GREAT PLANES Super Sportster 20, Instruction Book

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Basically all we've done here is to measure how long your new mount and engine is, allow for some spinner
back plate space and subtract the distance shown on the plan (A) from the distance you measured (B). The
result gives you the distance you have to move the firewall back. (See Figure 3 and 4 also.)
Let's do an example to show this. Refer to the drawings on page 35 showing the OS FS-61 in the Sportster
40. We are going to use a Goldberg Spinner. Our "B" measurement is 5-1/8" (measure from the front of the
thrust washer to the rear of the mount). By looking at the information in Chart 1 on page 29, we know that we
have to subtract 3-7/8" (distance A) from our distance B. The result is 1-1/4". We move the Firewall back
1-1/4".
Example: Work Area:
YourB
Your A
Move Firewall
Back
Step 5
Mark the new position of the Firewall on the inside of each Fuselage Side. Make sure your line is perfectly
parallel to the front of the fuse sides. Using the firewall as a spacer, install the balsa doublers cross grain
on the inside of the fuse side. Custom cut these pieces to fit. Do not glue the Firewall in at this time. Use
epoxy glue or thick CA to glue the doublers in place. The doubler should extend past Bulkhead #3. Carefully
trim the doublers to the fuse side shape.
Step 13
Draw the corrected bulkhead #1 position on the top view of the plans. Next still working over the plans,
glue Bulkhead #1, the Firewall, in place. Then add 3/8" (or 1/4" depending on your mount width- see Figures
4 and 5) scrap balsa filler blocks, vertically grained, forward of Bulkhead #1. Sand the blocks flush with the
fuse sides. Add 1/4" balsa filler below the Firewall later when you remove the fuselage from the building
board.
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