C1 Chaser TVBS Conversion Instructions

This details the conversion of my existing C1 Chaser flying wing to an ArduPlane TVBS (Twin motor Vectored Belly Sitter). A tailsitter which lands on its belly instead of its tail.

Starting from a flying C1 Chaser, you will need the following:

  1. Two 5" mini-quad style motors. I used the Cobra 2204/2300Kv motors.
  2. Two 18A ESCs. I used ZTW Spiders running standard PWM.
  3. Two Gemfan 6x3 Props. These have a lot better effeciency than many others.
  4. 3D printed tilt rotor motor mounts. These can be used on normal Tilt rotor or Vectored Tailsitters (either +/-45 deg or 0-90deg). I use these on my Tricopter Tilt Rotor MiniTalon with much more powerful motors:
  5. GPS and Compass. I used a BN-220 micro GPS (see notes regarding operation with EKF3 below) and a GY-271 discrete compass IC breakout board (https://www.amazon.com/gp/product/B008V9S64E)
  6. A flight controller capable of running ArduPlane 4.0 or later with 6 servo/motor outputs and GPS/compass. I used a MXK, MatekF405-Wing clone just because I had it laying around. I had gotten it since it has integrated BT module on SERIAL1, but, unfortunately it has custom firmware that prevents it from connecting with a ArduPilot GCS (was targeted for iNav and SpeedyBee Android app)
  7. 4S Battery Pack. I used 4, IMREN 18650 3000MAH 20A/40A LiIon batteries with 20A continuous current rating. But a 4s Lipo pack of ~180g -200g total weight will balance the same…it can take heavier. Now it cruises at 3-5A.

The conversion is very straightforward.

Flight Controller setup and calibration instructions (beyond following the wiki for Quadplane and then vectored tailsitters):

  1. I set the “level” point of the plane for about 3 degs of angle of attack of the wing chord line. This is a nice sedate cruise attitude.
  2. I then corrected this for VTOL modes by setting Q_TRIM_PITCH to -3 deg, lest it drift backward when nose up hovering. Both #1 and #2 are items I usually tweak in the first flights.
  3. Be sure to get the motor direction and assignments correct, as well as the tilt servos directions. This can be checked by changing mode from QSTABILIZE to FBWA and back in horizontal flight attitude. Motor direction can be checked in the Mission Planner Motor screen.
  4. I find that LIM_ROLL_CD of 4500 is too lethargic…I use 5500.
  5. I find that Q_TAILSIT_ANGLE of 60 vs 45, and Q_TRANSITION_MS = 6000, is a smoother transition.
  6. I like Q_LOIT_ANGLE_MAX of at least 30 and Q_ANGLE_MAX of at least 4500 are needed to be able to fight even light (<10mph) wind in STABLIZE and LOITER modes. Of course, even with these values, moderate to heavy wind is always an issue for a Tailsitter.
  7. Since I am using a micro GPS, if using EKF3, it may take a while(>10min) to have pre-arm AHRS consistency checks pass. I worked around this by upping the EKF3_CHECK_SCALE to 120 from 100 (not recommended for Copters!). And there is a change pending to “latest” firmwares for plane (only) to set it to 150 like EKF2 is in plane. Or you can use a bigger GPS or aluminum foil backplane enlargement under the GPS (Careful not to short electronics!).

One final note: first takeoff to VTOL upright, don’t be bashful about moving the throttle quickly to mid-stick (actually a bit above, ~65% ). Otherwise, it will scoot across the ground until there is enough vertical thrust component at 45 deg motor angle to rotate the plane. Faster is better!


Very nice…Tail sitters are the way to go…

Great post! We’ve made a tricopter C1 Chaser as we found the tricopter set up better suited for high wind situations.

You want to share some more details and pic of your setup?

I imagine its like my tricopter conversion of the miniTalon

First, great write up. The community thanks you. Do you have any advice on forward flight as well? For example, thrust vectoring and tilt for yaw and pitch in coward flight. Seems there are many, many parameters but just wanted to hear what you thought was the most important.

I would just set everything (RUD_DT_GAIN, Q_TAILSIT_VFGAIN ) at 0 to start and fly it like a normal twin…once setup, tuned, and trimmed (use SERVO_AUTO_TRIM), then RUD_DT_GAIN = 10 (percent) helps carve the turns on any flying wing…

note using VFGAIN will change your tune for roll and pitch

Great job with the writeup. Nice structure and all details included. I have a question for you, though: Are the tilt servos strictly necessary? At what angle should i fix the motor nacelles to the wings if I dont want to take the servo vectored thrust route? I know that would make the plane a normal tailsitter but I would like to have as little complexity as possible for VTOL.

you would set the motors at zero angle or a little negative angle with respect to vertical cg of plane…like any twin engine plane…

you will find that getting good performance from a non-vectored, non copter-style, tailsitter difficult…its possible, but with much effort and sometimes redesign…there are a couple of sucessful commercial designs…I suspect the C1 would be a poor candidate…its elevon area is much. much too small to control pitch and yaw in VTOL stance from prop wash…usually they have to be >1/3 the chord…look at 3D style foamies…HUGE control surfaces for control when hovering…

I would stick with the vectored setup

Ok, good to know. Would you mind sharing the .step fiels for the 3d printed parts? I would like to modify them in order to use round carbon tubes.

Design is here…I use TinkerCAD online for designing

Well thats very unfortunate. Tinkercad doesnt have any real export functions (for high level geom.). I guess I will have to make the engine nacelles myself.

Exactly what I hoped to find. Thank you very much for your patient work to share. That’s what the best of this forum is all about.
I’d like to know more about transition and potential problems with board orientation when sitting on the tail.
Spent a lot of time reading the developer’s thread, a while back, but finally decided I was biting off more than I could chew. Gonna take another whack at it this winter.
Thanks again.