Alternate air door

May 1st, 2011

The factory provides parts to build an alternate air door, which can be opened in an emergency to provide an auxilliary source of air to keep the engine running if the normal engine air filter is blocked by ice, snow, debris, a bird strike, or something else. I don't particularly like the design, since it's kind of hokey and it is a one-shot deal that requires removal of the cowl to reset it, meaning you can't test it before takeoff. But, after several rounds of making cardboard prototypes I couldn't come up with a better one that would a) fit within the restricted space available, and b) not increase the risk of having the engine ingest metal parts if any part of the alternate air door assembly were to fail (talk about the cure being worse than the disease). Some people have omitted the alternate air door entirely, but I decided it would be a good idea to just install the factory kit as provided. Every certified airplane has to have one, after all, and most of the requirements in Part 23 exist for a reason.

I cut a hole in the side of the air duct in the specified location, and match drilled holes through the steel mounting ring. Per the plans, I angled it 10 degrees clockwise relative to the top of the air duct.

Using hand seamers, I folded the retaining tab at the top of the ring:

Then I crimped the forward end, which provides a stop for the door when it reaches the closed position:

The door itself is a piece of 0.063" alclad that pivots around a screw and is actuated by a pull cable. This is the open position:

And here's the closed position. It took a bit of grinding to get it to close securely and move without interference.

The surface of the air duct is not flat all the way across, so you have to build it up to match the mounting ring. I mixed up a big batch of epoxy and flox and laid down a fillet, trying to as much as possible to squeegee the excess material away from where I didn't want it to go – this stuff is tough as nails and therefore hard to sand.

Here's the end result of the first round of sanding. I wish Van's would modify their mold to have a flat-topped bump on the side for this thing – sure would be a lot easier (and lighter!) to do it that way.

As I did other stuff with fiberglass this weekend, I'd use the leftover epoxy from each work session to mix up some microballoons that I'd apply to the alternate air door here and there, just to make it look less nasty. I know it will be hidden under the cowling, but I would like it to at least be a little bit professional looking.

I installed the pop rivets and polished the heads smooth to keep them from hanging up the door, or pushing it outwards and causing a gap, aka an air leak.

Here's the finished product, minus the cable that will be installed later once I figure out where to put the cockpit control for it. It's pretty ugly right now, but it will look better with a coat of paint.

Posted in Firewall Forward

Tool interlude

April 3rd, 2011

I went to fire up my drill press the other day, and absolutely nothing happened. I figured it must be the switch, which is a really low-quality plastic thing, but tested okay on my ohmmeter. So, I tore the thing apart trying to diagnose the problem.

Long story short, it actually was the switch – it was just making intermittent contact, and when I tested it the first time, it gave the false impression of being okay. But while I had the machine disassembled trying to pinpoint the problem, I managed to crack one of the cheaply-made pot-metal drive pulleys. Argh!

Since the cost for replacement parts was going to be about $50, and the whole drill press cost me less than $250 to begin with, I thought hard about just junking it – after salvaging the motor and chuck, of course – and buying a better-quality drill press to replace it. But after pondering it further, I decided I really want my next drill press to be a floor-stander, and I just don't have the space for one of those right now. So, fifty bucks and a few days later, I had a new switch and pulley in my mailbox.

Here's what the drive mechanism on this model of drill press (Delta DP350) looks like. The pulleys on the motor and spindle are actually split into two halves, and control of the spindle speed is effected by varying the spacing between the pulley halves, thereby changing the effective pulley radius as seen by the belt. It's partially clever and partially hokey. On the one hand, this is the only low-end variable-speed drill press that doesn't require you to stop the machine and change belts to alter the spindle RPM; on the other hand, the low-end speed is too fast to use with a fly cutter, the special belt is expensive to replace, and the speed-change mechanism sometimes gets bound up. By the way, getting the collar and snap ring back onto the motor shaft while compressing that big spring was a fun chore.

All back together and working again. One thing I will say for this drill press, it does have a very good depth stop (which I hardly ever use), the chuck is easy to adjust, and it generally seems to run true. It's just too bad about all the cheap internal parts. Hopefully by the time it breaks again, I'll have more space for a bigger and better model.

In other tool-related news, I bought a lathe! I've wanted to get one for a while, and when I saw that somebody in town was selling a Sherline 4400 on Craigslist, I couldn't resist.

I got a pretty good deal, if I do say so myself. In addition to the basic lathe itself, I got a three-jaw chuck for the headstock, both a 1/4" Jacobs chuck and a 3/16" Albrecht chuck for the tailstock, a dead center, a live center, a lathe dog, a steady rest, a milling vise, and a collection of carbide cutting tools. And probably some other stuff that I don't even know how to recognize yet!

I have scant experience using a lathe, so this will be fun to learn on. I doubt I'll do anything fancy with it, but it should be great for making tubular spacers and the like. However, one thing I didn't like about the lathe when I got it was the fact that with speed knob dialed all the way down to zero, you don't have any indication that the power is switched on. I figured it would be bad to start messing with the chuck while accidentally leaving the lathe in a state where it could unexpectedly start turning, so I took apart the motor control box to see if I could rig up some kind of reminder lamp.

I bought a 120VAC neon bulb from Radio Shack, mounted it in the plastic cover, and connected it downstream from the power switch. I used high strength loctite to secure the mounting nut on the lamp, so it can't work its way loose and fall down onto the speed control board and cause a short.

Voila, an obvious red light now reminds me to shut off the power before adjusting the lathe.

The lathe will get put to work soon to turn out some spacers for the oil cooler mounting bolts. Meanwhile, I have been working quite a bit on reinforcements to the baffles where the oil cooler will be attached, but I still have some work to do before I'm satisfied with the design.

Posted in Misc

The old paper clip trick

March 13th, 2011

Once the baffles are roughly trimmed far enough to generally fit under the top cowl, you then have to trim them further so you get a nice even gap all the way around the engine. I used the "old paper clip trick" to figure out what to trim and what to keep. I didn't invent this method, but here's how it works…

You start by putting a whole bunch of paper clips on top of the baffles. The jumbo size seems to work best.

Then, very carefully put the top cowl on, and push it down onto the paper clips. Here's a view of what's happening inside, looking in from the oil filler door:

This is another view looking aft from the spinner opening:

Then you carefully remove the cowl, and if you're lucky you're left with an impression of the inside face:

Mark a line the desired distance down from the top of the paper clips – without bumping them out of position! – and proceed to trim, file, and deburr.

Then, repeat a dozen more times! Seriously, I did this for hours, tweaking the fit a little more each time. The baffles came off and went back on many many times, which I didn't bother to take pictures of. But the finished product looks something like this:

Not a very interesting picture, I know. So here's a shot of the whole airplane with the cowl on, which is more fun to look at:

Posted in Baffles

Baffle trimming

March 7th, 2011

It's finally time to start fitting the baffles to the top cowl. The upright baffle parts all start life being extra-tall, and then they get trimmed down so they end just short of the top cowl. You don't want too small of a gap between the fiberglass cowl and sheet metal, which could cause the shaking engine to damage the cowl; you also don't want too large a gap because it prevents the rubber air seal material from doing its job.

But, since the cowl isn't transparent, how do you know where to trim without a lot of tedious trial and error? I used a method suggested by someone else, which begins by using wood strips to elevate the top cowl some known distance above its usual position:

After making sure the gap is even at each corner, you then reach through the gap to trace the contour of the top cowl onto the baffles.

It just happened to work out that a popsicle stick is just the right length to give me a 3/8" offset from the inside cowl contour. I notched the end to locate the tip of a sharpie, then used this contraption to draw a line on the baffles inside the cowl.

This is a picture looking in one of the cowl inlets, showing the line I traced. This worked pretty well and wasn't too difficult, other than requiring some painful contortions to get the pen onto every corner of the baffles while working in a limited space.

After removing the top cowl, I retraced the wobbly line so I could see it while cutting:

Then I removed all the baffle parts (quite a chore).

This is the first time in quite a while that the engine has been baffle-less:

The bandsaw made quick work of the initial trim cuts. I stayed outside the line, since there is some error inherent in this process, and I didn't want to cut off too much too quickly.

Everything goes back on for fitting, for the nth time:

After only one cut, it almost fits! Still a little trimming left to do in the aft corners, where I had a hard time making sharpie marks.

Here's a view into the right inlet, with the top cowl in place. I'll have to trim the cylinder baffles down a lot further once I glue the upper inlet ducts in place, but that will come later. For now, I'm glad I haven't attached them yet, since they would just be in the way.

Next: Yet more trimming…

Posted in Baffles

Cylinder 1 baffle

March 5th, 2011

It's relatively easy to remove and replace the engine baffles when they're still individual pieces clecoed together; it's tougher to do it when the various pieces are assembled into big unwieldy shapes. I am trying to design for future maintenance as much as possible, so I am trying to ensure that the baffles can all be removed without drilling out any rivets. In certain places this might require using screws where rivets are specified – we shall see. I know that the finished baffles are all going to have come off at least once before the airplane flies, since there is already at least one airworthiness directive on the cylinders that will need to be addressed.

With that in mind, I tested my ability to remove the baffles attached to the #1 cylinder as a complete unit – cylinder head baffle, inlet ramp, and crankcase baffle all clecoed together. With the flywheel removed, it's just possible to twist and turn it enough to get it off the engine and put it back on. So, all the parts you see here will eventually be riveted together as a single assembly.

I riveted the stiffener angle and the various brackets to the inlet ramp. Some rivets could be squeezed, and some had to be driven. Surprisingly, considering I haven't used the rivet gun in ages, none of them look too bad. Note the use of flush rivets between the three screws that will eventually attach an air dam in front of the #1 cylinder.

Then I riveted the cylinder baffle and corner gusset to the inlet ramp With careful planning, these rivets can all be squeezed if you do it in the right order. You can see where I used some red RTV between the mating parts to fill some gaps that would otherwise be air leaks.

I'll wait to rivet the crankcase baffle until I finish fitting the baffles for good, since not having it permanently attached makes it a lot easier to remove and reinstall the baffles… and unfortunately there's still a lot more of that to come.

Posted in Baffles

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