We’ve seen it time and time again: a giant-scale airplane noses over with enthusiasm and the engine and firewall are ripped out of the fuselage requiring major repairs. For the engine installation in my 85-inch version of the Ziroli Skyraider, I came up with a removable, through-the-firewall setup for engine attachment for a couple of reasons. This design makes it much easier to work on your power system after the model is complete, and if there is any unforeseen damage to the firewall, removing six bolts greatly simplifies removal for a workshop fix.
I used my CAD program to draw up a simple box structure with an attachment rim and then I had my buddy Pat at LaserCutUSA.com cut the parts for me. Of course you can just as easily make a similar design and reproduce similar the parts with a band saw and a drill press. The assembly is very easy to install and it took me about an afternoon to hang the engine.
I am using a Fuji 43cc with electronic ignition to power the Skyraider so I made the engine mount box/tank shelf long enough to accommodate the tank and throttle servo, with the idea that I would attach the ignition module and battery pack in front of the firewall.
For strength, I made the sides of the box from 1/4-inch ply and the top and bottom from 1/8 inch lite-ply. I added notched the fronts of the four sides to key the 1/4-inch firewall face into place. Alignment tabs on the firewall and the aft bulkhead fit the slots as shown.
I made the firewall just wide enough to fit the engine attachment bolts, and you see here the blind nuts have been ground back to clear the sides. I also doubled up the firewall with a second layer of 1/4-inch plywood to properly seat the blind nuts. The total thickness of the firewall is 1/2 inch and the two layers are laminated together with Zap 15 Minute Epoxy. The firewall is also glued to the plywood sides with epoxy.
Looking through the cockpit opening, the two aft attachment points will use blind nuts and bolts to anchor the box to the fuselage’s vertical side structures. The bolts can be easily accessed through the wing saddle should you want to remove the engine and fuel system for maintenance.
For more information on the Douglas Skyraider, go to: Nick Ziroli’s Website.