Quad-Y-Copter (V-Tail) Specs
To answer a few questions about the specs of the Quad-Y-Copter (V-Tail) here are some details on the config;
Motors: All four are rctimer.com 750kV
http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=123&productname=
ESCs: Turnigy 30Amp. I haven't measured the current draw but I'm sure 30A is overkill. http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=2164&aff=257015
Props: 3 Bladed 9 x 5 regular and counter rotating.
http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=5250&aff=257015
http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=5248&aff=257015
MultiWii Software 1.9 http://code.google.com/p/multiwii/downloads/list
Y4 Config as per http://www.multiwiicopter.com/pages/multiwiicopter-wiki
The frame is 12mm x 12mm Tassie Oak arms and 3mm ply.
Inverted V-Tail arms at 20 degrees.
Custom mixing.
#ifdef Y4
motor[0] = PIDMIX(+0,+1, -1/2); //REAR_R
motor[1] = PIDMIX(-1, -1, +1/5); //FRONT_R
motor[2] = PIDMIX(+0,+1, +1/2); //REAR_L
motor[3] = PIDMIX(+1, -1, -1/5); //FRONT_L
#endif
QuadYCopter video and performance review
If you do nothing else, skip to 48 seconds in the video below and watch the crash when I hit a tree and land right side up. I ask you how many types of models / electric helis can you punish with that poor piloting and keep on flying? If that had of been an electric heli, I would probably have been up for new blades, boom and miscellaneous other bent rods along with the time to take apart and re-assemble. The QuadYCopter is made from ply, 12mm x 12mm Tassie Oak booms, cable ties, 3mm aluminium and 3mm bolts (both nylon and steel). The two front arms are held in place with cable ties which means in the event of a crash, there is a fair bit of give without breaking anything.
The photos show the main booms are quite a lot longer than needed. That's the result of experimenting with the position of the motors and moving them closer into the centre. The unexpected benefit of this is protection for the propellers. I haven't broken one prop yet with this config.
The Y configuration of the booms is definitely easier for me to fly and keep orientation when compared to the symmetrical Quad. The bright red anodised aluminium landing gear at the rear is also a great help. In the video at about 4 minutes I do some "high-ish" altitude video and maintain good visibility, smooth flight and it descends quite well. Some configs I've flown haven't coped well when descending rapidly and wallow quite alot that sometimes ended up in uncontrolled oscillations that didn't end pretty. Not so with the QuadYCopter. I could descend quite rapidly and even up up the throttle quite aggressive when it got low and it would recover quite well maintaining a flat attitude.
The V-Tail config has good authority with plenty of yaw control. To be honest, in all my RC flying (mostly planes) I've never used the rudder (equivalent) until I started flying multirotor. The QuadYCopter can do some quite tight turns and makes a great prop noise when pushed hard. With a lighter touch, it will do graceful arcs producing some nice stable video footage.
Stability is good in "aerobatic" mode but in "stable" mode I'm still getting oscillation. I reckon some PID tuning should sort that out.
So for some ratings out of 10;
Durability: 9
Stability: 7.5
Aerobatic: 8
Visibility: 8
Simplicity: 9
Things to do;
1. Sort out the "Stable" mode.
2. Install FPV gear
3. Maybe install some LED strips.
Optimising the QuadYCopter
There's a few things I'm trying out to get the QuadYCopter as stable as the QuadCopter is;
- Three Blade props
- 4S Battery
- Shorten the front arms
- Restore the PID values to default
- Custom code.
By far, the biggest change / improvement has come from using Three bladed props and a 4S battery. Some might argue a well balanced two bladed prop is just as good as a three bladed but in my experience, the 3 blades seem much easier to balance. The 4S battery also makes a huge difference to the throttle response. It's much zippier and for example when I jockey the throttle on 4S, the Quad actually jumps and dips. When I do the same on a 3S battery, the response just isn't there and it's sluggish.
I'm trying modified code specifically for Y4 V-Tail I found in the forums here http://www.multiwii.com/forum/viewtopic.php?f=8&t=665
The default code is;
#ifdef Y4
motor[0] = PIDMIX(+0,+1,-1); //REAR_1 CW
motor[1] = PIDMIX(-1,-1, 0); //FRONT_R CCW
motor[2] = PIDMIX(+0,+1,+1); //REAR_2 CCW
motor[3] = PIDMIX(+1,-1, 0); //FRONT_L CW
#endif
The modified code is;
#ifdef Y4
motor[0] = PIDMIX(+0,+1,-1); //REAR_1 CW
motor[1] = PIDMIX(-1,0, 0); //FRONT_R CCW
motor[2] = PIDMIX(+0,+1,+1); //REAR_2 CCW
motor[3] = PIDMIX(+1,0, 0); //FRONT_L CW
#endif
Today I moved the two forward motors back on the arms so they're equal distance from the centre as the two rear motors. Initially I had them further forward to counteract the weight imbalance of having two 'sweptback' arms but with proper battery placement and I want to use a GoPro up front, I don't think I need more weight up front.
I restored the PID values to default but still get some osciallation in "stable" mode. before I lower P any more, I need to see what the effect of moving the motors has.
QuadYcopter Maiden Flight video
There's a few different names that might be applicable like a Y4, Inverted V-Tail, Y-Copter but I think I'll call it the QuadYcopter and it flies! All I had to do before this video was reverse the direction of two motors and it flew better than expected considering the lengths and angles are all eyeball precision.
I expect the QuadYcopter should deliver the best aspects of Quads and Tris;
- No mechanical tilt needed on the rear motor (tri negative)
- Larger unobstructed field of view looking forward (tri positive)
- Non symmetrical airframe which should be easier for orientation at long distance (Quad negative)
Notice in the video, it's quite twitchy in pitch. I reckon that's related to the CoG which I've moved forward by mounting the battery all the way forward. This isn't a big problem as I'll probably mount a GoPro up front which will further help with balance. The underside mounting for the battery means it can be moved all the way from the rear to the front to compensate for camera / cameraless flight.
I still need to mount a low voltage buzzer on the airframe. The problem is, I've started using an 1800mAh 4S battery whereas most of my other batteries are 3S. I've had to order a couple of voltage 2S-8S detectors that let the voltage be set via a jumper so I can fly with either 3S or 4S