look forward to seeing this fly, good luck!
Good point, I'm hoping the inverted-V step will help - the CG will probably be nearly in the middle of the step and where the beginning of the hull starts to slope up towards the tail. It'll be interesting, at least! I figure it's a good project to learn a lot about scratch building and problem-solving in the hobby. Even if it only flies from a hand launch, I'll still have fun with it!


Elite member
That looks like it's coming along rather nice.

You might be able to just play with the length of rear motor struts to dial in the thrust angle until it behaves to your liking.
I had a really good elevator/flaps mix on my FT Bushmaster that allowed for really easy handling on short takeoff and at super low speeds, I may do something similar here.

I should mention - the horizontal stabilizer and the wing are fixed at a very slight (and I mean 'very' slight) downward angle, and the wing itself has a trailing edge that extends a bit below the base of the wing (almost like permanent flaps). These weren't exactly design decisions that I made, but really just things that came about while mocking the model up late at night - In thinking about how these elements might play together I feel it may have a good amount of lift and decent characteristics at lower speeds (seems to me that it will just have a static amount of flaps and elevator trim built-in).

My thrust angle may end up being only a few degrees nose-up at first. Trimming might be very exciting on the first flight - and it'll really dictate most of the adjustments I make to any following iterations. I'm hoping to get some more work done today!
I just sheeted the bottom of the plane with some 1/32" balsa. Originally I wanted to do the whole thing in foamboard, but perhaps future hull plans will be simplified to allow for this. I was able to get it to conform pretty nicely and it was easy to hold in place while the glue set up. I trimmed the excess after the fact, but when I did the second forward hull section I did have to do a few test fitments and cut the inside to match the existing edge of the other side before gluing.
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Still deciding on the smaller side pontoons, the main hull is fairly narrow and I'd like to work out some pontoons that cut and keep the wing from diving just as well. I saw Localfiend had some wing floats on his Zero as well as plans that looked like they might work, haven't had time to build them yet.
Thanks, Ricci!
I was able to get a little bit of work done tonight! I mounted the motor pod to the wing, fairly high, to clear a 10" prop (currently mocked up with a B motor, waiting on the C motor to arrive). As I build this initial model I'm keeping some notes on how I would simplify the more difficult things I've come up against, for a more pleasant, faster, easier build at the expense of needless (in this case) scale details. For example: a slightly shorter forward fuselage with drastically simplified hull shape, flat on the bottom and at the step; a flat top for the forward fuselage and a very simple battery/electronics compartment lid (more prop clearance and lower motor pod mounting); one set of straight-through dowels for mounting the wing and motor pod (less parts and easier alignment); and simplified tail stabilizers (horizontal stabilizer on fuselage tail, rather than cruciform).
I'm excited to develop some beta plans! Anyway, here's the motor mount:
Here's a super simplified and shortened fuselage (I built three other simple fuses before settling on this size and shape - I switched to a hightail on the fuse and every side is simply flat, the height and length are a little shorter as well) - it should print just 6 landscape tiles, 2x3, on 8.5"x11", I like to use a 0.25" overlap with cut marks enabled in Acrobat Pro. I haven't included a removable cover for the battery tray area yet, but I'd like to do something similar to the FT3D canopy, but even easier as it only needs to be flat.
Here are the plans for the wing that I used, and will probably continue to use for the simpler versions.
I'll update the plan files as things are implemented or clarified.
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