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Radio Control Forum => Electrics => Topic started by: Tiger Tim on June 10, 2018, 03:48:13 PM

Title: What motor/prop/esc?
Post by: Tiger Tim on June 10, 2018, 03:48:13 PM
Hi all,

A couple years ago I picked up a very oldtimer-y ARF kit at an auction and this summer I want to get flying it.  The flying part is easy enough, I have experience there but it’s mostly on much smaller stuff.  What I’m struggling with is powerplant selection.

The plane itself is about a four foot wingspan, all balsa truss with film covering and it’s a three channel.  I figure at that size it should probably carry the same battery and servos as the Hobbyzone Super Cub but for the motor I want to go brushless and that’s where I get stuck.  I need to order some other stuff from HK soon, what motor and ESC should I tack onto my shopping list?

Title: Re: What motor/prop/esc?
Post by: Konrad on June 11, 2018, 09:59:25 AM
Well, the first thing I'd want to know is what charger set up (power and cell count) you have.

Second is what kind of performance you want. There are two basic set ups for old timers. That is the classic high wing pylon set up were they go almost straight up (often looping into the ground >:( ). And then there is the lazy climb of the cabin type model.

What does the ARF kit recommend for power, be glow or electric? And do you know how much nose weight you will need to reach balance? Also the All Up Weight (AUW) would be helpfull in selecting the motor (power system).

All the best,

Unlike glow, most electric motors have a much wider power band. Meaning that a motor will have more suitable aplications than what we had with the obsolete glow engine.

Title: Re: What motor/prop/esc?
Post by: Tiger Tim on June 11, 2018, 06:06:28 PM
First thing’s first, I intend to use 3s 1300mAh batteries for power.  I have a handful of 2200mAh packs but I’d rather keep it light.

It’s a cabin plane and I’d prefer to treat it as a sedate evening flyer.  Long takeoffs and slow climbs don’t bother me at all.

I’m out of town at the moment so I don’t have access to the instructions but I believe it called for a geared 400-size motor.  The nose is relatively long compared to the average snub-nosed oldtimer so I’m not too worried about nose weight just yet, plus I can push the battery pretty far forward if need be.

Title: Re: What motor/prop/esc?
Post by: Konrad on June 12, 2018, 11:21:49 AM
The Speed 400 7.2v was a seven cell (1.2v), 70 watt set up (8.4V x 9 amps). Some of the 6v or 4.8v rated motors could be set up for a bit more power. As the set up is for a geared motor you will want a low KV outrunner brushless if going direct drive. If you want to stay with a gearbox a photo of the model's firewall (mounting system) would be helpful. Heck if the kit came with a gearbox we might even be able to find a direct replacement for the Speed 400 motor. Does the manual give a suggested prop to use with the geared Speed 400?

The 1300 mAh 3 cell (3.7v) 11.1V is more than enough. Most of my "Speed 400" models are now using 480 mAh cells. Based on your performance desires you might want to aim for a power to weight of 60 to 80 watts per pound.

Title: Re: What motor/prop/esc?
Post by: Tiger Tim on June 14, 2018, 04:37:04 PM
I’ll have to get you the rest of the specs when I get back home but in the meantime I see you mention watts per pound.  I’m a little stumped at how I would sort that out.  Watts are a unit of power and power gets soaked up by the prop so I guess that’s a start.  Is there a rule of thumb for prop diameter as a fraction of wingspan for best results?  If there’s an ideal tip speed then it can be used to determine RPM and with chosen battery I could get a Kevin rating.  When I used to fly CL I seem to remember often getting best results with a pitch/diameter ratio of about 0.5, should it be around that for electric too?  That still doesn’t leave me with an obvious load on the prop that I can use to calculate amp draw, though.

I’m genuinely interested in learning to calculate this stuff for myself while I plod along through this exercise.

All in all this electric thing seems much more complicated than, “Use an .09 with about a 10x4.”

Title: Re: What motor/prop/esc?
Post by: Konrad on June 15, 2018, 07:05:04 PM
Watts per pound is a good point to start your power calculations.

I find that with electric it is much easier to come up with a suitable power system than the way we were forced to with glow. Because glow (I.C. engines) has a rather non-linear curve (very peaky) we were often force to stick with tribal knowledge that was passed down. Like your 1.5cc engine equates to a 10x4.

With electrics the power curve is rather linear. Even the efficiency curve is rather flat. This means that an electric motor can be run efficiently over a much wider set of parameters than what we were able to with the I.C. engine. Unlike IC where we need to build a dyno to find the engine's power curve, with electric the shape of the curve is easy to find with Volt and Amp meter.

What I'm getting at is that unlike IC engine the electric power planet is much more flexible and tolerant  as to how it is set up.

Tip speed in RC is used to determine prop noise in FAI F3A aircraft. As long as we stay away from the Mach rise it isn't a variable often used in selecting sport props.

In RC, heck just about any thrust at speed calculation a pitch/diameter of less than .5 should only be used to stir paint.  If looking at this in more detail I like to aim for a pitch speed of at least 2.5 time that of the stall speed.

Title: Re: What motor/prop/esc?
Post by: Konrad on June 20, 2018, 09:12:51 AM
Keeping with the idea of "Teach a man to fish and he eats for a life time. Rather than give him a fish and he eats for a day" I should have posted this link.

This is a great on line calculator.

All the best,

Title: Re: What motor/prop/esc?
Post by: TimWescott on June 20, 2018, 09:04:18 PM
I see you mention watts per pound.  I’m a little stumped at how I would sort that out.

It's a rule of thumb.  If you want a certain level of performance, go with a given watts/pound.  50W/lb is very relaxed, and was more a brushed motor thing because it was what you could attain.  80W/lb is a nice semi-relaxed point, and you can always throttle down.

If you don't mind spending the bux, just get an eFlight 400 from Horizon, and use props in the recommended range.  For electrics, your best bet is to recognize that props are cheap, and show up at the field with a handful of them for the first flight.  Try things out until you like how the plane flies.  You can either measure the battery current on the ground at full throttle to get an idea of the loading on the engine, or just run it for a minute and feel the motor, ESC & battery -- if they're not uncomfortably warm, then you're in business.

Title: Re: What motor/prop/esc?
Post by: Konrad on July 05, 2018, 08:52:09 AM
I see where I've left out an important piece of the puzzle. That is how to find the stall speed without actually test flying the model.
Here is an approximation I use. This was publish by Keith Shaw decades ago:

Stall speed = 3.7 x square root of wing loading

speed in mph
wing loading in oz/sqft

Sorry the units are english, I've yet to convert them to the universal standard of metric units

All the best,