uav project update - fixed blade coaxial rotor

One of the uav projects I started last summer was a fixed blade coaxial rotor flyer that was steered by shifting battery weight in the style of the old Hiller Flying Platform . I put the project on hold while waiting for some lighter-weight brushless coaxial motors from Maxx Products (CR2805), and then got further sidetracked by the X3D-BL quad project.

This past week, I restarted the project, adding a mount for an SRV-1 Blackfin Camera board set, and writing the code to drive four servo channels - two channels for the E-flight electronic speed controllers, and two channels for the servos that gimbal the battery pack using a hobby helicopter swashplate. The servo interface is working quite well, and I have a slightly modified version of the java console used for the X3D which shifts the battery weight and controls the throttle. Horizontal translation is controlled by shifting the battery weight (as seen in the following videos), and yaw is controlled by trimming the difference in speed between the two prop motors.

In the first video, you can see how the weight shift mechanism works. However, I found that my battery pack was not producing sufficient output, so the flyer never got very far. In the second video, the battery had been replaced, but clearly some work is needed on steering. The crash at the end shortened one of the props by 0.5", but everything else was fine.

I think the basic structure is there, so the next step is to add accelerometers to aid in stabilizing attitude. I have both a 2-axis Analog Devices ADXL202E and a 3-axis LIS3LV, so I'll add one of the other. A gyro is clearly needed as well for yaw control, and I'm not yet certain which to use - I have a few with SPI interfaces that require 5V, or I might go with an analog output and use a voltage-to-frequency converter to capture the signal, since I don't have any A/D channels.

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3D Robotics
Comment by Chris Anderson on February 3, 2008 at 5:03pm

What a cool concept! It's not totally clear from these vids and pics how the counter-rotating motors work--do you have a close-up of that?

Also, why have the battery at the top and the propellers at the bottom? Wouldn't it be better to reverse those positions, so it's inherently stable?
Comment by Howard Gordon on February 3, 2008 at 5:20pm
Chris -

Here's a link to the CR2805 data sheet -

I don't have a closeup shot at the moment, but some explanation may help. The top prop is an APC 12 x 6 E, and the bottom prop is a 12 x 8 P (pusher). The reason for using different prop pitches is that the lower prop sees higher incoming air velocity, so it needs a higher pitch to generate comparable lift.

When I started building this last summer, my first designs took a top-prop approach. Intuitively, you would expect that having the props on top would be more stable, but it actually doesn't work out that way in practice. The problem is that the hanging weight acts as a pendulum which is really difficult to dampen. With a high COG, if the airframe starts to slide sideways, there's a side force caused by air resistance that tends to make the craft self-righting. You'll find that successful hovering designs such as the Honeywell MAV likewise have a high COG. It's also easier to balance the high COG - an analogy I've seen is the exercise of balancing a baseball bat on your finger - it's a lot easier to do when the meat of the bat is on top.


3D Robotics
Comment by Chris Anderson on February 3, 2008 at 5:26pm
Ah...makes sense.
Comment by Howard Gordon on February 4, 2008 at 7:54am
Chris -

I just added a couple of close-up photos that show the coaxial motors and the servo assembly. They're in my photo gallery on this site.
Comment by Howard Gordon on February 5, 2008 at 11:59am
Just posted a photo showing a 2-axis accelerometer (ADXL202E) attached to the bottom of the radio board and routed to Blackfin GPIO lines. Next step is to add support for this to the firmware.


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