How's this approach for selecting motor+prop? - RC Groups
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Nov 14, 2005, 01:13 PM
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ecormier's Avatar

How's this approach for selecting motor+prop?

As we're approaching winter, I realized I won't be able to use my histart in the cold temperatures. Winches seem like too much of a hassle, so I decided to install a motor in the Laser Arts Majestic sailplane I'm currently building.

I'm new to electric flight, so I tried to learn as much as I can on rcgroups. My education is in EE, but I've been mostly a programmer professionally. I can understand most of the technical jargon, but I'm certainly not an expert when it comes to motors.

I've downloaded MotoCalc to try to eliminate the guesswork in sizing the motor + prop. I'd like to run my selection approach by you guys to see if it makes sense.

Approach to follow in next message...
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Nov 14, 2005, 01:34 PM
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Decide on a rate of climb you want and experient with different motor cell and prop combinations with MotoCalc. Use 100Watts / lb as a starting point, you can go up or down.
Nov 14, 2005, 01:38 PM
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ecormier's Avatar
Design goals:
A. Minimize the added weight to maintain low wing loading.
B. Maximize the number of "launches" per full battery charge.
C. Launch to 300ft in less than a minute, preferably around 30 seconds.

Approach using MotoCalc:
1. Enter my sailplane aerodynamic data
2. Pick a 3 cell LiPo battery pack for the best energy density.
3. Following the 50 to 100W per pound rule of thumb, I pick a motor that'll handle on the order of 200W.
4. Pick a gear ratio
5. Select a range of prop diameters and pitches that covers the range of props I can buy from my LHS plus a few other online shops.
6. Generate a static analysis report, for all of the diameter/pitch permutations. Save it as a csv file.
7. Using some speadsheet magic, I open the csv file and filter out the impossible diameter/pitch combinations spewed out by MotoCalc (I wish I could setup MotoCalc to do this for me).
8. In the spreadsheet, I multiply the Rate of Climb by the flight Time, to get the total climb capacity in feet per charge.
9. I sort the results by my calculated total climb, in descending order. I keep the top one or two results for that particular motor+gear. I append those results in my master list of motor+gear+prop setups.

I repeat this process for different motor + gear ratio combinations.

I'm at my work computer, so I don't have access to my results spreadsheet. When I get back home, I'll post my results.
Last edited by ecormier; Nov 16, 2005 at 12:20 AM. Reason: Tried to clarify a bit... I'm a horrible technical writer :)
Nov 14, 2005, 01:47 PM
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ecormier's Avatar
General findings:

From memory, I can give you a general overview of my findings:

[EDIT: My memory was off. See actual results below.]

I'll post my actual results when I'm home.
Last edited by ecormier; Nov 16, 2005 at 12:13 AM. Reason: Deleted false findings recalled from memory
Nov 14, 2005, 01:55 PM
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ecormier's Avatar

- Is MotoCalc good at predicting the aerodynamic and energy performance of the airframe + motor + prop? Am I asking too much out of this software?

- In general, I see that people tend to use motors with higher kv and smaller props (but this is generally for smaller planes than what I have). My results show that low kv and larger props give more total climb per charge. I have a hard time finding electric setup examples for open-class sailplanes.
Nov 14, 2005, 02:04 PM
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ecormier's Avatar
Originally Posted by rfb
Decide on a rate of climb you want and experient with different motor cell and prop combinations with MotoCalc. Use 100Watts / lb as a starting point, you can go up or down.
Thanks rfb. I have seen somewhere a rule of thumb stating 50 to 100 Watts per lb. I used that to crop down the huge list of potential motors.

I'm not picky on the rate of climb, as long as I can get a good 300' climb in 30 to 60 seconds. What I'm more interested in is how many climbs I can get before I have to pack my things and go home.
Nov 14, 2005, 09:33 PM
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ecormier's Avatar
Attached here are:

1. Comparison table of various motor and gear ratio combinations (only the best diameter/pitch combinations have been kept for each motor).
2. Table of diameter/pitch combinations for the Medusa 028-040 1200
3. In flight analysis report by MotoCalc for the Medusa 028-040 1200 and 17x13 propellor.

The tabs for the text files are aligned at 8 spaces.

The results I gave earlier from memory for 18x16 vs 17x13 @ 1200RPM/Volt were a bit off. The 18x16 has better climb rate, but the prop stalls at airspeeds < 10mph, whereas the 17x13 stalls at airspeeds < 5mph. I don't really know what airspeed a 3M ship has after a hand toss, but 5mph seems to be reasonable. I hope I don't have to rig up a short bungee launcher for this config.

Note to other newbs who may be reading this post: These results are for MY airframe! Your results may vary. Grab MotoCalc and do your own computations.
Last edited by ecormier; Nov 16, 2005 at 12:24 AM. Reason: Added clarification
Nov 16, 2005, 12:03 AM
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ecormier's Avatar

I would greatly appreciate it if an experienced EP guy can confirm that my reasoning here is valid. Anyone? I'm itching to finalize this so I can assemble my glider before it gets too cold outside. We just had our first snowfall today

It worries me that I end up with such a large prop (17x13) and low kv (1200), compared to what others say they use in their setup.

EDIT: I just reread all of my mumbo-jumbo above, and I'm not surprised no one answered. I think I write too tersely.
Last edited by ecormier; Nov 16, 2005 at 12:28 AM.
Nov 16, 2005, 01:43 AM
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Briggs's Avatar
i would like to see the answear also .
Nov 16, 2005, 07:44 AM
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Andy W's Avatar
I tried to read it and could not follow your logic/reasoning. There are too many misconceptions/assumptions in your thesis above for me to sit here and respond to every point. Methods for choosing a particular power system for a model have been covered here many times, also in a 6-part series posted by the site owner Jim Bourke back in 1998:

Finally if you're itching to get a model set up but unsure of a particular system, post here and others will pitch in with what they've used in this or a similar model (it helps to post specs on the model - I've never heard of a "Laser Arts Majestic" sailplane).

I'd give you 10 points for effort though!
Nov 16, 2005, 08:08 AM
soholingo's Avatar
I think the entire thing is good for a mental exercise and learning setups. But the best way to pick motors/batteries/props and to get as many as possible and fly them. The next best way is to ask someone who has flown the model. In a word experience is the best way to learn.

Example I had two 16 cell setups recommended to me. Hacker b50-9s (in motocalc and pretty accurate) and Lehner 19308d (not in motocalc). Lehner was more expensive, than the hacker, and I questioned the value. But once I flew it there was no question the lehner was worth the premium. There are no numbers collected about the lehner system, it just works.

Nov 16, 2005, 04:00 PM
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ecormier's Avatar
Thanks! The feedback is greatly appreciated. That tuturial is exactly what I needed.

Sorry for not giving the specs for the glider. They can be found here (click on the specifications tab).

I did not have EP in mind when I purchased that glider. The idea of EP only came up when I realized my histart would be useless in the cold (this is my first season as an RC flyer -- can you tell? ).
Nov 16, 2005, 06:04 PM
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ecormier's Avatar
I've finished reading the tutorial... really well written and easy to understand. It was helpful to see the prop power equation to understand the relationship of the prop with the rest of the power train. It also made me aware that motors have a torque limit -- I didn't check for that while I was compiling my comparison table.

Andy, I can't see what "misconceptions" I had in my reasoning. All I basically did was throw a bunch of setup combinations in MotoCalc, and prefered the ones that gave me the best total climb per charge. You understand what I mean by total climb per charge, eh?

total climb = rate of climb x flight time

Where flight time is the amount of time I get from the battery pack at full throttle.

I don't care how fast the glider zips up to thermal hunting altitude -- as long as it's reasonable (I don't do competitions). I just want to get the most "launches" per single battery charge. That number of launches is calculated by:

# lauches = total climb / launch height

The only misconceptions that I see I might have, is believing that the motor parameters are accurate and that MotoCalc is accurate.

I have admitted that my procedure, as written, was too terse and hard to read, but I can't accept that I had "misconceptions" without any justification on your part. If you believe I had misconceptions, you therefore must've been able to follow me up to a certain point.

As for relying on what works for others... I don't know. In general, it seems that people are more interested in climbing fast rather than getting the most launches out of single charge. That's what motivated me to use MotoCalc.

I now understand why my methodology ends up favoring small kv motors. It's because I arbitrarily decided to use a 3 cell lipo. I'll crunch the numbers again using a 2S2P lipo -- I bet the larger kv motors will end up being in favor.
Nov 16, 2005, 08:59 PM
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ecormier's Avatar
Here's a comparison table using my procedure when I impose a 2S2P lipo. As I suspected, motors with higher kv tend to be favored with this battery configuration.

I forgot to add the motor amps in my comparison tables, so some of the motors might be drawing too much current. I also was a bit sloppy when factoring in the weight of the gearbox. I'm going to recompile my comparison tables this weekend when I have fresher eyes.

Once I validate my MotoCalc selection with the real motor, I'll clean up my spreadsheet and put in some macros to avoid the tedious import, copy, paste, and sort that I have to do for each static analysis files MotoCalc spits out. I can then post the spreadsheet, if anyone is interested.
Last edited by ecormier; Nov 16, 2005 at 09:00 PM. Reason: Oops. Forgot attachment.
Nov 16, 2005, 10:46 PM
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Andy W's Avatar
Sigh.. you're determined to put us thru this exercise, aren't you..

Motor selection:
Originally Posted by ecormier
In general, I see that people tend to use motors with higher kv and smaller props
No. In general, most people realize that a larger prop is much more efficient. You can swing one by gearing a high kV motor, or by running a motor with a lower kV but lots of Torque (the newer outrunner style motors). I prefer the geared motors myself - much higher efficiency.

Prop stall speed:
Originally Posted by ecormier
blah blah .. The 18x16 has better climb rate, but the prop stalls at airspeeds < 10mph, whereas the 17x13 stalls at airspeeds < 5mph. I don't really know what airspeed a 3M ship has after a hand toss, but 5mph seems to be reasonable.
A stalled prop still generates PLENTY of thrust, and the numbers you've produced are irrelevant to your application. Your model will fly right out of your hand with even the lightest toss with the 18x16. With enough power, however, you'll also rip the wings off!


Disclaimer - I am not trying to dampen your enthusiasm..

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