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Author Topic: Wagner’s Dakota — RC conversion  (Read 259 times)
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TornadoAlley
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« on: February 28, 2020, 02:01:03 PM »

I’m planning to convert the BMJR kit of Wagner’s Dakota free flight to RC. Anyone out there familiar with the airplane who can suggest an electric motor size, prop, and ESC? I was thinking an 1800 KV motor with a 5x3 prop, and a 30A ESC.

I am concerned with overall weight, so much so that I’m thinking about making it rudder only. (The potential weight of an electric RC Dakota is substantially more than the weight of the .020 Dakota free-flight I built and flew in the early 1970s.) Easy retrieval is more important to me than the added control that an elevator buys.

I have very little experience (and luck) configuring electric motor setups, so thanks in advance for the help.
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Konrad
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« Reply #1 on: February 28, 2020, 02:59:44 PM »

1800Kv doesn't tell us anything about the size of the motor. If you are thinking of 30 amp ESC you are looking at a lot more power than what was originaly found with the Cox 020 Pee Wee and still more than what we got with the Cox Tee Dee.  Please give us an idea as to what level of performnce you hope to have. I'm thinking you may want 2 to 3 cells drawing around 10 amps. As a rule of thumb I like my sport ships to have 50 to 80 watts per pound of all up weight.

All the best,
Konrad
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TornadoAlley
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« Reply #2 on: February 28, 2020, 04:01:31 PM »

Thanks Konrad. I’m looking for enough power for a slow, relaxing climb similar to that of my FF Dakota with the .020. Nothing hot or slippery — a goal here is to introduce my grandkids to building and flying balsa models.

As an aside, seems like no matter how much I read about electric flight power systems, I find it all pretty baffling.

Thanks again for the help.
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Konrad
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« Reply #3 on: February 28, 2020, 04:48:54 PM »

Ok, not sure what your budget is. On the higher end of thing the power systems found on the Horizon Hobbies 2 cell UMX models should be more than enough power.  Take a look at what comes with the UMX Timber. The Horizon Hobbies stuff works well together. If looking for a lower cost but higher risk of failure there is the Hobby King path. Let us know what are your constraints.

Not sure what the issue is understanding electric power. Unlike glow the power curve is much more linear. Glow has that "peak" we all are trying to match. Now unlike glow were generally the smaller the prop the more power the engine produces (until you go past the peak rpm). Electric power the power band slops the other way. The larger the prop the more power an electric motor produces. Once you have this visceral understanding I think tuning for electric power should be easy. Also as the power slope is rather linear it is rather easy to predict the power with some simple algebra (a great learning tool for the kids!).
https://www.ecalc.ch/motorcalc.php
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Re: Wagner’s Dakota — RC conversion
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strat-o
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« Reply #4 on: February 28, 2020, 05:10:22 PM »

There are some simple absolutes that can help.  For example, if you take the same prop that a .020 turns and put in on your electric motor and simply match the .020's typical R.P.M., even though the electric may not be working up a sweat, you are still getting the same thrust that an .020 would be giving you.  So long as your motor doesn't weigh too much, perhaps you can just limit its output?

Also, to get horsepower, take watts and divide by 1000 (you'll be in the ballpark, actually divide by 746).  A run of the mill .049 outputs 42 watts.

Amps as a unit of measurement usually kind of sucks because you can't tell what the power is unless you know the voltage.  It kind of works if you always use the same voltage for everything and I think that's probably why people use it and not run into too much trouble.  Watts = amps * volts so if you have a 30 amp esc, to get the wattage, multiply by the voltage you are running.  (The thing amps are useful for is the amperage gives you an indication about the thickness of wires you need to operate safely)

If you run 12 volts then 12 x 30 amps gives you 360 watts which is around 1/2 horsepower.  I think you want to be around the 25 watts or less range with your project.

Hope this is more helpful than it is confusing!

Marlin
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Konrad
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« Reply #5 on: February 28, 2020, 06:00:46 PM »

The only issue I see is in measuring the amperage. Just because the ESC is rated at 30 amps doesn't mean that the system is drawing 30 amps. The size of the prop for any given system will control the current draw. An amp meter is a tool you will need. Much like a tachometer is a tool one needs for glow engines.

On the subject of props. The magic of electrics is not needing or wanting to stay with the same inefficient prop as we used on the glow motor. Using a larger slower turning will get you a much better power match to the airframe. So with electrics you can fly better than you did on glow with less power.

I hate to say this but it is best to forget what you know about glow and prop selection. It is best to think from a clean sheet. Much like you did when you first learned about glow. Watts per pound is a good start. That is easy to measure with some simple electric tools like an amp meter and volt meter. A gram scale would also help.

I like a clamp on inductance DC current meter like this. Be aware that AC clamp on meters will not work for our toys.
https://www.amazon.com/Uni-T-B4Q094-UT210E-Current-Capacitance/dp/B00O1Q2HOQ/ref=sr_1_7?gclid=EAIaIQobChMI1ufb_az15wIVk6DsCh0oZg9kEAAYASAAEgLxAfD_BwE&hvadid=312544269329&hvdev=c&hvlocphy=9031939&hvnetw=g&hvqmt=e&hvrand=11425844366177774106&hvtargid=kwd-341958282878&hydadcr=24632_10399691&keywords=inductive+dc+amp+meter&qid=1582931011&sr=8-7#feature-bullets-btf

All the best,
Konrad

P.S.
Does anyone really have a visceral feeling for what a horsepower is? One HP ay 12k rpm feels a lot different than one HP at 16K rpm. I'm of the opnion that HP is one of the worst measures of power. So few folks have actually seen a horse in harness pulling a load. Besides it take a dyno to measure most engine outputs. With electrics some simple meters (one should have these around the house) and we can get some meaningful readings.
« Last Edit: February 28, 2020, 06:12:11 PM by Konrad » Logged

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Yak 52
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« Reply #6 on: February 29, 2020, 05:42:49 AM »

Anyone out there familiar with the airplane who can suggest an electric motor size, prop, and ESC?

I'm not familiar with the design but I just had a look at this thread: https://www.rcgroups.com/forums/showthread.php?1543848-Joe-Wagner-Dakota

The first thing would be to estimate your final all up weight. The guys here are talking about 9-10 oz (250-300g) which sounds very heavy for a 24" model to me - but there is a lot of wood in an all sheet design. This, by the way, is going to make it a pretty fast model! Also it looks to me as if the guys in this thread are overpowering somewhat so a lighter power system may be possible.

For something this size and a light wing loading, I would normally be looking at a 10g Hextronik brushless on two cells (7.4V) and about 300-500mAh. This would use a 6A ESC and run a 6x4 prop to give you about 100-120g of thrust. This would fly a model of 150g nicely BUT if the weight really is going to come out at 250g it wont be enough and you will need to go bigger. Say a 15g or 20g brushless with appropriate ESC (10-12A?) and a bigger battery.

I am concerned with overall weight, so much so that I’m thinking about making it rudder only.
I would definitely keep the elevator, even at the expense of an extra 3-5g of servo. Although it's only small it looks like it's going to be a fast ship if the wingloading really is so high. And fast is ok if it's fully controllable! Otherwise you'll be flying a fast bar of soap and fighting it all the way.

See if you can work out a target weight and start from there. And then you can get some more specific recommendations.
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Starduster
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« Reply #7 on: February 29, 2020, 08:13:02 AM »

Here, let me simplify things just a bit

This is the set up I've been using with success for .020 size airplanes for a couple years. Not the cheapest you'll find, but it works:

1) Motor: https://www.horizonhobby.com/airplanes/motors/park-250-brushless-outrunner-motor-2200kv-eflm1130  (The spec sheet will tell you what size prop)

2) Battery: https://www.horizonhobby.com/all-rc-batteries/batteries/1300mah-2s-74v-20c-lipo--18-awg-ec2-eflb13002s20

3) ESC: https://www.horizonhobby.com/airplanes/electronic-speed-controls/10-amp-pro-brushless-esc-efla1010

And, with the ESC, you will have rudder and throttle

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Konrad
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« Reply #8 on: February 29, 2020, 08:36:13 AM »

I have done a lot of this type of model. On HIP I have these threads that may be of help.

The Lady Bug looks close to the size of the Dakota. It is a dated build but she came in at 215 grams with 3 channel control. Most models like these I recommend full house. But the Dakota would do well on 3 channels. With my Lady Bug I'm set up on 3 cells (450 mAh to 1300 mAh) drawing around 4 amps.  (44 watts = 93 watts/pound at the battery) She is a lively sport model.
https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=12348.0

In this thread there is a bit on control set up. While these gummy band 18" to 30" are powered by 5 to 10 gram outrunner motors. You might notice that they turn large props, often in the 7x6 range. As I recall this is a 0.15 cid prop!
https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=12443.0

Some of my 24" full house conversions are taxing the power of the 10 gram outrunner. With the weight of the sheet Dakota I'd think a 15 to 20 gram outrunner would be better suited.

I recall seeing Dakotas flying as Free Flight on Cox Baby Bees (0.049) with the 6x3 prop installed backwards. I don't think the power of an 0.020 and RC is going to make a pleasant model.

Please do give us some more information to work with.

All the best,
Konrad

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TornadoAlley
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« Reply #9 on: February 29, 2020, 11:56:19 AM »

There’s a wealth of information in all of your responses and I appreciate the time and effort you took to provide it. I’ve begun to read the info in the links you sent. It’s all beginning to make sense. I’m also carefully rereading info about the basics of electric power systems. Believe me when I tell you that I regret screwing around in my high school electronics class instead of paying attention — there’s not much take-away from throwing vacuum tubes against a classroom wall just to hear them implode and annoy the teacher. Now I’m paying the price.

Anyway, I chose the Dakota because I have such fond memories of flying it as a free flight. I realize it won’t make the best flying trainer because of its weight-to-wing area ratio, but I can attest to its durability. I watched mine cartwheel on landing time after time without damage. It’s also a quick, straightforward build, and that makes it ideal for the short attention spans of my grandkids. On top of all that, just looking at Wagner‘s Dakota makes me happy.

I’ll add more as the project progresses. Thanks again, Ron.
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Konrad
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« Reply #10 on: February 29, 2020, 12:22:01 PM »

As a 3 channel trainer I think the Dakota will great. I fear far too many folk get hung up on weight. Weight, or the lack there of, is NOT the holy grail of fixed wing aviation. In the case of a trainer durability is! Even at the cost of a few persentage points in landing (stall) speed.
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TornadoAlley
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« Reply #11 on: March 02, 2020, 10:46:04 AM »

In my quest for basic knowledge, I ran across this excellent tutorial that explains how to determine motor, ESC, and battery requirements for a given airplane. https://youtu.be/ZkK4ag--tpQ

And here’s an entertaining video of a glow-powered, free-flight Dakota produced by my buddies Mike and Richie in western Massachusetts. This is the BMJR kit that I mentioned earlier.
https://youtu.be/mBQ1JyksR2k
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Konrad
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« Reply #12 on: March 02, 2020, 01:12:50 PM »

Hum, that's not the way I'd do it (Then again I don't like videos). That looked to me to be more like a justification for what he had selected. Did he cover why he selected 3 cells as apposed to 2 cells or 4 cells and the capacity he did?

One of the best  write ups I've seen was done by Matthew Orme (of Aveox motors) Called System Selection. It is a bit dated (before lipos) but volts and amps are still the same. So is air and gravity.

Another good reference is Bob Boucher (Astro Flight inc.) Electric motor Handbook. Again dated but the graphs and the section on the selection of props is good.

All the best,
Konrad
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