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SBach 300 - Repairs and modifications

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Ron Gray28/03/2020 22:42:12
1931 forum posts
803 photos

Not too sure if anyone is interested in this 'project ' that I've just started, so dipping my toe in the water, so to speak.

To cut a long story short, when flying this model last year (May / June) I suffered a dead stick whilst quite low which resulted in me putting it down in a ploughed field short of our runway. The damage was quite severe, the belly was ripped out and there was also some damage to the wings and tailplane. I put it all to one side but with the current C19 situation I decided to get to work on the repairs. I decided that I was going to re-engine it with a DLE35 RA which was going to be a challenge bearing in mind the fuse was not designed for a canister type exhaust.

This as it before the 'landing'

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And here are some photos showing the damage to the fuse

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Note that I've already started on the work to house the can by cutting out a hole in the bulkhead to allow the exhaust through.

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Now the challenge is not just to get the can in place but also to make sure that the U/C will fit, bearing in mind that it will have to be lowered to allow the can through underneath.

 

Edited By Ron Gray on 28/03/2020 22:50:45

john stones 128/03/2020 23:08:39
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11456 forum posts
1516 photos

Interested Ron, you gonna build the can in and repair around it ?

Ron Gray28/03/2020 23:10:33
1931 forum posts
803 photos

Hi John, the plan is to build a tunnel for the can so that it can be removed!

john stones 128/03/2020 23:12:32
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11456 forum posts
1516 photos

I'll be watching then, I've a Sukhoi could do with a mod.

Ron Gray29/03/2020 09:28:51
1931 forum posts
803 photos

The first part of the re-build was to build new formers / part formers as a lot of them had been ripped out in the crash (see photos above). A couple of months ago I bought a 3D printer so I decided to use my newly acquired skills to design replacement formers but where strength and heat were involved I would stick with traditional ply ones.

This is the basic shape that most of them followed, the legs allowing me to graft them onto the remains of the original ones.

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And the various different sized ones stuck in place.

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Ply formers used where the can is going to be located with cutouts for the tunnel.

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And the tunnel formed with thin balsa which had been wetted and bent around the can to give the basic curve.

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Thin balsa extended up the sides of the tunnel and longeron supports added plus end cap.

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I then glassed the tunnel with 2 layers of glass fibre cloth.

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And trimmed back.

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And then the tunnel was lined.

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Edited By Ron Gray on 29/03/2020 09:34:11

Steve J29/03/2020 10:23:53
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1841 forum posts
53 photos

It looks like the can is going to be in contact with the walls of the tunnel. I would suggest having a 5-10mm air gap.

Ron Gray29/03/2020 10:50:12
1931 forum posts
803 photos

The can is shown unsupported, when it has its rear support in place it will raise it to give an air gap although it will be quite close at the front of the tunnel. I'm not too concerned about it as the tunnel will be open to the air.

Ron Gray29/03/2020 11:15:17
1931 forum posts
803 photos

Rear support bracket fabricated and fitted, hopefully you can see the air gap between the can and tunnel.

fullsizeoutput_138f.jpeg

Ron Gray29/03/2020 14:28:08
1931 forum posts
803 photos

A bit more progress, this part of the re-build is very slow as I am really making it up as I go along plus replacing missing parts as I do. I'm also trying to build in additional strength where it is needed.

Talking of which, I added some 'load spreaders' to the rear of the U/C former:

fullsizeoutput_139a.jpeg

Some additional strengthening for the U/C bearer plate

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Which form a nice 'chamber' around the exhaust pipe (more on that later, but keep in mind that I do have a 3D printer!!wink 2)

Strengthening2_1398.jpeg

There is a former here which supports the outside fuse sheeting, you can just see it curving away in the background.

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Its opposite number was torn out in the crash so needs to be replaced.

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A new piece was cut out using the scroll saw (I haven't gone completely over to 3D printing!)

Scroll_saw_1399.jpeg

And stuck in place

New_former_139b.jpeg

Using the offcut as a clamping 'tool'.

New_former_clamp_139c.jpeg

Next up will be the sheeting in of the front section and side sheeting around the U/C. Unfortunately I have discovered that I'm out of 5mm sq strip so have ordered a load plus other bits from SLEC. C19 is a bit of a nuisance in that regard as SLEC are only about 15 mins drive from me so normally I would have gone and picked it up. Hey ho, lots of other things to do whilst I await its delivery, the wings need a careful inspection, I know one took quite a hit along its root, so I'll peel back the covering to see what we have to deal with!

john stones 129/03/2020 15:47:05
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11456 forum posts
1516 photos

Ah leaving it out in the air, hadn't realised. yes

Not much wood in a wing these days, as long as it's still true it'll be straight forwards ish.

Ron Gray29/03/2020 17:36:05
1931 forum posts
803 photos

I just found some more sq strip in a bundle of balsa etc that is was earmarked for another build, so on with the fuse!

Peter Jenkins29/03/2020 21:17:15
1526 forum posts
254 photos

Well done Ron. Repairing these lightly built models is not easy so using a 3D printer to make some of the formers is an interesting idea.

Which make of can are you using? My 35RA with a JMB can only failed the 82 db test on a windy day with the wind blowing into the back of the prop - an unrealistic scenario in the real world!

Ron Gray29/03/2020 22:11:16
1931 forum posts
803 photos

Thanks Peter, it certainly is a challenge, this Sbach was certainly built light!

Regarding the can, 'unfortunately it's the DLE one, I say unfortunately because I would have preferred a Zim one but I already had this one sitting, unused, in the workshop.

Ron Gray31/03/2020 20:24:49
1931 forum posts
803 photos

Continuing on with the repairs I finished off the fitting of the longerons and then decided to sheet the whole of the repaired section of the fuse.

fullsizeoutput_139f.jpeg

Next up the covering

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Now in a post above I mentioned that the 2 ply U/C formers made a nice chamber around the exhaust so I am utilising this as a vent chamber and have designed and printed an air scoop for it which will also vent the can tunnel.

fullsizeoutput_139e.jpeg

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Now time to re-fit the can

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And to cut down the exhaust deflector

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I'm quite please with the end result. Now i've got a couple of bits to fix on the inside of the fuse then it's onto the tailplane and main plane. Just noticed tonight that one half of the tailplane has been knocked back and looks like the root rib maybe damaged. Good job it's a bolt on, that will be the next job after the fuse.

Onwards and upwards!

john stones 131/03/2020 20:42:33
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11456 forum posts
1516 photos

Nice work Ron. yes

Ron Gray31/03/2020 21:14:58
1931 forum posts
803 photos

Thanks John, I’m hoping that the repairs to the tail and mainplane won’t be as fiddly as the fuse!

Gary Manuel31/03/2020 21:29:36
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2255 forum posts
1548 photos

Hi Ron. Nice repair work. It's a job I hate doing, so good on you for making such a nice job and improving the model at the same time.

I have a question though if you don't mind. What's the thinking behind your 3D printed air scoop? The way my mind works, it looks like it will push air into the fuselage, creating a high pressure area, preventing hot air from escaping - and possibly opposing the natural flow from cowl to tunnel. Wouldn't it work better the other way round, sucking hot air out?

Ron Gray31/03/2020 22:10:13
1931 forum posts
803 photos

Hi Gary

The problem with the way the 'plane is designed is that there is no air flow from cowl to tunnel, after all it wasn't designed for a canister type exhaust. Also there is no direct route for air to get into the fuse as the can tunnel is sealed off from the rest of the fuse apart from a small slot. So the logic is that I do want to force air into the tunnel, I don't see that being a problem as the can tunnel is completely open to the air but in forcing air into it, some will go through the small slot into the fuse directly onto the fuel tank which sits above the can tunnel. I will fit an extract hole in the rear of the fuse to allow a flow of air.

To be honest it's all guesswork and I haven't a clue if it will work or for that matter make any difference, after all my GP Giant Revolver and SU26 both run with DLE 55s exhausting through cans and neither have had any heat problems and they haven't got air being forced through them!

Gary Manuel31/03/2020 22:43:00
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2255 forum posts
1548 photos

Yep, guesswork on my part too. It just looks to be the wrong way round to my eye. At least you have several options to try for best results - both ways round, blank plate or left open smiley

Peter Jenkins31/03/2020 23:46:21
1526 forum posts
254 photos

Good job Ron!

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