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Old Sep 07, 2012, 12:00 PM
If I build it, it will fly
United States, NY, East Rochester
Joined Jan 2012
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This motor would likely work out, being from one of the cheaper manufactures it would keep conversion cost down. Is that 100oz current dry weight, or an est based on the website specs? Not sure what .40 glow engines weigh, so your current weight might be lighter.

~psguardian
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Old Sep 07, 2012, 12:42 PM
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Originally Posted by mybad View Post
I came up with

6S battery 1500 or better, 24C = 21.6V
APC-TE prop 17inch 12 pitch
Mybad, is there really enough ground clearance for a 17 inch prop? Most glow trainers I've seen don't have that much ground clearance. Often you can't fit anything bigger than 10" or 12".

Set the model on a level surface, measure clearance from surface to where the propshaft would be, subtract about 1 1/2" inches for ground clearance, double what's left, and that should tell you the biggest prop you can safely put on there without running the prop tips into the ground on every hard landing. Please let us know what you find.

Also, what is your intended "Flight Mission"? Do you want aerobatic ability?

I'm betting you don't actually have clearance for a 17" prop, and if I'm right, that 300 rpm/V motor won't work either. You'll need something with a higher Kv to swing a smaller prop.

So please get back to us with the maximum prop size and intended flight mission, and then we can home in on the other components you need. It would help to also know your desired flight duration - with big electric models, this is often a compromise, and flights may be somewhat short to keep battery size down.
Quote:
Originally Posted by mybad View Post
Q2: Where do I find such an engine?
Q3: What will this cost?
Hobby King seems unbeatable for budget priced motors. They're in Hong Kong, with a US warehouse for some products. If you don't mind waiting a couple of weeks for your parts, they're a good source.

You'll have better answers on cost once we home in on a good combination, which we can do once you provide the missing data (prop clearance, flight mission, desired flight duration).
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Originally Posted by mybad View Post
Is it worth the cost, considering the plane was free?
We'll have a decent estimate of the costs before we're done, but only you can decide if it's worth it to you. Most importantly, would you enjoy flying a big sedate trainer in the first place?

-Flieslikeabeagle
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Old Sep 08, 2012, 06:17 PM
Sure, I can fly after sunset!?
United States, MI, Novi
Joined Jan 2011
477 Posts
More information and assumptions

OK. I'm quite raw at this and am having a lot of problems coming up with a good setup.

I didn't mention that the plane my daugher got me had been in a crash. Everything is intact except that the motor and gas tank are missing, as is the nose landing gear. I am balancing the plane on the main gear and leveling it out to estimate how much prop clearance I can get. It looks like the top of the firewall is 240mm from the ground.

I assume that I would mount the motor as high on the firewall as possible.

Assuming the diameter of a motor will be around 20mm, that puts the center of the shaft 220mm above the ground. Again, assumptions.

OK. I believe the following prop sizes and clearances apply:

15" prop ... 1 inch clearance
14" prop ... 1.5 inch clearance
13" prop ... 2 inch clearance
12" prop ... 2.5 inch clearance
11" prop ... 3 inch clearance

I initially picked a larger prop, believing the best prop would be 1/4 wingspan. Looks impossible unless I drastically modify the landing gear.

Accoding to the website, the flying weight of the gas model is 6.25 - 7 lbs. Assuming the same, the electric setup with battery and esc will come to 7 lbs?
I used webocalc 1.7.6
I will shoot for 7 minute flights.

Flying style will be SLOW SPORT AEROBATICS

I will look for a cheap outrunner, which is less efficient.

I plugged this all into Webocalc

112 oz flying weight
69 inch wingspan
793 sq inch area
12 inch maximum prop

this gives me 61 top speed\
this gives me 112 thrust
I choose 7 minute flight and cheap outrunner

I choose 6s lipo
this gave me 21.6v and 41.9 amp

webocalc suggested 610kv max ... I got apc-te 12/6 prop but 0.89 gear ratio

I got 1.00 gear ratio by upping the kv to 680

Estimated Model Performance
WebOCalc Results:Flies Like: Advanced Glow Trainer.
Power Level:
(with white highlighted prop) Medium/Mild aerobatics.
50 degree climbouts.
Minimum Pilot Skill Needed: Intermediate.
Minimum Flying Field Size: 1200 x 860 feet.
Minimum Battery Size: 6S, 2500 mAh, 17 C, lithium polymer.
Estimated Flight Duration: 5 to 9 minutes depending on pilot.
Will vary with throttle usage.
Suggested ESC Rating: 54 A to 63 A.
Power Into / Out of Motor: 905.0 watts in / 633.5 watts out.
Power To Weight Ratio: 129.29 watts/pound.
Estimated Stall Speed: 22.1 mph.
Wing Loading: 20.34 oz/square foot.
Cubic Wing Loading: 7.26 oz/cubic foot.

So, I need a APC-TE 12/6 prop
680 kv cheap outrunner
905 watts in (633.5 watts out)
6s lipo

Am I on the right track?
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Old Sep 08, 2012, 07:17 PM
wood is good
loNslo's Avatar
United States, CA, Marina Del Rey
Joined Jun 2012
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Quote:
Originally Posted by mybad View Post
...I assume that I would mount the motor as high on the firewall as possible...
Uh...no.

Each model has its own optimum thrust line position. You'll want to discover where this on your particular model.
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Old Sep 08, 2012, 07:38 PM
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Australia, QLD, Booyal
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Made me shudder when i saw that. Good thing you asked before just bolting it on
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Old Sep 08, 2012, 07:47 PM
Sure, I can fly after sunset!?
United States, MI, Novi
Joined Jan 2011
477 Posts
Quote:
Originally Posted by loNslo View Post
Uh...no.

Each model has its own optimum thrust line position. You'll want to discover where this on your particular model.
I'm beginning to think that this is hopeless. There is so much I don't even know that I don't know.
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Old Sep 08, 2012, 07:54 PM
wood is good
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United States, CA, Marina Del Rey
Joined Jun 2012
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Don't give up so easy. You have to do your own due diligence--maybe find someone who has a set of plans to measure. Or contact Hangar 9.
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Old Sep 09, 2012, 01:14 AM
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Los Angeles
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Quote:
Originally Posted by mybad View Post
Am I on the right track?
Don't be discouraged, mybad, you're actually doing very well. Sure, there are details to iron out, but you're definitely on the right track, and close to an answer. (I'll add more on this at the end of this post).

The exact thrust line is very important for precision aerobatics. If you were building a pattern ship or an Extra or something similar, it is pretty critical. Get it wrong, and the model will have all sorts of bad characteristics which will make it useless for its intended task.

But you're building a big ol' trainer. These are not precision aircraft by any stretch of the imagination. The (high) wing is already mounted several inches higher than the thrust line. Every time you open the throttle, this sort of model will tend to climb. When you close the throttle, it will tend to drop its nose and glide. This is normal, and even desirable in a trainer.

There is more to mounting the motor than just height, however. A little down-thrust and a little side-thrust is also usually required. The down-thrust reduces that tendency to pitch the nose up every time you open the throttle. The side-thrust (aim the propeller slightly to the right) helps you keep the model going in a straight line when you take off - without it, most models will veer left, and some models are a real handful if you don't have right thrust in the motor mount.

This is not, to use a colloquial expression, rocket science. Something like three degrees right thrust and three degrees down thrust will do for a starting point. If this is not already built into the firewall (it may not be square to the fuselage, but intentionally angled slightly right and down), it is easy to build into the motor mount. In a pinch you can just put a few washers under the motor mount to shim it to about the right angle(s).

Okay, back to your power system. I ran your airframe numbers through WebOCalc and came up with something very similar to what you found (good job on your part!)

First, I made the assumption you could use a 13" prop. It may turn out you don't have enough ground clearance once you move the motor where it belongs, but this is just a first trial run, and it's always a good idea to use the largest prop possible on this sort of model. Thanks to WebOCalc, trial runs cost no money and only a very little time!

I came up with 6S lipo, 43 amps, 580 rpm/V motor, 13x6.5 APC Thin Electric prop. I fine-tuned the numbers around this inexpensive motor: http://www.hobbyking.com/hobbyking/s...0KV_2000W.html

The attached screenshot shows the other numbers WebOCalc comes up with for this model.

-Flieslikeabeagle
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Old Sep 09, 2012, 01:26 AM
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Having seen what can be accomplished with a 13" prop, I dropped the maximum prop size to 12" and tried again. The wizards in WebOCalc came very close to the correct answer, and a little fine-tuning got me to the attached screenshot, based around this budget motor: http://www.hobbyking.com/hobbyking/s...0KV_1295W.html

So this time we have 42 amps, 650 rpm/V motor, 12x6 APC Thin Electric, medium aerobatics.

In both screenshots I've kept flight times short, probably too short for a trainer. These sorts of planes put very little load on the pilot, so long relaxing flights are the norm, and a bigger battery would be a Very Good Idea (TM). If this were my model, I'd want at least double the battery capacity shown in the WebOCalc screenshots.

However this second screenshot shows what you get if you take two of the cheap, common-as-dirt 3S 2100 mAh packs, put them in series, and use them to power this model.

-Flieslikeabeagle
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Old Sep 09, 2012, 01:50 AM
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Looking up those motors for mybad's trainer, I found something new (to me, at least). Hobby King now has what they claim are actual motor dyno results of some of their motors. (A motor dyno measures power, rpm, current, and other characteristics of an electric motor. Done right, this is the gold standard for motor data.)

The attached image is one of these dyno plots. This provides a nice basis for discussing some of the characteristics of these cheap motors, that we've already been talking about lately.

First of all, notice the red motor efficiency curve tops out at 75% efficient (what'd I tell you, psguardian? ).

Notice also that the same curve droops down to about 60% efficient at 52.5 amps; at this point 40% of the power from the battery is being used to roast the motor, while only 60% makes it to the propeller!

Extrapolate that red curve a little further with your eyes - the horizontal axis is marked in steps of 7.5 amps, so the next mark would be at 60 amps. Project the red curve a quarter-inch or so to the right, and you can see that it will have drooped to maybe 55% efficiency at 60 amps. By 67.5 amps it will be around 50%. That's the point where you could say "this is an electric heater" and it would be just as accurate a statement as "this is an electric motor". Half the input power turns the prop, the other half cooks the motor to death.

Hobby King says this motor has a maximum current capability of 70 amps. The dyno data proves that it really makes no sense to take it beyond 45 amps. Beyond 45 amps, all you get is huge increases in motor heat, and very little extra power (black curve) to the propeller. This is a recipe for disappointing performance and a fried motor.

There is much more information that can be read off the graphs. For example, the motor heating is the difference between output power (black) and input power (purple). For instance, at 45 amps on the graph, the output power (black) is about 640 watts, and input power (magenta) at the same current is 960 watts. The difference - 320 watts - is being turned into heat, trying to cook the motor to death!

So even 45 amps should be used sparingly, in short bursts, with this motor. 320 watts is a LOT of heat, and the motor will overheat if run more than a few seconds at this power level.

Notice also that input power is 960 watts at 45 amps. The motor weighs 243 grams, so this works out to right around 4 watts per gram of motor weight. And the motor is working really hard...320 watts of heat. Our rule-of-thumb of 3 watts of input power per gram would have kept the motor cooler and happier and more efficient, at 720 watts input power, which occurs at roughly 34 amps motor current.

Kudos to Hobby King for publishing this real data. And Boo to Hobby King for still lying in the ad copy under the data! But at least the educated viewer can figure out enough to separate the lies from the facts.

And it is nice to know that our crude rules of thumb - 75% peak efficiency and about 3 watts per gram of power handling ability - are not far off at all!

-Flieslikeabeagle
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Old Sep 09, 2012, 05:04 AM
Sure, I can fly after sunset!?
United States, MI, Novi
Joined Jan 2011
477 Posts
Wow!

Thanks for all the information and advice! Again ... I didn't even know what I didn't know!!!

At least I did know about the dual angles in mounting the motor. But, I didn't fully understand the reasons, until now.

Oh, in your second example combining two 3S batteries ... the motor you selected specs out 4S and 5S batteries only. Using 6S will draw fewer amps than 5S (to get the same watts, that is), but would it be safe to use 6S in this motor? There must be some reason they didn't list 6S with this motor, but they DID with the motor in your first example.

It will take me a while to digest all this.
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Last edited by mybad; Sep 09, 2012 at 05:35 AM.
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Old Sep 09, 2012, 09:57 AM
That's a funny word
NE Ohio
Joined Apr 2003
3,684 Posts
First thing I did was to check the manual for prop size , it was 10x6 I think. Should the engine prop even be a consideration when converting to electric?

ps. I also did my stock e-stik and the factory prop was right on the money at 11x8.5 and since this plane is a bit too big and fast for me too the 11x8 and 11x 7 are great contenders too. Might try some experimenting.

back to mybad's trainer: it looks like a fun project. How much was it at garage sale?

I also have been doing some prop clearance measuments on all my foam stock stuff and most are about 2-3". ALL my props have grass stains on them too.
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Old Sep 09, 2012, 01:31 PM
If I build it, it will fly
United States, NY, East Rochester
Joined Jan 2012
655 Posts
I have been reading those graphs wrong.... *face palm* ok i'm better now. HKs 'efficiency claims' are "max efficiency" which comes well before max Watts out... And looking at the heat Watts compared to drive Watts is crazy.

This looks really good. A free bird with a belly rash & no motor, not a bad gift. Could you possibly weigh the airframe on a grams/ounces scale, the more accurate the numbers the less guessing has to happen. Batteries & motors list actual weight so we can add those to your actual airframe weight for a more pin point power train.

~psguardian
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Old Sep 09, 2012, 09:50 PM
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Quote:
Originally Posted by mybad View Post
At least I did know about the dual angles in mounting the motor. But, I didn't fully understand the reasons, until now.
The real reason for right-thrust is spiral airflow behind the propeller. Seen from behind the model, the air coming off the rear of the propeller is usually swirling clockwise, because that's the way 99% of our propellers rotate. That means the air is flowing left to right across the top of the rear of the fuselage - and the vertical stabiliser is in its path. So the vertical stab. gets blown to the right, and that in turn makes the model want to turn left.
Quote:
Originally Posted by mybad View Post
Oh, in your second example combining two 3S batteries ... the motor you selected specs out 4S and 5S batteries only. Using 6S will draw fewer amps than 5S (to get the same watts, that is), but would it be safe to use 6S in this motor? There must be some reason they didn't list 6S with this motor, but they DID with the motor in your first example.
Good point. I missed that in the specifications. There is an upper voltage limit for any motor - rotating parts will start to fall apart if the rpm goes too high. Usually the glued-on magnets will fly off the bell first.

That said, I don't know if this motor will actually overspeed to the point where it gets damaged if you run it on 6S. More likely the manufacturer simply didn't want people putting on a huge prop, hooking it to a 6S battery, and frying the motor because of excess current.

There are actually good reasons for using motors on higher-than-normal voltage. Up to a point, it improves their efficiency and increases the amount of power you can safely get out of them. (This is not obvious, it is something that emerges from studying the 3-constant motor model, and/or looking at actual dyno data.)
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Originally Posted by mybad View Post
It will take me a while to digest all this.
Well, good! Every time you learn something new you decrease the chances of getting dementia in your old age. Our brains need to constantly be challenged with new things, otherwise they don't do very well.

Back when we were nomadic hunter-gatherers we were challenged by new things every day. These days we can easily live such settled and un-challenging lives in our comfortable cities that our brains turn to mush as we age.

I just read the results of a medical study that found that men over 69 who used a computer had 40% less likelihood of developing dementia - isn't that inspiring?

-Flieslikeabeagle
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Old Sep 09, 2012, 09:54 PM
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Quote:
Originally Posted by gulio View Post
Should the engine prop even be a consideration when converting to electric?
Only in the sense that it might give you an idea how much prop clearance there is on the model.

Small piston engines have very poor torque compared to same-sized (outrunner) electric motors. So the props on glow engines are always too small and therefore inefficient, limited by the motors ability to generate enough torque swing them.
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Originally Posted by gulio View Post
I also have been doing some prop clearance measuments on all my foam stock stuff and most are about 2-3". ALL my props have grass stains on them too.
On glow conversions I usually use 1 1/2" ground clearance, because they are always designed for a too-small prop, and you need all the prop size you can get.

You do have to make gentle landings with 1 1/2" ground clearance - otherwise you'll have more than grass stains on the prop!

-Flieslikeabeagle
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