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Oct 11, 2021, 04:20 PM
Registered User
Thread OP
Discussion

IC engine driven fan units


This is in relation to using an IC engine to run a fan.
These range from 7500 rpm to 14,000 rpm.
Lets start with the 7500 rpm motor.
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Q: Can a ducted fan unit with an 8 inch or less diameter, be made to use up 6 hp to produce thrust?
at 7500 rpm, this takes a 12 inch 2-bladed propeller.
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A 3.5cc glow motor can run up to 30,000 rpm, and happily spin a 6 inch fan, though this will be less than 6 hp.

Looking at the VTOL, this needs 3x 90mm fans drawing about 80 amps each, 22V = 5280w in, at say 80%, this is 4,224 mechanical,
or 5.6 hp.
seeing as a petrol IC motor complete is available at around 1600g, a direct drive fan unit and a ducted system with choke valves, seems feasible.
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Q:
1) could I get a 4-bladed big pitch prop, gear it up a bit, trim the tips off, and run it in a duct?
Would this work?
It is hard to estimate the effectiveness using existing prop tables- like the APC ones.
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The gear train also gets the way of the fan. The motor-geartrain-fan unit would need to be quite long.

You can run a fan straight off a single cylinder RC glow motor, as the prop end is quite narrow.
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Q:
The 4.2 Kw is based on 90mm fans running at 30,000 rpm, which is less efficient than a bigger fan running slower, I think?
Thus I could possibly use a smaller motor with a 200mm ducted fan unit.
How much smaller?
We are still wanting a reasonable jet output velocity.
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A duct system with multiple outlets and branches may be a bit lossy, and any choke mechanisms cause more losses, even when open.
It seemed to work for the Russian EDF Harrier, though.
(I will copy the youtube references)
Rc Scale Harrier VTOL Models (9 min 4 sec)


- getting those kinds of ducts made and working would be a big hassle...

** as per usual for my posts, you actually have to be interested in the results to try and absorb my posts, and think about it.
I have marked actual questions with a "Q:" and underline "-----------------------------"
Last edited by Owen_bern; Oct 16, 2021 at 02:01 AM.
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Oct 11, 2021, 05:02 PM
Registered User
Please don't be shocked, but both model ducted fans AND electric motors actually reach peak efficiency at high speed:

1. High speed (RPM) raises the fan blade Re and thus greatly increases efficiency

2. High speed (RPM) increases back-EMF and greatly increases the efficiency (torque/Watt) of normal (vs clock motor type) electric motors

You are allowed to "theorize" yourself into as many circles as you wish around this or refer to non-scientific "results" but both 1. and 2. above have LONG-ago been proven experimentally
Oct 11, 2021, 08:19 PM
Registered User
Thread OP
I have bought a motor.
It was a pretty good price!
now to sort out an 8 inch fan drive.
- any ready made fan units? - I will look on ecalc.

This does not supersede the electric version, as I have most of the parts.

I will try to work out a chassis about the same size and proportions.
A major problem is piping air back past the motor, to get the cog and lift thrust lined up.
maybe a 6 inch fan is better, to give me room for air passages.
Oct 11, 2021, 09:09 PM
Registered User
Coupez's Avatar
What "motor"?

I see a picture of an engine. "Motors" are usually electric (or hydraulic).
Oct 11, 2021, 09:36 PM
Registered User
Thread OP
Engine.
I need:
1) 145mm fan unit-Aeronaut Turbofan 8000 or similar.

- Current European stockists don't send stuff outside the EU.

$US stockist is too pricy, and freight is currently suspended from USA.


I need a 3.2:1 step-up gearing, 5.5 hp-capable.
This brings fan speed up to 21,000 rpm or so.
I suspect peak power is a bit below 7500 rpm, so a little experimentation may be needed.

Possibly Delrin gears 10mm wide, with lubricant, will do.

This gives 6699 or so gf thrust;

-This seems to be plenty to me, given the weights of the components.

I think maybe Thrust/weight of 1.4 is needed to make a good transition without losing height.
(sqrt 2)
The Harrier-type vents look a little tricky to make at present.-
3d printing needed?

You ned the parts to rotate in a housing, and also resist blow-out

I will look at the videos.- they may give me some tips.

Internal top-hat collar, outer part slides over the collar and fastens into holes in the collar. -
Use lubricant grease-this sounds moderately easy.

Q:-------------------------------------------------------------------------
Should I post the "supply of duct-fan unit" query elsewhere?
-----------------------------------------------------------------------------
An interesting feature is to divide airflow into 4 streams, and double back past the motor-fan unit, then turn 90 degrees outward to the swivel vents,
and per the harrier layout.
Maybe 2 streams, and a shaped "T' at the ends?
Oct 12, 2021, 12:06 AM
Registered User
Thread OP
This really works best with the 145mm fan.
I checked a 120mm fan, and for 5.4 hp, 1790KV, 6s,
this uses 5.4 hp and produces 6000gf, which is about 90 % output. of the 145mm fan.
Q:-------------------------------------------------------------------------------
Is 31,800 revs too high for a 120mm fan?
if I presume that the motor wants to run up to 7,000 rpm, this
ratio is 4.54:1 step up ratio.
Q:------------------------------------------------------------------------------
Also, can I use a 40A 4S 1250 brushless motor as a starter motor?
I can run it geared off the big gear, and have a one-way bearing coupling.
I will look for such a coupling.
Are there any problems doing this?
What gear ratio should I run it at?
The engine only needs about 100 rpm to start it, if that.
How much torque do I need against compression?
-------------------------------------------------------------------------------------
Driving the engine with an external starter could be more complex.
A steel main gear would be good for engine flywheel as well- rather like a boat motor.
Oct 12, 2021, 12:16 AM
Registered User
richard hanson's Avatar
Do it
Try the geared up setups
Its a hobby and if you also learn, its even better
As an old hot rod engine guy and also model builder, , I suspect you will find that the ice turning a fan is nowhere as efficient, trouble free, easy to tune, as a dri t coupling of electric power plant
Its hard to beat a setup having only one moving part
It is simply far more efficient
For example: the Tesla S Plaid which weighs 5000 lbs out accelerates cars costing millions using incredibly complex ice and power trains
The Tesla uses three electric motors which can turn 20000 rpm
Extremely efficient setup
zero to 160 mph in under 10 seconds yet you can sip a coup-a tea whilst doing it
0
Oct 12, 2021, 01:39 AM
Registered User
Thread OP
1)I have found a 75t module1 delrin, 6mm wide teeth.
this is 3 inches in diameter, so needs to be backed off to about 30 degrees entrant angle
Tan relationship? - maybe 5 inches back- I will check.
2) 16T 5mm bore pinion.
The fan actually has a 12mm bore, but 5mm should take the torque with a 1.5mm rollpin.
regarding starter, probaly 1000 rpm starting under the fan load, otherwise it won't pick up.
1250, 4S =14,800 rpm at 80%, so ratio = 14.8 to one. - this is too much, so I can just run it
on a 16T as well, which will spin the motor up to 3,300 rpm
Will the 500w starter handle this?? It will take sone tine to accelerate the fan and flywheel.
I can hit the ignition before it winds the fan up to half speed.

I also n3eed a seperate flywheel for the engine.
I will look on line for a boat one.
maybe 2.5 inch diameter by 1/2 inch thick steel?
What is the centre hole size? - around 3/8" for a 60cc?
I will check.

This is turning into a monolog, so I will move it to my blog.

I have an 8mm bore one-way bearing, which I can step down to 5mm input and up to22mm output-
hopefully without a lathe!
the 22 mm then steps back to 5mm for the starter pinion.

Much ingenuity with bits of aluminium and epoxy glue.- wedge the 5mm shaft in place.
- in-line pin?
Oct 12, 2021, 03:03 AM
Registered User

Learn by DOING


Quote:
Originally Posted by richard hanson View Post
Do it
Try the geared up setups
Its a hobby and if you also learn, its even better
As an old hot rod engine guy and also model builder, , I suspect you will find that the ice turning a fan is nowhere as efficient, trouble free, easy to tune, as a dri t coupling of electric power plant
Its hard to beat a setup having only one moving part
It is simply far more efficient
For example: the Tesla S Plaid which weighs 5000 lbs out accelerates cars costing millions using incredibly complex ice and power trains
The Tesla uses three electric motors which can turn 20000 rpm
Extremely efficient setup
zero to 160 mph in under 10 seconds yet you can sip a coup-a tea whilst doing it
0
Good Answer, Richard! That is the best way to learn about hings .. by actually doing, rather than just thinking and reading about them. Science is about independent carefully repeated observation of the real world anyway.

When we were in the 4th grade in class we made our own electric motors using thin insulated wire, proper diameter nails and wood. A few years after that we made our own crystal radios.

The OP's personal observations seem more closely associated with the pretty plastic motorcycle display models he's made rather than real flying models where weight is a major consideration.

https://owenprojectsblog.wordpress.c...rer-rear-view/
https://owenprojectsblog.wordpress.c...e-improvement/



But even his idea of using his "new" Rube Goldberg concept in a boat doesn't fit modern ideas of boat design where weight is also a consideration, since the days of building great sailing ships from felled oak trees has led to more modern materials with far better strength to weight ratios.

Quote:
Originally Posted by Owen_bern View Post
.... I also need a separate flywheel for the engine.
I will look on line for a boat one.
maybe 2.5 inch diameter by 1/2 inch thick steel?
What is the centre hole size? - around 3/8" for a 60cc?
I will check
Oct 12, 2021, 04:06 AM
Registered User
Thread OP
Quote:
Originally Posted by xlcrlee
Good Answer, Richard! That is the best way to learn about hings .. by actually doing, rather than just thinking and reading about them. Science is about independent carefully repeated observation of the real world anyway.

When we were in the 4th grade in class we made our own electric motors using thin insulated wire, proper diameter nails and wood. A few years after that we made our own crystal radios.

The OP's personal observations seem more closely associated with the pretty plastic motorcycle display models he's made rather than real flying models where weight is a major consideration.

https://owenprojectsblog.wordpress.c...rer-rear-view/
https://owenprojectsblog.wordpress.c...e-improvement/



But even his idea of using his "new" Rube Goldberg concept in a boat doesn't fit modern ideas of boat design where weight is also a consideration, since the days of building great sailing ships from felled oak trees has led to more modern materials with far better strength to weight ratios.
Not in a boat!
I was thinking of ic model boat flywheels with a groove round the edge for a rope pull.
They don't use those any more.
I need to find out how much inertia a 24 inch prop has, and replicate that with a metal disk.
I would guess at 4 inches in diameter by 1.5 inches wide brass.
Also, the motor prop connection is a 4-bolt, so I need to replicate this in the flywheel, plus I need a drive for the 8mm bore Delrin 75mm
pcd gear wheel.
I would guess I need a pretty good grip on it. A roll pin won't cut it!
Possibly I need a modelmakers lathe, with a 4-5 inch diameter workpiece capacity. - those things are expensive.
buying one of those would upset the missus.
Maybe I should instead get something made up for a couple of hundred dollars?
This looks like a good monologue, so I will copy it to my blog.
Oct 12, 2021, 08:21 AM
Registered User
Coupez's Avatar
Quote:
Originally Posted by Owen_bern
I have bought a motor.
It was a pretty good price!
now to sort out an 8 inch fan drive.
The reasoning process here reminds me of "Ready - FIRE! - Aim..."

Why not "sort out" the 8 inch fan first -- then buy whatever turns it most efficiently...

?
Oct 12, 2021, 10:51 AM
AndyKunz's Avatar
Quote:
Originally Posted by Owen_bern
Not in a boat!
I was thinking of ic model boat flywheels with a groove round the edge for a rope pull.
They don't use those any more.
Yes we do!

Quote:
I need to find out how much inertia a 24 inch prop has, and replicate that with a metal disk.
My high school physics book is packed away right now, but it has the answer you seek. It also gives you ways to determine it for any shape. It's just calculus. (Physics = applied calculus).

Quote:
Possibly I need a modelmakers lathe, with a 4-5 inch diameter workpiece capacity. - those things are expensive.
Yup, but it's a hobby, so expense isn't something you worry about much. It's not like it has to earn its keep. A local machine shop would be happy to make what you need given good CAD drawings.

Andy
Oct 12, 2021, 02:28 PM
Registered User
Thread OP
Quote:
Originally Posted by Coupez
The reasoning process here reminds me of "Ready - FIRE! - Aim..."

Why not "sort out" the 8 inch fan first -- then buy whatever turns it most efficiently...

?
1) I now have a 127mm fan. It is not ideal, but if I spin around 31,000 rpm it will work, just 10% lower output per hp than the 145mm.
the 145mm is hard to get, for now.
I am not sure how strong it is. Testing is needed.

8 inch was too big. It didn't fit my fuselage scale.

For ducted fan jets, it is a compromise between efficiency, looks, and chosen scale.
- Even more so for ducted fan VTOL.

2) Gearing between fan and engine was always going to be needed.

6hp IC motors don't turn at 30,000 rpm. - maybe a 3.5cc race nitro motor would?
That still only makes about 2 hp.- not enough thrust for a VTOL.
- enough to make a smaller model go very fast, though.
Oct 12, 2021, 02:55 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by Owen_bern
...
I need to find out how much inertia a 24 inch prop has, and replicate that with a metal disk.
I would guess at 4 inches in diameter by 1.5 inches wide brass. ...
Handy table with formulae for rotational inertia:
wikipedia...List_of_moments_of_inertia




Vriendelijke groeten Ron
Without a watt-meter you're in the dark ... until something starts to glow
E-flight calculators watt-meters diy motor tips&tricks Cumulus MFC
Last edited by Ron van Sommeren; Oct 12, 2021 at 05:10 PM.
Oct 12, 2021, 03:55 PM
Registered User

Fighting to the .... and?


You are fighting Physics. Any bets who'll win?

Many decades ago when Bob Kress was designing and selling ducted fan units usually propelled by the likes of very high nitro piped Super Tigres and started with "long-stemmed" electric starters ....I pointed this out to him:
Quote:
Originally Posted by xlcrlee
.... both model ducted fans AND electric motors actually reach peak efficiency at high speed:

1. High speed (RPM) raises the fan blade Re and thus greatly increases efficiency

2. High speed (RPM) increases back-EMF and greatly increases the efficiency (torque/Watt) of normal (vs clock motor type) electric motors
He immediately turned his nose up and poo-pooed the idea. Then someone proved the correctness of that advice. And Kress joined the move to EDFs

The rest is history (for most of us)


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