The “Twofer” is a combination of old school control line techniques, modern flight controllers, and radio control.
It is made from two relatively inexpensive 3D EPP foam profile planes from HobbyKing.
https://hobbyking.com/en_us/hummer-epp-3d-plane-1000mm-kit.html
The first video describes the modifications to the 2 planes, which are fairly easy and minor. The ailerons are pinned straight. The rudder is pinned with about 10 degrees offset. The thrust line is adjusted to about 10 degrees “out-thrust”. The elevator control horn is lengthened to provide “normal” not “3D” throws. The wing tip plate for attaching the string is described in the video, along with the screws for tip weight. The battery and all the radio gear except for the servo is just held on with Velcro.
The radio gear is pretty standard. Most any receiver would do, but in this case I am using an Orange R620X V2 6 channel DSMX receiver with S.bus output. Any receiver with 5 or more channels would do and PWM output could also be used.
When using S.bus to communicate between the RX and the FC (Flight Controller) I use the S.bus adapter cable. S.bus is easier to connect and allows the FC to run more efficiently, though I don’t think that matters for this application.
https://hobbyking.com/en_us/zyx-s-s-bus-connection-cable.html
The FC is a KK-mini running the OpenAero-VTOL Firmware which is now available with the firmware pre-flashed from Altitude Hobbies
The regular KK2.1.5 with OpenAero-VTOL pre-flashed is also available
If you already have a KK2.1.5, KK-mini, or even a KK-Hard Case, you can flash it yourself with the OpenAero-VTOL firmware which is free. They would all work equally well. The only thing that would not work is an old KK2.0 board. Details can be found in the OpenAero-VTOL RCG thread. Be sure to check out the extensive user documentation which is available at the end of the first post in the thread.
https://www.rcgroups.com/forums/showthread.php?t=1972686
The development of the Twofer is discussed in detail in the RCG build thread.
https://www.rcgroups.com/forums/showthread.php?t=2739809
The detailed flight controller programming is available in post #73 on page 5 within the RCG thread.
https://www.rcgroups.com/forums/showthread.php?t=2739809&page=5
I provided a “Beginner Mode” that I used when first learning to fly, and an “Advanced Mode” that I used to do the stunts and “Circle Mode” flying in the attached videos.
The elevator servo is a standard 8.5g EMAX ES08A II, though almost anything would work
https://store.flitetest.com/emax-es08a-ii-8-5g-servo/
The motor is a Carbon Bird 1175kv, 225W, turning an APC 9x 7.5E prop, though almost anything similar would do.
http://www.multiwiicopter.com/products/carbonbird-motor-nmb-1175kv-reversed-pa
The battery is 1.3Ah, 3S, 25-50C, but anything similar would work.
https://hobbyking.com/en_us/turnigy-nano-tech-1300mah-3s-25-50c-lipo-pack.html
The ESC is a Turnigy Plush 18A with BEC, but nothing special is required.
https://hobbyking.com/en_us/turnigy-plush-18amp-speed-controller.html
I would like to offer special thanks to HappySundays, the creator of the OpenAero-VTOL firmware for the KK2. It may sound like it is just for VTOL (Vertical Take Off and Landing) but it is really a universal flight controller that can be used for just about anything.
I would also like to thank Walter Colby (Wcolby1) for the fantastic ground based Video, and NothingBut4k for the amazing in flight video.
The Twofer is not the easiest thing to fly, so I would not recommend it for beginners. Takeoffs are inconvenient as you need someone to help you launch. Landings can be challenging as you need to get both planes down softly. The roll response is a bit slow as you are flying a plane with a 25 foot wing span, depending on the length of the string. It is also difficult to start and stop rolls precisely and you have to get used to using opposite ailerons to stop an existing roll, as if it had a fair amount of roll inertia. Elevator response is quite normal, and yaw is normal considering the huge wing span.
Circle mode is actually easy but the controls are odd. Left aileron to climb, and right ailerons to descend. Elevator can be applied in time with the circle as a form of “collective pitch”, but mostly I just let the AutoLevel do its thing which means it will drift down wind.
A much more skilled and capable pilot could probably figure out all sorts of neat tricks. I don’t think I have developed it to the fullest extent. A much longer string could be used and then it really would become two control line planes flying in mirror image. You could finally do a “wing under” without smashing into the ground.
It’s an interesting and unique project. I would not say it is practical in any way, though there are theoretical possibilities. Mostly it is just fun, silly pointless fun.
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