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Dec 08, 2009, 12:14 AM
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Discussion

Bench Test of APC E-props (11x5 to 15x6), Watts, Torque, Thrust


OK, here we go. For those of you who want to know how much Watts is required to generate a certain amount of thrust and at what RPM level, I believe this thread will provide some answers. You will find experimental test results for the most commonly used electric props (from APC 11x5.5 all the way up to APC 15x6, and a few other popular name brand props such as XOAR, Zinger, MAS, and Top Flite).

As part of experimental projects in our ITP (Introduction to Profession) class in Mechanical, Materials and Aerospace Engineering department at Illinois Institute of Technology (IIT) I have built a static test bench (see the pics) and performed a series of experiments by using the following equipment:

Motor: Torque 2814T/820

ESC: Airboss 45 Elite

Battery: Hyperion Litestorm CX 18C 4S-2600 mah

Power Analyzer PRO (Medusa Research Inc.) has been used to measure Volts, Amps, Watts, RPM (via Brushless Motor RPM Phase Sensor), Torque (via Medusa Research’s Thrust Cell and our torque stand), and finally Thrust (via a dedicated high accuracy load cell and through Power Analyzer PRO’s AUX port).

I’m going to divide the discussion into subsections for ease of reading.
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Dec 08, 2009, 12:24 AM
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Thrust Measurement


As can be seen in the pictures the electric motor is mounted on an RC engine test stand (from PSP Manufacturing), which is mounted on a polycarbonate (PC) board on wheels. The load cell is mounted on the opposite side of PC board via aluminum L-plates such that the axis of load cell is aligned with the axis of motor shaft. I built the whole setup a a little bit beefy to test also glow engines in the future. The whole set up stands on a large aluminum square tube. When disconnected from the load cell the test stand is free to roll on wheels. For safety reasons we mounted an aluminum rod on the square tube to restrict any unwanted movement of roller stand in sideways (see pictures).

The load cell used is from Transducer Techniques and has 25-lb capacity. Although we bought the load cell calibrated, we have made our on calibration for tension loads up to 6-lb by hanging 1-lb dead weights through a pulley system (voltage-load relation is excellently linear). We also used a signal conditioner (again from Transducer Techniques) to provide a 10V excitation voltage to load cell circuit and to amplify the signal voltage from mV range to V range. In the process, we needed to offset signal voltage output about 0.3V at zero thrust because the AUX port of Power Analyzer PRO has a linear input range of 0.1V to 2.6V.
Dec 08, 2009, 12:40 AM
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Torque Measurement


For the torque setup electric motor is mounted to a cylindrical block, which is rigidly mounted at the end of an aluminum shaft. The shaft is supported by two ball bearings and a thrust bearing at back side of 2nd ball bearing to prevent the shaft/ball bearings slipping from their mounts due to propeller thrust. At the back side of shaft we rigidly mounted a 1-in long moment arm which pushes toward the Thrust Cell (basically a scale with linear and calibrated voltage output from Medusa Research) due to reaction torque (Newton’s first law: for every action there is an equal and opposite reaction). By using a small steel ball at the pusher end of moment arm we ensured a point contact and an exact 1-in moment arm (see pictures for detailed views).
Dec 08, 2009, 12:43 AM
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Stepwise Experiments for Steady State Performance


Experiments have been done in a stepwise fashion where RPM is increased in steps of a few seconds as shown in the following graphs. As it is clear from the graphs when the RPM is increased there is an initial peak in WATTS drawn because of the rotational inertia of propeller during this acceleration phase (although not shown in the graphs the similar initial peaks are observed for reaction torque as well). Therefore, we picked the data points (RPM, WATTS, THRUST, etc.) from approximately middle of each step to ensure that data points correspond to steady state regime at constant RPM.

These results will be shown in following posts.
Dec 08, 2009, 12:48 AM
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RPM-Watts-Thrust for various APC props


The first graph shows RPM vs. THRUST for various APC Electric props from 11x5.5 to 15x6. As expected, for a given RPM larger diameter or pitch results in higher thrust. Therefore, looking at WATTS vs. THRUST graph makes more sense because it directly tells how many watts you will need for each prop to generate a certain amount of thrust. Or alternatively, to look at how much thrust you will get from each prop at a given power draw. This graph clearly shows that larger the prop diameter higher is the thrust efficiency, i.e., thrust per watts for a given pitch.

One must also note that in static thrust measurements such as here higher pitch for a given prop diameter results in lower static thrust efficiency (e.g., compare 11x8 with 11x5.5, 13x6.5 with 13x8, etc). Don’t let this mislead you for real flight performance because higher pitch props would give higher performance at high flight speeds. You should also carefully look at RPM vs. THRUST graphs where higher pitch generates higher thrust, however, at the expense of increased power requirement, which in turn decreases the static thrust efficiency (thrust per watts).

It must be noted that maximum RPM values in the graphs for different props by no means show the maximum attainable RPM (or WATTS) for corresponding prop. Our intend was to look at general trends and relationships rather than maximum performance evaluation for a given battery prop combination, so we have not used the same maximum power for each prop.
Dec 08, 2009, 01:03 AM
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A Little Bit Simplified Theory


Before going any further and show you Normalized Thrust graphs I guess I should attach the brief write-up that I prepared for my students. You can skip the first 5-6 slides unless you want to refresh your memory.
Dec 08, 2009, 01:22 AM
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Normalized Thrust


By normalizing the thrust as described in the PDF file attached in previous post, and for the exponent m=0.7 that appears in normalized thrust term we obtain the following plot for APC E-props. As it is clear from this plot, all data points obtained from different pitch and diameter props nicely falls onto a master curve which can be fitted by a quadratic function of RPM (remember thrust is proportional to square of RPM). The best-fit curve corresponds to a Thrust Factor of KT = 0.05.

What is the meaning of all this? Well, it basically tells us that all APC E-props tested here have almost the same thrust factor, i.e., they use almost the same blade profile (airfoil) and variation of the angle of attack and chord length of airfoils follow the same pattern as you go from hub to tip.
Dec 08, 2009, 01:42 AM
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A Quick Comparison of Props from Different Manufacturers


In addition to APC E-props, we have used the following props to make comparisons:

XOAR 12x6 (Wood)
Zinger 12x6 (Wood)
Top Flite 12x6 (Wood)
MAS 12x6 (Wood)
MAS 12x6 E series (Plastic)
MAS 12x6 K series (Plastic), regular glow prop

Results are given in the graphs below. I will leave interpretation of results to you as it is getting late. But let me tell this much: XOAR 12x6 (wood) and APC 12x6 are my favorites. I will try to post reaction torque and motor efficiency plots tomorrow night.
Dec 08, 2009, 04:15 AM
Proud to eat Kraut ;-)
Julez's Avatar
Very nice, that is useful information!
Dec 08, 2009, 08:21 AM
JUNK RADIO
Flazo's Avatar
here
Dec 08, 2009, 09:04 AM
Giz
Giz
Tony Rogers
Giz's Avatar
Nice project.

It would be nice to to see rpm for 100 watts motor power output for a range of APC props like this table for Aeronaut props.

With this information you can make a reasonable estimate of motor efficiency just by measuring the rpm and power input. Alternatively, if you know your motor efficiency you can get an estimate for rpm. This is invaluable when tweaking motor caclulator prop constants to get more accurate results.
Dec 08, 2009, 09:32 AM
Registered User
I briefly scanned the pdf file, so if I misunderstood, I apologize.

I am not positive what exactly you are trying to show here---either a characterization of the motor, or the characterization of static thrust of the props. I believe you have data to do both, but what I see in a couple of plots (like thrust vs watts) seems to mix up the two issues.

I say this because I think the "watts" you are using in the plot are the input watts (from the battery to the ESC/motor), and NOT the output watts to the prop from the motor, which would just be the torque * w, (w=angular frequency or about rpm/10). Input watts of course mix in the inefficiency of the motor, which varies as a function of the power level it is running at. So if I really wanted to know what prop would hover me with the least watts supplied by the motor---I would prefer to see that plot using the output watts.
Dec 08, 2009, 09:42 AM
Newbie in training
What makes the APC the most watt/amp drawning props compaired to others?
Dec 08, 2009, 10:12 AM
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Dr Kiwi's Avatar
Quote:
Originally Posted by foresight3d
What makes the APC the most watt/amp drawning props compaired to others?
From the last graph posted it seems to me that for the amount of thrust generated the APC 12x6 consumes fewer watts than, say, the TF and the Zinger. In the upper part of the graph (about 360W), the XOAR, MAS K and MAS E seem the most efficient at generating thrust/watt. At lower input the XOAR seems the best.
Last edited by Dr Kiwi; Dec 08, 2009 at 10:17 AM.
Dec 08, 2009, 11:51 AM
Registered User
Quote:
Originally Posted by Alan Hahn
I briefly scanned the pdf file, so if I misunderstood, I apologize.

I am not positive what exactly you are trying to show here---either a characterization of the motor, or the characterization of static thrust of the props. I believe you have data to do both, but what I see in a couple of plots (like thrust vs watts) seems to mix up the two issues.

I say this because I think the "watts" you are using in the plot are the input watts (from the battery to the ESC/motor), and NOT the output watts to the prop from the motor, which would just be the torque * w, (w=angular frequency or about rpm/10). Input watts of course mix in the inefficiency of the motor, which varies as a function of the power level it is running at. So if I really wanted to know what prop would hover me with the least watts supplied by the motor---I would prefer to see that plot using the output watts.
You are right, the Watts in the plots are the input watts delivered from the battery and therefore you should read the data as characterization of the prop in combination with electric motor (Torque 2814/820 and ESC Airboss Elite 45).

In the next posts I will also plot output power for two different props. Unfortunately we haven't performed extensive experiments to measure reaction torque vs. RPM for all the props, but it will give an idea. Please note that Thrust and Torque measurement are different experiments and are not measured in the same experiment, perhaps I will have time to complete torque experiments during winter break so that I can post more detailed graps as you (and I, too,) would prefer.


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