Cowl modification

[Tim Flyer]

Just thought I would post up my modification job on a Wots Wort XL cowl I’m adapting for an old laser 200v . I decided to split the cowl into a top and bottom section . It fits together with threaded inserts and plastic bolts in glass tabs moulded along the full section of the joint. These joining tabs are easy to make simply by taping the cowl together after cutting and adding cling film in the upper half and glassing over with wing tape and epoxy to form a tab . At that stage I also add the wooden plugs to later take the 3mm threaded inserts.
I’m making baffle formers for the main vents from superfine fine lightweight aluminium mesh (same material as car body shops use). That is easily formed to shape, tacked in then covered with lightweight glass sheet. For the bottom baffle I’m simply using a triangular balsa section. To help cut the sheet Accurately I first make paper stencils.
here are some photos.

[Tim Flyer]

Here are the top baffles tacked in

[Ron Gray]

Nice work Tim and I don’t think you’ll be disappointed with the performance, when I had the 240v in mine it was a monster!

[Tim Flyer]

Cheers Ron . It’s such bad weather I thought I would get on with it 😊.

[Tim Flyer]

Here are the finished top baffles after glassing they angle the air down onto the cylinders. It’s quite easy to make a complex shape with wire mesh and glass. It’s also super light 😉. There is a triangular balsa baffle between the two cylinders below.

[Tim Flyer]

Here are the exhausts mane from bendy pipe with silver soldered brass end inserts and fittings. I use k&S brass thin wall tubes that fit in concentric sizes so they are easily soldered for a good gas tight fit .

[Tim Flyer]

Here is a side view of finished job. One thing some eagle eyed people may note is that my clunk tank has only 3 feeds. Two of them are carb fuel feeds and the third is a vent . What I have done to allow easy filling and draining is connect a small brass pipe which feeds one carburettor to the wooden engine mount spacer . This allows easy disconnection an re connection for filling and draining. Access is via removing the 4 plastic bolts holding the top engine cover . This cover can easily be removed without removing the prop/spinner or bottom cowl half. I always use plastic cowl screws as they are less prone to vibration loosening and cause less damage in mishaps!

[Tim Flyer]

I can’t wait to fire the engine up. I’m hoping it will give the “DLE boys” at my club something to think about 😉😉.

[cymaz]

That’s a great job . I like the way the cowl is kept mostly uncut with the heads.

[Tim Flyer]

Cheers Cymaz thank you. I know there are quite a few amazing builders on this site but I thought my simple mods might spring a few ideas for others . The “clamshell” tabs fit really well and are very easy to do . The two piece cowl makes cutting the holes much easier! Other than repairs and model boats, this is the first time I have used fine aluminium mesh as a glassing former on a plane and I certainly think it makes baffles and air channels easy to make. Its quite a versatile product.

[Jon H]

that should keep it nice and cool. Will you cut an opening at the bottom to let the air out again?

I like the mesh idea as well. Handy for making contoured baffles.

[Tim Flyer]

Thanks Jon . I was initially planning to cut out a small vent in the bottom at the rear of the cowl and put a small spoiler on it to act as an extractor rather like in my Chipmunk. . I’m not so sure now as the rearward facing side air exits already built into the cowl will provide good suction. ....However a little exit on the cowl floor would also ensure good airflow past the carbs.....hmmm maybe I will it only takes a few minutes and I can do it while the canopy glue dries 😊 it’s easier now to max out airflow than having to fiddle around later and need to degrease etc before fixing .

Ps I just ran up the gorgeous 200v and was mighty impressed. The DIY pipes sound really great and it’s certainly not noisy at all. I would doubt it’s any louder than a laser 180 on standard exhaust. It’s very quiet below full throttle with a beautiful idle and amazing pick up, with a screaming full throttle on the 18x8 APC. It’s an old engine but thinks it’s brand new ! It was very very easy to set up and tune despite being my first twin😊.

[Tim Flyer]

This is the mesh I used it’s very fine and light.  . Obviously it can be bought cheaply too in larger rolls.

Edited By Tim Flyer on 09/03/2020 12:09:06

[Tim Flyer]

Spoiler is done now . I just cut a bit of aluminium plate as usual. This will give good airflow out under the carbs.

[ASH.]

Thanks for that Tim. I've often wondered what best material to use for baffles. Now I know.

Nice metalwork!

[Tim Flyer]

Thank you Ash . Whilst balsa is a great material for the flat bits the mesh allows very easy production of compound curves etc . It’s easy to snip with tin snips too. I just tacked it in with a few blobs I’d glue ( plasticine or blue tack could be used to help) and then covered it with matting and filed it with my perma-grit file afterwards to tidy it up . Smooth curves and shapes can be made by light tapping with a mallet too before putting the mesh in . I always use aluminium plate for spoilers or scoops as it’s so easy to shape and also cuts so simply with tin snips😊

[ASH.]

 

I wonder what's aerodynamically more efficient for helping move hot air out of the cowl. When I come to do baffles I'll put the lip (ramp) on the inside. It believe it creates a vortex of fast air which then gets sucked out. Any aerodynamists out there.?

The light Ali mesh is a great idea as it can me molded to shape. Rather than matting I'm sure cloth or tape would do.

Tim, need to pick your brains on paint choice for your Chipmunk cowl. Did you do any tests on the fuel proof of it?

 

 

Edited By ASH. on 09/03/2020 20:28:19

[Tim Flyer]

Not sure about aerodynamic efficiency as when I’ve looked inside model jets there is often no intake ducting to the turbine....I’m Still not sure why?
What I do think for our piston engine models, is that directing air via a curved baffle to the engine might improve cooling airflow a bit ( it will create focussed area of higher density air to increase heat transfer) . However Air is not that dense and is very free flowing, so at the speeds our models do the curved ducting might not be that important for efficiency . Anyway I’m not an aerodynamicist!
Regarding fuel proofing my Chipmunk cowl . I am confident the RC colours polyurethane varnish will be fuel resistant for my Laser 5% Nitro fuel . I don’t want to test it until I fly it though and I’m afraid our runway is still a bog.

Edited By Tim Flyer on 10/03/2020 07:24:36

[ASH.]

Hi Tim, the baffles are fine to direct the air to the cylinders, in fact they are a 'must' on fully cowled engines. I will do the same as yours on my Chipmunk and another couple of models.

What I was referring to was the spoiler. You put yours on the outside and it looks neat in Aluminium but it can also be put on the inside. What I'd like to find out is which is more effecient.

Thanks for the info on the paint. I will try it out.

Edited By ASH. on 10/03/2020 08:39:57

[Tim Flyer]

Outside is much much better! The whole idea is the fast flowing unrestricted external flowing air over the spoiler creates a low pressure area behind the rear lip. That’s why a rear facing spoiler can be called an extractor. A front facing one is a scoop. If I put it inside it would be pointless as the already restricted (by baffles) outward air flow is much slower and does not need directing to the exit. A hole in the bottom of a cowl with no spoiler at all is also not so great as depending on shape and cowl geometry air can actually be forced in or very poorly vented. The idea of rear facing extractors is to point exiting air in line with the air stream then it’s actually “sucked out “. I also use a cowl extractor on my little Acrowot on the bottom of the cowl . It works very well.

Edited By Tim Flyer on 10/03/2020 08:43:50

Edited By Tim Flyer on 10/03/2020 08:47:52

Edited By Tim Flyer on 10/03/2020 08:55:41

[ASH.]

I disagree, I don't think it would be pointless to have it on the inside. You have fast moving hot air in there which can be trapped if the exit hole is not big enough.

Both ways work, it's just in a scale model I would prefer if it's hidden.

[Tim Flyer]

The airflow in the rear of the cowl is much slower

[Tim Flyer]

It’s moved from the small area in the front to big open area in the rear...high pressure into a low pressure zone. I guess on some models the rear cowl is tight so the air velocity won’t change much. There are loads of variables but I’m 100% certain that this set up will work. I was first shown how to use extractor venting on fast IC boats in the 70s and it certainly works 😉.
Ps I totally get your point for scale models but then you can use a wider smaller lip. Just 5mm is enough to disrupt airflow and U don’t need a big extractor spoiler. As this is a sport model it doesn’t matter . On my Chipmunk I used a small spoiler which meant I didn’t need a great big hole in the bottom of the cowl. 

Edited By Tim Flyer on 10/03/2020 09:04:06

Edited By Tim Flyer on 10/03/2020 09:08:15

Edited By Tim Flyer on 10/03/2020 09:08:34

[Tim Flyer]

Chipmunk spoiler ..it hardly notices but disrupts airflow to “suck it out “. I guess my sport planes often get the “boy racer” look ..”.Subaru Impreza special!”

Edited By Tim Flyer on 10/03/2020 09:14:58

Edited By Tim Flyer on 10/03/2020 09:24:01

[ASH.]

There's an awful lot of air in the slipstream to disrupt.

We need a fluid dynamics expert.. 🙂