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Use Venturi effect for cooling air?

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Nigel Sharp03/08/2020 14:14:34
39 forum posts
166 photos

I am seeking advice wether it is worth adding a cowl like cover with rear facing opening over the "Air Exit".

I don't particularly like the thought of a visible an moderate sized hole in the rear floor of my fuselage. So am considering if by creating a cover that the lowpressure venturi effect will actually help extract the warm air from the aircraft.

I have a Cowl with opening, inside I have smoothed the surfaces to gain as much airflow as possible. I am going to fit a ventilated spinner to this as well giving quite a large input for the motor.


The air passes into the forward fuselage through an opening in the lower firewall bulkhead.


Air then passes the ESC and up through battery bay to exit the fuselage just behind the wing.


Can you cover this exit with rear facing cowl which should generate a low pressure effect in forward flight creating suction?


Anyone used this before?

Was it successful?

Is it a waste of time?

Doug Moss03/08/2020 14:22:49
13 forum posts

A NACA type duct will do the same job but with much less drag.

Nigel Sharp03/08/2020 14:35:52
39 forum posts
166 photos

Hmm yes that is interesting... I can make the opening a depression smile and effectively hide the dark hole.

That's made it certainly worth asking the question.


Jon - Laser Engines03/08/2020 15:10:05
5621 forum posts
271 photos

I use ducts to create suction all the time and it works very well. I am often asked why my models dont over heat with so little apparent cooling.

Doug is right about drag, but at our scale with the power we have its not a major consideration.

Nigel Sharp03/08/2020 15:18:32
39 forum posts
166 photos


Great endorsement of ducts.

This is a biplane with rubber band wing fixing so has inherent drag... but no wires to sing.

I've already made it a duct just waiting PVA we shall see it was obvious really. I have high hopes for the solution.

Tis only balsa so can be easily removed/enlarged as necessary.


Nigel R03/08/2020 15:29:42
4082 forum posts
694 photos

I wouldn't bother with the vented spinner. Looks like a nice big intake hole already.

Here's an internal exit ramp / duct / etc I made earlier:


Same model front view - intake area around spinner is fairly small, just two vertical slots really:


J D 803/08/2020 15:51:51
1586 forum posts
87 photos

On some types the open cockpit can make for a good warm air exit.

Nigel Sharp03/08/2020 15:54:31
39 forum posts
166 photos

Hi Nigel,

Yes my new duct is very much like the example you have shown It will get a bit of smoothing once the glue has dried.

Spinner is already on order the one in the frontal picture (2 1/4" dia ) in my first post just wouldn't quite fit the top deck profile and I'm a bit picky about things like that. So I needed a new 2" one anyway, decided to go for vented to keep the motor as cool as possible.

Build pictures are in my album RCM Wayfarer for those interested.

Right off to cover the wings...



Mike T03/08/2020 16:18:05
553 forum posts
35 photos

I'd cover that with a streamlined exit shroud with a small lip at the rear, to promote a bit of turbulence:


McG 696903/08/2020 18:42:03
3368 forum posts
1285 photos

+1 for Doug M's Naca duct.

Inverted, of course, as the original design is to let cooling air 'coming in' and not 'extracted'.

Really simple and effective with no protuberance at all.

A quick Google for the right proportions and job's a good un...



Simon Chaddock03/08/2020 19:32:46
5776 forum posts
3055 photos


All in favour of ducts but to keep the openings and exit as small as possible is it important to duct the air to where the heat is being generated rather than just so it passes 'down the fuselage'.

It much better to arrange that as much air as possible passes through the motor than just around it. The same applied to the ESC which in many cases tends to be quite effectively insulated by a layer of thick shrink wrap!

Nigel Sharp03/08/2020 19:39:35
39 forum posts
166 photos

Thanks everyone.

I'm going for the simple shape (easier to construct, and cover with film) for initial build. If I find that this needs some more work...

Then I'll construct a proper NACA duct from 1/32 ply on a plate which can be simply retrofitted to the rear fuselage.

NACA Duct/Inlets


Mike T04/08/2020 00:16:54
553 forum posts
35 photos

NACA ducts are for intakes...

trevor wood 204/08/2020 01:22:16
59 forum posts
56 photos

image.jpegPerhaps someone should have told Colin Chapman that they are only for intakes before he designed his F1 Championship winning Lotus 49B...

Trevor Crook04/08/2020 07:30:14
1003 forum posts
71 photos

Many, if not most escs have a nice flat surface which the label is stuck to, and its tempting to stick velcro or double sided tape to this. Not a good idea as it's the metal sheet heatsink under the sleeving. Better to stick the velcro to the lumpy side and have the airflow routed over the heatsink. 

Apologies if this is obvious to most readers.

Edited By Trevor Crook on 04/08/2020 07:31:31

Edited By Trevor Crook on 04/08/2020 07:32:19

Richard Clark 204/08/2020 07:55:34
426 forum posts
Posted by McG 6969 on 03/08/2020 18:42:03:

+1 for Doug M's Naca duct.

Inverted, of course, as the original design is to let cooling air 'coming in' and not 'extracted'.

Really simple and effective with no protuberance at all.

A quick Google for the right proportions and job's a good un...



I've never heard of a NACA duct bring used 'backwards' for air extraction. It's meant as a non-protruding intake

Taking a close look at the duct's very precise design it would, being a 'hole', certainly let some air out but that's all.

Nigel Sharp04/08/2020 09:21:55
39 forum posts
166 photos

Every day is a learning day.

Here is a link to a interesting clip Pushing the envelope - reverse NACA duct

Whilst I'm not going to all the trouble to make a full facsimile I was after confirmation of the principle of using lower pressure to create suction.

I think this makes it good enough for me. As I said earlier if the simple shape proves not to do enough I can very quickly introduce a proper shaped NACA duct either with a ply version or a GRP/VAC formed version with only minor surgery that will not effect the film covering much.


Doug Moss04/08/2020 11:23:42
13 forum posts

Which is why I said Naca TYPE duct.

Nigel Sharp04/08/2020 12:31:22
39 forum posts
166 photos


Yes I'm on that same page I was just after you posted.

I'm trying to get as near to finished as possible on this build this week. My furlough ended last friday and I am on enforced holiday this week. Back to work next.

I can leave the rear lower fuselage uncovered for the time being and knock up a proper duct using the NACA proportions in fusion 360 to give me some patterns to cut out and fit.

But I wonder if using the simpler shape would give just as good a result at model scales.


Mike T04/08/2020 13:30:07
553 forum posts
35 photos
Posted by trevor wood 2 on 04/08/2020 01:22:16:

image.jpegPerhaps someone should have told Colin Chapman that they are only for intakes before he designed his F1 Championship winning Lotus 49B...

On the basis that neither you nor I know what CC's thinking was with this, I'm going to assume that he needed to get some air out using the lowest drag solution, not necessary a solution which promoted maximum airflow through the duct.

If you want to actively promote airflow, you need a turbulator to reduce pressure at the duct aperture, otherwise a NACA type duct is just a cool-looking hole... smiley

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