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Old Jan 22, 2007, 05:47 PM
kids ate my heli budget
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Marutaka Pitts Special S-2

Hey all.

My grandfather recently emailed me to say that he found a Marutaka Pitts Special S-2 kitset in his garage that he had forgotten buying. We're looking to build it up, but I will only fly electric. I have a couple of packs that I would like to reuse (6S1P 3700mAh and 5S1P 4400mAh), I would like to use the 5S packs if possible. I'm currently looking at a Hyperion Z40 motor, but I'm looking for suggestions.

Pitts Special S-2
Scale 1:4.8
Fuselage 1185 mm
Wing Span 1320 mm
Wing Area (upper) 28.4 dm2
Wing Area (lower) 25.0 dm2
Weight 2.8 to 3.5 Kg
Engine 40 (2 stroke) 60 (4 stroke).
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Old Jan 23, 2007, 01:24 AM
kids ate my heli budget
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Auckland, New Zealand
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My local electric RC shop says:

Quote:
Sounds like a great project.

For this sort of model I'd suggest that you need to be around 100
watts/pound minimum. If it were me I'd go for more like 150w/P and as you've
got a couple of good battery packs already, lets look at 150.

3.5kg = 7.7pounds

6S = 22.2V
150W x 7.7 = 1155W

1155 / 22.2 = 52Amps

So that gives us plenty of choices.

I'd look at a Z4025/10. This will run on either 5 or 6 cells easily and as
it will burst up to 1300W - we are easily in the money.

The new Titan speed controllers can now handle up to 6S so a Titan 80 OPTO
is your best bet there.

We also have a new product in today from Hyperion that looks great - An SBEC to save you having to run a separate receiver battery.

As for mounting, the basic motor mount would probably be your best bet and
at $29 they can save you a lot of work.
Does anyone have any thoughts on this?
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Old Jan 23, 2007, 10:21 AM
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Sounds like good advice to me. If both packs can hack the current, you still have the choice to either go for light and somewhat lower power using 5s, or real power with the 6s without changing anything but the prop. This is a great project. And as you're just starting, you can probably shed some weight as well by dispensing with some of the heavy wood (firewall, motor mounts, etc) and using some modern ingenuity and materials.
Lorenz
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Old Jan 23, 2007, 02:20 PM
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Hastings, Hawkes Bay, New Zealand (Aotearoa)
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The only other thing I would check is the prop size. What clearance is there on the model and what prop size is does the motor need for the recommended power level.

Marutaka kits are renowned for using pretty heavy wood so there may be a chance to cut some weight out by replacing some of the stock wood.

Most 40 size conversions I have done have flown well on 5s setups.

Heather
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Old Jan 23, 2007, 02:38 PM
kids ate my heli budget
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Auckland, New Zealand
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We're planning on building it as light as possible by replacing some of the stock wood with lighter alternatives. Does anyone have a link to a good article about choosing a prop for an electric plane? I can tell you all about gearing in electric helis, but this will be my first fixed wing.

(I just found this post about prop sizing and motor selection, which has explained a lot for me: http://www.rcgroups.com/forums/showt...TRIC+AIRPLANES)
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Old Jan 26, 2007, 04:08 PM
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Concord, MA
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Try PCALC at www.flydma.com It probably doesn't have your motor or battery listed, but choose a motor with the same Kv from the list and then a number of Nicads that will aproximate your lipo's same voltage at load. Most brushless motors will be about 80% efficient so it should work to get you in the ballpark.

I was looking at Neumotor ORK motors on NSP's site. They are pricey, though.

There is a chinese co that makes a multi pole inrunner too that has a cooling fan built in that maybe you should check out
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Old Jan 26, 2007, 04:20 PM
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Concord, MA
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FG-MBLF-580 Feigao Ferrous Loss Free Motor 580 6Pole 760kv



A new concept and motor from feigao.
The motor is Ferrous Loss Free, and very efficient.
It has a super low kv of 760, meaning it can be used as a direct drive to run a propeller without needing a gearbox.
Great for large planes and helicopters!

Shaft Size: 5mm
Kv: 760
Motor Class: 580
Motor Weight: 289g
Diam: 45m
Length: 56mm
Poles: 6


310grams
$59.20USD
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Old Jan 26, 2007, 04:31 PM
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That was from ultrahobbies in Taiwan. I have not ordered from them, but it's probably closer to New Zealand than here.
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Old Jan 26, 2007, 04:35 PM
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I think about an 11-7 or 8 on 5 lipo cells, based on a 780 rpm/volt motor on 5 lipo cells at 90% efficiency, yielding about 7 lbs thrust at 80 mph prop speed. I used an ORK that had this KV for the table, and a 5 cell battery of the biggest MAH lipos listed in the drop down menu
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Old Jan 27, 2007, 07:17 PM
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Thanks guys. What about the Z-Power motors? They're creating quite a splash with the RC helis at the moment:

Z-Power Z30A-800 Outrunner (w/fan)
kV: 800
Io: 1.48A
Rm: 34.1 mOhm
wt: 280gm
5mm shaft.
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Old Jan 27, 2007, 11:55 PM
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That motor's Kv is too high for direct drive on 5-6s. You would only be turning about a 9" prop with that.

You will need a motor more around the 500kv range.

Heather
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Old Jan 28, 2007, 10:40 AM
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I think you could use a lower pitch prop and still have the High KV, although 760 or 800 doesn't seem to be "high." I think 12-4 is a size avilable here. I got a prop speed of 88 mph for the 11 7 on PCALC for 5 cells which seems about in the range. Someone said you wanted a propspeed 2 1/2 times the stall speed of the plane. I know the plane barely flies if the propspeed is too low, and the second you bank to turn it falls out of the air.

My recent experience has been that you do not want to push the Lipo's too hard if you are interested in lots of charge cycles from them. If they say 20c, run them at 15, if 15, try 12, if 12, run them at 10c, etc. They want to sell you batteries, so they want you to run them hard for fewer cycles and then replace them. That is what is so great about free market economics. I had read on the web to not overcharge or over-discharge lipos. Then there are the A123 batteries... They don't go to 4.25 a cell fully charged, but the maanufacturer says they can be discharged to a much lower voltage. So you need different charge and ESC settings. But they are more crash resistant.
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Old Jan 28, 2007, 10:58 AM
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I tried to paste the data from PCALC. I guess I would agree with Heather that the AMPs are pretty high. If we say 100 watts per lb, (OK 220 per KG) then you want 600 to 700 for this plane anyway. For 5 or 6 cells this is 35 to 45 amps, which I would think is OK with these batteries. I had a polyquest 3s4000 12C and was very disappointed with it's longevity. It was in a plane that drew 40 amps at WOT. I am seriously thinking NIMH still for high discharge rates, since they also can be recharged in 20 minutes.

The manufacturer claims A123 cells can be fast charged, and they can support a very high discharge rate for a lot of charge cycles too. I think Killacycle, the electric motorcycle dragster may be running these cells now.
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Old Jan 28, 2007, 02:36 PM
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These particular 5S packs are 4400mAh 12-14C. They've been flown in a helicopter that according to the maths was drawing an average of 22 amps during every flight. They consistently gave me 12 to 15 minute flights, which is damn good. They haven't been touched since I upgraded by chopper to 6S and have been in the cupboard at half charge for at least 6 months. I also have a 5S emoli pack that I used to use in the same machine. Maybe it would be better to use my 6S packs in this machine - they're the same weight and much more powerful - Hyperion VX 6S 3700mAh 20-30C packs.
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Old Jan 28, 2007, 05:03 PM
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I did try 5s lipo and the 780Kv with an 11" diameter X 5" pitch two blade prop in the aforewmentioned PCALC and got numbers which should work:46.8 amps 724 watts output@ 91% efficiency (this was a neumotor spec)static thrust 99.4 oz. pitch speed 56.5 mph. I would reitterate my experience I had with 12 c lipos, and that your 22 amps on a 4400 Mah battery is 5C. If you have two, run them in parallel. At 44 amps you would get 5C again from 8800 2P 5S, and should again give you 12 to 15 minute flights. I remember with 1200 SCR nicads we used to be thrilled with 4 to 5 minutes.

Brushless motors do get hot too, I think from induction or resistance.

I have an Hyperion 2213 and I love it. Pulls a 2 lb plane almost straight up with an 11-6 on 3 lipo cells. They are really well made. The 6 pole zero loss inrunner stylee may be a little bit more efficient. They claim 90%. I would think a can motror may be easier to mount in a glow model than the outrunner. I have an AXi 2820-10 and love it too. Also a Jeti Phasor 30-3 which I think is 6 pole, around 1000Kv. It gets hot after 5 minutes at 400 watts on 12 volts. Actually it threw a magnet off the rotor one time and now has a kevlar wrapped rotor, running it in a belt drive with 15 nicads. You can run these motors hard but make some provision for cooling them or stop and let them chill onc and a while.
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