Thirty years after her last restoration my N2S-4, the Rag Doll, is in Tony Blum’s Turn & Bank Aviation shop in Deer Park, WA, undergoing yet another one. Since the ol’ girl is now 70 years old this latest restoration should last her to her 100th birthday!
One of the things I planned to accomplish while she was apart was to upgrade the avionics and rewire the electrical system. My old avionics package, good gear butoutof- date technology even when it was installed in 1982, was still functional but it was time for a change. In addition, the design of the old radio panel made it difficult to access and service anything. I’m replacing the old units with a Garmin SL-40 comm, a Garmin GX-327 transponder, a Sandia Labs SAE 5-35 solid state blind encoder and a PS Engineering PM-1200 intercom. The task before me was to design a functional avionics panel and in preparation for doing so I established the following design parameters:
- Design it to occupy the space between the fuselage frame and the birdcage on the right side of the cockpit. Avoiding the intrusion of avionics into the cockpit inboard of the frame would allow maximum unobstructed cockpit leg room.
- Locate the radios as far aft on the panel as practical to allow the best visibility of their respective displays and provide easy reach to the various knobs and buttons. Conversely, avoid placing the radios so far aft that the pilot can’t easily reach the aft-most controls while strapped in the seat wearing the “straight jacket” (mobility restricting multiple layers of cold weather clothes, parachute and locked shoulder harness).
- Locate all switches and the ammeter forward on the panel, within easy sight and reach.
- Utilize the aft-most real estate on the panel for circuit breakers, clock and auxiliary comm jacks – items that need to be visible but are seldom accessed in flight. • Use combination switch/circuitbreakers where possible to reduce the parts count and to simplify wiring.
- Provide a removable segment of the panel to allow easy removal of radios and the intercom as well as allowing ready access to other components.
- Provide service loops in the wiring to facilitate radio removal for maintenance.
While disassembly of the fuselage was under way I took careful measurements of my old avionics panel, fuselage frame and birdcage former locations. From these I created a scale drawing of the proposed new panel layout on my computer.
After several iterations I saved the final design as a PDF file and took it down to my local FedEx Office store to have a full scale print made. The print was then placed in the cockpit of a friends Stearman and I sat in the seat to double check the ergonomics of radio, switch and CB placement. Once reassembly of the fuselage was under way and the birdcages were back on I constructed a full-scale engineering mock-up of the new panel. I used foam-core poster board for “sheet metal” and full-scale dummy radios made from styrafoam blocksand images of the radio faces taken from the manufacturers’ websites. The mock-up was then temporarily clamped in the airplane to check size, fit, clearances and accessibility.
Using a mock-up ensured that all components were clear of the frame and birdcage and that everything was within easy reach. Of particular concern clearance-wise was that the radios would clear the diagonal frame tube on the side of the cockpit as well as clear the rudder cable below with full left rudder applied (when the cable rises to its highest). At one point I even “suited-up” and strapped in with all my winter flying gear and chute on to double check my ability to reach the appropriate areas on the panel that I would need to touch in flight. After ensuring easy accessibility to controls as well as fit and clearances I then cut out the proposed the cockpit as well as clear the rudder cable below with full left rudder applied (when the cable rises to its highest). At one point I even “suited-up” and strapped in with all my winter flying gear and chute on to double check my ability to reach the appropriate areas on the panel that I would need to touch in flight. After ensuring easy accessibility to controls as well as fit and clearances I then cut out the proposed removable portion of the panel to verify that the radios could in fact be easily lifted up and over the diagonal fuselage tube for removal. Satisfied that the design was functional Tony and I began to cut metal. For desired strength and stiffness we chose to make the panel out of .050” T6 aluminum. We started by cutting out a flat blank, trimmed it to shape then made the major bend between the side and top of the panel. This bend is about 7º short of 90º for two reasons: First it was necessary to cant the radio stack slightly so that the radio trays would clear the diagonal fuselage tube that would otherwise be in the way of the vertically mounted radios. In addition we felt that with the top canted slightly toward the pilot it would not only look better but would offer a slightly better viewing angle of the radios and other components.
Once the top was bent we bent stiffening lips on the vertical and diagonal ends. Fifty-thousandth T6 aluminum is relatively stiff so a generous radius was put into all bends to eliminate the possibility of overstressing the metal and inviting cracks. Next we fabricated the removable section piece and using it as a guide scribed lines on the main panel to outline the cut to be made to accommodate it. After the outline was marked we used a unibit to drill largeradius holes where the corners were to be, then cut along the scribed lines between the holes using a pneumatic die grinder with an abrasive wheel. The rough cut was then dressed until the new removable section fit well and had the proper clearance. Lastly, a doubler was cut, drilled and riveted to the main panel and floating nut plates were installed to accept the mounting screws of the removable section.