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Pushing the Limits?

Old 11-14-2006, 09:19 AM
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aerofart
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Default Pushing the Limits?

Hope this is the right place to post this . . .

Let me preface this post by saying that I am a newbie to electric RC, and RC for that matter, but have learned a tremendous amount of information from these forums. I never realized it could be so complicated, but you guys really know what you're talking about and I appreciate having such a wonderful resource.

So here goes . . .

I am running a Ripmax XTRA 2829/13 Brushless Outrunner on my Sport Flyers WASP (great little stick, by the way). Ripmax specs recommend using 10x6 or 9x5 props with a 2-Cell pack, or a 10x4.7 using a 3-Cell pack with this motor, consuming a theoretical 7A (@6,100RPM), 11.5A(@8,500RPM) and 10.2A(@6,100RPM), respectively. I was using a 9x4.7 prop (as I recall - it may have been a 10x4.7) and a 2s1p pack with miserably poor results. The plane was barely getting off the ground, which, now that I know a little more about all this stuff, just doesn't seem right, considering that about 85 Watts of power were being applied (per Mfr specs) to move an 18-21oz load (don't know for sure, but using the model's specs as a baseline) - this should have been almost enough for vertical climb, no?

As if that isn't bewildering enough, I've been using a JETI 08 Advance Plus (8A) ESC all along - well under-rated in all but one case, according to Ripmax specs! Now, I would never have attempted using the different props had I known what I was doing and the risks involved, because now I see that the only safe prop to use with the 2829/13 would have been a 10x6, drawing 7A, comfortably within the limits of the ESC. Yet despite these abuses, I haven't fried anything. But wait, it gets better. . .

In my quest to actually have a decently-performing flier - one that doesn't get shoved aside by passing butterflies - I have continued to vary the power source and props, hoping to get *some* performance out of this setup.

As I already mentioned, in my initial setup and test flights I was using a 1320-2s1p (12C) power source, but then I switched to a 3s1p and a 9x4.7 prop and, Voila! The plane flies beautifully, with commanding power and control, climbs steeply. I haven't attempted vertical climb, since I'm a virgin pilot, but I think she'll perform. So I am happy . . . but puzzled, and concerned as well.

How is it possible that I am getting away with breaking the rules here, or am I? If I had to bet, I would say that at some point in time during my experimenting with the setup, I have crossed the theoretical limits of the ESC and/or the motor - but haven't suffered any problems or sensed any signs of impending disaster, no overheating ESC or motor. So what gives, I wonder?

Is it just a matter of time before the Grim Reaper pays a visit and breaks my toys, or is there a more plausible explanation for why I am able to get away with any of this? For example, could the ESC be limiting an overcurrent situation, could there be a timing mis-match that reduces the power cycling to the motor? I might believe that were the case when I was using a 2-Cell packs and having crappy performance, but I find it hard to believe now that she is performing so very well. Oh yeah, did I mention I also programmed hard timings on the ESC, yielding an "additional 20%" gain . . . I must have been an overclocker in my previous hobby, you think?

I welcome and appreciate any advice and/or explanation for this phenomenon from any of you gurus and other experienced folks out there. I really would like to establish a credibility in the numbers . . .
Old 11-14-2006, 10:42 AM
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jdetray
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Default RE: Pushing the Limits?

I have no experience with your power system and plane, but here are a couple of general thoughts.

1. Any time you start experimenting with different props and batteries, it's essential that you use a wattmeter or power meter to actually measure what is happening. Then you will no longer be guessing -- you will know exactly what is going on. I'm referring to the type of meter shown here:
http://www.bphobbies.com/view.asp?id...=U054250&img=l

2. Heat is the enemy of your power system components. If the motor, ESC, and battery are not getting hot, you may be OK, for whatever reason.

- Jeff
Old 11-14-2006, 06:39 PM
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Fliprob17
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Default RE: Pushing the Limits?

As I already mentioned, in my initial setup and test flights I was using a 1320-2s1p (12C) power source, but then I switched to a 3s1p and a 9x4.7 prop and, Voila! The plane flies beautifully, with commanding power and control, climbs steeply. I haven't attempted vertical climb, since I'm a virgin pilot, but I think she'll perform. So I am happy . . . but puzzled, and concerned as well.
By bumping up to a 3S LiPo you have gone from the 7.4 volts of a 2S to the 11.1 volts of a 3S, therefore increasing the voltage to the motor, which then increases your total watt output, as watts=voltsxamps. As long as the motor and ESC are spec'd to handle that increase in voltage, everything should be fine.
Old 11-15-2006, 12:55 AM
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aerofart
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Default RE: Pushing the Limits?

The ESC is NOT rated to handle a current over 8A for more than a short burst - that is the whole point of my post . . . trying to understand how it is possible that, with the change in power source and various prop combinations, I have managed to NOT fry the ESC or the motor . . .
Old 11-15-2006, 01:16 AM
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aerofart
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Default RE: Pushing the Limits?

Thanks Jeff.

I have read and am interested in getting one of these wattmeters. Have you personally used one of these units? I just wonder how practical and/or user-friendly they are - and whether I need to pull out my scientific calculator or perform feats of magic to interpret the results . . . sure, I can read the mfrs description, but that usually doesn't give you a good sense of real-world use.

I know the basics about electricity and electronic components, and I can even understand somewhat complex graphs, but I am not an electrical engineer by any means. If you could point me to a good review or provide your own feedback on this, that would be great. Also, do you know of any (cheaper) alternatives?

Thanks again.
Sal
Old 11-15-2006, 02:09 AM
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Default RE: Pushing the Limits?

Hi Sal-

I personally use the very similar [link=http://www.astroflight.com/store/store-type-tem.html?item=products:af-101n]Super Whattmeter from Astro Flight[/link].

Either of these meters -- and others of similar design -- allow you to directly read voltage, current, and power (watts). No calculations are required. You plug your battery pack into the SOURCE side of the meter, then plug your ESC into the LOAD side. The meter is powered by your battery pack -- there is not even a power switch. It could not be simpler.

The only "setup" required is to install connectors that match those on your battery and ESC. Both the Watt's Up and the Astro Flight meters are available without connectors, so you can just install whatever type you need.

If you happen to be working with a very low voltage battery pack, such as a 1-cell lipo, you'll need to power the meter with a separate battery pack. With 2-cell and larger lipo packs, a separate battery for the meter is never necessary, so you will probably never use this option.

I found a my Astro Flight meter used for $40, but $45-55 is the price range for basic, new units. There are fancier (more expensive) units with memories and computer interfaces, but I have never used anything but one of the basic models.

- Jeff
Old 11-15-2006, 04:38 AM
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Default RE: Pushing the Limits?

That's great information, Jeff. Definitely on my wish list now. Santa, are you getting this?

I have one more concern . . . it seems that the process of inserting the wattmeter in-line would not be very hassle-free. Seems you would almost have to set up a test bench or some other testing-only configuration in order to splice the watt-meter into your setup and get your readings, a process that would appear to be somewhat time-consuming and cumbersome, not lending itself to quick-checking of the setup, say, while it is still mounted in your airplane.

Are there any switched plugs (connectors) available that can be spliced into the powerlines directly, bypassing the primary connection and enabling the wattmeter to simply be plugged in at a moments notice? I Googled a bit but didn't come up with anything. The connector I envision would conceptually look like the image attached.

P.S. - I noticed you are into Astronomy . . . I've dabbled and maintain an interest in astronomy as well. Used to own a couple of Taks once upon a time, a 6" reflector and a 102mm APO (see photo).
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Old 11-15-2006, 12:01 PM
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Default RE: Pushing the Limits?

ORIGINAL: aerofart

The ESC is NOT rated to handle a current over 8A for more than a short burst - that is the whole point of my post . . . trying to understand how it is possible that, with the change in power source and various prop combinations, I have managed to NOT fry the ESC or the motor . . .
If you do not have a wattmeter of somekind, there is no telling what each prop is doing as far as amps is concerned. A 9x5 could be pulling the same as a 10x4.7, you just don't know until you hook something up and get a reading. The pitch of the two props is slightly different, and the increase in length may not make that much of a difference, especially when the longer prop has less pitch. Those manufacturer spec's for the motor and prop combinations that you are using for a basis of this whole "frying" or "not frying" situation are more of guidelines. Who knows, you may only be pulling 8 amps. Plus, the 8 amp rating they are giving is for contiuous current, for max. bursts you can expect to get away with closer to 11-12amps. Which all of those props fall into, according to your specs.. As long as you did not hold the throttle at full for a minute, nothing would have fried. You also mentioned "now I see that the only safe prop to use with the 2829/13 would have been a 10x6, drawing 7A, comfortably within the limits of the ESC." The 10x6 is going to draw more amps than any of the props you listed, the 9x4.7, 9x5 and the 10x4.7. No matter what battery you put in it. It simply has more pitch, not to mention it is the longest, therefore it is going to draw more amps. After visiting the motor manufacturers web page, they have the operating current rated anywhere from 6-10 amps, with 12 amps as the point where the motor works most efficiently, so again, there is really no telling until you get somekind of meter.

You also said that you were "puzzled" and "concerned" that putting the 11.1volt battery in the plane made this happen: "The plane flies beautifully, with commanding power and control, climbs steeply". So that is the point I was making previously, just trying to settle your concerns. You increased the voltage to the motor, not necessarily the amps the motor pulls, you would think more volts = more rpm per kv rating which would then = more amps, but you just don't really know until the meter says so. So with the voltage going up, your watts goes up, and so does the performance of the motor/plane.
Old 11-15-2006, 04:54 PM
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Default RE: Pushing the Limits?

Thanks for clearing that up. I now have a better understanding of what you were saying and agree completely - and the point that I really do need a watt-meter has really hit home. There is no sense in speculating about the unknown . . .

Thanks,
Sal
Old 11-16-2006, 09:50 PM
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Default RE: Pushing the Limits?

Sal, you are just a little on the smart side duncha think ?

~Matt

ps. My 16 year old brain had to read over some of the things you typed twice to get a good understanding of the terms you were using to some extint.
Old 11-16-2006, 10:34 PM
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Default RE: Pushing the Limits?

Hi Sal-

Typically, you would use a wattmeter mainly during the initial setup of a given aircraft and power system. You might spend a few evenings trying out different props and/or batteries. But once you find a good configuration, you would not need to use the wattmeter for that power system again -- unless you make big changes or something goes wrong.

For that reason, I personally would not bother with the switched plug solution you describe. Yes, depending on the plane, it can be a bit cumbsersome to insert the wattmeter between the battery and ESC. But since you may only have to do this once when initially setting up the plane, it seems to me unnecessary to construct a quick check connection.

Now if you become interested in doing more extensive motor/battery/prop testing, you would want to build a dedicated test stand, such as [link=http://www.rcgroups.com/forums/showthread.php?t=505683]this one[/link]. Since I like to make my own brushless motors, I am currently building a similar stand.

- Jeff
Old 11-18-2006, 04:19 PM
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Default RE: Pushing the Limits?

Matt,

I'm not sure how to take your comment, but I'll give you the benefit of the doubt.

I was just trying to be thorough. I apologize if my writing was a bit technical/confusing - but then again, that's why the forum allows for questions.

-Sal
Old 11-18-2006, 11:33 PM
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Default RE: Pushing the Limits?

What connectors do you have on your battery/ESC - if (when) you get a Whattmeter/E-meter/WattsUp/Medusa Analyzer....just fit it with those same connectors and you can then measure all the parameters simply by plugging the meter in-line between your battery pack and ESC.
Old 11-19-2006, 02:09 AM
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Default RE: Pushing the Limits?

Thanks, Dr. Phil.

I came to that realization a short while ago (silly me). But you do win the award for being the only one who came to my rescue! Much appreciated.

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