Who Else Wants a 63" Lavochkin La7 kit?

[Ron Gray]

So reverting back to angled as others have done

[Ron Gray]

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 Bowlan]

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 Gray]

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 H]

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 Flyer]

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 Gray]

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 Cotsford]

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 Gray]

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 Flyer]

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 😉.

[Nigel R]

What's the "iron method" for doublers?

[Nigel R]

wait, does it involve literally ironing on the doubler using PVA?

[Ron Gray]

Thanks Tim, I can safely put my concerns to bed then and get the motor fixed!

[Ron Gray]

PVA applied to doubler, let go touch dry, assemble double and balsa side, then use hot iron on ply doubler and hey presto it sticks - brilliant.

[Nigel R]

Fantastic, I like it, thanks Ron.

Wish I'd known about it two days ago!

Edited By Nigel R on 14/12/2017 14:19:25

[Ron Gray]

@ Jon. As the centre line of the engine is also the centre line of F1, rotating the engine makes no difference to the exhaust outlet, it will always poke through the cowl. The only way to avoid this is to slightly rotate the exhaust as well but you cannot go too far as it will foul the carb. These photos should give you some idea.

This leads for an interesting route for the throttle cable and will mean the addition of a silicon exhaust deflector on the end of the exhaust which would flex as the cowl is fitted. Still not convinced this is the best option so will have to look back over the thread to once again investigate how others have mounted theirs.

[Jon H]

Turn the carb the other way up, that should make the throttle easier not to mention the fuel line attachment. Also ic an supply another one of those long outlet exhausts if it helps tidy up the exhaust. I would also point the exhaust straight down if at all possible.

[Ron Gray]

Turn the carb round and the main jet adjuster sticks right out of the cowl. Point the exhaust down and it fouls the carb!

[Jon H]

Posted by Ron Gray on 14/12/2017 20:27:31:

Turn the carb round and the main jet adjuster sticks right out of the cowl. Point the exhaust down and it fouls the carb!

ummmmm

how far does the needle poke out? it does need to be accessible after all. there should be a range of positions where the exhaust and carb miss eachother. I don't have an engine to hand to play with but I will look at some options tomorrow when I get back to work

[Ron Gray]

Hopefully these will show it better

[Jon H]

if the carb/exhaust were turned to match the red lines they should clear eachother I think. The lines aren't super accurate but I hope you get what I'm on about

[Ron Gray]

It actually doesn't make much difference moving them as the main needle still sticks out of the cowl by about the same amount but I think that it's the best I will get.

Looking at Peter's photos, he has mounted his in a similar way to that shown in my last photos but I assume that he removes the main needle then fits the cowl. As long as I count the number of turns, and let's face it the cowl isn't going to be coming on and off many times once the initial tweaking has been done, I may well do it that way.

[peter harvey 1]

That's what I did , I also removed circa 4 mm from the end of the silencer which helps considerably with fitting the cowl.

I have some silicone tubing , from just engines, which fits over the laser silencer and can if wished be fitted after the cowling is in place and doesn't look out of place. .

The throttle cable exits through the wall of the exhaust deflector .

cheers

Peter

[Ron Gray]

Many thanks for that Peter, I will continue along those lines.

[Ron Gray]

The deed is done! Actually quite pleased with the results. Got to cut out the hole for the exhaust exit plus when I get the spinner I will do the final front cut on the cowl.