B-26 Ma­rauder

A gi­ant-scale gas/ elec­tric hy­brid

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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.

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