A giant-scale gas/ electric hybrid
Iwanted a warbird that isn’t seen too much at the field, and the Martin B-26 Marauder medium bomber fit the bill. It’s the same class as the B-25 but just isn’t modeled as much. I wanted as big a model as I could easily manage, and with a wingspan of 142 inches, 1/6 scale seemed doable. The B-26 is a scratch-build from three views (and whatever information I could find in books and on the Internet), right down to the landing gear and wheels. The construction materials are less common and include foam, light ply, and carbon fiber, and the structure is completely fiberglassed. I tend to design and build on the fly.
My B-26 has flaps, retracts, and functional bomb-bay doors, which operate like those on the full-size bomber. Most of my models are electric and I’ve taken some flak over the years for them being too quiet, so I knew this B-26 would need to have the right sound. I wanted to do a twin engine but didn’t want to live through losing an engine and the heartache that might go with it, so building gas/electric hybrid engines came to mind. I picked up a Zenoah G38 at a swap meet and connected its rear shaft to a 3000-watt, 200Kv motor I already had. I knew from other builds that the electric alone would turn a 24x12 prop about 5200rpm, and with the G38 trying to get to maybe 7000rpm, the result would be somewhere in between. The hybrid setup produced a scary amount of power and sounded great when I demonstrated it out at our field on a sawhorse (firmly nailed to the ground!).
So far I’ve got six flights in on my scratch-built 1/6-scale B-26. I was hoping to keep the model less than 50 pounds, but in the end, it did not happen. At 64 pounds, it’s now signed off with a Large Model Aircraft waiver and has nine great flights as of this writing. I’m still working on the final scale details, including the invasion stripes and nose art, and I look forward to sharing more details in a longer article in Model Airplane News.
THE B-26 AT THE WOODLAND DAVIS WARBIRD FLY-IN.
I’ve been asked many times, “Why hybrid engines?,” and half a dozen reasons came to mind. First is the reliability: If the G38s die for almost any reason, the electrics will keep them and the props spinning, albeit at a reduced rpm. Also, the 4hp electrics can easily spin the G38s so that they can be started right from the transmitter. Besides the convenience, it’s much safer without needing to get near or in front of the props. In addition, because the props get spun up to about 4000rpm, no chokes are needed. One goal was the right sound, and with custom exhaust manifolds, no mufflers, and 16-inch dual exhaust pipes on each engine, it comes close to sounding like a real bomber. The last benefit I’ll mention here is that the G38s fit entirely inside the nacelles, but they might have trouble getting the plane off the ground by itself. Together, each hybrid will approach 7hp, for a total of 14hp. When I testbenched the hybrid, I included a boost converter similar to one implemented in an automobile to allow the gas engine to charge the batteries. This worked fine but would not be too useful in this application as the G38s alone would have to be working hard to keep the B-26 in the air without charging batteries at the same time.
I like to build from scratch and have found that three-view drawings and pictures available on the Internet are enough to build from without actual plans. Bulkheads and ribs can be blown up to the desired scale, printed out, and taped to the material of choice (foam sheeting, in this case) for cutting. I generated the print scaling numbers in Excel, taking into account the thickness of the quarter-inch underlayment foam I’ve been using. Structural critical parts were either sheeted with fiberglass of appropriate thickness or cut from plywood. The spars are capped with carbon fiber.
The engine mounts are thick-walled S-glass tubing with engines bolted to plywood bulkheads. The two motor shafts are connected by automotive hose, heavily compressed with a steel clamshell. To minimize vibration, alignment is critical. I use the Castle Phoenix 100-amp controllers and 8-cell 5000mAh batteries on each engine. Tuning was achieved by programming unique throttle curves for each of the four motors, matching rpms, electric currents, and head temperatures at all throttle positions. I’m not working the G38s too hard yet, enabling only about 60% throttle to keep their temperatures down. I will eventually increase their load as their temperatures are getting to only about 220°F in the air versus 310°F statically on the ground. Flight times will go up from 10 to 15 minutes as I use more of the 24-oz. fuel tanks and less battery.
THE COMPLETED HYBRID ENGINE
The landing gear and tires are all scratch-built from foam, S-glass, carbon fiber, and aluminum. I vacuum-formed the nose cone, canopy, top turret, and tail section from 40-mil PETG plastic. I am using my 14-channel Futaba for the flight controls and a Taranis Telemetry system to collect performance data.
You can see more detail on the build at rcscalebuilder.com, where you will need to register as a guest. It was more than a year into the project when I found this painting of serial number 296165. It’s flying season now, so it will be a while before I finish the painting the model.
Keep ’em flying.