project pattern
Redux PHOTOGRAPHY: DAVE LOCKHART
Producing a lightweight and durable firewall is one of the many factors to a competitive electric F3A. A combination of Forstner drill bits and fine tooth blade on scroll saw (above left) are used to lighten the firewall and tail ring structures. Fiberglass/balsa endgrain/fiberglass composite firewall (above center) with
W
ith the firewall and tail ring mounting structures completed, it is time to “swiss cheese” the structures to reduce weight. The overall shape of the firewall lended itself fair- ly well to a number of holes of various sizes (drilled with Forstner bits) while the tail ring former had room for some substantial “win- dows”. Forstner bits were used to cuts holes in the firewall and the corners of the windows in the tail ring former. After drilling the cor- ners of each window, the four sides were cut using a fine tooth blade on a scroll saw. An additional function of the holes in the mount- ing structures is to allow cooling air to pass through the fuselage. Should the powerplant of choice be of the “slimer” variety, this swiss cheese step should either be omitted, or thin facings of 1⁄64-inch ply can be used to “seal” the structures.
After a final iteration of dry fitting the motor in the plane with the firewall and tail ring mounted to the motor, to check for prop- er fit, it is time to glue the structures into place. Once again the spinner shim is put in place and the motor/mount/spinner are as- sembled with the spinner tightly taped to the fuselage. I then use a couple drops of thick CyA to tack the firewall in place, and
hardwood dowel reinforcements and lightening holes ready for installation. After generating a firewall template using “toothpicks” (above right), the composite fiberglass/balsa endgrain/fiberglass firewall template is generated for the building archives.
is prepared and a paperboard
then carefully remove the motor/mount/ spinner assembly. Any gaps between the
firewall and fuselage side that are less than 1⁄32-inch can be ignored.
Larger gaps I fill with scrap balsa sheet. My technique is to cut 1⁄16-inch balsa into ¼ × ½-inch strips, and then slide them into the gaps. A very effective technique for fill- ing the gaps larger than 1⁄32 inch, but not quite 1⁄16 inch, is to “smash” the balsa scraps using a ¾-inch dowel as a rolling pin. A small drop of CyA holds each scrap in place. The next step is permanently gluing in the firewall for which epoxy and microbal- loons are needed. Any good quality slow cure epoxy will fit the bill; in my case I still have a stash of Hobbypoxy in my shop (along with K&B microballoons). Current day options include BVM Aeropoxy, EZ Lam, ProSet, West Systems, and Gougeon Brothers, available at a variety of retailers including Aerospace Composite Products
www.acp-
composite.com and Composite Sales Tech- nology
www.cstsales.com.
I add just enough microballoons to keep the epoxy from being runny. While not easy to explain, the goal is to create a mixture that is thin enough to penetrate small gaps between the firewall and fuselage, and still
have enough structure to maintain a small radius fillet. A large fillet is not needed; a “pinky” radius fillet is sufficient. I periodi- cally check the glue joint as it is drying to add small amounts of glue as needed to maintain a good fillet (as the mixture soaks into any gaps) and clean up any excess glue that has run completely through the joint. I typically hang the fuselage vertically (nose down) when gluing to the backside of the firewall, and from the nose ring when gluing to the front side of the firewall. Installation of the tail ring is completed in the same manner.
One additional step I recommend for fuel powerplants is the addition of fiberglass cloth corner reinforcement of the firewall and nose ring (most fuel powerplants using soft mounts utilize a rear primary firewall with a stabilizing nose ring structure). Rela- tively light (1 to 1.5 ounce per square yard) cloth cut into 2 ×½-inch strips is what I find easiest to work with. After cutting the need- ed number of strips to size, I place them on waxed paper and drizzle a bit of laminating epoxy (relatively thin viscosity) on them, and then place another piece of wax paper on top. Then, using a squeegee (old credit cards work well), the epoxy is forced into the
by dave lockhart with algirdas ungulaitis You can reach Dave Lockhart via e-mail at
davel322@comcast.net
Completed firewall with Neu F3A-1 mounted on a Hyde softmount (above left). Note the small clearance between perimeter of Hyde mount and firewall which allows the Hyde mount to move freely as designed. With the
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spinner/motor/mount/firewall assembly firmly taped (above center) in place against the nose ring shim, the firewall is now ready to be tack glued into place. A mixture of epoxy and microballoons (above right) secures the firewall in place.
JANUARY 2013
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