by mark rittinger photos by mark rittinger … mark rittinger photos by mark rittinger the fuselage...

114 MODELAIRPLANENEWS.COM

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CONSTRUCTIONBY MARK RITTINGER PHOTOS BY MARK RITTINGER

the fuselage has a simple box cross-sectionmaking it quick and easy to build. Thoughbrushless motor setups are very popular, Istill like to use the cheap power availablefrom ubiquitous Speed 400 type “can”motors.

The model presented here has beenflown with two 7.2V Speed 400 motors, aswell as two Scorpion brushless motors(innov8tivedesigns.com). The choice is upto you.

CONSTRUCTION

I began my project by ordering all my sup-plies from Prop-Shop.com. I was able to getmy wood, motors, ESC, wiring, battery,props and adapters, and other hardware allin short order. Begin with the fuselage. Youwill need to decide if you are going to builda “glass nose” or a “solid nose” versionbefore cutting the fuselage sides to shape.Laminate the formers from cross-grained1⁄16-inch balsa. F2 and F3 are identical. Tracethe fuselage sides onto 1⁄16-inch balsa sheet.Cut two identical sides. Install the trianglestock on the bottom corners from nose to

DOUGLAS A-26 INVADERA Speed-400 size military twin that’s easy to build and exciting to fly!

SPECIFICATIONSWINGSPAN 42 in.

WING AREA 200 sq. in.

WING LOADING 18.5 oz./sq. ft.

LENGTH 30 3⁄4 in.

WEIGHT 25.5 oz.

MOTORS two direct-drive 7.2V Speed400 brushed

ESC JETI 35 brushed with BEC (wiredparallel)

PROPS 5x5 APC electric

RADIO REQ’D 3-channel (throttle, aileron,elevator)

DESIGNED AS A LOW-LEVEL attackbomber, the Douglas A-26 Invader first flewin 1942. Powered by two Pratt and WhitneyR-2800 radials, it carried a variety ofweapons from .50-caliber machine gunsand rockets to a cannon. During the end ofWW II, the Invader earned a reputation as ahard-hitting, durable weapons platform.The Invader soldiered on through theKorean conflict, operating mostly in nightattack roles, but by the early 1960s most hadbeen retired or converted into fire bombers.

When the Vietnam War broke out, the triedand battle-tested A-26 came out of retire-ment for a third tour of duty converted intocounter-Invaders, designated B-26Ks, usedmostly for counterinsurgency missions.

THE MODEL

I’ve always been impressed with the sim-plicity of the Invader’s design. It has fineproportions for an RC model, especially anelectric-powered one. The long, narrow andefficient wing has a tapered planform and

The author MarkRittinger poses withhis new A-26 Invader

all dressed up inVietnam camo paint.

Construct_Invade 1/5/12 10:19 AM 14

DECEMBER 2008 115

tail and on top from the aft end of the cock-pit to the tail. Add the 1⁄16-inch doublersbelow the rear canopy between F3 and F4.Since F1 through F4 are the same width,they can all be added to one side now, usinga square to ensure proper alignment. Lineup the opposite fuselage side and glue it tothe formers. Lay the fuselage on its side,weigh it down in the wing saddle area, andprop the tail up 7⁄8 inch on one side at F5 (orto measure 11⁄4 inch to the center of F5 fromthe board). Glue F5 in place and remove thefuselage from the board. Add the instru-ment panel and the nose joiner piece. Wetthe sides and glue the nose sheeting

together. Install the 1⁄16-inch doubler underthe wing saddle. Add the triangle stockabove the wing then carefully sand thebevel on the outer edges so that the sidescan angle inward. Glue the sides to the tri-angle stock above the wing. Sheet thefuselage top and bottom where noted withcross-grain balsa sheet. Add the cockpitfloor and rear cockpit sheeting, as well asthe lengthwise sheeting above the wing.

If you build the solid-nose version, addthe nose block now. For the glass-nose vari-ant, you will need to tack a block in placeand carve the mold. Round the edges, andthe fuselage is nearly complete. Carefully

cut the bottom hatch free of the fuselage. Iused an alignment tab in the front andsmall rare-earth magnets in the rear to holdthe hatch in place. Now, simply add the tailblock, bevel the stabilizer attachment areaand the fuselage is pretty much done.

TAIL SURFACES

The Invader has a unique dihedral stabilizerthat requires a bit of attention to installproperly. Cut out all the pieces and assem-ble the stabilizer. Block up one tip 3 inchesand join the halves. Reinforce the centerjoint with a strip of fiberglass tape andepoxy resin. Make the two elevator torque

THE MODEL IS A ONE-PIECEDESIGN AND THE WING PANELS

ARE COMPLETELY SHEETED. IT’SIDEAL FOR INEXPENSIVE SPEED

400 BRUSHED CAN MOTORS

The fuselage parts are easy to make and the design is very simple. Large trian-gular balsa stock fills in the corners for easy rounding later in the process.

Here you see the fuselage sides glued in place around the formers. This basicbox structure is easy to build straight.

Construct_Invade 1/5/12 10:19 AM Page 115


rods from 1⁄16-inch music wire inserted intoaluminum tubing before bending the endsto shape. The ends of the torque rodsshould hang inside the fuselage. Glue thetorque rod tubes to the stab, and assemblethe pushrod using 1⁄4-inch-square balsasticks and wire pushrod ends. Cut the verti-cal fin to shape, then set it aside untilcovering.

WING ASSEMBLY

The wing is quite simple to build. Build thewing in halves, and join them after sheeting.

Cut out all of the ribs and the bottomwing skins from 1⁄16-inch balsa sheet. The

CONSTRUCTION DOUGLAS A-26 INVADER

Visit RCstore.com to order the full-size plans, or call (888) 235-2021.

wing outline is the bottom sheeting’s out-line. Glue the 1⁄4-inch-square leading edgeon top of the bottom sheeting. Measure theheight of the root rib and tip rib, then cutthe spar from hard 1⁄8-inch balsa sheet tomatch. Now you can glue the spars on thebottom sheet. Add the aileron framing andspars as well. Make them taller than neededto allow for sanding. Add the ribs but omitR1 in the center. Note that R3 and R4 are 1⁄8inch apart to allow the nacelle profile to fitin place. Mark the slot where the nacellewill go and the corners of the ailerons bysticking pins through the sheeting. Cut theopenings for the motor wires in the bottom

sheet then, once all parts are installed, sandthe leading edge to match the contour ofthe airfoil. Taper the trailing edge to a sharppoint. Prop up the wingtip 11⁄4 inches andglue the center R1 in place square to thebuilding board.

Make the washout guide from 1⁄8 balsa (asshown on the plans) and place under thewing’s trailing edge. Pin the assembly to theboard and glue the top sheeting in place.The top sheeting should be 1⁄2 inch largerthan needed all around. Sand the leadingand trailing edges to shape and add yourwingtips (unless you are making tip tanksfrom Estes model rocket parts as I did; estes-rockets.com). Repeat the procedure for theopposite wing panel then cut wing joinersfrom 1⁄8-inch plywood and join the panelstogether with 11⁄4-inch dihedral under eachtip. Wrap the center with glass cloth tapeand epoxy resin.

Cut open the slot for the nacelle profileon the bottom wing sheet only. Cut theailerons free and bevel the aileron leadingedges. Mark the torque rod location, andcut a slot for them in the bottom sheeting.Glue the rod tubes in place so the rod endshang under the wing so you can hook upthe aileron servo linkage. Make sure therods can rotate forward into the wing soyou can slide the wing into the finishedfuselage. Slowly trim away balsa from thewing saddle cutouts until you have a good,tight wing fit.

ENGINE NACELLES

The nacelles are easy to build and installstraight. Cut the nacelle profile from 1⁄8-inchbalsa. Depending on the motor you areusing, determine if you’ll need a firewall ora clamshell motor mount. I used 1⁄64 plydoublers as shown on the plans and EstesBT55 rocket tubes to install my 7.2V Speed

The completed fusleage structure with tail sur-faces attached awaits the wing assembly.




fiberglass cloth and finishing resin. Sandand prime them so you can paint.

PAINTING AND FINISHING

I covered the prototype before assemblingthe major parts. My covering of choice isthin Doculam laminating film, but anycommon covering film will do. You canalso finish the wings and fuselage withresin and cloth. Just keep an eye on theweight; you only have about 200 squareinches of wing area.

After finishing the model, hinge the con-trol surfaces using your favorite method (Imake my hinges from used floppy diskmaterial). Assemble all the parts, install thecanopies with white canopy glue, and thenmask off the model for painting.

FINAL ASSEMBLY

Slide the wing into the saddle and epoxy itin place. Check the tail alignment andwhen everything lines up properly, slide theelevator pushrod into the fuselage and gluethe stabilizer and fin in place. Run yourtwisted motor wires through the wing cen-ter and out the holes in the wing. Threadthem through the nacelles, and glue thenacelles to the wing with epoxy. If youinstall brushless power, put the ESCs in thenacelles to free up fuselage space.

Use foam or balsa sheet to fill and shapethe area on the wing above the nacelle.Install an aileron servo and linkage, andthen the elevator servo. I used a Hitec HS-55 servo (hitecrcd.com) for elevator controlheld in place against the fuselage side with

400 motors. Use the plan’s side view torough cut your pink or blue foam to shapeand tack glue the pieces to the sides of thenacelle profiles. Slowly sand and shape thefoam to match the profile. Sand the nacellesinto an octagon cross section then, keepsanding off the corners evenly until youhave a smooth rounded shape. Start with80-grit sandpaper and finish with 240-grit.

After the foam nacelles have been shaped,pop them off the profiles and hollow themout. Glue them back onto the profiles andfinish sanding them smooth. Make surethey match and fit the slot in the wing pan-els. My model has balsa sheet engine cowlsbut you can easily make plugs to form plas-tic ones. Once satisfied that the nacelleslook great, finish them with 1- or 1.5-ounce

The engine nacelles are made of foam and are built around a central balsa pro-file. The profile is inserted into the wing through a thin slot cut into the wing’ssheeting and then glued into place.

Here you can see the motor mounts the author made using sections of card-board model rocket tubes.

The elevator uses a two-pushrod wire arrangement because of the horizontalstabilizer’s dihedral.

The wing panels are also simple in design and easy to build. All of the ribs andspars are added to the bottom sheeting and the panels are joined before thetop sheeting is installed.



double-sided tape. I also used an HS-55servo for aileron control. The brushed ESCfor the Speed 400 motors goes in the nosesection, and the Hitec Electron-6 receiverfits nicely at the trailing edge of the wingbetween the torque rods. Set the controlsurface throws and directions, and makesure there is no control system binding.

I used OD Green, Dark Green and FlatBlack Testors Model Master paints alongwith Krylon Khaki for my Vietnam colorscheme. Add your decals and markings and

you are nearly finished.Wire up the motors, check for proper

rotation direction, and install them. Iwrapped the motors with 3mm foam andlightly glued them into the rocket-tubemounts. Make sure there is a tight fit andthe thrust lines are correct. My Invader per-forms very well with APC 5x5 props usingGraupner prop adapters (apcprop.com;hobby-lobby.com). I used a Dualsky 11.1V1300mAh 25C LiPo pack (2dogrc.com) heldin place to the fuselage side with Velcro.

The battery can handle 22 amps constantly.Final weight of my model was 25.5 ounces.After a thorough motor-on range check, it’stime to fly!

IN THE AIR

Always perform a radio range check, withmotors running, and run through thethrottle range. Double check the controlsfor proper deflections, along with a lastquick check of the CG. The Invader takesoff from a simple hand launch quite easily.A couple steps and a toss into the breezewith the nose and wings level results in amild climb. Once you get some air underthe wings, get it trimmed out for straightand level flight.

The Invader will slow down nicely and isactually quite a floater. Cut power at altitudeto get acquainted with the stall. It will stallto the left. Aerobatics are not really what anInvader is for, but it will loop, roll and flyinverted nicely. It does a nice Split-S, Immel-mann turn and stall turns as well.

Setting up landing is where you’re slow-speed flight and stall testing will help. Justline the Invader up into the wind, cut thepower and bring it in smoothly. It might sur-prise you how well it glides in ground effect.

Built right, the A-26 Invader will pro-vide a lot of enjoyment for a smallexpense. It’s an exciting twin that will cer-tainly shake things up at any flightline. Ifyou have any questions, feel free to con-tact me at [email protected]. Good luckand good flying!


Tape is wrapped around the Speed 400 motor for a snug fit into the mountingtube shown in the front of the engine cowl.

With the wing and engine nacelles installed, the A-26 really begins to look likea scale attack bomber. The engine cowls are simply made of wood and carvedto shape and sanded smooth.

There are several paintschemes available for the A-26

Invader. Take your pick.


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