I just did a scatter plot of Thrust compared with Watts. It's below. I need to figure out why there is so much variation in the wattage data -- the problem is that I'm having to sample the data at 1,000 samples/second to get the Photogate/RPM data, but I think that's causing issues with reading the current (via a Hall effect device that converts the current to a voltage -- and there seems to be a lot of jitter in the Amps data -- and that's, or course, being used for the Wattage calculation). Reading between the lines -- well, dots, actually, it seems that the APC MR's might get more thrust per Watt than the regular APC...with the Xoar's somewhere in between. Andy
I agree. What are you doing this evening? Actually, I need to shield the Hall effect sensor and see if I can reduce the sampling rate (as well as using stats). Andy.
Totally agree with Steve on the thrust vs amps issue. Actually I like amps not watts since the voltage varies during the life of the flight. Mario's testing has always been interesting since you can look at his scale and power meter and simply stop the video where you typically see your power. That's why I always look at the 10amp spot. Close to our typical power draw and to my mind gives an apples to apples comparison of a given prop.
Apart from having dinner with you guv'ner, not much. Sounds like we need to burn some motors and break some props this evening!
Gary: Can you clarify your Amps/Watts comment, please? Normally one uses Watts precisely because the voltage will indeed vary and thus as the voltage drops, the current has to increase to accomplish the same amount of "work" (that is thrust) to keep the aircraft airborne. Given Watts = Volts x Amps, as the voltage drops the current will increase, but the Watts remain the same. That's why I'm left with that feeling that I'm missing what you're saying? Perhaps I should have pointed out that the graph showing Watts was created using a DC stabilized/regulated bench supply set to 16.0 volts? Andy.
Andy doesn't the MK screen show only amperage draw and mah used? I haven't looked at a GPX file in awhile but don't remember anything in watts. Elsewhere, like the specs on the TMotor site, are all given in amps for a voltage. For consistency from other sources wouldn't amps vs thrust be a more consistent of depiction?
I think you're right about MK Tool, but measuring a motor's efficiency in terms of electrical power input to the motor cf. thrust out is best measured in Watts because you don't know what the voltage is that is giving rise to the current draw in Amps. Of course, in this case, I'm trying to provide a measure of the efficiency of the electrical motor (which is a constant for these three tests) and different props. For example, see the following link and note the Electrical Motor Efficiency -- in this case, of course, I'm taking it one stage further and I've already converted the power output Kilograms of lifting force, but note the Pin value. http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html I'm ultimately hoping that I'll test other motors and props and try and understand which are the most efficient -- as well as which are the most responsive. I should also give acknowledgement to Old Man Mike, on rcgroups. http://www.rcgroups.com/forums/showthread.php?t=1006721 It was from him that I got the idea to try these tests. Note also that he expresses efficiency in a unit of Watts/oz at maximum Thrust. However, I've got no axe to grind -- if folks would rather see Thrust cf. Amps, I can do that without any problem. Andy.
Mario de Roos, one of out forum members, has a large set of videos of his testing, likely similar to what you are proposing. https://vimeo.com/user11307214/videos
Thanks. I had see some of those videos but didn't realize they were all conveniently accessible. Mario's work is great. I just wanted to plot out some graphs so I could see at a glance which props might give the most thrust from idle RPM to leap-to-the-stratosphere power. I was presuming that the Xoar's would provide the most thrust compared with RPM so, as I say, I was bit taken aback that doesn't appear to be the case. I'll be repeating the entire cycle of tests this week just to verify the data. Once I've verified the data from the test rig, I'd be happy to test other motors and props if people are curious about them. Andy.
I suspect there's a wealth of data there...it's just a matter of lining up all the numbers in a meaningful way. I appreciate the hard work, Andy.
I agree -- too much data, too little time! . I'm going to start by re-calibrating all the sensors, as well as getting cleaner data without any outliers (I hate outliers!). I also want to do some "sensitivity testing" -- that is waggling the throttle up and down at approximately hover power settings to see how quickly the props/thrust changes. I'd like to get an objective test of at least these the APC, APC MR, and Xoar just so we have some objective data to base our buying and flying decisions on. Andy.
My brain aches reading some of the discussions on these forums ... But I find myself coming back for more ... I must be a sucker for intellectual headaches. Great work guys! Please continue delivering the pain
I was commenting on your mathematical prowess, which is several neural zip codes away from your vision, dexterity, and equilibrium centers.
Aha. As in, there are 10 kinds of people. Those who understand binary and those who don't. Got it. There are parts of my brain that don't seem to have zip codes....well, if they do, there's not much zip.... Sebastian: For the headache: Red wine. Or Guinness.
[quote="For the headache: Red wine. Or Guinness.[/quote] Andy .... I think I'll start with the Guinness and finish it off with a fine Red.
Andy If I could get the raw data I could do a regression and statistical manipulations to see if I can find correlative terms in the power scatter plot. Also, you know it would be cool if you could do that same test with coaxial motors. I have been curious to see what the thrust differences in 1. Identical Diameter/Identical Pitch top and bottom 2. Identical Diameter with a coarser pitch on top then the bottom 3. Large diameter on top small diameter on the bottom same pitch. 4. Large Diameter prop coarse pitch on top/ small diameter prop shallow pitch. It would be interesting to dispell all of the myths rumors conjecture with the coaxial configurations.