I have a long history of being involved with R/C planes and helicopters and moving into drones is a natural extension of that. However, until recently I had little exposure to quad copters and I had the opportunity to see them fly at a drone meet. I got interested in them and decided to build my own instead of buying something off the shelf. The goal for the project was to develop a central motor variable pitch quad copter. The purpose of the variable pitch is to increase the flight envelope which allows things such as inverted flight as well as faster response to things like wind. The benefits of this were well documented in the work that MIT did and a video of some of that can be seen here http://youtu.be/VIkqqVr_u9U. The second portion of this project is to centralize the motor. The purpose of this is to get the heavy motor off of the ends of the quad to further reduce inertia and increase response rate. It also allows the user to gear down the motor to swing a larger prop at slower RPM. This increases the total prop area and reduces the disc loading which makes the quad more efficient. It is also my hope that the mechanical systems will be lighter than the motor/ESC setup which will reduce the overall weight. The downside to all of this is the added mechanical complexity and increased cost of components.
The design effort started with a traditional X configuration with a central spur gear that ran the power to pulleys. This design did work but it was REALLY noisy and had so many gears that it robbed efficiency. The support blocks were made from 3D printed nylon which I had done at shapeways. The gears were RC car parts and the variable pitch system was from a small RC plane system. I used an APM for the flight controller. This is version 1 and it can be seen flying here https://www.youtube.com/watch?v=ZrD4gScApQk.
A lot was learned from this design and we quickly moved to version 2 which is nearing completion. Version 2 features a more efficient and much quieter drive train as well as a V-tail configuration. The official reason for the V-tail configuration is that it has better yaw control but secretly the reason is that it just looks cool!
Comments
I would start by modifying a traditional heli apm firmware.... adding a function which would demux swash plate into 4 servos instead of three.
The V-tail has not flown yet and I don't have a flight controller setup and working. Right now I have an APM and a KK2 flight controller. I would prefer to use the APM and that is what I used on Version 1. I didn't do enough research yet to see if I have the skills to make the APM work. If anyone has suggestions on how to make the APM work with a V-tail I would appreciate it.
@ Fred Briggs which flight controller are you using? is it a modified apm?
On version 1 which is an X configuration I had to make significant changes to the PIDs and I had to reduce the PWM rate to 50Hz from the default 200Hz. Other then that I did not make any changes. On version 2 with the V-tail I will most likely have to make significant changes. I have been putting that off because I am not a great programmer and I am not exactly sure where to start. Once the hardware side is done and I don't have any excuses left I will try and tackle that.
What kind of changes did you make to the APM to control the copter?
I agree on all points. The driveline drag is probably enough to prevent autorotation. But just thought I'd point it out as an option, since lots of people like the idea of autorotation on a quad.
The drag is bad even on the Protos. Without blades on, the head stops very quickly. About 1-2 seconds. I haven't really tested it with blades yet. But for sure, it's not great.
That is a really interesting solution that I was not aware of. I think the only way to make practical auto rotation in a design like this is to put a one way bearing in each pulley on the rotor shaft so that you bypass the drive line drag. Even by doing that I don't think these blades have enough inertia to actually auto rotate. If the design scales up it might be possible but on this small scale I think it would be really hard.
Well, actually the way that you have the motor oriented now, it would result in a rolling torque, not a yaw torque, so that is much easier to resist. That's good.
FYI, you might be interested to know that the MSH Protos drivetrain, has the one-way bearing installed in the motor pinion, rather than the large gear/pulley on the main shaft. As such, you could use that motor/pulley in your design to allow the blades to over-run the motor. This, theoretically could allow your machine to autorotate.
I'm not sure if it really could auto well, since the drivetrain has so much drag, the blades might just stop anyway. But if you want to play around with it, it's another neat idea.
So, the Protos uses a special motor, shaft and pulley assembly you'd have to buy. But it's just an idea.
Thanks for the comments everyone! I am familiar with the Stingray. I have not seen it in person but the pics/videos I have seen are very impressive.
@R_Lefebvre, The rotational mass of the spinning armature and small pulley in the center is not enough to cause any meaningful torque which must be compensated for. There is plenty of efficiency loss in my design (i.e. V-tail, belts, gears, etc) but the torque imparted from a central motor is not a concern. The purpose of the V-tail is to improve yaw control rates which are fairly slow on a typical quad compared to a helicopter. The downside to putting the rear blades at an angle is that I will lose lift efficiency. I plan on playing with the angle that I rotate the blades to see what will give me the yaw rates that I want without sacrificing to much lift efficiency. I am starting with a 35 degree angle and will go from there. For people who are concerned more about the efficiency than yaw rates it is easy enough to 3D print a new block that spreads the booms and puts the rotors in a more traditional flat design.
@ikrase- I do plan on putting a prop with a proper under camber airfoil for more efficient flight. I would guess that with the blades I have on it now that flight time will be reduced by 40%. I went with the heli tail blades because it was the path of least resistance and I would like to try some 3D with it :)
Thanks for adding photos. Slick design.
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