Scale BUSA Super Cub

[TonyHallo]

The firewall from the kit is three layers of 1/8" 3 ply plywood laminated together. I plan to use 1/4" plywood for the firewall. The 5/8" was laid out and cutting commenced. After trail fitting, decided to flush mount the firewall onto the 1/2" balsa sides rather than inset it into the fuselage sides. Another 1/4" was taken off. The tongue sticking out is part of the support for the cabin roof, I left the 1/4" on it and it will fit into a slot on the firewall. I need to review the building manual and check the down thrust setting, I may to take a little more off to account for it.
After running the Champ engine on Tuesday, I knew this is the correct move putting the Roto 85FS in this plane.

 

[TonyHallo]

The new firewall will also act as the baffle between the high and low pressure zones inside the cowl. The shape of the cowl is quite a bit different than stock firewall. A dam was made out scrap wood, this was roughly fit using a contour gauge, packing tape was applied to the cowl for release, and the pieces of dam were held in the location of the new firewall with 1/4" thick balsa blocks affixed with two sided carpet tape. Joint was then filled with a mixture of epoxy and milled glass. Hopefully I can remove it one piece once cured. This should provide a good starting point for the shape of the new firewall.
Going flying this afternoon.

 

[TonyHallo]

The ring came off the cowl just fine. A new 1/4" plywood firewall was cut to the traced line then another 1/16" was removed to account for the glass thickness. The plans call for 2 degrees down thrust so that was cut off the fuselage sides. The new firewall was located by mounting the cowl with the firewall inside then pressing it into two sided carpet tape. The cowl was removed and the new firewall was temporarily held in place with 4 3/16" wood dowels.
Next up stand the fuselage on the tail and locate the engine.

 

[dhal22]

Can't wait on the Roto 85. The inline 85 is my thoughts on my future Tigermoth build.

 

[Snoopy1]

Got the bench cleared from the Champ repair and moved the Sub Cub fuselage onto the bench. First order of business clean off 14 years of dust. Next up determine if the Roto 85 with fit and what modifications are necessary. The plane was originally built using a 3W 56 CS engine, the exhaust was routed to the scale exit location with stainless steel header tube and fitting. Based on measurements the firewall needs moved back 5/8".
View attachment 121436

How did you like the 3w cs56 always look at them and seriously thinking of buying one but never did because of the price and how difficult it was to get exhausts for it.

 

[TonyHallo]

Never ran it, thinking about putting in the next Cherokee. I have the Johnson mufflers but cut the stacks off to fit the scale exhaust in the Super Cub. All in weight, engine, mufflers, and ignition is 4.7#, over 1# lighter than the D&B 3.7 I was using.

 

[TonyHallo]

Here's a short video from yesterday and the reason why I'm going down this route. It will idle just at 1000 rpms, I raised the idle here to keep the rpm telemetry active, using a Frsky SP-RPM sensor designed for electric motors so it drops to zero below 1000 RPM. The sensing wires are hooked to the tach output through a 20uf 25V capacitor and it works just fine.

 

[dhal22]

Love the 4 stroke sound.

 

[AKNick]

Oh I love it!

 

[dhal22]

2 stroke engines on scale airplanes should be illegal.