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Jul 27, 2019, 05:11 PM
Jer. 29:11
jeffsch's Avatar
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Build Log

Douglas A-26 Invader / B-26 Counter Invader 42" with plans


From Wikipedia [1]:

Quote:
The Douglas A-26 Invader (designated B-26 between 1948 and 1965) is an American twin-engined light bomber and ground attack aircraft. Built by Douglas Aircraft Company during World War II, the Invader also saw service during several major Cold War conflicts. A limited number of highly modified United States Air Force aircraft served in Southeast Asia until 1969. It was a fast aircraft capable of carrying a large bomb load.
[1] wikipedia.org/wiki/Douglas_A-26_Invader

A friend is building the Rittinger Invader [2], and I wanted a same size foam model to fly along with relatively simple construction. This is a thick-profile model -- the fuselage and nacelles are thick enough to hide the electronics -- and both fuselage and nacelles use a monoblock.

[2] rcgroups.com/forums/showthread.php?784279

This model has a 42 in. wingspan, uses two 300- or 370-class motors, a 2 or 3S battery, and comes in around 18 oz. all-up weight.
Last edited by jeffsch; Nov 11, 2019 at 08:17 PM. Reason: Added recommended control throws to the plans.
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Jul 27, 2019, 05:11 PM
Jer. 29:11
jeffsch's Avatar
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(This post reserved for a gallery and thread index.)
Last edited by jeffsch; Aug 13, 2019 at 05:54 PM. Reason: Added link
Jul 28, 2019, 06:43 AM
Registered User
rajazo's Avatar
Nice work! Simple and beautiful. Not a easy task in the profile format. The detail with the cockpit make a great difference.
I hope you post a video of its flight soon
Congrats
Jul 29, 2019, 05:48 PM
Jer. 29:11
jeffsch's Avatar
Thread OP
@rajazo, thanks for the encouragement. And very interesting work on your 3D printed hybrid Turbo Porter.
Last edited by jeffsch; Aug 07, 2019 at 06:45 PM. Reason: spelling
Jul 29, 2019, 05:56 PM
Jer. 29:11
jeffsch's Avatar
Thread OP

Flight Report


The first prototype (A-26 "Stinky") had significant zoom climb. Added two washers for down thrust and still needed some down elevator trim to fly level at 1/2 throttle. Lots of power with 370 motors and 7x5 counter-rotating props. Not quite enough elevator authority for a good landing flare on low rates of 70%. 3S 850mah battery was 2/3 full after 3-1/2 minutes of flying; setting the timer to 5 minutes. Needs 1/2 oz on the tail to balance. Battery and motors cool on landing.

The second prototype (B-26 "Alley Oop") is powered with 300 motors and 7x6 props, which feels 'just right' for power. It doesn't have any zoom climb and no elevator trim. Does a nice slow roll (but faster than scale) on low aileron rates; good (but not aggressive) roll on high aileron rates. Needs moderate down elevator when flying inverted, so CG may be a bit forward of where it could be. Stalls by dropping a wing; minor stall on landing; recommend flying it all the way down. 2S 1350mah battery fits mid bay to balance (w/ no added ballast in the tail). Battery is 3/4 full after 3-1/2 minutes of flying, so a timer of 5 minutes looks good for this one too. Battery cool on landing.

No flight video yet but will dig out the hat cam and see what I can do.
Jul 29, 2019, 06:43 PM
Registered User
Good Job
Good job on these two planes. Jeff keep up the good work.
Jul 30, 2019, 07:06 PM
Jer. 29:11
jeffsch's Avatar
Thread OP
Thanks @TrRC67. I'm prepping build instructions. Feel free to ask questions as they come up.
Jul 30, 2019, 07:26 PM
Jer. 29:11
jeffsch's Avatar
Thread OP

Shopping List


Here are some recommended parts to complete the model.

* 1 m x 5 mm carbon fiber tube (spar)
* Two 9 gram servos (ailerons)
* Two 6 in. servo extensions
* One 9 gram servo (elevator)
* One 9 gram servo (rudder, optional)
* Two ESC extension leads (approx. 12 in.)

For the 'regular' power option:

* Two 300-class, 1600 kv motors
* Two 10 A ESCs
* Two 7x6E propellers
* 1300 mah 2S battery

For the 'more' power option:

* Two 370-class, 1100 kv motors
* Two 12 A ESCs
* Two 7x5E propellers
* 850 mah 3S battery

And, if you want to use a 3- or 4-channel receiver, you'll also need:

* Y harness for aileron servos
* Y harness for ESCs
Last edited by jeffsch; Aug 17, 2019 at 06:42 PM. Reason: Upgraded spar to 5 mm carbon fiber tube
Jul 31, 2019, 06:39 PM
Jer. 29:11
jeffsch's Avatar
Thread OP

Building the Nacelles


A good first step is building the two nacelles. They are made from an inner monoblock (1 in. foam) and two outer laminations (1/4 in. foam).

For the first prototype, I used a cardboard template and a hotwire to cut the monoblocks. For the second prototype, I used a CNC hotwire machine; the nacelles were a little long for the machine, so I cut each nacelle monoblock in two parts, a front- and a back-half.

If you'd like, you can taper the back of the monoblock to get a tapered nacelle. I used a "surform", something like [1]. With light strokes, it quickly removes foam without tearing.

[1] https://www.stanleytools.com/product...t-plane/21-399

After tapering, you may wish to puncture the sides of the monoblock to help the glue penetrate, and then glue the laminations together, paying attention to the alignment of the motor mount slot and the alignment of the top edge of the nacelle. (Be sure to make one right and one left nacelle!)

The finished nacelle will look a little nicer if you round the edges. I used a 3D printed corner sanding tool to make quick work of this step. (See attached.)

Each motor mount is 1.8 in. x 1.4 in. x 1/8 in. Attached are some 3D-printable motor mounts for 300-class or 370-class motors. Before gluing the mount to the nacelle, verify that the mounts fit the motors. After gluing, cut a hole for the motor wires to pass through the monoblock into the nacelle. I used a sharpened piece of brass tubing to do this.

If you know what scheme you'd like for the finished model, now is a good time to paint the nacelles. On the first prototype, I also reinforced the bottom of the nacelles with some 1/2 oz. fiberglass and water-based acrylic. On the second prototype, I reinforced with 5 mil laminating film. (Not sure which I like best.)

The last step is to mount the aileron servos. Because the monoblock is thicker than a servo, I glued some small foam spacers in the servo pocket, behind the servo. Attach the servo extensions, thread the servo leads through the nacelle, and glue the servos into place.
Last edited by jeffsch; Aug 03, 2019 at 12:23 PM. Reason: Add picture of corner sanding tool
Aug 02, 2019, 07:19 PM
Jer. 29:11
jeffsch's Avatar
Thread OP

Building the Fuselage


The front of the fuselage is also made from an inner monoblock (1 in. foam) and two outer laminations (1/4 in. foam). The back of the fuselage is a hollow box with the outer laminations drawn together at the tail, aligned with a top and bottom (1/4 in. foam).

Some A-26 Invaders had both the top and bottom turrets, some only one, and some neither. If you don't want the top turret, cut it off the inner monoblock (or cut the pattern to omit the turret). Conversely, if you do want the bottom turret, cut the slot for it in the fuselage bottom.

Both outer laminations have an opening for a servo hatch. The right lamination also has openings for the receiver and battery hatches.

Glue the fuselage sides to the inner lamination, taking care to align the wing slot and the elevator slot. Before the glue sets, glue the top and bottom of the back of the fuselage in place and align carefully.

When the glue has cured, add the optional lower turret.

Now is a good time to paint the fuselage.

Thread the leads for the elevator and optional rudder servos through the fuselage, and glue the servo(s) in place.

There are several ways to secure the receiver and battery hatches. On these builds, I cut a small square of scrap foam, glued it so it protruded about 1/2 in. from the front end of the hatch, and used magnets -- one in the fuselage and one in the hatch -- to secure the back of the hatch.
Last edited by jeffsch; Aug 03, 2019 at 12:23 PM.
Aug 06, 2019, 10:21 AM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Nice photos and description! Really outlines your process quite nicely.

Not to disrespect the B-26, but I've always been partial to a B-25 Mitchell. When he was discharged from the Army (found out he was only 17) my Dad hitched a ride home from Japan in the nose of one. I'm wondering if I could apply this technique to building my own.
Aug 07, 2019, 08:26 AM
Jer. 29:11
jeffsch's Avatar
Thread OP
@Blacky's Boy, what a great story.

Should be able to apply these techniques to a B-25. There's a start on a B-25 in the SEMFF WWII thread:

https://www.rcgroups.com/forums/showpost.php?p=20140955
Aug 07, 2019, 03:03 PM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Quote:
Originally Posted by jeffsch
@Blacky's Boy, what a great story.

Should be able to apply these techniques to a B-25. There's a start on a B-25 in the SEMFF WWII thread:

https://www.rcgroups.com/forums/showpost.php?p=20140955
Thanks for the link! I'll take a look at this once I get out from under my current projects
Aug 07, 2019, 03:04 PM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Thanks for the link! I'll take a look at this once I get out from under my current projects

Quote:
Originally Posted by jeffsch
@Blacky's Boy, what a great story.

Should be able to apply these techniques to a B-25. There's a start on a B-25 in the SEMFF WWII thread:

https://www.rcgroups.com/forums/showpost.php?p=20140955
Aug 07, 2019, 06:44 PM
Jer. 29:11
jeffsch's Avatar
Thread OP

Building the Wing


The wing is a KF step airfoil, where the bottom of the wing is a complete outline, and the top of the wing is half chord.

Each half of the wing has a channel for wires between the nacelle and the fuselage. The wing bottom has a opening at the nacelle and at the fuselage for wires to enter this channel. I used a CNC router to hog out this foam, but a rotary tool would work just as well.

If you want to add the optional wing hard points, typical on the B-26 Counter Invader, then cut out the four slots in each half of the wing bottom.

In the plans, the wing bottom has a slot for a 1 meter, 5 mm carbon fiber spar.

On the prototype, I cut a slot in both the wing bottom and wing top, used two 1/8 in. poplar spars, and tried laminating in some fiberglass mesh tape. This wasn't stiff enough for my taste, so I recommend a springing for the carbon fiber.

If you want to add dihedral, it's easy to do while gluing the wing bottom to the wing top. I used 2 in. per side, and made a simple jig out of 2 in. thick foam blocks to hold the wing while the glue set up.

When the glue sets, shape the leading edge. I used a 3D printed tool to sand the recommended shape but it's fairly easy to get a close approximation with block sanding.
Last edited by jeffsch; Aug 17, 2019 at 06:42 PM. Reason: Upgraded the spar to 5 mm carbon fiber tube


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