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Reduction Drive

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cymaz03/03/2019 08:35:34
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A club member and I are building a reduction drive for an Evolution 33gx. We have a 1:1.82 ratio. So at 6000 rpm the prop will be turning at 3296rpm.

My thought is what size prop range would work? As my other 33gx uses an 18x8 do I just multiply both these values by 1.82 to give me a prop size.....or is there more complex maths involved idea. Using straight multiplying would give a prop size of 32x14surprise. Someone at the LMA has suggested 30x10, 28x10 which might be suitable looking at other drives on YouTube. I just don’t want to buy several large props unnecessarily.

Anyone good at power calculation?

Work in progress 

Reduction drive

Edited By cymaz on 03/03/2019 08:36:31

Edited By cymaz on 03/03/2019 08:37:26

GrahamWh03/03/2019 09:39:08
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Hi Cymaz. As I understand it, it would be the load factor you need to multiply by 1.82

The load factor is proportional to the diameter to the power of 4 and to the pitch to the power of 1 so multiplying both diameter and pitch by 1.82 would multiply the load factor by (1.82)^5 - by 19.6!

An 18x8 prop has a load factor of 840 using a table from an old edition of a model flying mag I have, so you need a load factor of 1.82 x 840 = about 1529.

If you want the same pitch speed the 18x8 gave you at 6000 rpm, you need a pitch of 8x6000 / 3296 (or 8*1.82)= 14.56....or 14 to 15 inches.

An 18x15 would do this as its load factor is 1575..

Alternatives with roughly the same load factor would be 19x11-12, 20x9-10, 21x8, 22x6-7 but hese would have a slower pitch speed.

Load factor = pitch x (diameter^4)/1000.

This would be a good discussion to have as an article in the RCMA mag by someone knowledgeable.

Edited By GrahamWh on 03/03/2019 09:42:56

Edited By GrahamWh on 03/03/2019 09:50:15

cymaz03/03/2019 10:32:53
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Thanks Graham. I should have some prop sizes you mention.

I’ve read on RCUniverse that it’s not about HP but thrust as this is what gets the plane moving.

I really appreciate your maths yes

Edited By cymaz on 03/03/2019 10:33:07

cymaz03/03/2019 10:43:49
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I’ve just found this, now I know how to ask the right question. Is it what I’m looking for?

Frank Skilbeck03/03/2019 11:12:51
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Not quite as I can see it is for replacing props that run at the same speed.

One thing you could do is use one of the electric motor prop calculators and then find a motor that turns an 18x8 at 6000 rpm and note the power, then substitute the motor/prop with one that uses the same power to turn a prop at 3,300 rpm

A 18 x 8 turning at 6,000 rpm takes around 980 watts a 24 x 14 turning at 3,450 rpm takes 1,000 watts. using ecalc.

BTW a 6,000 rpm on 18 x 8 seems a bit weak, from what I can find it should be doing 8,000 rpm on that prop.

Frank Skilbeck03/03/2019 11:16:38
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Also an 18 x 8 at 6,000 rpm thrust = 4.5kg

A 24 x 14 at 3,400 rpm , thrust = 6.2 kg

cymaz03/03/2019 11:30:28
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Sorry Frank, 6000rpm was the number that was put into THIS.

It was a useful tool on selecting the toothed belt length and pulley sizes. The one thing that was avoided was not to have a reduction ratio as a whole number. I’ve read that if you do this, all sorts of belt and vibration problems appear.

I will start up the 33gx without the reduction drive to double check the rpm numbers.

cymaz03/03/2019 12:31:33
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Frank et al, JE 18x8 7200 @ WOT.

GrahamWh03/03/2019 13:19:41
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Cymaz, your table has load factor as proportional to diameter^3 x pitch, the one I use has it as diameter^4 x pitch. It makes a lot of difference. Does anyone know which is right?

(MIne comes from RC Model World, Sept 2012)

Frank Skilbeck03/03/2019 14:21:37
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Cymaz, OK 2nd set of runs,

18 x 8, 7,150 rpm, 1600 watts, 6.4kg, pitch speed 54mph

using same reduction gearing around 3,900 rpm, 24 x 14, gives 1500 watts, 8.1kg , pitch speed 53 mph.

BTW a Zenoah 62 is quoted as 7,600 rpm on a 24 x 10 and using the Mick Reeves Torquemaster 1.75 :1 , 28 x 14 at 4,400 rpm

cymaz03/03/2019 14:36:11
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Frank, yes ! I could start with that. The engine and drive will fit on my engine test stand. New longer bolts ordered to bolt it all in. As soon as they arrive I will fire it up and have a test run on the 24x10.

cymaz03/03/2019 14:51:36
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I’ve found a couple of Xoar 26x12 in my stock .......does the maths add up? laugh question

Jon - Laser Engines03/03/2019 15:11:25
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My number crunching suggests a 24x16 but i did do some rounding based upon 16 being a common pitch but 15 not so much.

I have been meaning to give this a try on one of our twins but have not had the opportunity to give it go

cymaz03/03/2019 15:17:42
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Jon, how did you do the maths? I understand the slower the rpm the greater the pitch needed.

Edited By cymaz on 03/03/2019 15:18:16

cymaz03/03/2019 16:04:53
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Doing some more Googling I found this.....

Make some assumptions I’ve come up with some figures. Please tell me if I’ve got it wildly wrong

  1. standard set up . 18x8, 7200rpm, 68f, Static thrust 6.73kg, speed 54mph, 2.1hp needed
  2. reduction . 26x12, 3900rpm, 68f, static thrust 8.6kg, speed 44mph, 2.2hp needed
  3. reduction . 24x14, 3900rpm, 68f, static thrust 6.2kg, speed 51mph , 1.8hp needed
  4. reduction . 24x10 , 3900rpm, 68f, static thrust 6.2kg, speed 37mph , 1.3hp needed.

Now I’ve tried to do a fair comparison. It seems to me that the 26x12 might do, on paper. Any thoughts??

Jon - Laser Engines03/03/2019 16:15:02
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I rounded a bit.

I assumed 2:1 to make life easy, as you then just double the pitch of the blade for the same pitch speed at half the revs. I then cheated and just kept increasing prop dia until calculated hp equaled that of the stock engine as engine rpm is the same as without the drive so output power is the same.

Its quick and dirty but proved accurate enough when a customer modified a 200v

With the new rpm figures i calculate 3500rpm on a 26x16 but again with rounding errors 24x16 is likely to work well too at higher revs.

My lazy effort wont be super accurate, but it gets you in the ballpark. I would just grab a prop and try it.

Where did you get the belt from by the way?

Simon Chaddock03/03/2019 16:23:59
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cymaz

As I am sure you are aware any form of 'drive' has a rather hard time when driven by a single cylinder as each stoke gives it a torque reversal from compression to power

Of course the same happens with direct drive but 'power' props are fixed rigidly to the crank shaft and are designed to take it.

Important the belt (?) is set to exactly the right tension.or it will wear prematurely.

cymaz03/03/2019 16:25:13
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I got the belts and matching pulleys from HERE

bert baker03/03/2019 18:01:31
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My concern would be the stress on the beam mounts

jeff2wings03/03/2019 18:07:21
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Not just me thinking that then ,also the length of over hang from the rear mounts, given the size of prop the P factor will be that much higher and could over stress the die-cast mount

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