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!
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The variable pitch portion was an adaptation of an airplane variable pitch design called "showstopper". I then used a 450 tail housing from a heli to hold it on a boom. The center plates were custom cut on a CNC out of fiberglass plate. The design had 2 problems though. It was very expensive because I had the main blocks 3D printed at shapeways which cost nearly $200 in printed parts. The second issue was noise. With all of the gears it was terribly loud with a lot of mechanical friction which robs power. I flew it inside once and very nearly went and got ear protection. no kidding!! I am happy to share some of the CAD files but I think the design needs some work if you really want to do something with it. since I built this things like the Stingray have come to market which are superior designs and they might be better starting points for a variable pitch design.
Hello , it is very interesting. I want to build one, would you please send me the parameter of your version 1 with apm. imawolfking@gmail.com. Thank you.
I didn't think about the hollow shaft friction before, but this could be avoided in a custom design using bearings probably sliding ones since the diameters would be very small.
But the efficiency would probably be a non issue since the belt drive could be installed on a slightly more efficient motor (higher KV )
I think that the main reason for the lack of options is that people just don't care... most models are made for aerobatics and that limits theirs flight envelope substantially... and flight controllers that could monitor the prop pitch are also a new thing... and the advent of the electric drive is kind-of-new ;) and variable pitch on a gas motor is a whole different story because of vibrations.
But in an UAV I think it could be advantageous especially with a folding prop... when using a system with airspeed and prop RPM measurements I think it could actually get you pretty good efficiency boost in the edges of the flight envelope - like take-off and high speed flight :)
It is true that you will lose a little bit of efficiency with a belt reduction but it will not be a huge loss. Overall you would actually gain efficiency because you can swing a larger prop. Although your maximum airspeed would be reduced as a result. I never actually measured the loss on a belt vs the hollow tube but my experience with it suggests that the hollow tube is actually less efficient. It has a lot of friction inside the tube, at least on the e-flite system that is the case. Regardless of which solution you go with I am not sure you can increase the efficiency enough to compensate for the increase in weight. Most likely this is why you see so few options n the market.
The e-flite system is too small I think, so is the hobbyking one... But there could be a way to adapt those... Where Could I get a larger diameter hollow shafts like 4-5-6 mm ?
But I like the gear reduction idea... I am a bit concerned about the efficiency loss there and the complexity.
What would I like most of all is a "hollow shaft" system adapted to use with a folding prop. it would be a perfect setup... But AFAIK there is no such thing on the market...
A system like that wouldn't be too complex and could be a great addition to an autopilot system - pitch control depending on airspeed, also using it to control descent and breaking.
E-flite makes a variable pitch prop called showtime made for direct drive on airplanes. I actually adapted this for the first version of my variable pitch quad. Essentially it replaces the motor shaft with a hollow tube so you can run a servo actuated rod through the middle. This rod then controls the variable pitch from the top. For airplanes it works pretty well but for my quad I found that it had some limitations and I moved away from that design. If you want something larger it would be fairly easy to adapt the shaft and pitch slider from a heli tail rotor to a gear reduction drive system. You just need a front and rear mounting plate for the prop shaft which would be a tail rotor shaft. Then you put a pulley on it like this http://www.mcmaster.com/#mxl-timing-belt-pulleys/=qazfei. Then you mount the motor on the plate with another pulley and run the belt. Then all you need is a servo mount to move the pitch slider. The heli blade hubs would be thicker then the props you showed so you will have to put some washers to get the prop to sit correctly. For the props you can just cut one blade off of the hub and mount it, so you need 2 props to fit one hub.
I'm looking for a good variable-pitch setup with a direct or geared motor mount for use on fixed-wing aircraft. I was thinking of using folding props - since they seem to be one of the most available options and they also have added benefit of drag reduction when folded :) But in this configuration it seems there are no available pro-hubs on the market, nor any working projects...
Other idea is to adapt some props with "flat mounting" like so:
http://hobbyking.com/hobbyking/store/__42851__16x4_Carbon_Fiber_Pro...
and making a pair of props out of those... when using a heli-type prop hub...
Anyone have any ideas?
I know should be (especially if If I would've known where to start)...
Could you point me to location with descriptions of APM's functions, so I could start learning my way through...
Also am I correct that I will need to change MP to accommodate new settings or it will display whatever changes I do with the apm firmware automatically?
Artem, that is a great idea! The Roll/Pitch/Yaw/Throttle commands already come into the Heli controller in separate terms. The heli controller does the mixing. The heli controller had 4 servo objects you'll need to make this work.
I would suggest just copying the heli object, and then modifying it as you need, change the mixing, etc. Should be easy.
I have something similar in mind, but this would involve changing MP to be able to adjust those new settings... and there is where I am lost as I hadn't have time to look into MP source...
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