I wanted to do a few real world hover tests with different prop sizes to satisfy my curiosity.
I tested 3 sizes of some commonly used props, the RCTimer Carbon 10x4.7, 11x4.7 and 12x3.8, all carefully balanced. There are other very similar props branded differently.

The 'copter is a carbon Tricopter, battery used is a new Turnigy 3300mAh 30C 3 cell. The motors are the very good Sunnysky x2212 980kv. Weight is 1046 grams (37oz) and the altitude above sea level here in Johannesburg, South Africa is 1500m or 5500ft.

The method was to take a fully charged battery, connect a BNB Products "Digital Power Recorder 50" inline, takeoff to 2m (7ft) altitude, hit alt_hold and let it hover in the same place for 90 seconds, then land. I then trimmed the log in the software to show only the first 60 seconds starting once the aircraft was stable. Not terribly scientific but enough to get some interesting data.

A summary of the tests:

  Average Watts  Average Amps  Average Volts
 10x4.7  140.54  11.81  11.90
 11x4.7  144.97  12.14  11.95
 12x3.8  147.45  12.34

 11.96

First observation is that there is not very much in it, the 10x4.7's are about 3.1% less on the watts compared to the 11x4.7's and about 5% less compared to the 12x3.8's but there is noticeably less power with the 10x4.7's.

In fact, at the altitude I'm flying at I would not use the 10x4.7's as there really is not much power in reserve if you're coming down from a fast descent, it takes a second or two and 2m lost altitude to arrest a descent. It would be really bad with a payload.

I also expected the 12x3.8's to be lower watts than what they are, but this may be due to the larger surface area compared to the others and there must be more drag.

Here are the graphs:

10x4.711x4.7

12x3.8

Views: 1379

Comment by thomas Butler on April 18, 2014 at 10:08am

Perhaps compare them to the new APC multi-rotor 12x4.5 (MRP) props?!  The MR props are stiffer and have a beefier hub. There was lots of hype over the MR props, but they've been a disappointment on my machine.  Is carbon fiber worth the added cost?  I tried the MRP props, and I found they set up a resonance in my copter!  Went back to the SF props at $5 each, and all is well!

 

I'd be surprised if you didn't fry the 2212-980 motor with the 12x3.8 props!

Comment by Gary McCray on April 18, 2014 at 10:30am

Hi Graham,

Very interesting and the opposite of what I would have expected and what has been my own (admittedly limited) experience on small quads.

I also plugged you copter into Xcopter calc and the main numbers fell in the same ball park, except the prop diameter relationship to efficiency was reversed as I expected.

Although at max power both of the larger props fell outside of the acceptable level for permissible electric power.

Clearly, depending on prop design, and actual real world characteristics, these things do not always come out as you expect.

On my own DJI F450 Flamewheel (with stock DJI 2212/920kv motors), the quadcopter is noticeably more efficient with 11" x 4.7 Gemfan carbon filled props than 10" props either DJI or Gemfan props.

It is also much quieter in a hover with the larger props.

I get at least an additional two minutes of flight time with the 11" props all things being equal.

Mine would probably be over current at max thrust, but the copter has so much excess power that even if you do open the throttle full it doesn't reach max power and it would be out of sight in 4 or 5 seconds.

Clearly even though you only switched props of the same type from the same manufacturer, it is possible to arrive at results that are not as expected.

Best Regards,

Gary


Moderator
Comment by Graham Dyer on April 18, 2014 at 10:33am

@thomas, well the 12x3.8's only draw 200mA(!) more than the 11x4.7's so potentially frying the motor with a bigger prop like this is a common misconception. Each motor is only drawing 4.11A so they aren't even close to breaking a sweat.

Only if I do many full power climbs or full speed forward flight for the whole battery could I perhaps overheat the motors enough to fry them, but that's not my flying style.

I don't have any of the MR props to test yet but these carbons, while not the best quality, are very stiff and pretty good for the money in my book. They do need balancing though which is a drawback, some show signs of rudimentary sanding but it's often too much as the non-sanded blade is the heavy one.

I tested some Gemfan plastic 11x4.7's against these 11x4.7's a while ago and the carbons used about 3W less and the current variation was much less.

Comment by Artem on April 18, 2014 at 11:40am

oh yes, I love the 11.47 rctimers!  they are perfect for the money! my dt750 @4s and 11x47 rctimers drwas about 9-10amps (tri total) on hover (i know dt750 are officially rated for 3s but they do just fine for on 4s not even warm after 13-15 min of flights with a gopro and vTx).  and so much reserve power!  


T3
Comment by Ted Van Slyck on April 18, 2014 at 2:32pm

I'm surprised no one has posted the math yet, so I'll do it

10x4.7 = 7.44 g / watt

11x4.7 = 7.21 grams / watt

12x3.8 = 7.09 grams / watt

Honestly, I'd say for 5500' those are pretty good numbers. At sea level I bet they would be pushing 9 g/w. I'm in Chicago and with a density altitude near S.L. My DT700's with APC 12x3.8 clock in at about 8.9 g/w. The APC does not handle fast attitude changes or gusty winds well however. 

I believe that with propellers properly designed for static thrust we can do better than 9 g/w. 

Thank you for the info Graham

Ted

Comment by Euan Ramsay on April 19, 2014 at 11:44am

I've got the 1045 and 1147's in Y6 mode, and they seem ok. I tried 1045 top and bottom, but didn't really notice any difference. My rig is twice the 1kg of the OP's though...


They do break easy though - right at the hub. I broke/cracked two on a simple tip over on landing (tarmac).

Comment by Andrew Rabbitt on April 19, 2014 at 8:25pm
Nice to see someone doing some real quantitative testing which seems to be a rare thing despite all the effort many put into experimentation.

One comment though regarding the method is that there's nothing to identify repeatability. I assume you simply tested ABC. That you wound up with a linear trend could be real or could just be a methodical sampling error. You could identify this by repeated data points - i.e. ABCA, or randomize the measurement order, say CAB, or do both - ACBA. If you repeat one point three times you can estimate your confidence level, say ACABA, where the standard deviation of the A samples would tell you something about the statistical significance of B and C
Comment by Jethro Hazelhurst on April 20, 2014 at 4:46am

I have some 12" RCT Carbon Fiber Propellers on the balancer right now, I can't wait to test them out. Thank you Graham!

Comment by Hugues on April 20, 2014 at 8:34am

It would be interesting to compare wood and CF props. They are both rigid materials (much more so than APC or GEMFAN props) but I have an intuitive feeling that wood has more flexibility than CF and thus would be a bit more efficient (to absorb and manage fast vertical speed variations and vibrations). Would love some real measurements. 

Comment by Ravi on April 20, 2014 at 7:10pm

very useful test for most fliers. i wish there was a full data bank of all commonly available props. using efficient prop is like getting free extra thrust. a simple excel sheet showing various figure would be enough to start with. flutter (vertical movement of prop blades) becomes zero when plastic props are replaced with carbon props. the flutter sound is replaced by a high frequency whine and that indicates efficient airflow.

Comment

You need to be a member of DIY Drones to add comments!

Join DIY Drones

© 2014   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service