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Old Nov 06, 2001, 08:46 PM
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FlyingKiwi's Avatar
Melbourne, Victoria, Australia
Joined Mar 2001
27 Posts
Prop sizing

Hi All,

I was wondering if someone out there in the ether could help. I recently scratch-built a wee 4:1 gearbox for a s400 motor (6v). Now what I'm wondering is "what prop" to spin on this thing?

Also, for future reference, does anyone have a lookup type table matching prop/gearbox/motor for different desired performance envelopes?? (ie more thrust vs lower current etc)

Cheers
Andrew
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Old Nov 06, 2001, 09:03 PM
Tell that to George Zipp!
mikeb33's Avatar
United States, IL, Inverness
Joined Oct 2000
1,819 Posts
We're going to need to know a lot more:
Airfoil and plane type
weight
number and type of cells
what type of flying you want to do
etc...

MIKE
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Old Nov 06, 2001, 10:11 PM
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Melbourne, Victoria, Australia
Joined Mar 2001
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What COULD I spin on it? (rules of thumb), ie min size/pitch to max size/pitch. I mean I know that a 6X3 is about right for direct drive on 6 cells, what is about right for 4:1 geared on 6 cells?

I'm thinking of whacking it into a low wing loading, undercambered camera platform....

cheers
Andrew
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Old Nov 06, 2001, 10:14 PM
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Florida
Joined Oct 2001
178 Posts
I tried to use MotoCalc for this same reason. MotoCalc seems to get me in the ballpark, but far from accurate, in my opinion. My amp meter is dead right now but @ 3:1 on the sp400 6v. with a 10X7 slow flyer prop on 8X600AE cells, the 17-18 oz plane jumps into the air from the ground in 3 feet or less. If I hand launch, I merely have to extend my arm and it leaps into the air.
Another combo I used is 2.5:1 with an 8X6 slow flyer prop. I get very similar flights. Cruising around at 1/2 throttle for about 10min.
I also tried an 11X3 slowflyer prop at 2.7:1 and had very similar results.
With these combos vertical is about 100' , and level consecutive loops are no problem at less than full throttle.
Funny thing, I switched to a 9:6E prop at 2.5:1 and top speed was much better but in flight thrust was much less because it took approx 70% throttle to keep it aloft.
It really depends on what you are looking for. These combos are not for top speed 100 mph, but for me 30-40 mph top speed, low fly bys, loops, rolls and tooling around low at 1/2 throttle is cool.
I was looking for a 4:1 to use for this same setup and guessed at somewhere around an 11 or 12 X6 slowfly.

Hope I'm not off topic too much, but I have another plane I've built awaiting a motor. This one is a low wing, fully symetrical, acrobatic plane. Motocalc tells me 4:1 with a 10X8 , same batteries, using a Kyosho Atomic Force 17x2 ($20), 20 oz plane, will give 29 oz static thrust 50 mph prop speed at full throttle. 1/2 throttle flights at 27mph for 14min. A much more efficient motor I guess.

I hope my past experimenting will save you from buying a dozen props like I have to do.
Thanks,
Rick
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Old Nov 06, 2001, 10:30 PM
Heli Bouncer
Looooeeee!'s Avatar
At the Discount Clambake
Joined Feb 2001
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Kiwi

I've done a quick check on P-Calc and for a optimum, a 11x8 doesn't look bad, about 8 amps, 63 watt's in, 42 out, on 7, 600 AE's, 4300 rpm, 17.6 oz's of umph. Seems to be the best for thrust and duration, although a 10x8 doesn't look bad for a little less thrust and more flight time. This is on 7 cell's, which I think will do you better as you're carrying weight.

cheers

Looee
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Last edited by Looooeeee!; Nov 06, 2001 at 10:34 PM.
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Old Nov 07, 2001, 09:10 AM
Registered User
Terre Haute, Indiana
Joined Nov 2000
357 Posts
I have been using an 11x8 cam prop (folder) with a 6v speed 400, 4:1 gearbox and 6 or 7 cell ae battery on a 20 ounce duration model. I already had these from an old sailplane so I checked it with motocalc and it looked good. It draws about 8 amps with 7 cells and the motor efficiency is high for a ferrite motor. It gets about 15 minutes with 6 cells with careful throttle management.
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Old Nov 07, 2001, 09:44 AM
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United Kingdom, Bracknell
Joined Nov 2000
11,556 Posts
I'm using APC-E 11x8.5s with 4:1 gearboxes and 6V S400s on 8x2400 in my trike u/c Twin Star. Current draw (10-11A per motor) is more than MotoCalc predicted, but still reasonable. Performance and duration are very good.
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Old Nov 07, 2001, 10:12 AM
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Joined Oct 2001
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THE GENERAL RULE FOR SIZING A PROP FOR ANY GEARBOX IS !!
MULTIPLY THE SQUARE ROOT OF THE GEAR RATIO BY THE DIA OF THE DIRECT DRIVE PROP.
SO FOR A SPEED 400 THE NORMAL DIRECT DRIVE PROP IS A 6 INCH PROP. THE SQUARE ROOT OF A 4/1 GEARBOX IS 2.00.
6 X 2 = 12 NOW THATS A LITTLE TOO LARGE FOR PRACTICAL PURPOSES ON THE SPEED 400, SO SETTLE ON AN 11 INCH PROP, AND BASED ON THE LOW RPM OF THE PROP, START OFF WITH ABOUT AN 8 INCH PITCH.
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Old Nov 07, 2001, 10:47 AM
jrb
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Edina, MN, USA
Joined Oct 1999
11,417 Posts
Welcome to e-zone L.A.!

Hope you visit often.
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Old Nov 08, 2001, 01:55 AM
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FlyingKiwi's Avatar
Melbourne, Victoria, Australia
Joined Mar 2001
27 Posts
Many Thanks

HI Everyone,

Thanks very muchly for all your advice. The local hobbie shop didn't have any 11x8's in stock - the largest he had in was a 10x7.

Necessity being the mother of invention, I grabbed one of those (A$3.50) to use as a template for a ply rip-off. Scaled it up somewhat (ex 1/2" on the ends + a bit fatter through the middle). Now out with the trusty B&D wizard to do a bit of shaping, then over a boiling kettle to add a bit of twist & hey presto! - 1 scottish prop!

Now to mount on my home built gearbox (blades bolt to soldered on brass plate on output shaft) and up up and away!!!

Cheers
Andrew
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