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Apr 04, 2005, 12:32 AM
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Electric equivalent of .25 wet power x 4


I'm building a Palmer 1/12 scale B-24 and it uses 4 x OS .25's for power. I'm thinking of making it electric but need a little help figuring out the power setup.

It will weigh around 17 lbs with a wing loading of 32 oz/sq. ft. and should fly like a heavy trainer. I want to use an 11" 3 blade prop (scale).
I am thinking AXI 2814/12 with 3s1p 2000ma LiPo for each motor. That's around 25 amps per motor. Suggestions?

Should I use one big speed control or four 30 amp controllers.

- Mark
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Apr 04, 2005, 03:41 AM
Renewed interest
ozace's Avatar
Getting the mtors to sync would be difficult if not impossible with one controller. Safe bet is to use 4 controllers with some fancy mixing you should be able to get differential thrust to help in the turns, and use 2 motors for taxi.
Apr 04, 2005, 04:58 AM
hul
hul
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I think an AXI 2820/12 is a better choice. The 2814/12 will only turn an 8.5x6 (2 blade) prop on 10 cells at 25amps. The AXI 2820/12 turns an 11x7 prop on 10 cells/25amps (data from Model Motors website, www.modelmotors.cz). A 2 blade prop on 10 cells draws roughly the same current as a 3 blader with the same diameter and pitch on 3s Lipo.
25A out of a 2000mAh Lipo is 12.5C, a lot of current. You'd probably be better off with at least a 3000mAh pack per motor, about 8C. This is not necessarily heavier, see this thread:www.rcgroups.com/forums/showthread.php?t=286457
I run a brushless twin on one controller, but would go for 4 speed controls in your case, for starting reliability.

Hans
Apr 04, 2005, 05:06 AM
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Around 250W per motor is spot on.

Could be done with speed 600's as the most basic and cheap option.

Beware of scale prop diameters. Often these have scale pitch too And scale pitch is not enough to fly a model whose scale wing loading is far far higher than the full size.

For example, peering into a speed 600 geared setup I have to hand, on 3s LIPO an 11x10 prop pulls around 250 W static, but only has a pitch speed of 56mph. Thats OK if your bird stalls at under 20mph, but a lot of scale IC models are designed to fly at much greater weight than that.

I make its stall speed nearer 30mph from your figures. You need a pitch speed of at LEAST 70-90mph. To get that on any sensible 11" prop will generate enough thrust to pull the model up vertically.

It simply will not fly properly on 11" 3 bladers unless you have MASSIVE power into them.

You have two choices - go for smaller higher revving props (as the glow engines would do) or build it like a parkflyer and get the weight down by a factor of about 4...

On the sort of poqwer you are taking avout, a 7x5 prop would match it all better, spinning at about 14,000 RPM...
Apr 04, 2005, 08:46 PM
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Thread OP
Thanks for the help. I hadn't thought about pitch speed. I just want performance as good as the OS .25 FX's or better. The OS .25 turns a 9x5 to 10x6. Max revs are 19k so I would think the 10x6 is turning around 12k maybe?
I am thinking in terms of static thrust because that's what I'm used to and the OS .25's get about 3 lbs thrust.
My gut feeling is the AXI 2820/12 with an 11x7 , and 200-250 watts would put out 3 lbs of pull. Or should I pay more attention the pitch speed?
Now I have to wrestle with the cost! Around $1200 for the whole shabang.

Mark
Apr 05, 2005, 07:30 AM
hul
hul
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it is possible to have large diameter props and high pitch speeds; needs lower kv motors (or higher gearing) and higher pitch props. Basically what the F5B guys do.
188W output give about 45mph pitch speed with a 11x7 prop at 7100rpm or about 60mph with a 11x10 prop at 6300rpm.
Ramoser makes 3 blade 10.5inch diameter props with adjustable pitch up to 10 inches. They're not cheap but look good.

32 oz/sqft is a bit on the high side, a diet wouldn't hurt.

Hans
Apr 05, 2005, 07:48 AM
f5b-uk
Mike Seale's Avatar
Quote:
Originally Posted by hul
I think an AXI 2820/12 is a better choice. Hans
I'd agree that the 2820/12 is a good chioice. I saw an own design aerobatic model using this motor yesterday spinning 11x7 on 12 NiMH. I'd say performance was very similar to a 25 glow motor.

If you intend using a 3s lipo you might need a bigger prop (12x8?) or you could try the 2820/10. This motor, on 10 cells and a 12x8 was pulling a limbo dancer vertically with authority yesterday. Not sure what the current was but if it was too much for the 3s lipo you could cut back to 11x7 or 10x7 maybe.

Good luck with the project

Mike
Apr 05, 2005, 08:03 AM
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Quote:
Originally Posted by hul
it is possible to have large diameter props and high pitch speeds; needs lower kv motors (or higher gearing) and higher pitch props. Basically what the F5B guys do.
188W output give about 45mph pitch speed with a 11x7 prop at 7100rpm or about 60mph with a 11x10 prop at 6300rpm.
Ramoser makes 3 blade 10.5inch diameter props with adjustable pitch up to 10 inches. They're not cheap but look good.

32 oz/sqft is a bit on the high side, a diet wouldn't hurt.

Hans
This is all true, but by my calcs even 60mph is not enough on a 32oz/sq ft model.

I agree, if the model goes on a diet and stall speed comes down to 20mph or so, those combos are the bees knees. Not sure what wingloading that would be, but my guess is nearerr 15oz/sq ft.
Apr 05, 2005, 08:12 AM
f5b-uk
Mike Seale's Avatar
Quote:
Originally Posted by vintage1
This is all true, but by my calcs even 60mph is not enough on a 32oz/sq ft model.

I agree, if the model goes on a diet and stall speed comes down to 20mph or so, those combos are the bees knees. Not sure what wingloading that would be, but my guess is nearerr 15oz/sq ft.
The model can lose a lot of weight in the wing. An AXI does not need hardwood bearers like a .25 glow does. Also, the weight of an AXI with associated cables is less than a .25 with fuel tank. So the structure of the nacelles can be much lighter and the spar could also be lightened somewhat.

Mike
Apr 05, 2005, 04:18 PM
hul
hul
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Quote:
Originally Posted by MarkIam
Or should I pay more attention the pitch speed?
this article by Keith Shaw gives a good explanation http://members.aol.com/kmyersefo/scale.pdf. According to him, you get 20mph stall speed at 29 oz/sqft wing loading. I think that is still on the heavy side though. Surely that plane can be built lighter than that.

I think you're doing the right thing by investigating this as thoroughly as possible before spending that amount of cash and time on a plane this big (over 9ft).
You may also want Motocalc to give you an opinion.
And have a look at this thread https://www.rcgroups.com/forums/show...ncaster+marius , it's about a 12ft Lancaster

Hans
Apr 05, 2005, 05:03 PM
hul
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have a look at this thread too: https://www.rcgroups.com/forums/showthread.php?t=122092

Hans
Apr 06, 2005, 10:28 PM
Balsa Flies Better!
I'll go with the volume idea-- larger airplanes get away with higher area loadings because the wing has increased volume relative to smaller aircraft. Thus, I think your stall speed will probably be lower than some of the folks have estimated- but a little extra lightening wouldn't go amiss.

I've done a bunch of glow to e-conversions, and I'm worried that you're going to be a little undergunned. However, I doubt that an OS 25 is turning a 9" prop at anywhere near 19k rpm- that'd really be screaming. I think you're on the right track though, with an Axi 2820/10 on 3 cells spinning a 10" 3 blader- although another option is the Mega or Jeti 30/3- an oz heavier but they take more watts. But, I think you need to figure on pulling about 35A per motor- which will give you 350 watts per motor. All up 1400 watts which is about 80 watts/lb- a reasonable power loading for an airplane designed as glow, but intended to be flown non-aerobatically. This will make sure you don't feel shortchanged in the power department compared to glow- the larger props will help make up for any test stand disparity.

In terms of li-polys though- you're gonna need some fairly fat cells, because this airplane could need a heavy throttle stick. Thus, you want cells at 6-8C- no higher!. You're probably looking at 5000-6000 mAH per battery- this is not a cheap airplane. If you haven't started building though, by the time you get it done, the li-polys will be cheaper- not sure about the motors though. Definitely go with separate speed controls per motor- probably Phoenix 45s or Phoenix 60s. You want some headroom since you may be running the motors flat out a lot.

Sam
Apr 09, 2005, 02:46 PM
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Thread OP
Thanks for all the help on this. I really appreciate it.
The B-24 is built and I have the OS .25's ready to go but I just have this nagging feeling that it should be electric so I have been trying to decide before I start breaking in the engines. However I am going with the wet power, for now. It's just going to cost too much for my budget at this time. Those Lipo's are too damn expensive! Maybe if I buy one motor a month and then one huge lipo pack each month my wife won't notice! Amortize the cost a bit, eh?.
I built the nacelles with removable firewalls so I can change over to electric someday without too much trouble.

Hul - Excellent links you posted. That Mars and Lancaster are inspiring! They really make me want to go ahead and spend the money.

So if anyone knows a battery manufacturer who would like to sponsor a B-24J liberator, Let me Know!

Mark[IMG]Nac01130506.jpg[/IMG]
Apr 09, 2005, 08:08 PM
Balsa Flies Better!
D'oh!

Hold it- before you start breaking in some glow engines you may never use- a horrible thought has just occurred.

A couple of points- having had exactly one takeoff on a glow powered twin (and losing an engine, and having a long rebuild job) there's no way I'd recommend that you put an airplane with that much time into it into the air on anything less than a nice reliable electric setup. (Yes, electric twins have had issues too, but nowhere near as many.) And I'm a guy who now has 3 electric twins- including a '38.

This airplane may not benefit much from li-polys. I'll lay long odds that bird is going to need noseweight, and conceivably a fair amount. The electric motors are going to be a lot lighter than glow engines, which means that you've got even less weight in front of the cg. You're gonna need batteries or lead in the snoot in all likelihood.

Don't break the bank with li-polys-The newest NiMH cells are apparently doing quite well in terms of power/weight as li-polys, although their duration is still leaving something to be desired. But better to go electric in terms of motors, and upgrade batteries later if the lack of duration is really raining on your parade.

Basically you're looking at 2 700 watt packs (might be a pain to charge 40 cells) IIRC the new 2000 mAH GP cells weigh in at 1.5 oz- about 60 oz all up in battery weight. This might be about 20 oz more than li-polys- but on an airplane that size- you might not notice it so quick. And with some creative shopping, you might find some deals on these packs because people tend to go li-poly.

Sam
Apr 10, 2005, 04:18 AM
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vintage1's Avatar
I agree with sam. Te cheapest way to get this model in the air is a load on NiiMh in teh nmose, and four quality BRUSHED motors with gearboxes.

4 x good buggy motors or the neodym or cobal 600/700 type motors won't break the bank.

The speed contriollers won't need duplicating either. At a pinch you might fine a 120A brushed controller. Or a pair of 60A at the worst.


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