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: 4846

Comment by Daniel Allen on April 21, 2014 at 8:29am

- Gram - I can't believe you pushed a 12x4.5 on those tiny motors. I was pushing 10x4.5s and that would already pushing the limit. Do you think you can do some actual hover time testing? I think real world tests are always the best.

- Thomas, I'm glad you mentioned the APC 12x4.5 MRPs. I'm interested in testing those out, since my 12x45 Rctimer CF props have been a disappointment too. So why didn't you like the results besides the vibrations?


Moderator
Comment by Graham Dyer on April 21, 2014 at 12:10pm

Daniel - I don't know how you think 4 amps per motor (49W) in my real world hover test is pushing it on a motor that's rated at 150W? But this is such a common misconception, in fact each motor drew just 0.5A more in the hover with the 12x3.8's compared to the 10x4.7's.

The motors are also very close to their most efficient current band (5.11A) in the hover and are in no way "being pushed" or working hard at all.

If I do a full power climb out for 60 secs, drawing 19A per motor (57A in total from the battery) then yes, the motors (and ESC's and battery) would heat up and if that kind of abuse is continued the motor windings may discolor and eventually burn out. (I have burnt out more motors than I can remember in the pursuit of extra performance with planes and heli's).

As for hover time testing one can extrapolate from that current draw and get an accurate flight time, in fact there's an app for that: RC E-calc - so theorectically:

With the 10x4.7's: 80% of 3300mAh will give 13 minutes and 25 seconds.

With the 12x3.8's: 80% of 3300mAh will give 12 minutes and 50 seconds

So the 12x3.8's will fly for 35 seconds less than the 10x4.7's.

Comment by Daniel Allen on April 21, 2014 at 1:24pm

Gram, my apologies. This is what I get from reading the comments first since I was looking for the key word 12x4.5 MRPs. I was under the impression you were testing on a bench, yet without a scale to measure thrust. So sorry for that.

I actually did my own motor/prop/battery tests, 54 tests in total. The test used 6 different motors, 6 different props, and 4 types of Lipos (3-6s). It was a bench test, which was aimed at finding the most efficient combination. However in hindsight I didn't do a good enough job of making a real-world emulation. The fault was measuring thrust at an approximate PWM rating for all the motors.

I plan to redo the test with something more consistent. I'll make a rig that requires the motor to keep a certain weight at hover. It will be at least 6 feet off the ground to avoid ground effects. So 250g, 500g, 750g, and 1000g which represents different loads. I'll be recording Amps, Watts, Volts, and approximate PWM. 


Moderator
Comment by Graham Dyer on April 21, 2014 at 1:42pm

No worries. A few more tests like this would be really good in increasing our understanding of multirotor props.Please put your results up on a blog.

I did some tests a while back of some expensive carbon Tiger Motor props and they drew more current in the hover than the Gemfan plastic 11x4.7's. They did not require balancing though.

Comment by Daniel Allen on April 22, 2014 at 10:56am

Gram, I'll definitely try to get a post going in the future. I didn't do the best job in using a scientific method. That's why I want to redo some of my tests with the proposed method above. But if you want to see this temp video, here is a work in progress. https://www.youtube.com/watch?v=0pZm-cGef-o

Comment by David Jones on July 11, 2015 at 4:34am

I don't wanna be unpopular by resurrecting an old post.

But I really can't understand how you fly a 12" CF SF prop with those motors.

I used to run a tricopter (1040g without battery, using batteries from 300g to 450g) with DT750, I recently upgraded to SunnySky v2216 800. (yes they are genuine)

They can't handle the 11" CF SF prop, bought from HobbyKing, looks very similar to the ones in the picture you posted.

Yes they do fly with this prop, but when doing an autotune from APM2.6 it gives PID implying a significant lack of power, and it flies like it has a lack of power, is easily wobbled around by the wind and isn't anything like as stable as it used to be with the DT750.  (I'm almost at Sea level)

Obviously if I flew on a 100% calm day it might be ok, but its rarely like that here.

The x2212 is less powerful than the v2216 in terms of torque unless I'm very much mistaken, so how in the world does it handle 12" SF prop effectively?

I'm not talking about over stressing the motors but just having enough torque to fly in a way that is stable in the air?

Can I ask what your PID numbers look like?

At the moment I am having difficulty finding the right prop to suit my needs.

I was flying nicely with cheapo 10x4.5 sf, getting comfortable 16min flight times (5000mAh 3s). (although in summer APM was warning about lack of thrust, which I wouldn't have noticed, didn't seem like a problem) (BTW the DT750 only lasted 10 to 12mins to Sunnysky a huge improvement in that regard)

But after one of the props broke I switched to using HQ 10" thin with 4000mAh 4s, they work but a bit less flight time maybe only 14min to be comfortable, did last longer but I accidentally ran battery lower than intended.

However APM still complains of lack of thrust (even though it seems ok), so I am about to try APC 10x.45 MR as the HQ thin were really far too thin for my kv.

I was thinking of trying the APC 10x4.5 or HQ 11" thin, but read that MR more efficient.

Comment by David Jones on July 11, 2015 at 4:37am

Is the atmosphere really so much thinner that thats whats made the difference?

Comment by Artem on July 11, 2015 at 8:29am

Yes, air density plays huge role. In my case at sea level my tricopter takes 10-12amps to hover (depends on batt voltage) up 6700ft it was taking 14-17amps all else being the same.  Also do not try to use autotune on a tricopter with 3 AC later than 3.1.5 (I think it was the last one to provide decent PIDs for tricopters) 

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