Archive for the ‘Cowl’ Category

Oil door

Sunday, June 10th, 2012

One of the many places on the airplane where builders are given the opportunity to customize is the little door on the engine cowl that lets you stick your hand in and check the oil level. The standard RV oil door is a piece of fiberglass with a piano hinge and a couple of quarter-turn fasteners – nothing wrong with that, but I wanted something a little different. I had the fiberglass door on my last airplane, and it had a tendency to flex during high-speed flight, and would even pop open unexpectedly on occasion. The solution, of course, is to either add stiffness to the fiberglass door, or build the door out of something stronger. I had some 0.063" aluminum laying around, so I decided to make the oil door for this airplane out of metal instead of glass. I also decided that I wanted a different hinge and latching mechanism, for purely personal reasons.

I started by squaring up the edges of the cowl cutout and making the corners nice and round:

Then lots of iterative fitting, trimming, and bending to get the aluminum door to fit the opening. The cowl has a gentle conic section at this point, a shape which the aluminum doesn't naturally want to follow without some smashing persuasion.

Here's the aluminum door, shaped and trimmed, sitting next to the stock fiberglass door that came with the kit:

Instead of the standard piano hinge, I bought a "hidden hinge" from Nonstop Aviation (whose name always makes me think of an old Brian Aldiss novel, which by the way was not great). Lots of folks install this type of hinge, including plenty of certified airplane factories. It's just a piece of regular hinge with a sort of gooseneck extension riveted on, thus allowing the pivot axis to be tucked cleanly out of sight underneath the cowl – I could probably make my own if I had a decent bending brake. It works the same as a piece of piano hinge, of course, but it looks a little nicer. It's spring-loaded too, although I left the spring out while I was fitting the oil door so it wouldn't launch itself across the garage.

Here's what it looks like with the door closed – no visible hinge line:

It takes some adjusting to get the door to open properly without binding, and without the inside edge crashing into the cowl and scratching the paint.

The hinge is flat and the inside of the cowl is curved, so I made it flat by laying up an epoxy/flox mixture and letting it cure with the hinge clecoed in place:

I wanted to use a push-button latch instead of the quarter-turn fasteners called for by the plans, so the oil door can be opened without tools and without anything sticking out into the breeze. There are basically two choices here: the Hartwell H5000, which holds very securely but is hard to make look nice, and the Camloc KM610, which is easier to install but trickier to make latch properly. The tie-breaker for me is that the Hartwell latch when operated tends to spring open like a demented mousetrap, and I've bruised my knuckles on them too many times to want to go to the trouble of having one on my airplane.

Also, pay no attention to the prices in the preceding links – those are new-certified prices, which are ridiculous. There's a surplus place in the neighborhood that sells new-condition Hartwells for ten or fifteen bucks and Camloc pushbuttons for five, so I picked up a handful of the latter in various sizes to play with:

Since they were so cheap, I removed the spring from one and drilled a hole in the exact center of the button, thus turning it into a drill jig for properly locating the mounting holes relative to the main hole:

It was then no sweat to drill the holes to mount it, although you do have to be super careful about where you position it relative to the edge of the cowl cutout. Too far forward, and it won't latch; too far back, and it will latch easily but it won't hold properly. Patience counts here.

So does test-fitting:

Here's a view from the underside. Note how the latch tongue is pretty short as well as pronouncedly rounded, which is what causes the difficulty.

To get the clecoes out of the way and make sure the whole works was properly rigid, I riveted the latch and hinge to the door:

As pictured, the latch tongue bears on an un-reinforced fiberglass surface, which of course won't do at all. To fix this I fabbed a little striker plate out of some thin stainless steel material I bought from the K&S display down at the local hardware emporium:

A single flush rivet holds the striker plate to the cowl:

Once I had the metal parts built and the overall mechanism working (which was fun) it was time to make the fiberglass look cosmetically acceptable (which is never fun).

I protected the door with packing tape, then latched it in place and squeegeed a flox/microballoons/cabosil mixture into the gaps between the door and the cowl.

After a couple iterations of sand-fill-sand, the cowl was a good match to the shape and contour of the oil door. But I had a problem – there wasn't enough gap around the door to let it actually open. Hmm, obviously zero-clearance is no good here.

What I wanted was something I could put around the circumference of the door to provide a uniform separation between the metal and fiberglass for a cast-in-place operation. I experimented a bit with some of this nylon grommet edging material I had laying around, but it wasn't quite right. Kind of an interesting idea to keep in mind for the future, though.

In the end I hit upon the idea of using some silicone fusion tape instead. This is close to ideal, since it's fairly thick, resists epoxy, and follows the curved corners without wrinkling. I dremeled out a gap around the cowl opening, then wrapped the door with two layers of silicone and latched it in place.

Then I spackled in a new layer of filler:

The result, after puling the door out and sanding down the high spots, was a set of nice straight edges and round corners. I forgot to take a photo of it, but this technique gave me a fairly uniform 1/16" gap all around the door opening.

As I expected, the clearance was a bit less around the corners where the rubber tape stretched, but I got that cleaned up pretty well with a file.

I attached the hinge to the cowl with stainless screws and tinnerman washers, the better to keep the fasteners from pulling through the fiberglass. The grey splotch here is just a misting of primer I sprayed on as a guide coat to help me sand down the filler.

Inside, I made a little hinge pin retainer out of some angle stock and a leftover hinge eye I found in the scrap pile. It picks up one of the mounting screws and keeps the hinge pin from getting away.

Here you can see how the gap looks – it's a bit deceptive in a photo because the coloring is uneven where I sanded, but in person it looks pretty nice. I'll let the painter fix up the last 10%.

Here's a video I shot showing how the whole thing operates. Dig that solid latching action!

And now I need to clean up the garage before I do anything else. What a complete mess.


Sunday, May 6th, 2012

When I was a young lad, I had to suffer through years of braces and other painful torture devices in to correct an unsightly overbite. In fact, I only narrowly escaped being required to wear headgear to junior high (the horror!). So it was not without a certain sense of deja vu that I set out to correct a similar condition on my airplane cowling. It's a bit hard to see here, but the way my upper and lower cowl halves fit together causes the one to be out of alignment with the other by about an eighth of an inch:

Knowing what I know now, I could probably have corrected this early on when I was initially fitting the cowl halves together. But now that everything is trimmed and drilled, it would be impossible to shift the relative alignment of the upper and lower cowl without affecting everything else, including the parts that actually turned out pretty good. Still, no matter – even though it's a pain to work with, fiberglass can erase many sins. I first obtained some 1/8" thick closed-cell foam, and cut a half circle to fit the cowl. With a sanding block I matched the contours, then epoxied it to the lower cowl. (the notched-out area in this photo was repaired before subsequent steps}

A weighted board covered with wax paper ensured the foam would adhere evenly all around and leave a flat surface:

On top of the foam I laid up three plies of 8-ounce bid cloth. The foam is just a substrate, of course – the epoxy and glass will provide the actual strength.

After several hours of sanding and fitting, I had the excess glass ground away and the shape looking pretty good:

It took a lot of on-and-off fitting and work with the grinder before the top cowl would fit again:

Now the two halves are roughly even, but there's still room for improvement:

I stuck some packing tape to the top cowl as a release agent, and reinstalled both halves on the airplane. I then squeegeed a thin layer of microballoons and cabosil over the new layup, as well as the cowl joint behind the spinner:

After another couple hours of sanding, the surface is getting pretty smooth:

In case you were wondering, sanding fiberglass is dusty and hot and no fun at all. Not even a little bit.

Now we're talking!

Nice and flat all around:

As a bonus, the fit behind the spinner is greatly improved as well. Good enough to give to the painter for the final detail work.

This little mini project was one of those things that isn't strictly necessary to make the airplane fly, but it would have bothered me to leave it undone. I think they call that craftsmanship. I'm just glad the airplane won't have to wear headgear.

Cowl intake ducts 2

Sunday, October 23rd, 2011

Had a bit of a work interruption recently due to a painful injury that knocked me off my game for a couple weeks, but I'm back on top now. So, remember those great-looking cowl intake duct closeouts I made last time? Yeah, about that… it seems that it would have been a good idea to check the fit with the flywheel installed before I got all crazy with the fiberglass. Grinding away a week of work was no fun at all.

I traced the line of the crankcase baffles onto the cowl so I'd know where to build the shape back up:

With the inboard ends of the inlet ramps cut further back, the cowl fits over the engine like it's supposed to:

Here's another problem area I've been putting off dealing with. I had to cut the lower cowl farther forward in order to allow it to clear the engine air filter when it's being installed and removed, and in doing so I lost space for the third attachment screw.

I cut the upper cowl to match and sanded the edge straight. For now I'll just live with only two screws on each side of the spinner here. If it causes problems down the road, I'll come up with a fix. In retrospect, it was a bad idea to follow the plans and drill all three holes early on… I should have drilled only a single hole at the very front, and left the others for after I had the cowl trimmed to fit the baffles.

At least the inlet ducts are fine at the outboard ends. I trimmed the forward cylinder baffles to fit the inlet ramps using the paper clip method.

I built up the new inlet duct closeout shapes I wanted using floral foam. This time made sure to leave clearance around the flywheel, and to allow a more natural transition of the crankcase baffles as they run across the inlet ducts and up to the apex of the cowl.

I laid up three plies of glass over the foam:

Then I followed up with the usual sand-and-fill process. Once again, this looks like hell, but it's fairly smooth to the touch.

Here's a view looking inside the cowl to show just how close the flywheel comes to the cowl and inlet ducts. There's a gap of about 1/2" all the way around, now that I properly shaped the fiberglass to fit.

Now I guess I'm back where I was a month ago, except this time the cowl fits properly. I guess that's progress, of a sort. Oh well – if this was easy, everybody would be doing it.

Cowl intake ducts

Sunday, September 18th, 2011

The baffles have been fitted to the cowl, but the cowl isn't finished yet. I still have to attach the intake ducts to the inside – these will help the air enter the cowling volume and slow down before flowing through the engine.

I put the cowl on the airplane, then reached in through the inlets and traced the position of the baffles onto the inside of the upper cowl:

The lines make it fairly straightforward to locate the ducts on the cowl. I also trimmed back the lips of the cowl inlets slightly in order to make them symmetrical.

I drilled and clecoed the ducts to the cowl to temporarily hold them in place:

Then I glued the ducts in place using epoxy and flox:

I used more flox and microballoons to fair the ducts into the cowl. Even though you can't see this area from outside the airplane, the oncoming air sure can, so I figured a smooth transition would help cut down on cooling drag.

This area was particularly interesting… the cowl is relieved here to clear the #1 cylinder on the bigger 200 hp engine, which prevents the intake duct from fitting properly. I used a heat gun to reshape the corner of the duct, and filler to bring it all together.

The space between the cowl surface and the intake duct forms a tunnel between the high and low pressure parts of the cowl, which is known to cause a loss of cooling air and resulting high cylinder head temperatures. Many folks end up closing off one end or the other in order to force the air to go through the cylinder fins and oil cooler, where it will do some actual work. I figured I might as well go ahead and make this modification now while it's easy.

I started by cutting some endplates out of balsa, and coated the soon to be hidden surfaces with epoxy to make them resistant to oil and water.

Then I glued them in place on the inner face of each intake duct "tunnel".

Once the wood was glued securely, I laid up some glass over the top to provide the real strength and close up the remaining gaps. Hmm, since wood is composed of fibers that are made of carbon, do you suppose I can get credit for this as a "custom carbon fiber part"?

As I went along, I wiped down the inside surface of the cowl with whatever unused epoxy I had left over in each batch. Eventually the entire surface will get an epoxy coat to seal it, so I figured why let perfectly good glue go to waste.

Finally, I squeegeed on a coat of filler to smooth the edges of the glass layup on the endplates, and sanded it all smooth. It looks like hell, but it's smooth to the touch, which is the point. This might be overkill, but when you stick your hand in and feel around for how I did the baffles (which is something all builders do) hopefully you'll say "hey, craftsmanship!" Not to mention, maybe all this effort will make the airplane 0.001 knots faster… every little bit helps.

Next: Trimming the baffles to fit the cowl, again!

Center cowl joint

Sunday, October 3rd, 2010

On the inner faces of the air inlets, the cowl halves are joined together by three screws on either side of the spinner. Here I'm marking and drilling pilot holes using the spacing provided in the plans:

This is an area where you typically see a lot of wear on RV's that have been flying for a while – no surprise, since the screws only have a thin layer of fiberglass to bear on. I decided I wanted some reinforcement here, so I fitted a piece of 0.063" alclad to each side of the upper cowl. This will give the screws something else to wear against, and actually helps in another way by filling some gaps in the area where the top cowl overlaps the bottom cowl flange.

Then I glued each one to the inside of the cowl using an epoxy/flox mixture. After curing, it's now pretty much permanently attached.

After the doubler plates were affixed, I put the cowl onto the airplane and match-drilled the pilot holes into the lower cowl.

Oops! The dimensions in the plans will cause the aft-most screws to interfere with the engine flywheel and ring gear. No problem, I just filled up the holes with flox and redrilled new holes farther forward.

I figured it couldn't hurt to bond some more doublers inside the lower cowl too, so I did that before riveting on the nutplates.

Nutplates and doublers permanently attached to the bottom cowl flanges:

Test-fitting the screws – everything lines up, which is a relief. These screws are only barely threaded into the nutplates, due to all the stack-up of extra material. I'll probably have to use 5/8" screws instead of 1/2", but that's okay.