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Author Topic: Sub 600 Past Battery Limit  (Read 2801 times)

Offline Motorman

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Sub 600 Past Battery Limit
« on: May 29, 2016, 12:19:36 PM »
I have a 51 ounce 575 sq. in. plane that I have been using an APC 2 blade and turning 9570 rpm with Hubin fm9. The charger puts back 78% of battery power. I made a composite 3 blade that needs to turn a little faster to get the airspeed the plane wants.  The new prop is so much better, a night and day difference. I really want to use it but, The charger puts back 88%. Battery is 4S 2500mah Hyperion 36x36x106mm.

I've seen other guys with this size set up have the same problem, the prop/rpm that pulls best makes you hit the low voltage cut off. I'm already flying as slow as I can (5.35) for 5 min 5 sec 2 laps left over. I'd hate to put a bigger battery on it that's another 3.5 oz of nose weight.

The prop comes out of the mold 11.5 x 4.5 and I tweaked it to 5.5 pitch. Thinking of going up in pitch and down in diameter but I hate to compromise on the prop to much.

How do you approach a problem like this. More pitch and less rpm, lighter plane, less flight time, A little bit of everything? What helps the most?

Thanks,
MM 
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Offline Fred Underwood

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Re: Sub 600 Past Battery Limit
« Reply #1 on: May 29, 2016, 01:44:45 PM »
What is line length and battery weight?  How does the TP 4s 2700 compare

http://www.thunderpowerrc.com/Products/2700ProlitePlusPower/TP2700-4SP25_2
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Fred
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Offline Motorman

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Re: Sub 600 Past Battery Limit
« Reply #2 on: May 29, 2016, 02:27:42 PM »
65 ft handle to thrust line. Would shorter lines really get me extra laps?  The TP's are the same size and weight as the Hyperions. I was hoping to not spend $300 on new batteries. 
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Offline Fred Underwood

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Re: Sub 600 Past Battery Limit
« Reply #3 on: May 29, 2016, 03:03:19 PM »
There are 52 increments of 0.1 seconds in 5.2 seconds.  65 ft/52 increments is about 1.25’/increment, so if you shorten the lines by about 1.25’, you gain about 0.1 sec, or about 0.08sec/ft.   5 minutes and 10 seconds has 310sec/5.2sec equals 60 “lap equivalents,” my term, so you would save about 60 times 0.1 or 6 seconds/flight, or 1 lap gained/0.1 saved.  Sorry for the math, but just trying to show how it works.  If not correct, I am sure someone will correct.

That is a reasonable approximation to try, and yes, I did that to save battery for a while. 
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Offline Tim Wescott

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Re: Sub 600 Past Battery Limit
« Reply #4 on: May 29, 2016, 03:52:42 PM »
You should be able to tweak in more pitch and leave the diameter the same.  Aerodynamics is complicated and my understanding simple, but I'm pretty sure that going down in diameter will make it less efficient.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

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Re: Sub 600 Past Battery Limit
« Reply #5 on: May 30, 2016, 02:56:05 PM »
and increasing the pitch wont neccessarily save battery either, it depends on the system. If your system is more biased towards higher rpm, then you will loose efficiency by increasing pitch.
if you are close on battery, and the plane will tollertate it, I would shorten the lines,
with that size plane I would not hesitate to use something like 62 foot lines, that would give you 3 tenths to play with.
with the weight and wing area, you will need to maintain the airspeed,, I dont think repitching is the answer, I would try the shorter lines first
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Offline Motorman

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Re: Sub 600 Past Battery Limit
« Reply #6 on: May 30, 2016, 03:54:41 PM »
How do you bias a system for higher/lower rpm? I'm using Castle Ice Lite 50.


MM
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Offline NED-088

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Re: Sub 600 Past Battery Limit
« Reply #7 on: June 01, 2016, 11:11:17 AM »
A prop with more diameter and pitch will allow you to run a lower rpm.
More diameter alone will give you more efficiency already (I'm with Tim, here).
51 ounce roughly equals 1400 grams (now my metric mind has an idea  ;)), must be about a Vector 40 size?
I can picture one square inch, but 575 of them don't relate to something I can imagine....

I'd fly that one with a 12 x 6, but don't be surprized if a 13 x 6.5 works even better. The latter will need ~ 7500/8000 rpm.
The gain in efficiency can be up to 5%.
Goes without saying that your motor should not be too small. I see too many people with motors loaded to their max. Not good.

And for batteries, these gives you an extra 100mAh:
http://www.store.revolectrix.com/Products/PLATINUM-Label-LiPo-Batteries-XH-equipped/Revolectrix-2600mAh-4S-Lipo-Platinum-Label-GP-KO-JST-XH-Balance-Connector
Not too expensive...
 
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Offline Mark Scarborough

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Re: Sub 600 Past Battery Limit
« Reply #8 on: June 01, 2016, 11:22:59 AM »
Quote from: Motorman on May 30, 2016, 03:54:41 PM
How do you bias a system for higher/lower rpm? I'm using Castle Ice Lite 50.


MM
not sure I can give you a definitive answer, but basically, motors have a happy spot, KV and internal resistance and current draw, that coupled with prop load, rpm and battery size, its a system
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Offline Joseph Daly

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Re: Sub 600 Past Battery Limit
« Reply #9 on: June 01, 2016, 01:45:15 PM »
If you are looking for a little more head room with the batteries, you could go with 2800 HV Hyperion. They are about the same size and weight as the 2500 ones. The price is reasonable and they are of good quality.
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Offline Tim Wescott

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Re: Sub 600 Past Battery Limit
« Reply #10 on: June 01, 2016, 02:10:59 PM »
Quote from: NED-088 on June 01, 2016, 11:11:17 AM
I can picture one square inch, but 575 of them don't relate to something I can imagine....

One DM2 works out to 15.5 square inches.  I think that's how you metric-heads measure area, yes?  My cackle-later says 37 DM2, if I'm doing my math right.  Brodak lists the Legacy 40 as having 565 square inches, so we're basically talking the same sized plane.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Tim Wescott

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Re: Sub 600 Past Battery Limit
« Reply #11 on: June 01, 2016, 02:15:32 PM »
Re-reading your post, you've got a 4S 2500mAh pack.  A 5S 2200mAh pack, if you could find one that would fit, should be about 20% heavier, would hold about 20% more energy, and would give you oodles of voltage overhead with the slightly faster-turning prop (5S 2000mAh, all else being equal, should be -- equal.  But things never are).  If the prop is really giving you that much more performance, the degradation from a bit more battery shouldn't be a killer.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Mark Scarborough

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Re: Sub 600 Past Battery Limit
« Reply #12 on: June 01, 2016, 02:23:08 PM »
seriously, try shorter lines, 3 feet down to 62 foot lines ( are we talking your 65 foot lines are eylet to eyelet or centerlines)
that will allow you to fly the same airspeed in less time, or slower airspeed in the same time, or a combo of all ,, which should make a big difference
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Offline Motorman

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Re: Sub 600 Past Battery Limit
« Reply #13 on: June 01, 2016, 02:52:23 PM »
Airplane is Brodak Tanager. I'm surprised nobody said lighter plane. 
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Offline Fred Underwood

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Re: Sub 600 Past Battery Limit
« Reply #14 on: June 01, 2016, 03:30:12 PM »
Back to the beginning using your parameters.  Appropriate size battery alternatives could work, but aren't inexpensive.  The shorter line alternative is fairly easy to try.  Shorten about 3' and then slow the motor a little and you save battery and gain laps.  Actual rpm and flight speed go down, as does centrifugal force a bit.  Compromises like that are common as pocketbook, trim and power/battery meet.

And then next, the new plane and power system learning from this one :)
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Fred
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Offline Mark Scarborough

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Re: Sub 600 Past Battery Limit
« Reply #15 on: June 01, 2016, 03:33:53 PM »
Quote from: Motorman on June 01, 2016, 02:52:23 PM
Airplane is Brodak Tanager. I'm surprised nobody said lighter plane. 
why would we,, we are trying to help you solve a problem with THIS plane,
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For years the rat race had me going around in circles, Now I do it for fun!
EXILED IN PULLMAN WA
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Offline Motorman

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Re: Sub 600 Past Battery Limit
« Reply #16 on: June 29, 2016, 09:56:55 PM »
Well, I could take the monokote off of it and use something lighter. I ended up putting more pitch in the prop and reducing the rpm now it's ok. Those rpm use allot of juice.

MM
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