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Who Else Wants a 63" Lavochkin La7 kit?

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Bob Cotsford21/02/2018 13:20:47
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After a couple of diversions in the shapes of an electrified Wot4 Mk2 ARTF and the Easystreet it's back to the La7. The next task was a bit of a nasty one, cutting the cowl. After finding the cowl's centre I used a compass cutter to scribe through the resin on the inside and outside of the cowl then hacked the glass out inside the circle using a Dremel saw and finishing it with glasspaper wrapped around a roll of masking tape.

Big hole to fill in

Then it was a case of sliding the cowl over the engine and marking what needed cutting to clear the rocker cover. A small hole for the needle and a bigger cutout for the exhaust then it's time to get the vacum out and wash hands thoroughly. I still itch!

Some smaller holes

The next job will be to make a baffle plate to close off most of the cowl to direct incoming air just over the cylinder. 3/32" sheet should do for that.

Edited By Bob Cotsford on 21/02/2018 13:22:56

Tim Flyer21/02/2018 15:10:25
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Good to see you are getting on well there Bob. I have cut the front on my cowl too and will be doing similar on the baffle plate. I’m still debating whether to shorten the Laser 80s exhaust pipe as Peter did or just make a small panel in the cowl . I put my servo tray in the front last night, and am finishing all the Engine plumbing and control holes in the front before finishing the engine bay and fitting the wings. I will have to add a fair bit of fairing around the front of the wings to make it look ok. I must admit I’m enjoying it although my time is limited so just a little bit happens each day. I have changed to a 10oz tank from the original 12oz I had first planned. It’s on order at the moment but hopefully the smaller tank will give the servos more clearance.

Edited By Timothy Harris 1 on 21/02/2018 15:11:28

Ron Gray21/02/2018 15:58:21
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@Tim - the Laser fitted ok without cutting the exhaust on mine:

img_2890.jpg

Bob Cotsford21/02/2018 16:51:24
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Is the cowl polyester or epoxy? From the smells generated when cutting it I thought polyester. I'm going to let a couple of lengths of 7mm carbon tube in as gun troughs. I have ideas about using black tube for the tank filler and vent set into the tubes as dummy gun muzzles? My OS has an ASP exhaust which when fitted with a silicon deflector/extension will exit right up against F2

Tim Flyer21/02/2018 21:34:50
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Hi cheers Ron . My exhaust will protrude by 5mn approximately but with the close fitting cowl there isn’t much wiggle room. I will probably just simply cut a small removable panel. I’m not going to be lazy and cut the exhaust 😉 Good idea on guns Bob .

Tim Flyer22/02/2018 09:39:11
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Sorry Bob didn’t reply regarding the cowl question. Yes the cowl is formed from polyester. However epoxy is fully compatible with polyester and I will certainly be epoxy coating the Inside on mine plus the balsa baffling . I never use polyester resin on anything much these days as it’s so heavy and smelly (with the exception of car p47 body filler).

Ron Gray22/02/2018 09:46:21
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Tim, what's the advantage of coating the inside with epoxy?

Bob Cotsford22/02/2018 10:56:12
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I usualy put a 1" band of 20gm cloth around the inside rear of a glass cowl just to give the mounting screw points a bit more strength, plus a patch around any holes for cylinders, needles etc. to prevent cracks starting. With the need to slide over F1 this might be a problem on this one. I've noticed a few bubbles in the glass on this cowl, nothing on the outside but a full coat of epoxy inside might be a good idea to seal it.

Cuban822/02/2018 11:48:53
2911 forum posts
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I believe that the general consensus is that epoxy can be applied to fully cured polyester, providing that the underlying surface is well prepared. Working the other other way round is the real no no. I was reading up on this a while ago on a boat building website, but the consensus was certainly not 100% from those who had gone down that route for repairs to their polyester glass hulls (not models) and results were not always satisfactory and often resulted after a while, to problems with water ingress.

In my glider guiding days, the chap who used to produce polyester fuselages for Dick Edmonds and Sean Bannister told me that epoxy should never be used in conjunction with polyester as failure of the bond was likely. I ignored his advice and sure enough on one model, some epoxy glassed-in servo bearers popped out of position due to the bond failing, luckily the model could be landed safely by maximum retrimming but it could have been disastrous. I always went with like for like after that, and have never encountered a problem ever since.

We are talking about thirty years ago, so perhaps the chemistry has changed, but might be worth trying a test piece beforehand.

Tim Flyer22/02/2018 12:48:20
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The only caveat with using epoxy on polyester glass is that the polyester must be fully cured. If slightly sticky, fresh very smooth or greasy the bond will not be good. Critical bonds always need preparation. Epoxy resin is lighter less brittle and far far superior. It has much better moisture barrier and prevents osmosis and makes a smooth impenetrable barrier insid cowls. Epoxy  is routinely used to repair full size ( And model fibreglass boats) . Yes polyester chemistry has changed over the years and boat hulls are thinner now plus the glass woven fabrics available are better. I would not use polyester over epoxy....firstly polyester is more brittle so will crack easily. Full size boats are built with polyester resin due to cost but very very often repaired with epoxy. Just one extra thing to note .  Car body fillers eg isopon P38 etc are mostly polyester resins but can be used on properly cured epoxy as the are more flexible than standard polyester resin. 

Edited By Timothy Harris 1 on 22/02/2018 12:49:24

Edited By Timothy Harris 1 on 22/02/2018 12:50:40

Edited By Timothy Harris 1 on 22/02/2018 13:15:44

Tim Flyer19/03/2018 15:08:12
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Hi all ! I hope your builds are going we’ll. I have managed to find time with the bad weather to make more progress. I will be finishing Wing to fuselage attachment tonight after glassing the wings last weekend. I must admit I do like fiberglassing! Anyway after doing that I’m veering towards using brown paper to cover the body rather than Solartex . i think it will be more “ding”resistant. I wondered if Deluxe Eze Kote would be ok to use with paper covering or should I buy a tin of PVA and use that slightly thinned instead? . I will post some photos soon🙂

Ron Gray19/03/2018 16:10:07
1917 forum posts
799 photos

Unfortunately all of my building is on hold at the moment due to family issues so the LA-7 has not progressed. Looking forward to seeing photos of yours Tim but would suggest going down the PVA route, tests I did earlier this year showed me just how good brown paper and PVA is!

Tim Flyer19/03/2018 16:51:59
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Sorry to hear that Ron . I have managed to get a bit done here and there once wings fixed I will do wing fairing. It’s all bit by bit but I must admit it’s been quite therapeutic. I would have preferred a “built up wing “ and body rather than foam to be honest, but have still enjoyed it.

Glenn Philbrick19/03/2018 17:51:52
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I'm afraid my LA 7 is still in its box as I am building a Tony Nijhuis Lanc. I have started to experiment with brown paper using 50% PVA as suggested and am quite impressed by the results. I have painted poly-c over the brown paper covering to stiffen it up ready to be rubbed down for priming. I've also applied the paper in 5" wide strips to see if it looks like panel lines on the finished model. A couple of issues I've had so far, do not seal the balsa before applying the PVA it will not bond if the surface is too smooth, the edges of the covering tend to shrink and lift the paper off at the edges. There are some other threads on here about covering with brown paper that are quite useful.

Tim Flyer20/03/2018 09:10:06
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Hi Glenn, now the Lanc does sound a really big project but I’m sure it will be a superb model . Thank you for your reply and thank You for the tips. I must admit I have also done some experimentation. I had to sand off an unsightly tail fin bulge on the LA7 which meant I had to sand through the veneer to get it looking ok. I reinforced it after sanding by using glass covering cloth coated by gloved hand with neat deluxe aliphatic glue. I then used light filler over the top. It worked very well and the glass cloth was as smooth as when using epoxy but obviously aliphatic is less messy and easier handling. Good luck with the Lanc😊

Edited By Timothy Harris 1 on 20/03/2018 09:11:07

Tim Flyer20/03/2018 09:53:17
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4972b0a6-874e-463a-bd1c-fc4c681384bc.jpeg

Tim Flyer20/03/2018 09:54:41
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Wing located showing cowl slot and strong ply tank floor fitted

Tim Flyer20/03/2018 09:55:35
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4c614762-6b60-4323-a94a-c0e400fbfec8.jpeg

Tim Flyer20/03/2018 09:56:51
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Underside minus wing. I chose to make a traditional mount for rear wingbolts on f3

Tim Flyer20/03/2018 09:59:27
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a90d4e9c-3fed-4360-9c7f-bd65faca488c.jpeg

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