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My next trainer must be electric... So what do I do now?

Old 09-19-2009, 12:45 PM
  #1  
Speedtorque
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Default My next trainer must be electric... So what do I do now?

Hi, there I recently moved locations and found that my new local flying club cannot accept glow powered flight, electric only.

Now both flying and electrics are not new to me, but I am still a beginner. I could solo at my last club, meaning they thought I was safe, but I am still not great at landings (without breakages).

For this new club, I have told them my position and that I feel I need some more tuition. Although confident that I will be able to solo soon enough, I/we feel that I'm still at the stage where I need a trainer.

They advised my to look at this: http://www.servoshop.co.uk/index.php...&area=Aircraft

And why not? It's a standard trainer, it's sensible and cheap. it looks as ugly as my last trainer (Thunder Tiger 40), but that looked nice in the air. I'm sure this will to. However, nearly all manufactures are now suggesting that any of their range is "potentially electric" and there are nicer trainers out there.

When I looked at the recommended power set up for the E-Pioneer, I became very confused. Moto calc gave many suggestions, but most involved using multiple packs or 3 or 4 cells lipos and lots of strange prop sizes. I'm totally confused by it. The reviews used 4 cells and a 42mm out runner, but said it was too much and suggested 3 cells. http://www.peakeff.com/PickMotor.aspx suggest 5 or 6 cell with really small motors and props.

So, before I buy anything. can someone help me through my ideal set up please?

My ideal criteria would be something that flies like a 40ic trainer with a 46 motor in it, but I would like good duration. I'm not at the beginning stages where I tire from a 10 minute flight. I need some good time in the air so I can get on with practising my touch-and-goes and get lots of hassle free air time.

Is this the right airframe for this? if so: Can I achieve that kind of performance and duration through peek efficiency and battery type?
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Old 09-19-2009, 03:11 PM
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Sky Sharkster
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Default E-Pioneer Trainer?

Hello Speedtorque, Welcome to Wattflyer!
I'm not familiar with that particular model, but it looks like a decent trainer. You're starting at the large end of electric power (well, maybe the upper-mid range, now!) but since you're familiar with this size model (whether glow or electric) you should do fine.
The sizing of electric motors is very confusing, especially with the variable of voltage, mAH, cell count, prop size, etc.
But a simple rule of thumb may help;
100 watts per pound.
This is a good general size for average "Sport" or lively trainer performance. Since the weight range of the E-Pioneer is listed as 4.0 to 4.8 lbs, let's choose 4.4 lbs as a medium.
4.4 lbs is 2 kilos. or 2000 grams.
So, we want a motor that's capable of generating 100 watts per pound of model weight, or in this case, 440 watts. That's without running at top speed all the time, it should be the upper range, but not the top. So, let's look for a motor that's capable of, say, 500-600 watts (top end) so it's not working too hard at 440 watts.
I couldn't find the recommended JP C35-14 on the motor page of the Servo Shop listing, and the other JP's were out of stock. I'd be careful about purchasing any motor that's not easily available, either as a replacement or spares.
So, we'll go for a few well-known brands and see how they compare.
http://robotbirds.com/catalog/produc...f983f415a06573
Hyperion have a good reputation as sport motors and they make a few higher-performance versions as well. As you can see, it fits our 500-600 watt output quite easily.
Her's another in the size we're looking for;
http://www.ripmax.com/item.asp?itemi...tegory=070-020
As a general rule, the higher the input voltage, the smaller the prop. 3s LiPos are 11.1 Volts, 4s is 14.8v, 5s is 18.5v, etc. LiPos are nominally 3.7 volts per cell in series.
So, a motor of about 500-600 watts output on 3s or 4s, a large-diameter, low-pitch prop like a 12" x 6" or so will yield a slower, steady flight. A smaller, high-pitch prop will generate faster flight speeds and usually, shorter run times.
All things being equal, duration is mainly determined by mAH. For a given size motor, prop and input voltage, a 3200 mAH battery will run nearly twice as long as a 1600 mAH battery. Why not exactly twice as long? Because a 3200 mAH battery is heavier than a 1600 mAH battery. So, you would need a slightly higher throttle setting to maintain the same flight performance, thereby using up your mAH at a faster rate.
I hope some of this is useful, any questions, ask away!
Ron
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Old 09-22-2009, 09:52 AM
  #3  
Speedtorque
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Thanks for the advice Ron. Especially with regards to parts availability.

I think I understand the watts per lbs rule and that mah equates to endurance, but what confused me is the data from the calculator programs.

For example: The manufacturer recommends 3 cells and a 1100kV motor running a 12x6 prop, making 386W.

So from the prop, side of things it looks similar to what a 46 engine would run?

One example from the peak efficiency calculator was: 4 cells, 1260kV and an 8x4 prop.

Now assuming that both set ups produce the similar watts (the both make about 1.4kg of thrust) , I know that the larger prop will offer more speed control and the smaller prop will offer more max speed. The former being more appropriate for a trainer.

But, Motocalc suggested, in one example using 2 packs of 6 cells on a 320kV motor turning a 12x6 prop (when I specified the prop size).

What is the advantage of running high Volts and low kV compared to the manufacturers suggestion? I'm guessing that somewhere along the line there is a trade off between efficiency (which seems to correlate with high rpm) and what is familiar to those coming from glow engines. By that I mean, a 12x6 will make a familiar hum to the same prop powered by a 46 glow, minus engine noise. Whereas a 12x3 does appear much more efficient by theoretical calculations, but to make the same thrust it must scream at high rpm. I know of many glow users that would consider that noise to be some kind of voodoo magic.

In short:

Should I use the manufacturer's suggestion even though it is a little underpowered?

Or, should I use one of the unusual suggestions from the calculators?

- John
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Old 09-22-2009, 11:50 AM
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Sky Sharkster
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Default Prop Sizing?

Hi John,
The prop size/voltage equation is a little more complex than I first described, when you add in the other main factors of motor efficiency, thrust output and amp draw. Rarely does the most efficient prop/voltage combo yield the best static thrust, or if you find a prop with the greatest thrust, it may have an unreasonably high amp draw (hence duration).
If you specify a certain size prop for an application, the calc will try to adjust the voltage and Kv to obtain that result. But, in the case you presented, it may be a fictious answer. I don't believe there is a 320 Kv motor, I've never heard on one that low (Kv). The low end is usually 500-800. It may exist, but not that I've seen.
If you find a motor that has the (advertised) power range you need for the application, I'd start with the manufacturer's recommended prop size (and battery voltage) and purchase a couple spares that are 1 size larger and smaller. If they recommend a 12 x 6, add a 12 x7 and a 12 x 5 to your flight box. Or possibly an 11 x 7 for more speed, a 13 x 5 for slower flight but more thrust. Another change you can make is to try different brands of propellers in the same size, they can be quite different in use.
The important thing to remember is that calcs are theoretical. Only as good as the input data and occasionally quite different from real-world experience.
They can only suppliment what happens in practice, or offer a suggestion of prop sizes and voltages. The true answer is a wattmeter. This is the single best investment an electric flyer can make and will pay for itself many times over in money saved by not burning up ESC's, motors, batteries. Once you've selected a motor, battery size (voltage input) and prop range with the calc, then you can fine-tune the combination with a wattmeter.
I'm sure this is not a complete answer to all your questions, but there are still a lot of variables. The calc will get you in the neighborhood, but a wattmeter will provide an exact answer.
Ron
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Old 09-22-2009, 06:23 PM
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Matt Kirsch
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Do you still have your old glow trainer?

If not, convert it to electric! It'll cost you a fraction of buying a whole new plane, take you a fraction of the time to set up, and you'll have an airframe that you're famililar with.

A .46 glow engine will fly any .40-size trainer. In the same vein, an electric power system that flies one .40-size trainer well, will fly them all well. It doesn't have to involve reinventing the wheel for each airplane.

One of my club's members has a Hobbico NexStar EP, which is just the glow NexStar with an electric motor bolted to the front. From the experience we had with that plane, 3S is not adequate for a .40-size trainer. The plane flew well on 4S, but took a long time to get off the ground. We tried one of my Rhino 4900 5S packs, and it easily matched a glow NexStar move-for-move. I no longer own any Rhino 4900 5S packs, if you catch my drift

They easily get 12 minute flights out of the Rhino 4900 packs, with plenty of capacity left for as many go-arounds as necessary to get a good landing approach.

There is no such thing as "too much power," only pilots who are incapable of using their left thumb. With the affordable brushless motors and batteries on the market today, it makes absolutely no sense to skimp on power.

One other piece of advice: Ignore Motocalc. When you give it free reign and tell it to recommend a power system for you, it goes nuts. You end up with hundreds of weird combinations of out-of-production motors running insane cell counts at outrageous currents.

For outrunners, I use what I like to call the "AXi standard" to choose a power system. It starts with a loose calculation of Watts per pound. Next I find an AXi combination that produces that power by looking through tables on the ModelMotors.cz website. When I find the right motor, I write down the dimensions, weight, and Kv rating, which I then use to shop for motors with similar specifications. Final tweaks can be made to the power output using different props, as above.
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Old 09-22-2009, 07:03 PM
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Murocflyer
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Check out this e-powered trainer:

http://www.3dhobbyshop.com/detail.aspx?ID=2097

It comes highly recommended. I don't own one, but it looks like a nice plane.

Discussion thread here

Frank
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Old 09-23-2009, 12:17 PM
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Speedtorque
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Ron: I see your point with efficiency, I guess I was asking too much from the calculators. As for the Watt meter, I bought one only a week ago. I have been itching to use it on my mugi (http://www.mugi.co.uk), but since the move I haven't seen my Lipo bag anywhere? I can see that tool being a great help though.

Matt: Great idea, but the Tiger Trainer really has seen better days. So, I gave it to a pal who was R/C-curious and told him to use it until it dies (lets hope he catches the bug). 5 cells?!? I don't know why, but that size of pack makes me nervous, but I see your point on power. There are many nice trainers and I'm sure that they all fly great on electric, but the E-Pioneer (http://www.jperkinsdistribution.co.u...20Seagull%20EP) is designed as an electric so weighs in at around 5lbs flying weight. I think my old 40 trainer was more than that with a glow. Am I right in thinking electric power systems weigh more for the same power and duration. I'll try that AXI standard, that seems a good Idea.

Frank: That model looks really smart, but I can't find it here in the UK and at 2lbs with a 53" wing it would not be able to fly that often over here. At my old club every flyer had a least one 5lbs + hack as you just never know what the wind will be like at the field. When you get BBQ weather over here, it's nice to bring out some light weight models, but they rarely get flown. I think that's a big problem with electric flight here in the UK there are not that many days when a 3 cell aircraft, eg: 450 heli or a 36" spitfire, can actually fly. Hence I'm looking at a 61" 4-5 cell trainer.

Thanks for all your advise guy, it has been most useful and I feel I understand this world of sparks better now.

- John
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Old 09-23-2009, 01:48 PM
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Matt Kirsch
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5 cells, oh yeah

That NexStar on 5 cells is over a pound lighter than with the original 16-cell NiMH battery. Plus, it's a whole lot more powerful. It wouldn't even take off with the NiMH if the grass wasn't mowed down to almost bare dirt.

I'm actually finding a correlation between cell count and glow displacement. My planes tend to end up with about one LiPoly cell for every .10 cubic inches of 2-stroke glow engine displacement.

.25-size = 3S LiPoly
.32 to .40-size = 4S LiPoly
.46-size = 5S LiPoly
.61-size = 6S LiPoly
.91-size = 9S LiPoly

It just seems to work out that way. I never use less than 3S on any plane, and I do have some exceptions to the rule, such as my SSC combat planes which normally run a .15 glow engine. These use a 6S LiPoly pack because I need the voltage to spin the class-standard 8x3 prop at the class-standard 17,500 RPM.
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