Two Planes, One Pilot, The “Twofer”

by Ran D. St. Clair | November 15, 2016 | (5) Posted in Just Fun


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.

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.

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.

The development of the Twofer is discussed in detail in the RCG build thread. 

The detailed flight controller programming is available in post #73 on page 5 within the RCG thread.

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

The motor is a Carbon Bird 1175kv, 225W, turning an APC 9x 7.5E prop, though almost anything similar would do.

The battery is 1.3Ah, 3S, 25-50C, but anything similar would work.

The ESC is a Turnigy Plush 18A with BEC, but nothing special is required.

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.


udo789 on November 21, 2016
Amazing! Very clever.
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minimo on November 21, 2016
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Sirglider on November 22, 2016
This is really amazing and brillant! Congratulations!
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Jordan3367 on November 22, 2016
Wow! This is really cool!
Can you post your KK2.1 settings? I'd like to try this with a few of my own planes.
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Ran D. St. Clair on November 24, 2016

The detailed flight controller programming is available in post #73 on page 5 within the RCG thread.
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Yogenh on November 23, 2016
I would like to know how you came up with doing this!
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Ran D. St. Clair on November 24, 2016
I don't have a simple answer. I spend a lot of time actively trying to think of new ideas. The vast majority of that is recycling old ideas, or new things that upon a few moments evaluation are impossible or impractical. Occasionally, often early in the morning, something occurs that is seemingly possible, and from there it is a much more predictable path to implementation.

Ultimately all ideas are derivative. Almost all new ideas have been thought of before. New technologies often make old impractical ideas possible, or at leas accessible to the common man. Li-po batteries, flight controllers, brushless motors, EPP foam airframes, Carbon Fiber spars, are all examples.

Creativity is not a gift or special talent. It is pounding rocks by the ton to find the occasional nugget. The inclination to do that is possibly a gift.
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Yogenh on November 25, 2016
I think that you have a great gift!

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Ran D. St. Clair on November 26, 2016
FYI, development of the Twofer continues. I have been flying in circle mode with 100 ft. line, and leaning how to lean the circle into the wind and hover. I even managed a control-line style loop which means inside loop on the right, and outside loop on the left.

I also tied the right plane only via. the 100 ft. line to my transmitter and flew it as a control-line plane. It flew great. I did multiple loops, lazy 8s, inverted flight, etc. It was actually really easy to fly that way.
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HilldaFlyer on March 5, 2017
Wow, this is one of the most amazing things I've seen in a long time. My hat is off to you.
Well done, and a big congratulations.
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Two Planes, One Pilot, The “Twofer”