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Who Else Wants a 63" Lavochkin La7 kit?

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Jon - Laser Engines12/12/2017 11:09:57
5203 forum posts
237 photos

Yea i can see that now Bob. The cowl will probably need to be cut to mirror that flap, shouldnt be too tricky though. Really nice woodwork on yours as well. 

I agree too that you need a baffle plate in the cowling, something like this fine effort in a customers fw190 should do just fine.

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Edited By Jon Harper - Laser Engines on 12/12/2017 11:11:02

Bob Cotsford12/12/2017 11:31:01
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8248 forum posts
454 photos

That's the idea Jon, force the air through the fins with nothing wasted. With the reduced air volume coming into the cowl the exit should be plenty.

Jon - Laser Engines12/12/2017 12:29:55
5203 forum posts
237 photos
Posted by Bob Cotsford on 12/12/2017 11:31:01:

That's the idea Jon, force the air through the fins with nothing wasted. With the reduced air volume coming into the cowl the exit should be plenty.

Yup. Often people seem to forget about getting a good flow going and assume that if its sat there surrounded by air all is well. Not the case as you need the flow to get rid of all that warm air. My P39 runs into trouble with this as im able to cool the engine but cannot get rid of the hot air. Its up for some serious modding this winter to resolve the issue.

Also when you choose your spinners get a nice big one as the full size only has a small gap around its spinner. Something around 3/4 inch smaller than the hole in the cowl is likely to be about right

Bob Cotsford12/12/2017 16:39:15
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8248 forum posts
454 photos

Nice pic Jon, it shows off the shutters in the cowl, the size of the exhaust flap, the two metal cowl bands, the thick glass protection behind the pilot, the guns and also emphasizes how shallow the gun blisters are. Now I'm curious as to what the small blister just outboard of the U/C leg is?

I make the spinner about 4" - I think I may have one around that size in stock smiley

Edited By Bob Cotsford on 12/12/2017 16:40:42

Jon - Laser Engines12/12/2017 16:57:08
5203 forum posts
237 photos

I think im using 4.5 inch on my 80'' model so 4 might be a little big. Without all the bits in hand though its hard to say. I know my 80'' seafury uses 5 inch. in essence though, just make it look right and you wont be far wrong

Anyway you are right about the cannon blisters, they seem to have gone which is a little weird. The bumps outboard of the u/c are hardpoints for 100kg bombs. Alas i cant find a photo showing it though so ill just put my model up again :D

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Ron Gray12/12/2017 17:19:40
1682 forum posts
458 photos

The cowl measures 7" diameter at its widest so I reckon a 4" spinner would be perfect! The Cowl outlets are as shown below, the first where the tapered exhaust outlet has been formed and the other is the opposite side where I plan to form something similar but a bit more hidden.

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Trouble is, with a 4" dia spinner the area of air intake = 11.25" (approx), the exhaust area = 3" (using both sides of the cowl). As the ratio of exhaust to intake should be roughly 4 / 1 we need to place some serious baffling in the cowl and / or have another cut out underneath the cowl where the silencer goes.

Bob Cotsford12/12/2017 17:28:45
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8248 forum posts
454 photos

There are lots of photos in the book I have, Lavockin La-7 by Milos Vestsik, and the 2 gun version blisters really do look very shallow on all of them. The 3 gun variant blisters look to be more prominent, longer but narrower.

I've just measured the spinner on the 1/72 drawings in the book at 8mm, for a 135mm span which near as I can make it is 3.6" for this model, so 3 1/2" is a good size. I bet I don't have one of those, though iirc I did buy something near that for a Ki61 that never got used.

Ron Gray12/12/2017 18:58:40
1682 forum posts
458 photos

Just been looking at fitting the engine and had been thinking about an angled mounting was the only way to go. However, just been playing around and an inverted mounting could well be the best fit, especially as another cut out needs to be made for the exhaust and I can make the carb intake draw air from behind F1. Sure there needs to be some careful cutting of the cowl as it spans back to F2, but the section behind where the exhaust cut out will be could be made as a separate piece with small, scale like screw fixings holding it in place.

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Jon - Laser Engines12/12/2017 19:03:18
5203 forum posts
237 photos

Ron if you baffle the cowl as per my post at the top of the page you wont have any trouble with cooling as the area in becomes only as large as the gap around the engine.

Also inverted looks nice, but where will the fuel tank go? The carb looks really close to the firewall too which will cause issues.  I think side mounted will be your best bet as the carb can go in the tunnel, the tank position should be easy, cooling is easy and the exhaust can point straight down. Should be nice and tidy

 

Edited By Jon Harper - Laser Engines on 12/12/2017 19:04:53

Ron Gray12/12/2017 19:49:04
1682 forum posts
458 photos

Bu**er forgot that I wasn’t dealing with a gasser! Glad I did decide to post pic on here first.

Edited By Ron Gray on 12/12/2017 19:49:47

Ron Gray12/12/2017 21:01:02
1682 forum posts
458 photos

So reverting back to angled as others have done

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Ron Gray14/12/2017 06:50:57
1682 forum posts
458 photos

I have a dilemma and have no doubt that the learned sages on this forum will be able to help. With the engine mounted as shown in the last photos, canted over, the carb intake is in the purpose made exhaust outlet. When flying won’t the cowl create an extract effect on this outlet thus fighting the carb intake?

Piers Bowlan14/12/2017 07:41:39
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2045 forum posts
53 photos

I have probably missed something here Ron but what is your objection to mounting the engine at 90 (80?) degrees, side-winder fashion? It will probably run better, the glow plug will be easier to get at for starting, and the cowl will cover some of it. The bit poking out will have 'Laser' stamped on it, so will be cool!

Edited By Piers Bowlan on 14/12/2017 07:42:21

Ron Gray14/12/2017 07:53:37
1682 forum posts
458 photos

No objection whatsoever! The problem would then be that the carb intake would be through the firewall, again I haven’t got a problem with that as I like the idea of drawing sit from within the frame but choking it would be a problem as would fuel spit back as I would have to make a duct to the outside to get rid of it! The cylinder head should be completely covered by the cowl whichever way I fit it!

Jon - Laser Engines14/12/2017 08:32:28
5203 forum posts
237 photos
Posted by Ron Gray on 12/12/2017 19:49:04:

Bu**er forgot that I wasn’t dealing with a gasser! Glad I did decide to post pic on here first.

Edited By Ron Gray on 12/12/2017 19:49:47

You arent alone. I would say that running issues caused by poor tank position is 90% of the troubleshooting calls i get. Its a common mistake.

If you rotate the engine a little more towards the horizontal how far can you get before the exhaust no longer pokes out the bottom?

If you are able to rate it to a position where the exhaust outlet is about 3/32 inch inside the cowl you can then fit/remove the cowling without having to manhadle it round the exhaust. The gasses will easily jump the small gap and the whole thing will be nice and neat.

If the carb is then fouling the top of the exhaust vent just cut out a small section and back fill it so that it blends into the rest of that exit tunnel.

Tim Flyer14/12/2017 11:53:19
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1218 forum posts
231 photos

My carb intake is in the air exhaust vent and it will be fine. It is blended in with the vent tunnel just as Jon said above and as you can see in the photos I posted earlier. It was easy to do with scrap balsa and will work well . I also blocked the cutout I made for the exhaust in the firewall at the rear so there will be no oil ingress and no carb spit behind F1.

Edited By Timothy Harris 1 on 14/12/2017 11:56:08

Ron Gray14/12/2017 12:12:46
1682 forum posts
458 photos

But won't the edge of the cowl work as an extract and try to draw the air from the vent tunnel, the reverse of what you want for the carb?

When I get home tonight I'll look at rotating it further so that the exhaust actually exits into the exhaust vent (smoke coming out off that will look cool, although will have to make provision for mucky oil residue) but then I have to see where the carb will be.

Edited By Ron Gray on 14/12/2017 12:20:29

Bob Cotsford14/12/2017 12:20:05
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8248 forum posts
454 photos

I would have thought that the cowl will always be at a positive pressure Ron, unless you add a lip to the cowl rear which would act as an extractor. With the cowl in place the tunnel cross section is squeezed a bit at the rear so I don't see it getting any negative pressure. Seeing as the airflow is always the 'wrong way' with the downdraft setup on Lasers I doubt it should make any difference to the running.

Just as an aside, I see you are using No More Nails Express wood glue, what type of glue is it?  Does it set quicker than other white glues such as the 5* aliphatic and Resin W that I'm currently using?

Edited By Bob Cotsford on 14/12/2017 12:21:12

Ron Gray14/12/2017 12:24:31
1682 forum posts
458 photos

Good point Bob and maybe I'm worrying unduly!

No more nails is a pva type glue, waterproof and v quick drying. I use it on builds where pva is the ideal solution and it worked extremely well with the doublers on the LA-7 where, for the first time, I used the iron method which I will definitely be using again. No More Nails plus Gorilla Glue are my glues of choice oh yeah and Uhu Por.

Edited By Ron Gray on 14/12/2017 12:26:37

Tim Flyer14/12/2017 13:42:15
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1218 forum posts
231 photos

Carbs are often right in airstreams but the velocity stack( the tube on outside) helps keep negative pressure differential in the barrel. We would have to work hard to get negative pressure in the outlet even with an extractor tab. All we are doing by adding a tab would be to create a difference in pressure between front and back of cowl air in order to move the hot air outside. A number of my planes e.g Wot 4 xl have airstream runnning over the carb . It doesn’t matter ( unless we are talking really super fast planes 😉.

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