Per several requests:
This details the conversion of a Wing Wing Z-84, 845mm flying wing to an ArduPlane TVBS (Twin motor Vectored Belly Sitter). A tailsitter which lands on its belly instead of its tail. This is the samllest TVBS I have build so far. Only about 650g AUW, 2.5A cruise, 10A hover on 4S, 3000-3500mah Liion battery for ~45 minute duration with VTOL takeoff and landing.
I started from a NIB Z-84 kit, but you can convert your existing one if you are willing to do a little cut job on the motor enclosure. You will need the following:
Two 5" mini-quad style motors. I used generic 2204/2300Kv motors.
Two 20A ESCs. I used standard PWM, no need for BLHeli32/DShot/etc, but okay if you have them
Two Gemfan 6x4 Props. These have a lot better efficiency than many others.
3D printed tilt rotor motor mounts. I re-designed my tilt mounts to be the smallest, possible. Smaller than I had on my C1 Chaser and MiniTalon Tilt Tri. They seem to hold up well and when they are broken, only the pivot block breaks, not the wing mount.
STL files: https://www.thingiverse.com/thing:4580761
4, 693ZZ 3 x 8 x 4 mm ball bearings (https://www.amazon.com/gp/product/B07DZCZWB3 2)
4, 3mm bolts for the motors
A method to couple the tilt servos to tilt mount. I prefer ball links, especially on my MiniTalon, but on this lighter plane, I just use one ball link and a Dubro quick link on the servo arm.
2, EMAX ES-09MD dual ball bearing micro digital servos (plus two for the elevons if a new build
2, 3mm x 30mm bolts for tilt mounts
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 1)
A flight controller capable of running ArduPlane 4.0 or later with 6 servo/motor outputs and GPS/compass. I used a OmnibusF4Pro clone just because I had it laying around.
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 cruises at 2.5A.
The conversion is very straightforward.
Remove or dont build the pusher motor and ESC. I mounted my FPV video transmitter in the tail area vacated and attached the motor foam cowling to the normal hatch as one piece.
Assemble and mount the twin tilt motors on the wings approximately 140 mm from the balancing dots under the wing. I just hot glued them.
Mount the GPS and compass in the rear.
I added a nose skid to add clearance for the props in grass.
I used a standard micro mini-quad FPV camera (Runcam Sparrow) and 600mw 5.8Ghz VTX in the rear.
-Connect and setup the FC.
Flight Controller setup and calibration instructions (beyond following the wiki for Quadplane and then, vectored tailsitters):
- 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.
- I then corrected this for VTOL modes by setting Q_TRIM_PITCH to -8 deg (since the flight controller is mounted in an area with a little slope it needs a bit more than -3 to compensate for the 3 deg positive “level” above)), lest it drift backward when nose up hovering. Both are items I usually tweak in the first flights.
- 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.
- I find that LIM_ROLL_CD of 4500 is too lethargic…I use 5500.
- I find that Q_TAILSIT_ANGLE of 60 vs 45, and Q_TRANSITION_MS = 6000, is a smoother transition.
- 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.
- 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!). (in master this is now defaulted to 150 already…you can leave it if it is). 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 . 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! But you will find that it only takes about 17% stick to actually hover once up…this will be reset automatically to mid-stick if you switch to QHOVER for a few minutes with Q_M_HOVER_LEARN=1. Then hover will be at mid-stick in all VTOL modes.