Cowl modification

[Tim Flyer]

We probably need to borrow a wind tunnel if we really wanted to test different set ups , but I normally get away with just “overdoing”what I initially intended so there is plenty of margin. I’m sure an “efficient” system in winter might not cool enough on a hot summer day with thinner air etc . On our boats we often changed water cooling from winter to summer. In the winter I would run water through the manifold before reaching the engine so it wasn’t over cooled. Many of these things aren’t totally necessary and there are many different ways of achieving the desired result, but I guess its all that fettling with set ups etc that makes modelling fun! 😊

[Tim Flyer]

Sorry to bring up boats again but a reverse scoop or extractor works a bit like a self- bailer in the transom of a boat . A self bailer is simply a hole in the transom at the bottom of the hull below the water line. A one way valve is fitted to prevent taking on water when stationary. As the boat moves faster and lifts, the water rushing under the hull helps grab exiting water (due to water’s viscosity I believe) and the bilge is cleared fast . I believe air also has similar properties and exiting air meeting faster air going in the same direction receives some sort of pull due to its viscosity...ooh dear that’s as far as I can try to go 😳

Edited By Tim Flyer on 10/03/2020 13:54:07

[ASH.]

Tim, this a model flying forum - it's no place for 'transoms' or 'bilge drain valves' on here. Please have some respect! This is big boys stuff..

As you say the air inside is moving to a larger lower presuure area. The lip is to speed up and distrupt the air causing it to be sucked out into the fast slipstream. If it's a simple rectangular exit hole then a half inch triangular Balsa stock glued on l/e of hole should do it. It it's a complex cutout like on your XL then a thick piece of string following the shape will achieve the same.

To get maximum extraction perhaps a lip on the inside and out would work best.. Or they may cancel each other out,

I will get to the bottom of this, I know some aerodynamic graduates. That's what makes this hobby fun for me too - I can get really anal about things!

Ps. Would love to know how you do water cooling on model marine engines!.

BTW. Fins on outside do work like on the Sukhoi cowl.

 

Edited By ASH. on 10/03/2020 15:44:17

[Martin Harris - Moderator]

I can't help with the aerodynamic theory but I do know that I put the basics to the test some years ago in a 1/12th scale combat Hellcat. There were two holes in the front of the cowling - one just big enough for the prop driver and the other a 3/4" x 1/4" slot just in front of the cylinder of the Irvine 25 inside.

I made a hole just larger than the cylinder head in the bottom of the cowl, slightly elongated to the rear in an egg shape to accommodate the virtually flush head - just the glow plug protruded - and used the back of my covering iron to form a small lip on the front edge.

The cowl was mounted tightly to the fuselage at the firewall so there would have been insignificant leakage and with the exception of a tight cut-out for the silencer exit and a small hole for the needle valve, the only air exit was around the head through the aforementioned hole.

I ca honestly state that this arrangement, tested in 7 minute mostly full throttle combat bouts, never showed even the slightest hint of overheating.

Edited By Martin Harris on 10/03/2020 17:15:14

Edited By Martin Harris on 10/03/2020 17:17:21

[Tim Flyer]

That sounds a fun plane Martin! I think that does underline the fact that practical experience is best rather than all sorts of rules which can often over complicate things . One thing i should add which I guess we are all aware of is that as models have “supersized” over the years ( along with many things in society) so the cooling of engines has become a bit of a tougher proposition. Larger engines have less surface area to volume so do require more cooling . I remember many modellers complaining of loose head bolts when OS launched the 120ax . It seemed many users were stretching the head bolts after allowing the engine to overheat . I think it would be fun one day to make a fluid cooled warbird rather like the full size . I understand some of the supersize models already have that .

[Don Fry]

I remember seeing a Hawker Hart with a marine watercooled head. Builder had made a working scale radiator ( not as difficult as it sounds), and circulation was effected with a Perry pump I think. It worked

[Tim Flyer]

I did the maiden flight on the Wots Wot XL today. It was superb the 200v really makes it very fast indeed. Wind was 28mpb but no problem flying although I did have to put the wings on inside our clubhouse,! I was alone at the field so no flying pics I’m afraid . Here is a video of the engine running on the stand , **LINK**

Edited By Tim Flyer on 20/03/2020 22:31:48