I had a lengthy conversation with Jefito yesterday evening and ended up spending some time doing a bit of CAD work. He brought up motor cooling; specifically, the fact that I might be able to get better performance out of my brushless motors if I could figure out a way to cool the stators during operation. He had a few ideas beyond simply increasing airflow, including some novel thoughts featuring dry ice or liquid N2. Definitely worth considering, as GimbalBot's flight time probably means the few minutes of coolant I could carry would be adequate. It's been many years since I've used LN2 in a personal project and I've been looking for an excuse to find a good MN supplier.

However, we ended up discussing air cooling as a simpler alternative. I came up with an idea for directing additional airflow into the back of the motors, taking advantage of the fact that they already have large cooling passageways machined into their backs (with vents opposite those holes on top of the motor rotors):

Using the hollow CFRP motor mount as a duct, I used a series of diagonal slots to orient three CFRP plates to deflect airflow. I also cut a few holes where the motor sits (and in the CFRP motor mount plate, not shown) to direct airflow into the two motor cavities:

Here's a confession--I've never done a 3D printing project. I've certainly played around with 3D printed stuff, and hit the button to run a stock program or two; in college we had a Z corp SLS setup that we used for resin bonded sand, but it was being used for things quite a bit so I set my sights elsewhere. Since then, I've always gotten hung up on details like machine/material cost and the structural deficiencies of commonly available rapid prototyping compounds. In this case, however, 3D printing is the perfect fit for a few nozzles:

Yaay fillets!

I dropped the opacity of the motor down a bit so the fit between the nozzle and the motor/mount makes a bit more sense:

I need to tweak the dimensions a bit, mostly because my original measurements of the motor vents weren't intended to be highly accurate. But you get the idea:

Sorry about the crudely annotated drawing. Green is motor rotation, blue is cold air (with length somewhat proportional to velocity), and purple and red are air getting progressively hotter as it travels through the motor. The idea is that the air velocity below the propeller is not uniform; instead, the velocity increases as you get to the perimeter of the blade:

source: http://www.gorhamschaffler.com/system_effects.htm

Or at least, that's what my understanding of propellers suggests. This design is intended to capture some of that high velocity air and send it back through the motor. Think the pressure above the motor will be low enough to cause a decent amount of air to move? Or am I naive to assume that the velocity profile of the air coming off the fan blades can be described so simply, without accounting for the myriad of factors (distance from propeller, airflow disturbances caused by the frame, propeller shape, lack of duct compared to the picture above, etc) involved in a good simulation? If I follow this plan, I'd cut a fairly large hole in the CFRP motor mount and run instrumented cooling tests at a constant thrust (or current) with various inlet opening sizes and locations. 

So here's another thought: What if I design a venturi that will pull air through the motor and exhaust it through the CFRP motor mount? I'd still use the same nozzle, but the motor frame might look more like this:

Then it's time for a bit more 3D printing:

These little guys fit into the motor mount like this, with air velocity now represented by line thickness instead of length:

If you aren't familiar with the venturi effect, take a read through the Wikipedia article. Essentially, Bernoulli's principle says that conservation of mechanical energy along a streamline (along with a few assumptions about the fluid, such as incompressibility or at least fairly low speed) means potential and kinetic energy can change, but must sum to be the same figure. If you do the math, you'll find that the velocity of a fluid traveling through an orifice is proportional to the square root of the pressure drop across that orifice, corrected for fluid density: 

source: http://en.wikipedia.org/wiki/Venturi_effect

where p1 and p2 are the high and low pressure ports, rho is the fluid density (assumed constant throughout the system--i.e. incompressible) and v1 and v2 are the velocity at the narrow and wide point in the system. So... I've got a high velocity jet of fluid (air) being forced through the venturi by the propellers, and the four low pressure ports are near the vena contracta. The pressure differential between this point and the top of the motor stator should give me some flow through the cooling system. Again, I'm hoping that the holes above the motor will provide an adequate high pressure side to generate flow through the venturi.

So--I'd love to hear some thoughts on this before finalizing dimensions and sending the 3D printing order out. I'm actually fairly comfortable sizing a venturi, since I spent a few years selling industrial differential pressure based flow meters (amongst many other types). I'll need to make a few assumptions about things like velocity, but I should be able to get a rough idea of beta ratio (the ratio of the constriction diameter to the entrance diameter) and port cross sectional area. I think it's a better solution than the flow diverter model since it should have a much less severe effect on the propellers, and I'm much more confident that I'll be able to achieve an adequate pressure differential across the stator to generate some airflow. Having said that, the turbulence and general sketchiness of the airflow that close to the propeller could cause all sorts of unpredictable effects that make the system fail miserably.

More to come on this tangent, I assure you! Comments?