KR2S Engine Cowl work 08

First fitting ... I'm really pleased with the lines

Oil access door ended up exactly where I wanted it.

Mmmmmm....

Compare this ...

To how it started. The two spinners are the same article.

KR2S Engine Cowl work 07

Easter long weekend, great excuse to do a little more work. Last post was the upper deck of the engine cowl, this time I tackle the lower half.

This first shot shows the upper deck repositioned in the mould with double sided tape. The edges were taped to ensure the joggle would release, I will be laying 1.5" over the top deck edge. I found this stretchy duct tape shrank away from the corners so I ended up replacing it with masking tape.

I decided to leave the 3rd mould part off to begin with to make it easier to reach the nose area. This proved to be a good decision and ensured a good layup in this crucial area. Note I have covered with peel-ply to assist with bonding in the next process.

All the materials ready to lay the outer skin ... fine layer of glass to start followed by a single layer of 200gsm CF (45' biased), peel ply, bleeder (vac bag not shown).
Ready to lay up the outer skin.
Outer skin done (sorry, no shots of the vacuum process this time). Note the joggle being formed in place.
Same as last time, marking up for the 3mm foam core-mat. The circle is where I will make a cutout for the carby ram air scoop and will be hardened with CF only (4 layers). The tombstone shape will be a cutout for the nose gear leg.
This foam is quite dense and very rigid but can be moulded slightly by applying heat, as seen here.
Here the foam bonded in with a micro slurry. I did this the same way as last time, weighed down with damp sand over a plastic sheet. Also as before I have left 3" around the edges to harden them in preparation for the fixings. These will have a total of 4 layers of 200gsm when finished.
The end result ... gorgeous ... weighing in at 2.9kg for a total so far of 4.7kg. With a little more trimming to do and the addition of the ram air scoop I'm right on my 5kg target. Note the slight damage demoulding it. Actually there were 2 scares getting it out of the mould, the second was that, with the small round air inlets, it didn't want to let go of the nose mould. Remembered a trick a wise old gent told me and let it soak in water for a few hours ... little more persuasion and out she popped.
This shot shows how well the joggle is formed and those pesky round inlets.
The joggle trimmed up, I will be using a piano hinge along the sides and just one screw each between the inlets and the spinner. This CF sandwich method sure is stiff when finished.

KR2S Engine Cowl work 06

Yes it's been a long time between posts, life seems to have a habit of getting in the way.
This next series of shots is the start of laying up the actual engine cowl from the mould I made earlier. You will remember I made extensive modifications to a donor cowl because I didn't like the lines, inlets and general fit. I then took a 3 part mould off this plug and am now making the final part(s).

I have decided (based on the advice of another builder) to make the cowl in 3 parts. First the oil access door, then the upper deck with a built in joggle for the oil door and last the lower cowl part, once again with built in joggle for the upper deck. Goal weight 5kg (11 pounds) For a comparison see Mark Langford's @ 3.7lbs http://www.n56ml.com/cowling.html. I think even Mark would agree this is a little too light as it gives when you sand it.

Here you can see the results of vacuum bagging the oil door. I took measurements from the oil filler neck on the engine by extending the line of the neck with a broom handle and measuring where it would meet the surface of the cowl. This became the centre of my door.

Door trimmed to size and then covered with several layers of tape to allow for the thickness of paint later on.

First attempt at vacuum bagging the top deck. I failed to get a proper vacuum on the bag as I slopped a little resin on the tacky-tape in one corner. You have to be impeccably clean when you do this. I could have salvaged it but it would have taken a little more filler and was already resin rich. A good learning experience ... isn't that what it's all about.

One of the other things I didn't like about the first attempt is that I filleted around the door edge with flox which subsequently cracked when I lifted the deck out of the mould. This and the sharp corner at the prop flange would take a little more care next time.

What I did, around the door and the prop flange, is to first lay some rovings tight into the corner. I then built the surrounding material up close to the height of the door with some scrap.

I then laid one complete layer of 45' bi-axial carbon (stitched not woven like cloth)
This time (nervous I guess) I didn't attempt to vacuum it, rather spending the extra time ensuring the right bond and resin ration by hand. Didn't come out as well as you would have expected from Vacuum but still acceptable.

In this shot I have given the peel-ply surface a light sanding and marked it up for the core-mat.

I used 3mm high density foam as my core-mat as it does not soak up resin and remains light. As you can see, very stiff right out of the box. The agent said it could be moulded with a heat gun.

Well he was right, it does mould nicely ...

And holds it's shape once cool.

I bonded the foam in with a light micro slurry and weighted the whole thing down with wet sand. This worked remarkably well.

The final (inner) layer was built up as follows:

- Around the edges 2 strips of 45' CF

- Through the centre 2 strips of CF uni

- Then one more layer of the 45' bi-axial CF over the lot.

This time I successfully vacuum bagged this stage and the result was fantastic

As you can see I have 5 layers of CF around the edges and through the centre and 2 layers with 3mm foam everywhere else. The result is a very stiff assembly that only weighs 1.8kg (under 4lb). I'm happy with that.

Oh and the door looks real flash too.

KR2S O-200 Second Engine Run

No more sitting in the plane pretending and making vroooom noises.

I have real vroom noises.

KR2S Prop Clearance

Here is my Propeller clearance check ... this shot with the NLG tyre deflated and showing a healthy 7" clearance.


FAR Part23 section 23.925 states that “There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) … between each propeller and the ground with landing gear statically deflected and in the level takeoff, or taxiing attitude, whichever is most critical.” I have 9.5" so I think I'm sweet.

FAR 23 goes on to say “In addition, there must be positive clearance between the propeller and the ground when in the level takeoff attitude with the critical [tire(s)] completely deflated and the corresponding landing gear strut bottomed.” Positive clearance ... I still have 7" with the NLG tyre completely deflated so I'm sure this passes.

KR2S Engine Cowl work 05

Well it's been a lot of work but I finally pulled the mould off the plug I made. This is where I left off last time, the plug almost finished in primer.


Next I added a few coats of gloss to act as the base for waxing. This really shows the imperfections. I wet sanded this down to about 400 grit then polished with auto cutting compound then several coats of mould release.

Here I am setting up the flanges for the top 1/3 of my 3 part mould. I'm making it 3 part for ease of handling but the final cowl will be only 2 parts. As suggested by a fellow KR builder I will build the oil door first in the middle of the top 1/3 of the mould. Once set I'll trim it to size then hot glue back into the mould. Attach the nose 1/3 to the top 1/3 then I'll then build the upper cowl with a joggle over the oil door. Once again I'll remove the top cowl and trim it to size then return it to the mould, now adding the final 1/3 and then lay up the lower cowl with a joggle over the edge of the top cowl. A lot of up front work but it will save time down the road.

Notice I have used builders plastic to protect the forward deck, this will bite me later.

Here I have added non hardening hobby clay to the gaps in my formwork and trimmed it to form a perfect edge to the plug. The white dots are domed plastic hemispheres that have nails in them. They are used in the legs of furniture to protect them and make ideal locator sockets in the first mould flange. Once removed a corresponding dome is formed in the mating surface of the second mould flange when it is laid up.

Tooling gel-coat added, this one was spray on.

After the gel set up I filleted the acute angles with micro then 3 layers of CSM. Notice the polyester has wrinkled the builders plastic. lucky this is only in the flange area added for bagging and will not affect the final part.

Here I have added formwork for the lower flange. Again I have used the domed plastic hemispheres to make locators in the mould half.

I did not take any photos of laying up the lower 1/3 but trust me it was a real b!tc#. Wet layup upside down is no fun at all, resin drips everywhere, gets on your skin and runs down your wrists. I had those plasticated overalls on and the resin still soaked through them and burned my back. I found a neat 3" mini paint roller that was fantastic for wetting out the glass. The hairy ones are only a couple of bucks each and will wash out in acetone without falling apart.

First 1/3 removed, cleaned and trimmed. Notice the neat indents from those dome things.

Top and bottom thirds all finished.

The nose 1/3 was a simple layup but because of it's complex curved it was difficult to remove it from the plug. So much so that I had to partially destroy the plug to get them apart (hope the moulds are good ... no going back).

All 3 parts keyed together with neat little domes and corresponding sockets.

Now all I gotta do is pull the final cowl, yeah?????

KR2S Engine Cowl work 04

Back on the cowl again. If you have been following this blog this is where I left off last time. I was given a basic O-200 cowl and took to it with a saw as I thought it was draggy and ugly (and it didn't fit my mount length either). I'd narrowed the inlets from over 5" to 3" and tapered the spinner face from 13" to 10". All in all it was starting to take on a shape I was pleased with. BUT ... I am far from taking a mould off it yet.

Here I am checking the symmetry of the sides just behind the inlets. Made a template out of MDF then flipped it over to check the other side.

Close but just a little off. I made indicator blocks out of MDF ...

Then glued them in place to show me where to fill.

Started filling (I'm going to soon get sick of that word ... filling)

Some time has passed and I have the new profile on the underside of the belly (other post).

This is the first time I have offered the modified cowl up to the fuse. I have been relying on a template on an engine stand. The shape of the cowl bottom does not quite line up with the belly so some more adjusting is needed.

Experimenting with potters clay. This produced a remarkably good profile quickly but had the disadvantage of drying out. Later on I found I had to cut this out in strips and back fill with a polyester filler.

Resulting profile taking shape.

And the finished clay model. Note the plastic to keep it moist.

I've now finished with the belly mods and have flipped the KR back onto her wheels ... well the 2 mains anyway. I left the nose leg off so I could finish the cowl plug without it getting in the way. Trial fit the prop to check the ground clearance. I have 3.5" here but she is sitting in a nose down attitude. Hope to get 6 to 7" clearance when finished (even if it means trimming the tips again).

Another view of the plug taking shape.

Strapped an RV-6 Ram air inlet to see how it looks. later I will fit this but will be tapering the training edge flush with the belly.

Some final shaping of the bottom of the cowl. Even with the 2.5" lower belly the oil tank on the O-200 protrudes a little. Eventually this will be covered by the ram air scoop but I still need a reasonably good shape to pull the mould off. You never know, somebody else may want a cowl out of the mould and choose not to add the Ram air.

Sanding and filling and sanding and filling and sanding and filling and sanding and filling and sanding and filling. Gawd it makes an awful mess. See how the dust is picked up in the flash.

WOW! now I'm getting closer. Put a coat of fast dry primer on it so I can see all the flaws invisible when you have the mottled appearance of many layers of fill and old paint.

Certainly starting to look the business, still some minor fill jobs to make it perfect.

And just to remind you why I am doing all of this, here is what I started with.