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Cut A Perfect Cowl

This article was originally published in Fly RC’s February 2016 issue.
By Scott Copeland

Everyone knows that the “A” in ARF stands for “Almost”. For some kits, that can be a pretty big “A”! While the majority of ARF’s have most of the major components built, there are significant variations in the amount of time and skill required to finish them well. There is also the issue of instruction manuals being, shall we say, “loosely” translated Chinese building instructions. All of a sudden the “Big A” looms very large.

One of the daunting tasks when assembling some ARF’s is mounting the cowl. For electric airplanes, the chore is somewhat easier since the motor is completely enclosed within the cowl. Internal combustion engines are decidedly trickier, since the engine cylinder and muffler usually protrude from the cowl and there are secondary access holes needed for the needle valve and glow plug (for glow engines).

In any case, the cowl must be mounted such that there is appropriate space between the cowl and spinner, the engine shaft must be centered in the cowl opening and all of the appropriate holes must be cut accurately in the correct spot. I was faced with such a task while putting together my Phoenix Model Cessna Skylane. The manual said something like, “ Measure and mark all cowl cutouts; remove cowl and make these cutouts”. Huh? That’s sort of like saying,“To drive a car, insert the key and manipulate the pedals”. My intent here is to eliminate the guesswork and inform the Fly RC readers exactly how to mount a cowl.

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The first step is mounting the engine per instructions. The spinner back plate and prop were bolted on temporarily and the distance from the firewall to the back plate measured. In the case of the Skylane, the distance was 4.5 inches. The cowl was then placed on the workbench and the length of the cowl was measured through the face opening. The Cessna’s cowl was 4.75” long. We want a gap of approximately 1/8” between the spinner back plate and cowl face. If we subtract the length between the firewall and back plate from the cowl length, we get a measurement of 1/4”. We then add the desired cowl-spinner gap to this number and come up with 3/8”. This is the amount that the cowl should overlap the fuselage when mounted in place

Next, I set my compass to 3/8” and carefully traced a pencil line around the entire front of the fuselage. I then wrapped a strip of vinyl electrical tape around the fuselage following the reference line. We have now established the cowl overlap so we now need to mark the cowl cutouts. Since the Skylane was glow powered, there were 4 cutouts I needed to make; the cylinder head, muffler, glow plug access and needle valve. The procedure is the same for all cutouts. A piece of stiff paper (manila file folder) is cut and taped along the electrical tape reference line. The bottom edge of the paper lines up with fuselage trim stripe. Make certain the edges of the paper are at 90 degrees from one another using a draftsman’s square. The paper is then rested against the cylinder head and the cylinder head is traced onto the paper with marker. The paper is then removed and a cutout is made in the paper where the cylinder head was traced. The paper is then taped to the cowl, using the cowl edge and trim stripe as positional references. The cylinder opening is then traced onto the cowl. The same procedure can be followed for the muffler.

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The openings can now be cut carefully with a Dremel cutting disc. After making the cutouts, we must mark the location of the mounting tabs so the holes for the cowl mounting screws can be drilled. On the electrical tape, I marked the tab centerline and measured to the center of the tab (3/4”). The prop and spinner back plate can now be removed and the cowling slid into place over the engine. The cylinder and muffler openings may need to be widened slightly since we want a slight air gap around the engine protrusions. I used a Dremel sanding disc to gently widen the cowl opening, but use caution and a light touch since it is very easy to remove too much cowl. Remember, you can always take more material off, but you can’t put it back on! Once satisfied with the cowl openings, tape the cowl in place using your electrical tape reference line, making sure that the trim stripes line up correctly. Re-install the spinner back plate and check the spinner-cowl gap. When satisfied that everything lines up correctly, drill the mounting holes.

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“But my cowl doesn’t have a trim stripe for reference”, you say. “How can I be sure that my cowl will be properly aligned”? The answer involves one more step. With the engine mounted as previously described and the cowl overlap reference line established, tape individual pieces of file-folder paper along the electrical tape reference line wherever part of the engine will protrude from the cowl. Mark the cutouts on the paper. Leave the paper in place and tape it securely along its back edge. Now tip the paper templates back so they rest against the fuselage; a rubber band will be helpful to keep them from moving. The engine is now dismounted and the cowl slid into place up to the electrical tape reference line and held firm by masking tape. The templates are now tipped forward and rested on the cowling so the openings can be marked. The cowl is then removed, the openings cut, the engine re-mounted and the procedure followed as above for final cowl mounting.

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The whole process can be time-consuming, especially if the engine must be dismounted. Don’t rush. Follow the time-honored adage “Measure twice; cut once”. Fit, sand and re-fit until everything aligns properly. The time spent will be reflected in a perfect cowl mounting that looks great and adds to the beauty and functionality of your finished model.