My friend and I are building a small direct drive helicopter. We both have a background in mechanical engineering and we found it very difficult to obtain good specs on brushless motors and propellers. Some websites provide a results database, but these databases are frequently for a combination of both motor and airplane propeller, which makes building something different very difficult. When we started rewinding motors, we built a system to measure torque, current, rpm, voltage and temperature. With torque, we can actually completely decouple the motor and propeller in the results, which really speeds up the development and allows calculating ideal gear ratios or find the correct propeller, in order to maximize efficiency.
The code is based on a modified MultiWii system with a series of connected sensors. Currently, the tests are entirely automated to test different pitch and torque combinations. There are safeguards in the code to stop the tests based on measured temperature, rpm and current. The code automatically generates graphs of the results.
That got us thinking. Would people be interested in a community-based website with motor and propeller specs? Our code is already open source and we could make our test rig available for purchase. The rig could be sold on kickstarter for approximately 150-250$ if we sell 40-100 copies. We don't expect this project to generate money really, but the goal is to help the community obtain better motor and prop tests. The system would include torque, thrust, current, voltage, rpm and temperature sensors. The instrument would include a mount for most standard motors. People with test rigs could upload the motor and propeller results on the website and specify the brand, spec or custom winding, etc.
Below are a few more images of our tests. The helicopter is a modified FBL100 and we tested quite a few motors and propellers. Our tests demonstrated that it is very difficult to make a small and efficient direct drive helicopter. It flies though.
If you are interested, please reply to this survey!
Comments
@Brian Martin Thank! I would add that the design for the ''control volumes'' is also even more difficult as the propeller needs to be match to the motor.
Right now, the whole motor and propeller assembly is mounted on bearings. There is an arm attached on the motor mount that pushes on a load cell that looks like this . On the load cell, there are small strain gauges and the geometry does a really good job at measuring only the force in a single direction. The arm length times the force is the torque. We calibrated it with precision weight, and it is quite precise and linear. I will look more into the response time of the load cell.
oops i just read your first comment your dyno sounds very similar to a prony brake. I see that you have to wait for the load cell to settle. You could link the BEC of your ESC to a micro-controller (like arduino) and write a script ramp-up the throttle response very slowly so your load cell has time to settle. I'm sure they make fast response load cells, maybe omega.com would have something. A carefully designed inertial dyno i think would yield smoother results although their is some data that you cant get. my 2 cents :-)
-Brian
Great Job!! Like many other who have commented on this post i also have wanted to do this. I have always viewed a multirotors drive train as 3 separate and discrete systems (control volumes if you will).
But anyway as i said before i also wanted to build something like this (you guys beat me to it :-)). What sort of dynamometer did you design; inertial, prony brake, or some other type? My preliminary design will use an inertial dyno with a hall effect sensor to measure speed with an arduino but i have not built it yet. I would be very interested to learn more about your project, and sign me up for one of your testing rigs.
-Brian
I would be more inclined to build it myself maybe with the option of purchasing any custom or difficult to locate parts from you guys, but if that were not an option I would be willing to buy a full setup. ..
@Chris Paulson You are right about the noise. Since the tests were automated, we could afford to measure a very high number of points, so artifacts show quite will. We could run the tests a few times to do a statistical analysis. Think some of the noise may be due to the backlash in the servos and swash plate, which caused imprecise pitch.
Our test parameters (different pitch and input combination) were all in a spreadsheet. Our program opened the spreadsheet and looked for tests not performed yet. That was useful as we could easily add test points in a zone of interest without redoing the whole test. Having integrated protection in the code also allowed us to run the test without close monitoring, as the program would skip test points that required current or temperature too high. We even had the helicopter detach of the test rig once and the test stopped as the rpm detected was abnormal.
@F1P You are right, we should have specified, the tests were done at a constant voltage.
@Hugues The max efficiency is out of the graph. For the blades we tested (we tested about 10 different types, with length of 5 to 20 cm, symmetric and asymmetric), the peak efficiency is at very low thrust and RPM (under 30g force). We were designing for a 175g helicopter, so our graph is centered on the relevant part.The colors also get very difficult to read when including the very high efficiency at low RPM.
@ Adam Kroll The scale in the first graph is motor efficiency, unitless, from 0 to 1.The second scale represent the thrust per unit of power, in g/w. Quads usually have real life performances of 5 to 12g/w. It would certainly be useful for aircraft endurance, but mostly for the motor. The prop on aircraft has to be tested with airflow, although a static test can still serve to compare propeller profile.
The noise seems quite high in the motor torque direction. Can you elaborate on how you make the measurements? A little smoothing may help the plot. Also, I assume these are static tests? Would it be possible to mathematically model an estimate of dynamic thrust efficiencies?
That's very interesting! Especially since you got it all automated. I also built a test-setup and make the torque measurements with a leverage arm on a pivoting attachment point for the motor. I use different propellers on a motor/esc combination to measure the efficiency:
This graph could also be plotted as a function of RPM and Torque, but pwm and current are easier to measure for mostly. I concluded from this graph that ESC's lose efficiency at low pwm ( throttle setting ) which makes sense.
What are the z scales ie colours referring to in both graphs? This project would be really worthwhile for developing aircraft for endurance.
Interesting. A question: in your graph above "efficiency map for blade T" title states a max efficiency for values that are out of the graph. Bug or misunderstanding?
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