HOW TO: TRIM YOUR ENGINE COWL

Jul 10, 2011 1 Comment by
9 EASY STEPS TO A PERFECT FIT
The engine cowl on this .60-size Lanier Stinger fits perfectly around the engine. Follow the nine easy steps outllned below, and your engine cowl will fit this nicely.
Making cutouts in a fiberglass engine cowl has always been difficult for me. I used to zip through a plane’s assembly until it came time to install the cowl; then, my progress slowed to a crawl. I tried many techniques to become more confident that my first cut would be in the right location. If the cowl fit over the engine, I would eyeball it and estimate where the engine needed to exit. Having marked the spot, I’d cut a little at a time with my Dremel tool. I constantly test-fit the cowl on the plane to make sure that I didn’t make the hole too big. If I couldn’t fit the cowl over the engine, I held it above or below where it would be attached to the fuselage and guessti-mated where I should start cutting. Again, I would cut a little at a time and then sneak up on the final size of the opening. Both methods were time-consuming, inaccurate and frustrating.
Then I learned to use card stock to measure precisely where a cowl should be cut to suit an engine. This technique is especially useful when the cowl can be slid onto the fuselage and doesn’t have a predetermined position. Follow along and see just how easy it can be to cut and install a cowl.
  1. 1: Here are the tools you’ll need. I use card stock or a manila file folder to make the templates and masking tape to hold them in place. I always use a Sharpie pen to mark measurements on heat-shrink covering and painted surfaces; these pens can be used on any surface, and their marks can easily be removed with rubbing alcohol.
  2. 2: I remove the cylinder head from the engine and draw around it on the card stock. Then I follow this outline to cut out the exact shape of the engine head for a perfect cowl fit.
  3. 3: I have two templates ready to tape to the fuselage. The narrow one is for the hole where the needle valve will protrude through the cowl. I use the wider one with the �handle� to determine the position of the cylinder-head opening. If this template didn’t have a handle, it would follow the fuselage’s curvature when it was taped to it and would distort the cutout.
  4. 4: Before I attach the templates to the fuse-lage, I measure from the firewall to about 1/8 inch behind the drive hub/washer. I use this measurement as a reference point to properly position the cowl on the fuselage.
  5. 5: I tape all of the templates to match the positions of the engine parts that protrude through the cowl. Notice the Z-bend on the cylinder-head template; this is necessary because the cowl is much wider than the fuselage. The Z-bend will allow the template to lie flat on the cowl and will help to determine the correct position for the cutout. With the templates taped into place, I now remove the engine.
  6. 6: I transfer my earlier measurement from the firewall to 1/8 inch behind the drive hub/washer to an alignment jig that I’ll use to align the cowl with the fuselage. I made the jig out of scraps of stick balsa. The jig’s arms must be at a 90-degree angle to the shaft (the part I’m holding).
  7. 7: I slide the cowl onto the fuselage and put thick CA on the front of the jig’s short arms. I then insert the jig through the cowl’s front opening, making sure that all three of the jig’s short arms contact the firewall to keep everything aligned for the 10 seconds it takes the CA to cure.
  8. 8: I pull the cowl forward until it touches the jig’s three long arms. This aligns the front of the cowl with the firewall and ensures that it’s in the proper location. I then tape the cowl down around the fuse-lage and use the template to transfer my cutout measurements to it.
  9. 9: Using my Dremel tool, I start at the center of the area to be cut out and slowly work out to the outline I drew following the template. I check the opening against the engine to ensure that it’s the right size. After making the cutout for the cylinder head, I start work on the needle-valve access hole.
Using the head cutout as a guide, it’s easy to make the other access holes for the Pitts-style muffler. I also use this technique to find the positions where my cowl hold-down screws should go into the firewall.
CONCLUSION
Since I’ve been using this template method to position engine-access holes in cowls, I don’t hesitate to cut openings in a fiberglass cowl. This technique will work on even the most complex installations. Try it, and you’ll never go back to your old method.
Featured News, John Reid
John Reid

About the author

West Coast senior editor About me: I’ve been involved with RC aircraft since high school and have flown just about everything. I started my RC career with scratch-building, but now like many pilots I rely on ARFs to get me in the air. My main focus is on pylon racing, aerobats, combat and scale warbirds.

One Response to “HOW TO: TRIM YOUR ENGINE COWL”

  1. Reply
    Jeremh says:
    October 31, 2012 at 4:09 AM

    Awesome idea. Can you email me? I would like to discuss with you a few small steps I am unsure of myself on.

    Thank You,

    Jeremy

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