Step 13.9, Cowling

The cowling is the fiberglass enclosure around the engine.  It is analogous to the front quarter panels and hood of an automobile.  Theoretically, this cowling was made using molds taken from a perfectly fitting cowl from yesteryear.  If this cowl ever fit anything, it would be surprising indeed.  More on the fitment later.

The cowling is split laterally and each piece is supposed to be over sized such that careful trimming would result in one's own perfectly fitting cowl.  The first difficulty, though, is that no exact reference point exists in any dimension that would allow one to know where to trim.  However, I do know three things: first, that the back of the spinner must be about a 1/4" from the front face of the cowl. Second, that the opening for the prop shaft and the front is supposed to be 13" in diameter, and third, that the cowling is longitudinally symmetrical.

Since I didn't have the prop installed when I was fitting the cowl, my first datum, the back of the spinner, didn't exist.  No problem, I'll just mount the spinner back plate without the prop.  Now how thick is the prop hub.  Hmm, this could be trouble.

Here is the beginning of the fitment problems.  The upper and lower cowl halfs do not meet along the horizontal parting line.

No problemo, I'll just add more "side"to the lower front cowl.

Doesn't fit along the side either.  More cowl, please!

The upper front guides are glassed in.  These smooth the transition into the plenum.

A bit of spot putty fixes most of the unevenness.

I made these clips to hold the heater hose away from the exhaust pipes.

Now to cut the openings in the lower cowl for cowl flaps.  These openings allow additional air to flow, providing better cooling under high load or elevated ambient temperatures.

Now on to more sanding.  The reddish power is spot putty being sanded off.  The spot putty is primarily being used to fill the trillions of pin holes in the fiber glass cowl.

With the pin holes filled its time to apply the primer.

As it turns out, not all of the pin holes got filled.  I still have 150 billion to go.  I'm going to need more spot putty.

After one more round of primer, the cowl is finally ready for paint.

The next step is to install and test the cowl flaps.

Getting near the end now.  Adding the heat reflective foil to the inside of the cowl.  Hopefully, this will prevent the paint from bubbling on the outside.

Shush!  Sleeping puppy.

I'm not real happy with the final fitment of the cowling.  It seems that the fit is now different since the cowl has been painted.  Since that doesn't make a lot of sense I think I'll blame it on tolerance build up with the quarter turn fasteners.  Either way, the cowl doesn't fit as well now as it did before when it was just attached with clecos.  It's not obvious in the pictures, but there are some areas that will require additional work.

Also, the spinner is too close to the cowling so I will need to address that before first engine start. There are a lot of details to wrap up as I inch ever closer to finishing.  The old adage that 90% of the work occurs in the last 10% of the project applies here.

It seems that I am testing the Greek philosopher Xeno's paradox who noted some 2500 years ago that before one can finish his airplane he must first finish half of his airplane,  And before he can do that he must finish a quarter of his airplane -- and so on, in an infinite number of ever smaller steps that seem to prove that it is impossible to finish an airplane.  

Xeno was clearly ahead of his time!