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-   -   Semi-scale B-25 (https://www.wattflyer.com/forums/showthread.php?t=72990)

dgjessing 01-17-2014 12:49 AM

Semi-scale B-25
Ever since my wonderful wife arranged for me to fly in a B-25 last summer (this one: http://www.champaignaviationmuseum.org/#!b25/cgmb ), I've been vaguely thinking about building a model of the plane.

Today I've started thinking about it in more detail, and started on drawing up some plans. This will be my first twin engine model, so I'm not planning on getting overly involved in scale looks, but rather just going for a basic flyer, to learn the ins and outs of doing a twin. So the fuselage and engine pods are going to be squared off foamcore structures - sort of a 3 dimension "profile" thing :rolleyes:.

I found some 3-view drawings somewhere on line and established a scale conversion factor to lay out the plans. It's going to have a 42" wingspan and about 32" over-all length.

More as things progress! :)

fhhuber 01-17-2014 02:05 AM

Slight downthrust and outboard thrust both sides even with counter-rotating props.

And don't count on electric preventing single-engine-out. I've seen planes throw the prop hubs and other strange things which electrics are not immune to.

dgjessing 01-18-2014 12:15 AM

OK, off we go :D


Got the wing halves cut. The plan is to glue in top & bottom spars, then cut the angles (the B-25 has a gull-wing - dihedral in the center, flat beyond the engine pods).

dgjessing 01-19-2014 12:22 AM

And here are the four wing sections, with spars glued in and the angles cut:


And here's the whole thing glued up:



baz49exe 01-19-2014 08:09 PM

Looking good:- can't wait to see this model develop!:D:D


dgjessing 01-19-2014 11:18 PM


Nothing to show today, but I did put a coat of primer on the wing and ordered motors & speed controllers. I'm going with two of these: http://www.headsuphobby.com/Emax-CF2...otor-H-445.htm with 5 x 3 three blade props. That ought to do it (I hope) and the 3 blade props will look good :). I got both left and right hand rotation props.

I'm anticipating about 26 - 28 oz AUW.

I'm going to need to run both the aileron servo wires and power lines for the speed controllers out to each engine pod. Would there be any issues with interference to the servos if all those wires run in the same groove?

DHC Beaver 01-20-2014 12:36 AM

I have a rat's nest of wires in my FW Ente,but have no problems with interference.I did run the motor wires in a separate paper tube though.IMHO, you should think seriously about keeping the motor wires separate,as they would be the "noisiest".
Did you put any dihedral braces in the gull wing part?Look forward to developments.[popcorn]

dgjessing 01-20-2014 03:09 AM


Originally Posted by DHC Beaver (Post 937556)
Did you put any dihedral braces in the gull wing part?Look forward to developments.[popcorn]

Yes - there is an 1/8" square pine spar top & bottom, then vertical webs of 1/16" ply at the joints, glued to the pine spars. All epoxied together. Should be pretty strong, I'm thinking.

fhhuber 01-20-2014 03:33 AM

Depends on expected weight and style of flying if that will be adequate. A butt-joint of a spar may as well be silly putty if it doesn't have a good doubler.

The glassing may or may not be enough... You can get away with a lot with a smaller model such as yours. The same techniques with larger model I'd expect the wings to fail.

dgjessing 01-22-2014 12:11 AM

And the wing is pretty much done:


It's covered in packing tape (which doesn't do compound curves) so we've got a lot of dark triangles where the tape is doubled up :roll:. But I think once the fuselage, engine pods and invasion stripes :rolleyes: are in place the dark spots will be much less noticeable.


Originally Posted by fhhuber (Post 937568)
Depends on expected weight and style of flying if that will be adequate. A butt-joint of a spar may as well be silly putty if it doesn't have a good doubler.

Apparently I did not describe the wing joints adequately. The two (top & bottom) spars are joined in lap joints with a plywood gusset with the angle of the wing cut into it, and extending an inch each way into the joint. The foam is a butt joint, but not the structural stuff. I'm sure the joints are considerably stronger the the straight sections.

On to the fuselage! :D

fhhuber 01-22-2014 01:14 AM

OK... looking good so far.

You can use heat with some of the packing tapes to get them to do compound curves :) Works well for getting out wrinkles too.
Test on tape applied to scrap foam. Sometimes the foam deforms before the tape shrinks or stretches.

dgjessing 01-24-2014 10:44 PM

I like the stripes :):


The white is "Duck Tape", the black is electrical tape. I cut the white to size by placing it on wax paper and cutting with a razor blade.

On to the front landing gear. On the real B-25 the front wheel is just a caster - it is not steered directly. Ground handling is done with the rudders, brakes, and differential thrust. On my model I'm skipping the rudders and making the front wheel steerable through the rudder channel. So I need to make a nice sturdy pivot point, and I'm going to mount it in rubber to adsorb some shock & hopefully protect the wire strut.

dgjessing 01-28-2014 12:14 AM

And starting on the fuselage:


That wooden thing is the mount for the above-mentioned front wheel pivot.

dgjessing 01-28-2014 11:49 PM

And it's kinda taking shape:


I've run a couple of 1/8" sq spruce strips from the nose wheel structure back to the tail (sort of visible in the picture).

baz49exe 01-29-2014 07:55 AM

"Looking good" Dave.:D
I still use spruce for spars and fuselage keels and it works very well for me too!


dgjessing 02-02-2014 12:55 AM

A little more work on the fuselage, and the engine pods roughed out:



DavidR8 02-02-2014 01:45 AM

Nice work...looking forward to seeing the progress

fhhuber 02-02-2014 03:38 AM

Watching... planning to scale up and build one "a bit" larger...

dgjessing 02-02-2014 05:19 PM

I'm getting close to needing to run the wiring out to the engine pods. Here's the plan:

In each pod will be an ESC for the motor and a servo for the adjacent aileron. I'll have a separate UEBC in the center fuselage. So as I understand it, I'll need to run a + and a - wire at battery voltage to power the ESC, then a + and a - wire at servo voltage, and two signal wires (one for the servo, one for the ESC). So a total of 6 conductors, two of heavier gauge.

Does that sound right?

fhhuber 02-02-2014 08:59 PM

The amount of wires sounds right.

dgjessing 02-02-2014 09:31 PM


Originally Posted by fhhuber (Post 938948)
The amount of wires sounds right.

Thanks :)

dgjessing 02-02-2014 10:10 PM

It's starting to look like something:


I put in 1/8" plywood plates for the rear landing gear, gorilla glued directly to the foam of the wing. (Inside the engine pods) Hope that's strong enough :eek:.

I think it is...

pmullen503 02-03-2014 12:23 AM

Don't forget to disconnect the red (+) wires from your ESCs if you use a separate UBEC. Though at this size, it might easier to run ESC's with built in switching BEC's and just disconnect one of the ESC's red wires.

CHELLIE 02-03-2014 12:38 AM

Looking Good :ws: Subscribed

dgjessing 02-05-2014 12:20 AM

Here's one of the rear landing gear legs in place:


Most of the work was in making a stamping tool to form those hold-down straps. I've since slathered epoxy over the straps and screws to hold them in place, as there is not going to be any easy access once things are buttoned up.

The motors and ESCs and stuff arrived today! (They were in postal limbo since being ordered the 19th of last month :eek:)

After piling everything I could think of on the scale, it looks like it may hit the high end of my estimated AUW of ~28 oz. We'll see :rolleyes:.

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