Small Unmanned Aircraft Research (sUAS)

Andy Johnson-Laird : Last updated March 3, 2015

 

sUAS Propulsion Systems

Multi-rotor helicopter sUAS are usually known as "copters" -- the name helps separate them from "helicopters" -- which can either be sUAS or full-scaled manned helicopters.

 

Copters rely on several subsystems to fly safely:

1. A flight controller that has nano-scale accelerometers (which measure acceleration along three axes) and gyroscopes (which measure rotation about those axes).
2. Lithium polymer (LiPo -- pronounced Lie-Poh) batteries that provide power for the motors that turn propellers.
3. Electronic speed controllers (ESC) that translate the motor speed commands from the flight controller into heavy current power to spin the motors in the correct direction and at the right speed.
4. So-called "brushless motors" that are driven by the ESC and to which propellers are attached.

 

In 2015, we really still are in the "Wright brothers" era of small unmanned systems. The FAA has only just now proposed some aviation regulations to govern how pilots can fly copters in the USA and it is unlikely that we will have final rules until, oh, say, 2017 (yeah, yeah, we're about five years behind Canada, the UK, Australia, New Zealand, and Europe).

 

Worse yet, from the point of view of someone buying or building a copter, the only folks out there who are publishing test data on the  actual performance characteristics of the LiPo batteries, the ESCs, and the propellers, are the manufacturer's themselves. There are no agreed-upon standards, or little or no data published about how the manufacturers are doing their testing. And, until we were daft enough to step forward, no independent testing companies.

 

You really see the problem if you ponder, for example, if Vendor ABC says that at 100% throttle, their SuperDuper 4991 motor with a Grynnflynk 14 inch propeller produces 2.2 kilograms of thrust, that statement alone raises more questions than it answers. What do you mean 100%? 100% of what? Does the amount of thrust include or exclude the weight of the ESC, motor, and prop? Or not?

 

So, suckers that we are, we decided to start testing LiPo's, ESCs, motors, and propellers. That meant we had to design and build our own apparatus from scratch using our knowledge of physics -- and half-a-dozen calibration weights of different masses. We built our first apparatus and it worked fine, but it was apparent that new motors were coming on to the market that were larger, more powerful, capable of spinning larger props, and with an increasing desire, were we to run them flat out, to rip themselves off the Mark I apparatus and head for Eastern Oregon or the Pacific Ocean (depending on which way they happened to be pointing). So we started all over again with a bigger, definitely stronger, Mark II apparatus that would resist a big motor and 28" prop's desire to be elsewhere even at the highest possible throttle setting. (We hope.) We also built a wire mesh safety cage around the apparatus and when it's running we watch from behind a large piece of 1/4" thick polycarbonate clear safety screen.

 

We also had to write our own software to take the data that the new apparatus captured and crunch it down into meaningful values. And test. And test. And test. Did we mention we had to do lots of testing? And suddenly, almost with out warning, we realized that we were done testing ready to get down to business. We now knew enough about our apparatus to know it was accurate. We had calibrated it (we had to develop cunning ways to calibrate it too!) and were ready to move through the various ESC, motors, and propellers, capturing the data, and writing reports. We didn't dare add up the costs, but it had taken us almost two years and close to $40,000 in terms of apparatus, data acquisition hardware, software, ESC's, motors, props, consulting time, and caffeine.

 

But before you dive into the results really need to read an introduction to the topic -- it sets the scene, defines some terms that you need to understand our results, and generally provides the intellectual life support you are going to need. So read this Introduction first, please.

 

And also check out our independent testing of LiPo batteries too. We suffered a bit to get that to be good quality science too.