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Most power i can get from a 6" prop

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Christopher Morris 202/06/2020 11:59:08
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185 forum posts

Hi, whats the Most power i can get from a 6" prop. So i am not limited on the motor size but i am limited by a 6" prop because its in a tube. Any suggestions for prop pitch & type of brushless motor to use. Also, any pros & cons going with a 2-3-4 bladed Thanks.

Chris Walby02/06/2020 12:18:17
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1251 forum posts
308 photos

How about 12 blade edf type, then the limit will be how fast you can rotate it and its pitch. Might be a limit to the tip speed on the fan (prop).

PS I think a 2 bladed EDF will be very noisy and not very efficient.

Shaun Walsh02/06/2020 12:45:29
327 forum posts
49 photos

If you definitely want a 6" prop and not an edf:

Rev limit for 6" APC E prop is 150000/6=25000rpm

Assuming a 4s Lipo motor KV needed to achieve 25000 rpm is 25000/14.8=1689Kv

So you are probably looking at something like this

**LINK**

or this

**LINK**

Approx 400watt

If you want an EDF then something like this at 2100W may work

**LINK**

Not sure what the comparison of thrust would be like though.

It all depends upon what you want it for and how big a battery you want to use.

leccyflyer02/06/2020 12:56:07
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1481 forum posts
324 photos
Posted by Christopher Morris 2 on 02/06/2020 11:59:08:

Hi, whats the Most power i can get from a 6" prop. So i am not limited on the motor size but i am limited by a 6" prop because its in a tube. Any suggestions for prop pitch & type of brushless motor to use. Also, any pros & cons going with a 2-3-4 bladed Thanks.

What's the model?

SIMON CRAGG02/06/2020 13:01:31
575 forum posts
5 photos

Have you tried punching the figures into e.calc?

There are virtually limitless options, I have used it countless times.

Excellent bit of kit!.

Shaun Walsh02/06/2020 13:05:55
327 forum posts
49 photos

You can see the thrust for APC props here:

**LINK**

Christopher Morris 202/06/2020 13:58:20
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185 forum posts

Ah! Thanks, guys, i am typing this again because i clicked one of your kind links & lost my text, lol.

This is all for an experiment on lifting a hovercraft with forcing air down a central 6” tube out to a perimeter of 4” tube with holes underneath behind the skirt & hoping to lift the craft. Forward propulsion for the craft will be a separate thing.
I liked the EDF route but was getting a bit expensive at over a hundred pounds.
Now the other links are helping as i already have some 80amp esc & spare 4s 5amph batteries & it is just getting down to motor & prop.
Am i right in thinking a 6x4 prop is going to drag the most air in? As the very detailed prop chart has confused me in that it looks like its part imperial & part metric ??.

IE: When i get to 6x4 props i can understand this. Then it goes on to 6x42E-3 starting to get confused.
But i am looking to lift about 9ibs/4klg.
At worst, it could be fitted with 2 motors & blades but would like to get away with one. Thanks

Shaun Walsh02/06/2020 14:38:43
327 forum posts
49 photos
Posted by Christopher Morris 2 on 02/06/2020 13:58:20:

Ah! Thanks, guys, i am typing this again because i clicked one of your kind links & lost my text, lol.

This is all for an experiment on lifting a hovercraft with forcing air down a central 6” tube out to a perimeter of 4” tube with holes underneath behind the skirt & hoping to lift the craft. Forward propulsion for the craft will be a separate thing.
I liked the EDF route but was getting a bit expensive at over a hundred pounds.
Now the other links are helping as i already have some 80amp esc & spare 4s 5amph batteries & it is just getting down to motor & prop.
Am i right in thinking a 6x4 prop is going to drag the most air in? As the very detailed prop chart has confused me in that it looks like its part imperial & part metric ??.

IE: When i get to 6x4 props i can understand this. Then it goes on to 6x42E-3 starting to get confused.
But i am looking to lift about 9ibs/4klg.
At worst, it could be fitted with 2 motors & blades but would like to get away with one. Thanks

The abbreviations can be confusing, 6x4.2 E-3 translates as 6" diameter, 4.2" pitch, Electric 3 blade.

This thread might be of interest.

**LINK**

Have you any idea what the total mass of the hovercraft will be? I don't think that the thrust will need to exceed the mass of the model but have no idea of the relationship between mass, thrust, air volume etc required for effective hovering.

Shaun Walsh02/06/2020 14:40:09
327 forum posts
49 photos
Posted by Shaun Walsh on 02/06/2020 14:38:43:
Posted by Christopher Morris 2 on 02/06/2020 13:58:20:

Ah! Thanks, guys, i am typing this again because i clicked one of your kind links & lost my text, lol.

This is all for an experiment on lifting a hovercraft with forcing air down a central 6” tube out to a perimeter of 4” tube with holes underneath behind the skirt & hoping to lift the craft. Forward propulsion for the craft will be a separate thing.
I liked the EDF route but was getting a bit expensive at over a hundred pounds.
Now the other links are helping as i already have some 80amp esc & spare 4s 5amph batteries & it is just getting down to motor & prop.
Am i right in thinking a 6x4 prop is going to drag the most air in? As the very detailed prop chart has confused me in that it looks like its part imperial & part metric ??.

IE: When i get to 6x4 props i can understand this. Then it goes on to 6x42E-3 starting to get confused.
But i am looking to lift about 9ibs/4klg.
At worst, it could be fitted with 2 motors & blades but would like to get away with one. Thanks

The abbreviations can be confusing, 6x4.2 E-3 translates as 6" diameter, 4.2" pitch, Electric 3 blade.

This thread might be of interest.

**LINK**

Have you any idea what the total mass of the hovercraft will be? I don't think that the thrust will need to exceed the mass of the model but have no idea of the relationship between mass, thrust, air volume etc required for effective hovering.

Sorry, just read it's 4Kg.

Peter Christy02/06/2020 15:10:16
1823 forum posts

Many, many years ago, I had one of these:

**LINK**

A Jetex Mobo hovercraft! It was powered by an AM .049 glow motor (un-silenced!) fitted with a 3-blade 5x4 prop. (may have been 5x3 - it was a long time ago!) Under the prop were a number of fixed vanes - around 6 IIRC - angled so as to counter the torque of the engine.

I'm guessing that motor ran somewhere in the region of 12,000 rpm. It produced enough lift to make the hovercraft fly a little on the high side, so the air cushion became unstable! Sellotaping an old penny to each side of the body, in line with the engine, loaded it up enough to maintain stability!

It was VERY noisy! Peak power was reportedly .052BHP at 14,000rpm **LINK**

.052 BHP translates to roughly 39 watts.

Bear in mind that the Jetex Mobo was "free-flight", very light, and went wherever it felt like! Enormous fun for an 11 or 12 year old at that time, but not popular with the neighbours! wink

Anyway, those figures may be of interest. Or maybe not! laugh

--

Pete

Christopher Morris 202/06/2020 15:18:59
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185 forum posts

Thanks, Shaun, a handy & helpful link, Like many things its can be a bit of a black art. If i end up to heavy, i will look at the building of it in a way that i could use it as a mould for glass fibre. or even the Froogle paper mache version.. I will look into those motors & purchase one making sure i have another build it will suite if not enough power.
You have also made the props a lot clearer. I think that PDF needs a small update for newbies. frown
The reason of this as an experiment was getting the air direct to behind the skirt with no chamber as many have.
The props i will have to test as some are better & some less noisy

Simon Chaddock02/06/2020 15:30:35
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5717 forum posts
3034 photos

Christopher

A prop is good at shifting 'free' air but very quickly looses effectiveness against any sort of back pressure no matter how small.

A fan is intended to operate against a pressure although it is still very small.

Even an EDF mutli blade fan might struggle against the sort of duct pressures found in a hovercraft but it will still be more effective than a two blade prop.

The important features are the total weight and the area of the cushion. Your air supply has not only to be able to provide the required pressure but to do so whilst still providing a sufficient volume to compensate for the losses from the 'free' hover height.

What area has to support your 4 kg?

brokenenglish02/06/2020 17:00:01
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572 forum posts
30 photos

The technical links given are all no doubt valid, but I seriously doubt that a 6" prop will lift 4kg under any conditions.

Imagine a 4kg model maintaining a vertical climb on a 6" prop... and I doubt that any "duct" effect will increase the thrust of a 6" prop to 4kg !!!

Christopher Morris 202/06/2020 17:01:54
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185 forum posts

Hi Simon, approximately 800x600mm. But because its all of a lightweight tubular frame that is slotted together i will duct tape the joins for now & possibly reduce the size/weight if necessary.. Its looking like i might have to.
The duct idea was to reduce building a chamber to save weight. Ah! its an experiment & a bit of fun.. 

Edited By Christopher Morris 2 on 02/06/2020 17:10:43

Peter Christy02/06/2020 18:00:10
1823 forum posts
Posted by brokenenglish on 02/06/2020 17:00:01:

The technical links given are all no doubt valid, but I seriously doubt that a 6" prop will lift 4kg under any conditions.

Imagine a 4kg model maintaining a vertical climb on a 6" prop... and I doubt that any "duct" effect will increase the thrust of a 6" prop to 4kg !!!

But it doesn't have to lift it! It just has to maintain a cushion of air under the vehicle at greater than atmospheric pressure.

The lift motors on the cross-channel hovercrafts couldn't have "lifted" a fully laden ferry, but they didn't need to. They just needed to make positive pressure underneath, and maintain it when it "leaked".

Having said that, I tend to agree that a 6" fan of any kind would have difficulty maintaining the required pressure for a 4KG load. But maybe two, back to back, like a contra-rotating prop? That would also solve the torque issue.

--

Pete

Simon Chaddock02/06/2020 18:36:03
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5717 forum posts
3034 photos

Christopher

Lets say 24" by 31" - 744 sq ins . It has to lift 9 lbs or 144 oz.

This means a pressure differential of 144/744 = 0.2 oz/sq in.

A 6" prop has an area of 28 sq in so it would have to generate a 'pressure thrust' of 4.8 oz to lift 9 lbs. On the face of it this looks quite achievable but what would be airflow against this pressure?

It is the pressure that lifts the hovercraft but to achieve a clearance this pressure has to be maintained along with an appropriate airflow.

Your hovercraft has a circumference of about 110". A 1/4" hover 'gap' would take virtually the entire 'free' airflow from a 6" prop with nothing left to create any sort of pressure.

With hovercraft It all comes down to a trade off between pressure and airflow.

I hope this helps.

Richard Clark 203/06/2020 02:36:01
295 forum posts
Posted by Christopher Morris 2 on 02/06/2020 11:59:08:

Hi, whats the most power i can get from a 6" prop. So i am not limited on the motor size but i am limited by a 6" prop because its in a tube. Any suggestions for prop pitch & type of brushless motor to use. Also, any pros & cons going with a 2-3-4 bladed Thanks.

It's an "How long is a piece of string? question.

From my personal collection of such things. 3.5 inches diameter + 50,000 RPM + £500 = approx 5 horsepower

Edited By Richard Clark 2 on 03/06/2020 02:36:32

Richard Clark 203/06/2020 02:54:32
295 forum posts
Deleted - irrelevant.

Edited By Richard Clark 2 on 03/06/2020 03:05:53

Piers Bowlan03/06/2020 06:13:28
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2154 forum posts
53 photos

Christopher, I built a small hovercraft in my youth with a .049 Cox Babe Bee for lift. It used a 6x4 prop and a vertical duct made from coffee tins! There was masses of lift and it would hover 2in above the ground but like Peter Christy it was laterally unstable and also span around like a top, due to the torque reaction.

Firstly, for a 24in x 31in hovercraft I can't see why it is going to weigh 4kg, what are you planning to build it from, what size battery etc? Also, I would think that it would be more efficient to move a large volume of air slowly rather than a small volume quickly, so why not use a bigger prop? Two blade props are more efficient than three or a ducted fan for that matter. I realise that you are ducting the air around the circumference within the skirt, but where there is ducting there will be losses. Some hovercraft I have seen duct the air into a plenum chamber beneath the lift motor with large holes around the circumference within the skirt.

Good luck with your project, it will be interesting to see how it develops. yes

John Lee03/06/2020 10:00:31
747 forum posts
73 photos

From my days of playing with a one man hovercraft we used a Villiers 9E engine which developed about 8hp (6Kw) to lift about 300lbs with power to spare. Juggling the units that equated to less than 20 watts/lb

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