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Old Oct 17, 2006, 03:36 PM
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Originally Posted by skiermike123
somebody please tell me if I'm going to burn up any of my components with my current setup!?
Mike, sorry I couldn't answer sooner, I've been busy slaving away at my job.

Your components are safe - no worries there, you're not hurting anything. You are running your batteries at less than their rated maximum current, and with the small (12") prop, you are likewise running your electric motor at well below its capabilities. As I said earlier, my first recommendation to you is to increase your prop size:
Quote:
I suggest you try an APC 13x6.5 Thin Electric prop - that will be about as much as this motor can handle on a 3s lipo pack. I figure about 75 oz thrust and 50+ mph pitch speed (and around 45 amps current draw, but that will depend on your battery, ESC, altitude, air temperature, and whatnot, so expect a few amps variation there).
I haven't flown an Acromaster yet, but if the ground clearance numbers posted earlier in the thread are correct, a 13" prop should clear just fine. If not, slap on some bigger wheels, and if that doesn't get you enough clearance, bend up some music wire to make slightly longer landing gear. Not a big deal, really. You NEED a bigger prop with your Power 25 motor and existing 3-cell lipo pack, otherwise you are getting less power out of it than the smaller Hacker or Himax.

Now let me try to answer some of your other questions. Firstly, power is the product of current (amps) and voltage (volts). So you can generate 400 watts of power in many ways - you could have 40 volts and only 10 amps, or 20 volts and 20 amps, or 40 amps and 10 volts, or any number of other permutations.

The Himax 3516-1130 recommended for the Acromaster goes the last of these routes, with the recommended 11x5.5 prop, it pulls nearly 40 amps from a 10 volt (3 cell) lipo battery to generate its power. The alternative Hacker A30-16M is similarly designed, but has slightly less weight and a slightly lower maximum current rating. The Power 10 is in the same category, but it is less efficient than the Himax or Hacker, and I would therefore not recommend it (unless you have one already, or got a screaming deal on one).

Now, your Power 25 is designed more for the middle road I outlined above - it is optimized for voltages higher than 10 volts. It does not handle any more current than the Himax (less, actually), but it will handle more voltage, and thus end up squeezing out more power - provided it gets the higher voltage it wants.

Prop size factors in in a big way also - you can put lots of power into a small prop by revving it to high rpm, or put an equal amount of power into a bigger prop that turns slower. The smaller Himax and Hacker motors for the Acromaster rev higher, and need to spin smaller props. Your Power 25 revs lower, and needs to spin a bigger prop.

I understand how confusing it must be when you first start out with electrics. Part of the problem is that the manufacturers do not do a good job of telling customers what they need to know. If it's any consolation, I would be totally lost if you asked me about glow engines (what the heck is a clunk? Is a double clunk twice as bad, or twice as good? If the engine has a "good idle", does that mean it sits around all day watching Oprah like my no-good cousin thrice removed?).

-Flieslikeabeagle
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Last edited by flieslikeabeagl; Oct 17, 2006 at 05:13 PM. Reason: fix typo - replace "higher" with "lower" in 2nd last paragraph
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Old Oct 17, 2006, 03:38 PM
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Quote:
Originally Posted by Jeremy Z
Good Lord, look at this post. Are you proud of me Beagle?
Jeremy, I'm proud of you, writing a thousand words in a good cause - to help out a fellow RC modeller getting into electric planes.

Now, doesn't it feel good?

-Flieslikeabeagle
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Old Oct 17, 2006, 03:49 PM
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Quote:
Originally Posted by Vantasstic
I use the UBEC on three different planes with no problems.
I'm sure you do, Vantasstic. The Spektrum (and other 2.4 GHz RC equipment) is less likely to be bothered by a switching regulator than conventional 72 MHz equipment.

This is because the electrical noise from switching regulators is confined to lower frequencies, possibly including the 72 MHz band. However, there should be negligible noise at 2.4 GHz from any switching regulator, as the switching frequency is usually at least ten thousand times lower than 2.4 GHz, and maybe a hundred thousand times lower.

-Flieslikeabeagle
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Old Oct 17, 2006, 04:18 PM
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Upstate NY, USA
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My backordered AcroMaster shipped yesterday. I haven't ordered a powerplant yet. However, lying around (originally intended for a mothballed scratchbuild) I have a PJS 3D 800E motor, APC 10X5 prop, Jeti Advance 18 plus controller, and a 4-cell "Twenty" TW-2100XP-4S.

I'm wondering if I can expect this equipment to power the Acromaster adequately as a basic traditional acro trainer, deferring 3d capabilities for future pilot/powerplant upgrades.
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Old Oct 17, 2006, 04:25 PM
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Ah... that's like scratching an itch, beagle -- a straightforward, comprehensive answer. Satisfaction.

I think I'm going to stick with the power 25 and get a bigger prop. I just flew the thing since my last post (lunchtime) and am more and more satisfied with it. I was looking at my batteries and they report a 55A continues and 80 burst. I don' tknow how many watts that would be, but I'll figure it out the next time I get a chance.

The thing is pretty nice as-is, I'm excited to put a bigger prop on it. I think the real question now is landing gear! I just broke it off twice in one day! (simple fix).

btw, I get almost 10 minutes per battery and they're only 2200 mAh. Go figure.
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Old Oct 17, 2006, 04:39 PM
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Jeremy Z's Avatar
Northern IL
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Quote:
Originally Posted by skiermike123
The thing is pretty nice as-is, I'm excited to put a bigger prop on it. I think the real question now is landing gear! I just broke it off twice in one day! (simple fix).

btw, I get almost 10 minutes per battery and they're only 2200 mAh. Go figure.
Are you going to get the folding prop, or a fixed prop?

The landing gear is already on the flimsy side for this model, IMO. You've got a heavy setup, which makes it even harder on it.

Get that folding prop I recommended and ditch the landing gear. You can revel in the glory of belly-landing when you can fly out of fields with long grass and no one else can.

The only thing that I've found with belly-landing mine is that the empty landing gear bracket catches the grass, and it stops instantly when it touches down. I can either tape over it, or just pull it out.

Jürgen is right that the airframe is stressed more when the plane is heavy. But I don't believe that a light plane handles the wind as well as a heavier one does.
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Old Oct 17, 2006, 04:44 PM
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yeah, this is a pretty solid plane, too, frankly. I'm not worried about the stress on the airframe. It might be a *little* hard on the landing gear, but... thats superfluous anywyas. Honestly, the landing gear is too for any weight of that plane--you couldn't make your plane light enough to handle it, period.

The weight definitely helps in the wind, in my opinion.

Is a folding prop just as good as a standard one?
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Old Oct 17, 2006, 05:26 PM
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Quote:
Originally Posted by skiermike123
Is a folding prop just as good as a standard one?
Depends on the brand - but from what I hear, the Graupner and Aeronaut folding props are high quality items.

One thing to watch out for is that two props of the same nominal dimensions (14x7, say) can actually vary quite a bit in how much they load a motor down. Propellers with wide blades (especially out towards the tips) will load down a motor more heavily than propellers with narrower blades. (Some of the Graupner CAM props have blades that narrow a great deal towards the outer tip of the blades).

The two prop blades in the first picture Jeremy Z posted are quite good examples of these sorts of differences in prop designs. You can clearly see the Aeronaut Carbon blade is much wider than the Aeronaut CAM blade, and that Carbon prop will almost certainly draw more current from the motor than a same-size CAM prop.

Also, a 13" APC prop is fairly inexpensive - if you don't like it, you're only out five bucks. Folders easily cost $20 - $30, so there's a bit more pressure to "get it right the first time", as mistakes cost quite a bit more.

-Flieslikeabeagle
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Old Oct 17, 2006, 05:40 PM
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The LHS still had it, so in the end I did buy an Acromaster. I took the box home and test-fitted the airframe together to see how it fits. My wife commented that the thing was "Huge! As big as a pet!" . Lots of fuselage side area there, no doubts about it.

Skiermike, Jeremy, thanks for the warnings about the undercarriage. One quick look at that silly little plastic bracket glued under the fuse was enough to tell me this was an afterthought - that's no way to design LG attachment for a 1kg foam plane. Because foam is so weak, the loads need to be spread out over a much larger area of foam, otherwise the LG will rip out easily. I guess a landing gear redesign will be part of my build. I have more fun with planes with landing gear, so I'd rather not go the belly-flopper route.

-Flieslikeabeagle
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Old Oct 17, 2006, 05:42 PM
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I think I'll try the 13" prop to start out. what do you think about doing a 13" prop with an especially low pitch--or maybe a 14" prop and a really low pitch. I fly a sig something extra (gas) with an APC 12.25x3.75 and it rips. Not that the powerplants are comparable, but that low pitch provieds tons of low speed thrust. What do you think about a 13x3 or something similar?
Mike
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Old Oct 17, 2006, 06:14 PM
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Quote:
Originally Posted by flieslikeabeagl
Skiermike, Jeremy, thanks for the warnings about the undercarriage. One quick look at that silly little plastic bracket glued under the fuse was enough to tell me this was an afterthought - that's no way to design LG attachment for a 1kg foam plane. Because foam is so weak, the loads need to be spread out over a much larger area of foam, otherwise the LG will rip out easily.
You know, I think they just copped the landing gear bracket from the MiniMag and maybe opened up the clamps to accomodate the thicker landing gear wire. It is exactly the same design. On the MiniMag, the result was that it has the beefiest landing gear of any (foamy) plane in its class. On the AcroMaster....not so much. Let us know how you redesign it, and be sure to post pix.

Another thing to watch for is friction in the pushrod tubes going to the tail. I built mine carefully, but I didn't notice this until it was glued in. I even put a drop of 3-in-1 oil in the rudder guide tube, and it's still loaded down a bit. It's the routing somewhere. I just gave up and used a nice switching BEC.

I was happy to see that MPX included proper hinges on this plane, and that they did a good job of the tail wheel design.

Keep an eye on the amount of current your BEC will need to supply. I blew up my cheezy E-Flite 40A ESC because I overloaded its BEC. (probably thanks to the aforementioned friction in the tail pushrods)

Since you were mentioning putting the servos close to the tail, take some CoG measurements before you make anything permanent. Maybe you can buy skiermike's Power 25 off of him, and that will counterbalance the extra weight you're going to put in the tail.

...OR Use your 2 lb. inrunner with planetary gearbox. That will DEFINITELY counteract the tail heaviness. Oh, and don't forget the 17" prop!! (OK, I'm teasing a little now, hehehe)
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Old Oct 17, 2006, 06:20 PM
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Jeremy Z's Avatar
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Quote:
Originally Posted by skiermike123
I think I'll try the 13" prop to start out. what do you think about doing a 13" prop with an especially low pitch--or maybe a 14" prop and a really low pitch. I fly a sig something extra (gas) with an APC 12.25x3.75 and it rips. Not that the powerplants are comparable, but that low pitch provieds tons of low speed thrust. What do you think about a 13x3 or something similar?
Mike
The Somethin' Extra probably runs at a MUCH higher RPM than your Power 25 will. Start with the 14x7. Even if it is a slight overload, it's not going to matter because you won't be running it at full throttle constantly.

You're underpropped now. The idea is to get you to where you're utilizing the torque that your motor makes. If you are really that worried and want to start small, do yourself a favor and buy a set of the big blades too.
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Old Oct 17, 2006, 06:33 PM
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cool. I wasn't going to go with the lower pitch because I was worried about the load on the engine--I was looking for increased thrust at low speed, but you're right, the gas engine is at much (**much**) higher RPM.

And you know what, I think I'm going to like my Power25, thank you very much.
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Old Oct 17, 2006, 06:33 PM
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Quote:
Originally Posted by skiermike123
I think I'll try the 13" prop to start out. what do you think about doing a 13" prop with an especially low pitch--or maybe a 14" prop and a really low pitch.
<snip>
What do you think about a 13x3 or something similar?
Mike
Nooo! Don't do it! Your AM will barely fly even at full throttle, and chances are very high you will stall it and destroy it.

This is another big difference between glow and electric planes. Glow engines turn at too high an RPM to match the model aeroplanes they are installed in. So one can use a low-pitch prop, and still have plenty of pitch speed to fly. By using a low pitch, one can use a bigger prop, so the trade-off works out well: more thrust, and still plenty of pitch speed (even though it turns out low-pitch props are terribly inefficient - when the pitch drops to less than half the prop diameter, lots of energy is wasted churning air, rather than pushing it smoothly backwards. Good for thrust, bad for efficiency. Good for helicopter rotors, bad for aeroplane propulsion.)

These electric outrunner motors are very different - they turn at a lot lower RPM. If you used a 3" pitch, the pitch speed would be so low that the AM would probably never get off the ground. You would have ample thrust, yes, but at too low a speed for flight. Imagine an elephant trying to push a Piper Cub down the runway fast enough for it to fly, and you'll have the right mental image!

I put some thought into recommending that prop size to you (I ran a simulation using motor simulation software), and 13x6.5 is what I suggest.) It will give you about 50 mph pitch speed, and plenty of thrust. Try it, I'll be very surprised if you don't like the results.

Edit: I see Jeremy got there first. The 14x7 prop might work, but my simulation software thinks you will go over 50 amps current draw, maybe 55 amps. That's pushing the motor pretty hard, since its rated for 44 amps maximum for 15 seconds. Efficiency will drop like a dead duck, heat generation will soar, battery life will decrease, and I wonder if its worth it.

-Flieslikeabeagle
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Old Oct 17, 2006, 06:46 PM
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you guys are awesome. Thanks for all the help. I could ask questions all day long... moving from gas to electric has been rather exciting. With gas is sooo much simpler: do you want adequate power, or ridiculous power? hmm...

of course they're not convenient, the make a mess, can be difficult to tune... but electric setup is much more complicated.

I want to get a couple of different motors now just to fiddle around.
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