Chapter 23: Installing the Cowls


In 2002, if you wanted cowls you had three choices: (1) buy pre-fab cowls from Featherlite; (2) buy pre-fab cowls from AeroCad; or (3) make your own.  I didn't want to make my own, and I knew I'd have to do major surgery to get the Featherlite cowls to fit my engine.  The Featherlite cowls were made to fit Nat's Lycoming O-360 narrow deck engine.  I have the IO-360 wide deck engine.

It just so happens that at the time, AeroCad was making cowls for an IO-360 wide deck engine.  The top cowl was really just a duplicate of the Featherlite top cowl.  (AeroCad just popped a mold off the Featherlite top cowl.)  Even though it was meant for a narrow deck engine, it would fit my engine just fine with the minimal one-half inch clearance around the cylinders.   The bottom cowl would fit except for clearing the aft intake pipes.  It was molded to fit a "B" sump engine and I have an "A" sump engine.  The intake pipes extend farther down and outboard than the ones on the "B" sump engine.  The cowl would clear the forward pipes, but the aft pipes would actually poke through the lower cowl cheeks.  So I knew that at some point I'd have to perform minor surgery to widen the cheeks and extend them aft a bit to provide clearance for the aft pipes.  (Addendum:  I changed the A-sump to a C-sump.  The intakes pipes clear the lower cowl now.  So, the lower cowl surgery is not required any more.) 

Knowing this, I bought the AeroCad cowls anyway.  I'd rather spend a little time performing the minor surgery than spend large amounts of time making my own or getting the Featherlite cowls to fit.

Step 3:  Cowling Installation

The cowls don't come with tick marks or centerlines drawn on them, so I took a few moments to determine where the centerline was on the forward and aft edges of the cowls.  This helped me to align the cowls to the firewall, turtleback, and engine ring gear.  

Top Cowl Bottom Cowl

  

Turtleback, Lower Firewall, and Spar Flanges

I started the cowl installation in 2003.  I made the turtleback flange as per plans, contouring the foam as per Figure 6 (page 23-3) and laying up the 4-BID flange onto the turtleback.  I instinctively knew that we were supposed to add BID strips from the turtleback flange overlappping onto the firewall, but I could not find this step anywhere in Chapter 23.  So I emailed Nat.  He confirmed my intuition but could not confirm how many layers to use.  So I guessed and laid up 4 BID on the underside of the turtleback flange overlapping onto the firewall.  I did the same thing for the lower firewall flanges (4-BID) and cosmetic pieces (2-BID).

Stiffening Rib

While the turtleback flange was curing, I floxed the stiffening rib to the top cowl as per plans.  The plans don't say anything about BID taping the stiffener, but Nat emailed me and said we are supposed to implicitly BID tape everything (Chapter 3 Education).  So I'm going to tape the stiffening rib to the cowl with a layer of BID. 

 

(Addendum:  If I were doing this again, I'd definitely, definitely, definitely  wait to install the stiffener until closer to first flight.  First, it's easier to fit the top cowl to the bottom cowl without the stiffener in place.  Second, the cowls have a natural tendency to curl up a bit side to side, especially when stored on a shelf for long periods of time.  Well guess what?  The stiffener also curls up and exacerbates the problem!  I'm convinced that the stiffening rib has "curled up" after sitting in storage over the past two years.  I believe it contributed significantly to the loss of clearance between the top cowl cheeks and the cylinders.    I wouldn't leave the cowl stored for long periods of time with the stiffener in place.)

Fitting the Cowls

The next step was to fit the top cowl.  The goals are simple -- (1) center the cowl with the fuselage and engine; (2) have adequate clearance (1/2-inch minimum) between the engine and the cowl; and (3) end up with the cowl flange a quarter inch from the firewall.  Well, this is another one of those "herding cats" exercises.  Not too bad though.  Getting #1 and #2 were easy.  I placed some pieces of 1/2-inch foam on the top and sides of the cylinders and on top of the starter ring gear.  The cowl centers itself side to side by default! :-)  But I did move it and tweak it a bit to ensure the cowl matched up to the turtleback centerline and was centered of the engine's starter ring gear.  Getting #3 was an iterative process.  Because of the angle that my top cowl made with the fuselage, the cowl lip was closer to the firewall in some places than others.  I found that I had to sand the lip alot to adjust the fit, especially at the 9-10 o'clock and 2-3 o'clock positions.  So, it was cowl on, check fit, mark it, cowl off, sand.  Cowl on, check fit, mark it, cowl off, sand.  Cowl on.... Well, you get the point.  I kept trimming away until the cowl fit like a glove!   I marked out where the cowl screws would go, but I didn't install the cowl screws just yet.  I need to contour the fuselage and the cowls first.  Cowl screws would just get in the way.  Instead, I drilled some 1/8-inch holes and used clecos to temporarily attach the top cowl to the turtleback.  

 

With the top cowl cleco'd in place, the bottom cowl was easy to fit.  All you really do is match up the cowl's centerlines to the centerlines on the fuselage bottom and ensure the aft end is centered on the engine.  I marked out where the cowl screws would go, drilled the small holes through the cowl lip and the firewall flanges, and used clecos to hold the bottom cowl in place.  

Cosmetic Pieces and Flanges

I cut and shaped the cosmetic fairing pieces as per plans.  There's been a running debate in the group whether to route the vent lines on the inside of the turtleback, or on the backside of the firewall.  The plans aren't very specific about it except that the pictures at the end of the chapter show the vent lines on the back side of the firewall.  The hole through the cosmetic piece is clearly shown, too.  I guess it's six of one, half dozen of the other.

  

Wing Root and Spar Flanges

The next steps are laying up the flanges to attach the cowl to the wing roots.  Here are pictures of the plans process.  The first picture shows the top cowl being positioned to the wing root with bondoed popsicle sticks.  The bondo will stick better if you roughen the surfaces first.  The "white" on my cowl is gelcoat.  AeroCad used it as a release agent  so that the cowl can be removed from the mold.  My bondo did not stick to the gelcoat.  The second shows the various spar and wing root flanges.  The third picture shows how I used the cowl to force the 1-BID lay-up to conform in place.  You must do something like this, or else the BID will bubble up because it won't lay flat in that 90-degree corner where the flange meets the wing root. The last picture is of the cowl flange that fits under the spar flange.  I had to make this flange twice because the cowl didn't fit well after the strakes and wing roots were contoured.

 


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