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Old Dec 05, 2012, 06:28 AM
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Joined May 2010
25 Posts
Question
IC to brushless conversion. .09 IC

I apologise for the seemingly common question but i cannot find a simple and conclusive answer so i am stumped and requesting some assistance from more experienced modellers.

I am looking (and have been for some time) to start a winter build.

I have a plan for a DHC-2 beaver thats old...It suggests a 0.075 engine (doesnt state nitro, petrol or diesel e.t.c.) for RC or .15 max for free flight.

I was thinking 150-200W brushless outrunner would suffice but what about Kv?

I imagine (having no experience) that the IC engine will rev fast.

As its a 40" span model its pretty small so running 4s or even 3s maybe out of the question weight wise so i am guessing (and it IS a guess) circa 3000kv? That will give me a max rpm of 22,200 on 2s.

Any other suggestions?

I have been scouring hobbykings website for a suitable motor if anyone knows of a suitable one they can point at?

I am in the UK so tower hobbies e.t.c is a pain shipping wise.
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Old Dec 05, 2012, 08:04 AM
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scirocco's Avatar
Australia, ACT, Kambah
Joined Feb 2001
3,227 Posts
It might seem harder this way, but rather than think " I want an .09 IC equivalent", you're better starting from scratch and thinking "hmm, lovely 40" Beaver, nice big wing xxx sq in maybe 3-4lb flying weight, fit props up to 11", what electric power system would work for moderate sport aerobatics?"

If you have access to the plans, you should be able to determine wing area, take a decent stab at flying weight, and figure out what diameter prop will fit with say 1" clearance on its wheels in flying attitude. And you can decide how you want to fly it. That is enough information to get in the ballpark for a power system.

You've already said 150-200W. Is that based on say 3lbs at 50-75W/lb, or 2000 x glow capcity in ci? In any case, for a reasonably low wing loading gentlish flyer like a Beaver around 75W/lb gives you your first clear goal, but how to get there?

First off, cell count and pack size. For sport flying, most folks are going to want to get at least 6 minute flights. Allowing for using a max of 80% pack capacity for battery health reasons and average power usually a fair bit lower than peak, a pack big enough for 6 minutes (1/10 hour) at the max power works out to be a reasonable size for 6-9 minute flights depending on throttle management. Using a pack in 1/10 hour is a discharge rate of 10C, where 1C is the rate (current) to discharge a pack in 1 hour. 10C also happens to be a nice conservative discharge rate, again, good for pack health. So, if you want 200W, current on 2S (7.4V nominal) is 27A, and on 3S (11.1V nominal) is 18A. 6 mins full rate at those currents is 2700mAh and 1800mAH respectively. If it turns out you want/need a bit more power, current and required pack size to maintain flight time naturally increase. At this point, I'm going to cheat and apply experience rather than analysis and suggest that 3S 2200, probably the most common pack size available would be a good starting point. While a 2S solution would be quite workable, I disagree with your assumption that 40" is too small for 3S or 4S. 40" is smack bang in the middle of 3S territory, and at this power level there is probably a wider choice of motors to suit 3S than 2S.

Next, motor overall size (weight actually). Motors of the same Kv but different weight have signficantly different power handling capability. Bigger motors can dissipate more heat, and can hold heavier wire with less resistance. Anyway, another guideline, again fairly conservative, is that average outrunners can readily handle about 3W power in per gram motor weight. So if you're in the 200W range, motors around 70g should be capable of handling the power. Going a bit heavier if needed to balance, likely on a short nose radial engined model is OK, maybe even desirable.

Now the hard part - prop and Kv. Actually prop should be fairly easy - for many scale glow conversions, as big diameter as possible while maintaining ground clearance will be preferable.

There is a calculator to help narrow down the Kv vs prop size dilemma - WebOCalc. And Ken Myers has taken an alternatevbt detailed approach you can find via http://www.theampeer.org/. Once that gets you in the ballpark for a generic motor, you can start to look at specific systems using ECalc or Drivecalc.

So, if you can provide the critical info of weight and max feasible prop size (budget too), you're sure to get a range of specific suggestions, and hopefully also a better understanding of how to approach the problem.

PS. You could go for a super high revving 2S solution with Kv = 3000, but you'd have to turn a very small prop, but it would just not suit the Beaver. Again, cheating, you're likely to end up if 3S, Kv between 900 and 1200 depending on prop size (8-11"), and if you went 2S, Kv 1300 - 1800.
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Old Dec 05, 2012, 02:14 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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I'd suggest this one for 3s and a 10"-11" prop: http://www.giantshark.co.uk/a22l20l-...otor-p-49.html
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