Marty d's CH-701 build log

[Marty_d]

Thanks Bob, I talked to Blueadventures and Nomadpete, and they both agree with the gel coat too.

[Marty_d]

Been a while!!  I finally bit the bullet and did a test piece from gel coat + 2x layers of carbon fibre + peel ply, using polyester resin.

 

I bought an oven tin so I didn't destroy any of my wife's existing ones, did the gel coat first with a paintbrush (might use roller for bigger pieces) then let it cure for a week or 2 before doing the rest.  

I used 6 x coats of wax, buffed back each time, then a spray release just before the gel coat.  It worked a treat - popped out of the mould perfectly.

 

While I'm very happy with the mould release and the gel coat, I put in way too much resin and the part is heavier than it should be.  In one photo of the interior you can see one perfect bit where the carbon fibre is showing a shiny coat of resin, the rest is all too thick.  I will be far more sparing in application on the real thing.

 

 

[Marty_d]

First attempt at the top cowl.

 

I did 6 applications of wax, then a gel coat.  Unfortunately I left it too long before doing the next step and it fully cured.  I did soften it with acetone before applying 3 layers of carbon fibre and poly, but I had a bloody hard time releasing it from the mould and the gel has come away in places.

 

Regardless, I think I can use the piece still.  I'm going to but in another couple of carbon strips inside to stiffen it a little, and some kevlar and fibreglass strip along the edge where it'll join to the lower cowl.  I'll do some bog and sanding and see how it comes up.

 

Weights:  

Gel coat:302g

Carbon: 572g

Poly resin: 338g

 

Total weight (before trimming, bogging and stiffening):  1212g

 

 

[Marty_d]

It worked!!!!!

 

I just popped the lower cowl out of the mould.  It came straight out, no sticking, no lumps and bumps and divots.  (Well maybe a couple of minor divots!)

 

Decided not to use gel coat this time, just 4 layers of carbon fibre and equivalent weight in polyester.  The difference - thanks to some excellent advice from several people, especially @nomadpete - was using PVA sprayed and brushed into the mould first, and a bead of silicone around all the opening holes and the edges.   Yesterday I used a small foam roller to put on a coat of poly, followed by carbon, followed by poly, followed by... you get the picture!

 

I am also extremely grateful to the forum member (I'm not sure if he wants to be named) who donated more than enough carbon fibre to do this cowling.

 

Sorry about the dodgy photo, obviously taken in the dark.  The piece has daggy bits of PVA hanging off it but that peels off easily.  Also looks like an evil halloween pumpkin from the front, especially with those strands of CF in the "mouth" - but hey, it worked, and I'm stoked - and relieved!

 

Will take more pics when I've trimmed everything and test fitting over the engine.

 

 

[planedriver]

See Mate!, we told you you could do it.

We had every faith in you.

Just get it all finished before it's time to push the daises out of the ground.

You are an inspiration to others.

Rgds Planey.

 

[Marty_d]

Hey folks,

 

As promised - some photos.  I took off the muffler today and started test fitting, need to work out where the exhaust is going to exit the cowl and cut accordingly.

 

Haven't got the prop yet but bought a spinner that bolts on the front of it, temporarily bolted that on with some BA (Bunnings Aerospace) bolts to get some idea of what it'll look like.

 

By the way - thinking about how the top and bottom pieces will go together - planning to use aluminium piano hinge on the sides and maybe Dzus round the firewall connection.  Any advice (and pictures) of the best way to do this?  Overlap? Rebated piece under on one surface?  Butt join?  Thanks!

 

 

Edited June 9, 2021 by Marty_d

[IBob]

That's looking great!

 

My experience of cowls is entirely limited to what came in my kit, but I'll pitch that in:

 

The cowls (in the Savannah) generally go inside the al fuselage skins (though some do fit them outside). Apparently this used to be a pretty tight fit, but it does not seem to be an issue with the later models. The hugely experienced pilot who test flew my aircraft grabbed it by the holes at the front and tried to move it up and down: I think he was checking against the possibility of the cowls coming off in flight. I now give them a wiggle as part of my preflight, and I like the idea that they are tucked inside the skin, though no doubt there are perfectly safe ways to fasten them on the outside.

 

The upper cowl overlaps the lower, which is stepped or rebated in.

 

The fasteners are Dzus. 3 on each side for the lower cowl, of which the top one is also shared by the upper cowl. Then a further 3 along the join between the cowls at the side, plus a further 1  (per side) close to the spinner. The ones supplied were aluminium, but they bind so I bought steel replacements.

 

There is also a hinged hatch over the oil tank. Assuming you intend burping and checking the oil level of the 912 during preflight, some sort of hatch will save a lot of messing around.

[nomadpete]

Hey Marty.

Looks like you've got an aeroplane in your shed!

[Marty_d]

Thanks Bob, Peter and Planey.

 

Bob, that's interesting that the Sav has the cowl inside the fuse ally.  Everything on the 701 is built so the overlaps are downwind, ie the bit that's towards the front always goes over the bit behind it.  I always assumed that cowls would be like that too.  

The installation instructions for the factory cowl (attached) have it overlapping the fuse and screwed to it, with the top overlapping the bottom and screwed together.  I especially like the bit on page 8 that says "Note: on our demo, we push up the bottom cowl to fit around the fuselage and ignored the beveled flange along the top of the bottom cowl."  Maybe I should do that - not worry about trying to bevel the joint, the top cowl is only 3 layers of CF thick anyway so it wouldn't hurt it to sit over the bottom cowl.

Actually it's interesting that they just use countersunk flat head screws into flush riveted nutplates on both the fuselage and joining the cowls.  Maybe I should do that - sounds easier than trying to run a hinge along it!

 

Yes access to the oil tank - definitely will have a door there.  

r912s-ring-6.pdf

[nomadpete]

On my lightwing, cowls overlap the fuselage. It only had two clips on the top section. I'd use dzus wingnuts. They should be easy to visually check, easy to release/refasten without tools, and difficult to drop in long grass.

The lower cowl was fastened to fuselage by three csk screws on each side, with csk washers and captive nuts.

This setup adequately kept the cowlings in place.

[IBob]

13 hours ago, Marty_d said:

Thanks Bob, Peter and Planey.

 

Bob, that's interesting that the Sav has the cowl inside the fuse ally.  Everything on the 701 is built so the overlaps are downwind, ie the bit that's towards the front always goes over the bit behind it.  I always assumed that cowls would be like that too.  

The installation instructions for the factory cowl (attached) have it overlapping the fuse and screwed to it, with the top overlapping the bottom and screwed together.  I especially like the bit on page 8 that says "Note: on our demo, we push up the bottom cowl to fit around the fuselage and ignored the beveled flange along the top of the bottom cowl."  Maybe I should do that - not worry about trying to bevel the joint, the top cowl is only 3 layers of CF thick anyway so it wouldn't hurt it to sit over the bottom cowl.

Actually it's interesting that they just use countersunk flat head screws into flush riveted nutplates on both the fuselage and joining the cowls.  Maybe I should do that - sounds easier than trying to run a hinge along it!

 

Yes access to the oil tank - definitely will have a door there.  

r912s-ring-6.pdf 521.22 kB · 3 downloads

Marty, I have an idea that the cowl inside/outside question is (another) one of those regional things (and eventually it will be possible to tell where a NZ  Savannah was born by these local variations).

What a great set of instructions. The CS screw setup looks good, and I'm pretty sure that is how c185 cowls are fastened.

About all I would add is that I had my cowls off quite regularly in the early days, checking for chafing and leaks etc. And I still take the top one off every few weeks. So I would want to be able to do that quickly and easily, and the Dzus fasteners do that: it would be about a minute to remove, and 2 minutes to put back on.

[Marty_d]

Yes it's starting to look like DZUS are the way to go. Someone gave me a link to an Australian supplier so will calculate how many I need and put in an order. 

[IBob]

Marty, they come in various lengths, you'll have to work out which you want.

Ideally, you'd look at an aircraft that has them, and use that as a basis for your length measurement.

I also spent some time tweaking the S springs they get hold of, in some cases bending or packing those out a bit, to arrive at the sort of action I wanted. But my cowls are fibreglass with almost certainly variations in thickness.

[Marty_d]

Thanks Bob. Being a scratch built cowl, there's no guarantee that other planes would have the same thickness. 

I'll measure as best I can to select the right length. 

Great advice - keep it coming!

[IBob]

FWIW the ones I got that go through just 2 layers of f/glass

were:

Model                           Cat #                  

121J-540-Z3C             AJ5-40 

You probably need something just a bit shorter.   

There are a few guides online as to how to measure up for them. 

I found it quite a pleasant job adjusting them  by packing or flattening the S spring a bit, though it's best done before paint.

I guess the advantage of the screw and nut plate is that you don't have to adjust for thickness....          

[IBob]

PS if you go with the Dzus, get a spare: as someone already mentioned here, they're easy to drop and lose in grass. I've done this a couple of times, been fortunate to find them, nowadays I'm very focused when taking them out and they go straight onto the cockpit floor just forward of the seat/s where they can't wander..)

[Marty_d]

That's the thing isn't it.  I can imagine that the cowl is on and off like a bride's nightie while the plane is under construction, FWF is being sorted out and everything adjusted.  But after you start flying, how often do you remove it? 

[Marty_d]

7 hours ago, IBob said:

PS if you go with the Dzus, get a spare: as someone already mentioned here, they're easy to drop and lose in grass. I've done this a couple of times, been fortunate to find them, nowadays I'm very focused when taking them out and they go straight onto the cockpit floor just forward of the seat/s where they can't wander..)

Bob, I've just been watching some youtube stuff about dzus.  Apparently some have a reduced section on the shank so you can slip a circlip over them which goes between the parts to be held.  This would keep the dzus attached to the outer part when they separated.  Like below: these ones from Bullant seem to have a nylon washer (1.3mm thick), not sure how it goes over the large part to be held on but it'd certainly stop accidental droppings.

[nomadpete]

Marty, I'd be taking off the bonnet every time I prefight the aircraft.

[IBob]

Marty, that's good info. And I would think the washers are just a tight fit over the lower part?

My fasteners have that undercut too. However on the bottom fastener for the lower cowl my test pilot filled the undercut with a piece of tube shrunk on, as the small amount of slop there was allowing too much movement at the pointy end for his liking.

Some of my fasteners do have to come all the way out, but I'll look at getting some washers.

 

And I agree with Nomadpete, certainly initially as everything beds in: you will be looking for oil and coolant levels and leaks, exhaust leaks, exhaust spring breakages and any chafing of pipework or wiring due to vibration or contact with the cowls. Plus you will be visually checking throttle synchronisation at max and min, with some initial adjustments of that until it settles.

 

 

[IBob]

(PS before setting up throttle cables, set up carb throttle stops, also idle screw, as per the Rotax manual. It's quick and simple, just so many turns, but I found mine were set up very rough from the factory.)

[Marty_d]

2 hours ago, IBob said:

(PS before setting up throttle cables, set up carb throttle stops, also idle screw, as per the Rotax manual. It's quick and simple, just so many turns, but I found mine were set up very rough from the factory.)

My engine came out of a well maintained training aircraft so I might leave the adjustments as they are initially!

[Marty_d]

Quick update - I finished trimming the lower cowl today (apart from where the exhaust will exit), located it properly and did a 1/8" hole on each side so I could cleco it.  

 

Unfortunately I'm not going to be able to use the top cowl - it got a couple of cracks when I was trying to lever it out of the mould, and when I try to press it down in position it creases along the crack lines.  I think I have enough pieces of carbon left to do another one, and this time I'll use PVA - I'm a convert Peter!

 

I did some weighing today - the trimmed lower cowl is just on 2kg, the top (which I can't use) is just over 1kg.  This is fairly light compared to fibreglass - there's a top cowl the guy who lent me the mould gave me made out of chopped strand and poly, that weighs over 2kg.

 

[Marty_d]

Laid up another top cowl today (hardly the weather for it, but I knocked up a cover with a lava lamp bulb in the top to keep it warm).

Had enough scraps of carbon left over to do 4 layers.

 

Hoping for a successful release in the next couple of days!

 

 

Edited July 4, 2021 by Marty_d
spelling

[planedriver]

Sounds great! keep at it mate.