I am an engineering student at Montgomery County Community College, Blue Bell, PA, US and I am part of a quadrotor research project (website: quadforge.net). We are using a DJI F450 with the APM 2.5 and this site, along with the hardware and firmware, has been invaluable. Thank you.
We are starting to test various prop and motor combinations to see which one gives us the greatest g/W. There a lot of threads with advice about which combinations are the best but not a lot about how to test which is the best. We have come up with a simple test rig:
The motor/propeller would be free to move in the notches cut out of the PVC. Everything would be connected to the APM 2.5 (with power monitor), we would set a few specific throttle levels and see which combination generates the most thrust over how much power it is using. After we figure out which one generates the most g/W we're going to fly and see if we are right.
What do you think of the rig?
How do you test out which motor/prop combination is the best (so you know, we are not looking to change frames which limits our prop diameter to 12 inches)?
Do you have any advice as to what motors and propellers to purchase (other than the stock DJI ones we have)?
I'd recommend Graupner E-Props or Xoar E-Props for the best efficiency. Also get a quality motor with quality bearings; if you can afford ceramic bearings, get them as they will give you the best efficiency.
I'd be interested in seeing your results, will you be posting them to the website that you mentioned?
I assume you are going to incorporate a graphing watt meter into the test stand. That's a critical piece of equipment for prop and motor testing. There are plenty of relatively inexpensive ones out there.
I did a lot of testing a few years ago when I started electric flying. Used a simple L-shaped thrust meter pressing on a digital scale and a Hyperion Emeter connected to a PC running Logview (www.http://logview.info/). Peaks were recorded both by the Logview program and by taking a photo of the Emeter and scale at full throttle.
The best though is not to use a battery if possible but rather an adjustable power supply, that way variations in measurements can be avoided. (Different LiPo batteries under load can read anywhere between 10.8 and 11.4V which messes up all the measurements.)
g/w is good for different prop sizes, rpm/w is good for the same prop size.
These are some of the peaks:
We are using the 3DR power module and another voltage/current meter to double check it. We can graph the voltage/current data from the APM's .tlogs.
We'll be posting the information to the website and we might also post it here.
Thanks for the tip on the props.
For props and motors you have to take the common lowest denominator, i.e. which are most commonly used. There is such a massive variation of props but APC slowfly are fairly ubiquitous, others are rare in many regions. Other variations like altitude and temperature can make a difference too.
We have a power supply that I didn't even think about. Thanks for pointing it out!
Why is RPM/W a better efficiency rating when looking at the same propellers?
Not neccessarily more efficient but it just makes it easier to compare motors, you can actually leave out a thrust meter as the formula for thrust is "ThrustVal = 0.00000000128199 * RPM.value^2 * Diameter.value^4 * Density.value/29.92 * CF.value", For a simple version ignore Air Density/29.92 and CF value which are both very close to 1.
Quadrotors also use relatively low watts in the hover, mine uses about 45W (4A) per motor so 180W for all 4 but can go to 450+W in a full power climb. the peak efficiency point varies from motor to motor, mine are supposed to be around 5.5A.
This site is good if you like the numbers: http://www.ecalc.ch/xcoptercalc_e.htm
Where did you get your equation from? and what is the CF value?
Also, where did you find that your motors peak efficiency is 5.5A? Is that number from the manufacture or from testing?
We are using ecalc but some of its numbers don't seem to add up. In the bottom columns it gives a voltage, but the RPMs (gotten by multiplying KV by voltage) don't match. We are also looking at this calculator: (http://www.drivecalc.de/).
When you did your tests, did you have the motor connected to an APM and then use a controller for the throttle or did you script it?
I did a Google search for "propellor thrust formula" for an Android app that I created (here).
My motors peak eff. came from the ecalc.ch website but note that Kv is only valid for a motor under no load.
When I did my tests was before APM's came along so I just had an ESC, batt and motor. I was testing for fixed wing aircraft and tested every motor I got to see how they'd perform, (the manufacturers specs were often way out).
I even spent too much time rewinding motors to change their characteristics, a friend who now also winds motors for fun said one well known cheapie brand had 4 different number of winds on it's 10 stators, they're obviously s'posed to have all the same.