FT Sea Otter thrust angle

[iceking1152]

After perusing the (numerous) existing threads on the topic, I wanted to start a new one and make it a compendium for the knowledge about this design. Previous posts determined 105mm was the optimum value for the height of the prop shaft from the rear fuselage. Here mine is 115mm. I just want to confirm that it’s too much and the motor should be placed more downwards?

 

[iceking1152]

After doing some basic thrust tests it is obvious that it will pitch down harshly with acceleration. I’m trying to fix it now!

 

[iceking1152]

Sadness… I trimmed about 1cm total from the motor pod, and now 8” props won’t clear the fuselage even though I have the correct shaft height now. Is my only solution to remove and remake the whole motor pod?

 

[AIRFORGE]

The nacelle support mark should be aligned level with the top of the fuse.
Your nacelle appears to be bent downward in the rear, which will create upthrust and decrease prop clearance. You want the thrust to be level or slightly down.
You can put alcohol on the glue to separate the nacelle support from the fuse if you want to rebuild/repair/replace the nacelle/support. It might take a bit of patience to work it free, though.

 

[iceking1152]

I remade the nacelle, leaving the original nacelle support in place. The prop shaft sits 5mm lower than my first attempt, and the thrust tests feel ok. I might as well “send it” at this point. Hopefully I can do the maiden in a field on Friday.
Does the sideways thrust angle look too extreme?
Thanks for your help, AirForge.

 

[AIRFORGE]

I remade the nacelle, leaving the original nacelle support in place. The prop shaft sits 5mm lower than my first attempt, and the thrust tests feel ok. I might as well “send it” at this point. Hopefully I can do the maiden in a field on Friday.
Does the sideways thrust angle look too extreme?
Thanks for your help, AirForge.
View attachment 256093

The side thrust is designed into the nacelle and should be fine.

 

[RickHunter]

@AIRFORGE is right. You need that side thrust on this plane. When I first built it I thought it was crazy, but still had to add a little aileron trim as well. Once you dial it in, it is a pretty good flyer. It is one of my go-to 3s planes. I fly it on a 2826 2200kv with a 3 blade 7inch prop.

 

[iceking1152]

@AIRFORGE is right. You need that side thrust on this plane. When I first built it I thought it was crazy, but still had to add a little aileron trim as well. Once you dial it in, it is a pretty good flyer. It is one of my go-to 3s planes. I fly it on a 2826 2200kv with a 3 blade 7inch prop.

Thank you for the words of encouragement. I was planning to use a 3s 2200 mah with an 8x6 prop. That isn't optimally efficient as I'm on c-pack motors - 2218 1180kV, so I also got an 8x4 and and 8x8 to experiment around with. Even if/when it crashes, I can see myself rebuilding it so I always have one. I have eyed this design since I first became interested in the hobby a year ago and began my mini sea duck.

 

[RickHunter]

That motor should work fine. It's a light plane so even a little under-prop should be okay. I used that because it was the only motor I had sitting around at the time that would fit with the prop clearance on 3s. One of the reason I love the foamboard planes is that you can crash and repair/rebuild without breaking the bank. It's a great attitude you have for this hobby. I always say, "you have planes you've crashed and planes you're gonna crash.

 

[iceking1152]

I finished up the build, now all I need to do is program rates into my TX, then I’m headed to a field for some testing. I made magnet mounts for the hatch, they work alright but I will probably still tape it.

 

[RickHunter]

Nice! May you have clear sky's and soft landings.

 

[iceking1152]

I had neither clear skies nor soft landings. The maiden of the Otter went well, especially after turning rates down to 75%. Unfortunately I drilled it into the ground on the 2nd flight and the main damage was that the elevator servo stopped working. The wire is hard to access/replace, so may just cut the wires and solder a new one on instead of messing around with the extensions that go through the tail.
As for the mini sea duck, this one always made a high pitched noise at high rpm, a member of my club said the props were too loose, but it still did it after I tightened them so I thought nothing of it. Well, the right prop had a rapid unscheduled disassembly during takeoff on the 3rd flight, and the 2/3 blade prop imbalance likely put too much torque on the motor mount which caused it to break off and strike the leading edge of the wing. Luckily I was still low and slow, so it wasn’t too hard to minimize the damage on the crash landing with 1 motor.
My girlfriend got a video of the maiden of the otter which I am about to upload to YouTube and link here.

 

[iceking1152]

I made it out like a bandit with the damage on this. I checked to see if the extension was unplugged from the elevator servo, and that was it. I cut a little access hatch and will be taping this connection so it doesn’t happen again. So besides a little nose squish, this bird is almost ready to go again. I had so much fun! Can’t wait to fly again.

 

[RickHunter]

Nice. I just finished up a build on the Mini duck myself (except for electronics). This is the first Mini duck I've done. The regular Sea Duck is one of my favorites, and I did a 200% Duck for the 2025 Monster build challenge which was fun while it lasted. https://forum.flitetest.com/index.php?threads/ftfc-25’-monster-scale-challenge-rickhunter.79360/
Can't wait to fly the mini.

 

[iceking1152]

Nice. I just finished up a build on the Mini duck myself (except for electronics). This is the first Mini duck I've done. The regular Sea Duck is one of my favorites, and I did a 200% Duck for the 2025 Monster build challenge which was fun while it lasted. https://forum.flitetest.com/index.php?threads/ftfc-25’-monster-scale-challenge-rickhunter.79360/
Can't wait to fly the mini.

That looks awesome, it's friggin huge! I really like the 3d printed cowlings and engine mounts. Would you mind sharing the .stl for those? I also see you have an engine test stand, if I had one it would help ward OSHA away from my plane "factory". The mini sea duck was my first build and this otter is the 2nd, so I don't have the larger versions to compare the characteristics to. Are you putting a functional rudder in yours? I opted not to on mine and instead programmed 3 diff thrust modes, which seems to be working ok.

 

[RickHunter]

No rudders on any of the ducks. I use differential thrust. They do amazing flat spins with diff. thrust on. The cowling and motor mounts are not my design but I will attach their links here. I'm just finishing some skid plates for the Mini duck which greatly increases the life of the plane since i land on asphalt or hard packed dirt only. Also some motor mounts for the mini duck. So I'll share those when done too.

https://www.thingiverse.com/thing:3925766

https://www.thingiverse.com/thing:2542362

These are for regular size flite test planes not mini. I increased them by 200% for the Monster build. Motor mount should be printed in PETG or ABS. PLA tends to melt with engine heat. I'm working on shrinking the Sea Duck Cowl to fit the Mini Duck, but my first attempt (60%) was too small, so i will let you know what % to use when i figure it out.

 

[RickHunter]

And the thrust stand was made by @Houndpup Rc and works great, but I think it only covers big motors at this time. Hint, Hint Houndpup 😁.

Part - 3D Printable Thrust Stand

This is a fully 3D-printed thrust stand I designed for testing larger RC motors, including 50xx-size outrunners. It’s built strong enough to handle serious torque without flexing, and it can be printed on any basic FDM printer. Materials &...

forum.flitetest.com

 

[iceking1152]

No rudders on any of the ducks. I use differential thrust. They do amazing flat spins with diff. thrust on. The cowling and motor mounts are not my design but I will attach their links here. I'm just finishing some skid plates for the Mini duck which greatly increases the life of the plane since i land on asphalt or hard packed dirt only. Also some motor mounts for the mini duck. So I'll share those when done too.

https://www.thingiverse.com/thing:3925766

https://www.thingiverse.com/thing:2542362

These are for regular size flite test planes not mini. I increased them by 200% for the Monster build. Motor mount should be printed in PETG or ABS. PLA tends to melt with engine heat. I'm working on shrinking the Sea Duck Cowl to fit the Mini Duck, but my first attempt (60%) was too small, so i will let you know what % to use when i figure it out.

Thank you, I really appreciate you sharing those - I didn't think to check thingiverse. Have you tried printing the motor mounts for the mini duck? If not, I'll start up a test print at 60% scale and report back if it needs additional resizing.

 

[RickHunter]

Thank you, I really appreciate you sharing those - I didn't think to check thingiverse. Have you tried printing the motor mounts for the mini duck? If not, I'll start up a test print at 60% scale and report back if it needs additional resizing.

I'm printing the motor mounts now. My motors are zing 2208 1800kv with 16x16 mounting. If they fit I will share it. If your mounting is different let me know i can pry mod them for yours real easy.

 

[iceking1152]

I'm printing the motor mounts now. My motors are zing 2208 1800kv with 16x16 mounting. If they fit I will share it. If your mounting is different let me know i can pry mod them for yours real easy.

The flite test c pack 2205 2300kV that are on my MSD also have 16x16 mounting. Are you using the base .stl or one of the ones with specific mounting holes?