Low Kv motor needs SF prop? - RC Groups
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Feb 06, 2013, 12:43 PM
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FlyBoy20's Avatar

Low Kv motor needs SF prop?

Looking at the APC site and it's clones, and there seems to be a bewildering range of prop designs. I've been wondering for months about SF props, and what they are specifically used for? Obviously for slow flying planes like 3Ds (?), so I'm assuming they're for fitting to low-Kv motors? Or else maybe they're for rubber-powered planes? Just dunno...

(Likewise I'm assuming a 'sport' prop is for high-Kv motors...then there is the APC 400 props - most confusing..)

Anyway, I'm looking for a 12x8 for my 910 Kv motor, and to run it with max. efficiency - for a future FunCub. Haven't seen a 12x8 APC type in this size yet, but there is an SF I could get, so should I get it?
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Feb 06, 2013, 12:59 PM
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flydiver's Avatar
You are a paddler in a boat, You have a choice of oars, a big fat one, or a skinny one. You can paddle the fat one slowly with high resistance, or the skinnier one fast with less resistance.
They'll both work.
If you try to 'force' the fat one too fast the weak handle will break.

SF props are RPM limited (small hubs, skinny roots-they CAN break), pull more amps, put out more thrust (fat blade). If ALL that is appropriate for your flying use, then use them.

You can get pretty much the same watt draw from a skinnier but larger blade, if that's what you want. After all the mucking about with a wattmeter is done, you use the prop that suits both the motor and how you like the way it flies. I proved to a buddy with dynamic wattmeter tests that a skinny GWS HD prop was the most efficient one he could use for his plane. He tried it and didn't like it so went back to over propping with an APC SF. He couldn't wrap his head around 3/4 throttle with the GWS HD provided the same thrust with LESS amp draw, than 1/2 throttle of the APC SF. You can lead a flier to information.....but.....

BTW-irrespective of the designation props are NOT all the same. For example, GWS SF are floppy and weak, where the APC SF of the same size are stiff. Clone APC props (lots now available) are generally not the quality of APC. I've seen some so poor they were impossible to balance.

Do you have a wattmeter? You need one to have a hope of figuring out prop efficiency.
Plus you don't specify your motor. 910KV is about as descriptive as 8-cylinder. That could be a 40g motor or a 400g motor, which would use entirely different props.
Last edited by flydiver; Feb 06, 2013 at 03:33 PM.
Feb 06, 2013, 01:40 PM
An itch?. Scratch build.
eflightray's Avatar
If you really need a 12 x 8 prop, look for an APC 12 x 8 Thin Electric.
Feb 06, 2013, 01:50 PM
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pda4you's Avatar
SF props are used on low power systems. They are best when spun slower and could be appropriate for your cub depending on power. As you can see from the APC site they have a max RPM and you should not exceed that.

I use very few of the SF type props as they are just not very efficient IMHO.

What size plane? How much power will you be using (watts)?

If it is the Multiplex FunCub you need a APCe type prop as recommended above.

Feb 06, 2013, 10:04 PM
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Piece's Avatar
The idea with a slow flyer prop is to generate a lot of thrust for planes with low-power, lightweight power systems. They're normally used at low RPMs and flight speeds are accordingly slow. Basically, you're using most of the motor's power to generate lower speed and higher thrust than a "normal" prop.

Think of it like the gearing on a bicycle: You might have a 21-speed mountain bike. Three chainrings (30, 36, 44) and seven cogs (14, 16, 18, 22, 24, 26, 36). The six lower cogs are your "normal" riding gears - pavement, grass, small hills, big hills, whatever. Those are like normal props. You can get a very wide range of thrust and speed for a given power input.... But then you have that 36-tooth cog that you only use when you're going straight up a cliff. You might be putting a ton of power into it, but only a tiny bit of speed is coming out since the bulk of it is going toward pushing you forward... Ok, well, it made more sense in my head.

The motor's Kv value is a secondary concern. It's really a disservice to yourself to think of a motor's RPMs or power in terms of Kv alone since it doesn't really tell you much at all. I could use a slow-flyer prop on a motor with a Kv value of 500 or 3500 depending on the motor's size and the input voltage.
Feb 08, 2013, 11:36 AM
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Anyone know the max Rpm of a GWS slow fly prop?

Also....are they so floppy and weak that its best to avoid?

I bought a couple different sizes but dont have to use them if they suck.
Feb 08, 2013, 11:37 AM
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pda4you's Avatar
They stink. I would recommend the GWS HD direct drive props - they rock.
Last edited by pda4you; Feb 08, 2013 at 12:30 PM.
Feb 08, 2013, 12:28 PM
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flydiver's Avatar
Originally Posted by wopachop
Anyone know the max Rpm of a GWS slow fly prop?

Also....are they so floppy and weak that its best to avoid?
They are OK for a Slow Stick and low RPM, low output motors.
GWS does not publish RPM limits. If they did likely no one would ever buy the SF again. As I said above "GWS SF are floppy and weak".
The HD's I like a lot.

If you try them and over rev you can hear the prop blades distorting as they get pushed too far. They make kind of a flock of birds taking off sound.
Feb 10, 2013, 06:54 AM
Registered User
I use 8" and 9" APC slo-fly props on a lot of models in the one-pound range, eg. GWS parkflyers, a light 48" Cub, 30" balsa 3D planes, and 40" EPP foamies. I use a 10" APC Slo-fly on a 2 pound balsa Katana clone.

Not really appropriate for anything larger than that.

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