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Gearvbox/Prop Size help

Old 09-01-2011, 04:12 PM
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pinky_d_brain
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Default Gearvbox/Prop Size help

Hi all I need your expertise
I'm looking for gearbox/prop size for motor/esc/battery I already have

motor
http://www.e-fliterc.com/Products/De...rodID=EFLM3005

ESC Thunderbird-36:
http://www.castlecreations.com/produ...nderbirds.html

Battery
Flightpower EVO-20 (1200mah 20C) 3s LiPo

want to go with a folding prop and hub so it can be modded
Now I have to confess as it will affect your selection this is not for a plain as of right now it's for a very light hydro style airboat with as little drag as possible
this has never been done in the airboat world so I am asking for your help
I know it will not be as fast but if I can get half as fast I will be very happy
Right now I need to beet 80 sum mph as it is the top speed of are little compaction at this point, I hope to go over 100mph
I have been looking around and think the gearbox round is the way to go for the thrust to weight ratio
I appreciate any and all thoughts on this even negative ones
Old 09-02-2011, 08:35 AM
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Dr Kiwi
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Default RE: Gearvbox/Prop Size help

Let's look at the math in round numbers.

100mph pitch speed needs 21,000rpm from a 5" pitch prop.
Loaded, you might get 90% of Kv x v....

3s (call that 11v) and 4600Kv gets you 11x4600 x 0.9 = 45500rpm

if you need 21,000rpm then you can only gear the motor ~2.2:1

Now can a motor rated for only 7A/12A burst spin say a 5x5 prop at 21000rpm... no!
Old 09-03-2011, 05:19 PM
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pinky_d_brain
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Default RE: Gearvbox/Prop Size help

See this is why I am asking
So what do you suggest
Old 09-03-2011, 06:11 PM
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Dr Kiwi
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Default RE: Gearvbox/Prop Size help

What was missing from your equation was how big a prop can you fit on this boat... knowing that we can find a motor which might get you 100mph.

An ARC 28-37-2.5 (83g, ~2900Kv) on 3s with a Zagi Carbon 5.1x4.9 prop gets you 883g @ 112mph pitch speed from 34.10A/385W
Old 09-03-2011, 07:05 PM
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pinky_d_brain
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Default RE: Gearvbox/Prop Size help

Any size working on design I build around power plant / prop

so 105600 per min
so
4900rpm kv motor 11v = 53900rpm
6.6:1 gear box
= 8166 rpm
so 105600/8166=13
so I need a 13 pitch prop with that set-up but what diameter if it would even be doable

If I could afford it I would get this
Motor: Mega 16/35/1
Gearbox: KPG27 5.2
Prop: 16x17 RFM
ESC: ICE100
Battery: 5S

or something similar
Old 09-04-2011, 01:14 PM
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Dr Kiwi
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Default RE: Gearvbox/Prop Size help

It might be worth your while playing with this [http://www.badcock.net/] to get an idea of how much power a certain prop requires to get to x rpm.

If you geared your 4900Kv motor at ~3.3:1 to get ~15000rpm, you could use a 7x7 to get 100mph pitch speed... but that needs 250W-in to do it.... so your present motor can't cut it.

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