Polaris Seaplane Parkflyer
Here's my latest design to share with the forum, a foam parkflyer adaptation of Laddie Mikulasko's beautiful Northstar seaplane. I named this design Polaris to pay homage to the original (for those that don't know, Polaris is the astronomical name of the real North Star). I owned a Balsa USA Northstar kit back in the late 1980s and have loved the design ever since. This design was intended to be as true as possible in outline to the original, but I did make several modifications to adapt it to simple sheet foam construction, electric propulsion, and to better suit the parkflyer role. The most obvious changes are larger empennage, larger control surfaces, and a slightly deeper hull (see the comparison pic below), all of which improve low speed handling for better parkflyerability.
This model handles BEAUTIFULLY both in the air and on the water, and is a real joy to fly. It's well-mannered, smooth and stable at all speeds and aerobatics are a breeze, but to me one of the best things about this model is the amazing speed range—top speed is in excess of 70 mph, yet the model slows down well and can plop down into the water at high alpha under full control at less than 10 mph. This allows the model to be flown in very tight spaces if desired, or if you have a bigger flying site you can open it up and enjoy the speed. It handles wind and waves remarkably well, too. This model also flies very well off of grass—takeoffs require only 10-20 feet and it can even do easy touch and goes off grass.
Here are the technical specs:
Wing area: 343 sq in
Weight RTF: 20 oz
Wing loading: 8.4 oz/sq ft
Motor: Grayson 2212-06
Battery: Thunder Power 2100 mAh 11.1V Prolite
Prop: APC 6x4
Current: 22 amps
Watts: 240 watts
Power loading: 190 watts/lb
Speed control: Grayson 30 amp
Receiver: Berg 7P
Flight controls: Elevator, ailerons, rudder
This model was designed to use a Grayson 2212-06 motor and 3s 2100 mAh lipo. At 20 oz AUW, this motor provides 190 watts per lb, 1.3:1 static thrust-to-weight ratio for unlimited vertical, and 70+ mph top speed—and all at a very light wing loading of 8 oz/sq ft. Plus the small 6" diameter prop minimizes torque effects and allows for a low thrust line to minimize thrust-induced pitch changes. And to top it all off, this motor is very inexpensive ($39.99 for BOTH the motor and ESC!), so any worries about potential water damage are greatly reduced. Other motors that could work well on this model are the Littlescreamers Park Jet (with 6x4 prop) or Super Park Jet (with 7x5 prop). In fact, with the LSPJ motor and 1320 mAh battery, AUW would be only 17 oz—resulting in an extremely low wing loading but still over 1:1 thrust-to-weight ratio for excellent performance. This model was designed to accommodate up to a 7" diameter prop—just mount the motor at the bottom of the firewall if using a 6" prop and at the top of the firewall if using a 7" prop.
Like most seaplanes this model has a high thrust line, however, there is very little pitch trim change with throttle. That's primarily because both the motor and horizontal stabilizer are installed with -2 degrees incidence relative to the wing. The only time I've noticed a pitch trim change with throttle is during VERY slow high alpha flying, where going to full throttle quickly will pitch the nose down. Since the elevator is directly in the prop blast there's plenty of control power to correct, but if you just advance the throttle smoothly you won't even have to worry about it. But most of the time when flying this bird, you don't have to worry about the high thrust line at all.
One of the secrets behind the fantastic slow speed controllability of this model is the location of the prop right in front of the elevator and rudder. That not only gives this model great slow speed control, but also gives it a degree of control similar to my thrust vectoring park jets. At slow speeds, the model will loop and yaw VERY quickly if you give a quick blip of throttle along with the control input!
The finish on my model is just bare foam with some colored packing tape for trim. This keeps the model very light, quick to build, and also makes it easy to repair. Plus the packing tape is strategically placed in areas that need to be strengthened anyway for a seaplane—the bottom of the fuselage and the leading edges of the wings and tail. Hence this paint scheme!
EDIT (4-30-09): For those that want to save time over scratch building this model, a complete laser cut kit is now available at Model Aero. Here's a link:
And here's a link to a fantastic review by Jon Barnes of the Model Aero Polaris kit. LOTS of great photos and videos in this review!
Here's a link to a nice writeup from Daedalus66 that summarizes some helpful tips for setting up the power system of the Polaris and preventing overheating problems.
Here's a link to a very helpful post from Daedalus66 that summarizes the recommended improvements and changes to the basic Polaris design, all learned from years of experience and thousands of models being built:
Here's a YouTube flight video of this model flying off water:
And here's a video of flying off snow:
Here are some photos showing the construction details for this model. If you've built any of my pusher-prop park jet designs before, you won't see anything new here.
This model features all internal servos and linkages to make it as watertight as possible. The only places for water to enter the hull are the 4 pushrod exits, which can all be sealed with grease or Vaseline. The elevator servo is mounted inside the nacelle, and a channel inside the vertical tail allows the motor power wires and elevator servo wires to be run internally.
The #1 glue I recommend for building this bird is epoxy mixed with microballons, since it's lightweight, cures quickly, and perhaps best of all, is pure white to match the color of the Depron perfectly. That makes most of the seams invisible. I also applied beads of epoxy/microballons at all structure intersections to waterproof the model.
Plans and Construction Guide
Plans for this model are posted below in the usual tiled and untiled formats. I'd recommend taking the untiled plans to Kinkos, who will print them for only a few dollars.
I hope others enjoy this model as much as I have. But remember, the only rule is—if you build it, you gotta post pictures!
EDIT (9/20/08): An illustrated construction guide has been added to the attachments below.
EDIT (11/7/08): Revised plans and construction photos showing a new nacelle design option are posted in post #392 (link below):
Great video! That plane is a thing of beauty. I had a Northstar that was about 50% complete when my house was struck by lightning and most of the items in my workroom got cooked, including the Northstar. I never got around to replacing it. Maybe now I will.
What kind of foam did you use? It looks like Depron, but I can't tell.
Your experience has been very similar.
I had one back in the 80's and loved it.
I lived on a lake in Michigan at the time.
I'm now on the Columbia river just south of you !
After noting the Twinkle and a similar design here in Rcg
I've been meaning to dig out the old plans and have a Foam Fest of my own
but THANKS to YOU I dont have to design it myself now :-)
This'll have to wait till winter but it's definitely on the build list :-)