Swappable B25 Mitchell Twin-Engine Bomber

UPDATE -  I started a thread in the forum on this build if anyone wants to post upgrates or pictures of their versions that would be great to see. 

http://forum.flitetest.com/showthread.php?12028-Swappable-B25-Twin-Engine-Mitchell-Bomber&highlight=b25

The basic design of the FT Spitfire is readily adaptable to a number of other planes. I have previously posted adaptations of The Messerschmitt bf109 and P51D Mustang, both of which flew well and looked great (see links at the bottom of this post). The whole idea behind these was to change any parts needed to make it resemble the plane of choice, but not to change the joints between the parts so you know it will fit together. 

This basic configuration allows for almost any WWII fighter to be adapted, but there are two big exceptions the would also allow a lot of really fun planes from the same era to be made. The first is gull wings (e.g. Corsair or Stuka), which seems like it should be doable with only a few changes. The second is big multi-engine planes, like bombers. Here, I show how to make a popular bomber using essentially the same parts as FT Spitfire or the other planes I have designed. I picked the B25 Mitchell bomber for a few reasons. It is an iconic plane, its shape lends itself well to foamboard construction, and it was no sluggard in the air, having been adapted for a number of purposes beyond droning around and dropping bombs. I hoped this might be the case in a scaled up Swappable twin engine version, and this proved to be the case. The plane flies like a big plane, but is lots of fun and looks pretty good. 

Below are some videos of the maiden flights showing some of the main characteristics, and also instructions to build your own. I should say, this is not a plane for beginners. Both flying it and also building it requires some experience. I have not been quite so step by step with my build instructions, because I assume if you try it you have some experience under your belt, and already know the basics. Much of the design was made up during the build, and many of the parts I made up as I went, and built to fit. Still, if you know how to build a Spit or bf109, you could do this from the below instructions. 

But first, I recently got to go to the RAF museum in London (http://www.rafmuseum.org.uk) and I highly recommend it. It is an excellent museum, and happens to have every plane I have ever designed, and some I am just working on. So I have updated all my old posts to include picutres of the real thing, and where possible will do this in the future too. 

Here is a shot of the dintinctive cockpit and nose. 

This one has a different tail turret. You can also see the interesting partial dihedral on the wing (just past the nacells)

Here is a nice shot of the distinctive rudder. 

Here are two short movies showing how the plane flies and looks. The first one has a clip from the BombCam that we set up, which is kind of fun (the camera faces back so you can see the plane as it falls). The second on is from a friend Ed (thanks Ed), and shows a few clips from the prototype flight before it was finished. 

BUILDING INSTRUCTIONS (PLANS BELOW):

The first step is the wing. Where the wing meets the fuse is the same as the Spit and my other builds, and all I did was extend the length so that each wing half uses the whole length of a pieces of Adams foamboard, and extended the spar accordingly (did not bother making plan for this, just make it longer).  The overall shape is just estimated based on pictures of B25s. Ailerons were standard 1.5” X 10” and the servos were installed on the spar, as usual. The wing halves are attached like those of the Spit, but I reduced the dihedral by 50% since the B25 only had a bit of dihedral between the fuselage and nacells. This seems to be fine and did not affect the stability.

Next is the fuselage itself. You have to install the wings BEFORE adding the nacells (or the wing can't be forced into the wing holes in the fuselage). Again, the overall shape, including the height and width, is the same as the Spit, but the length is extended to the rear to use the whole length of the foamboard. None of the notches for the power pod need be cut in the top. The holes for the wings are the same, but raised so the wings emerge roughly half way up the fuselage (once the top formers are added).  I also inserted a triangle at the top rear because I made my whole tail assembly removable for storage, but this is optional. At the extreme rear the two sides are not glued directly to one another, but three small pieces of foam are glued between them as spacers (where the rear gun turret will be). 

Wing installed in fuselage

The motors are housed in two adapted power pods. The front end is identical to a normal powerpod, but at the rear they are tapered on the sides and bottom to form the trailing edge of the nacells. Paper was glued to top of the back 2”, which covers the part that shows behind the trailing edge of the wing. I cut a notch about 3” from the back to allow the wires to be fed from the powerpod to the fuselage (this is an extension of the servo wire, and I soldered a Y-connector for the positive and negative power wires with bullet plugs). I tucked them into the gap in the trailing edge of the wing to keep it tidy, and left all the connections outside the powerpod so they can be disconnected before removing the pod. The power pods are mounted under the wing in two A-fold boxes that fit tight to the power pod dimensions, which are glued to the wings (with notches cut in the wing bottom) parallel to and 5” from the fuselage (to easily allow 9” props). If you make this tight you don’t need the skewers at the front, just one through the side. 

Marking nacell positions

To make the nacells look more realistic, it would be nice to make them round. So to trick the eye, I made round covers for the front of each. These are easy. First take a short length (like 1”) of 3” foam pipe insulation. Then use these to form a tube from a length of something like thin plastic or paperboard (I used the packaging from a thermarest) that has been cut in the shape shown to allow them to fit over the leading edge of the wing when they are formed into the tube. Then I added some velcro to the back to hold them on the plane and put a little square of tape where the cover meets the top of the wing. It works well - the eye sees a round nacell. (this piece is included in the plans below)

Nacell covers, trick they eye into seeing a round nacell

These are removable (to take the pod out) and attached by velcro

The power system I used consists of two Turnigy 2217 1050kv motors, two 35amp ESCs and 9X4 SF props. This provided plenty of power. I used a standard 2200mah 3S battery, which is situated near the front of the nose to balance a pretty tail-heavy design.  

To finish the fuselage, formers are added as usual. Three former shapes were used. Formers are numbered from front to back on the main fuselage (1 & 2 forward of the cockpit, 3, 4 & 5 behind. Former 3 is repeated twice. Forward of the cockpit formers 1 & 2 set up a slight downward angle. The usual paperboard is glued and taped down over the formers to the rear of the cockpit, as usual. I just cut this to fit after the formers were installed and cut two slits at the last former to taper it. Leave the paperboard forward of the cockpit for now, it is cleaner to install that later. 

Formers and paperboad installation - Formers 3, 3 & 4 are already covered here

In the bottom I cut out a section to make bomb doors, and also provide access to the electronics. I aim to drive the bomb doors with servos at some point, but am posting this before I have gotten around to figuring out how. This proves to be a bit tricky. With servos with huge throws it would be simple enough, but if anyone has any good ideas to McGiver something with standard Hextronic 5g or 9g servos, I would love to see a comment. For now, I satisfy my want to bomb things with a simple bomb release mechanism on the gear channel (see picture). 

 Bomb Dropper

BombCam (the yellow loop attaches to the dropper - it is a slice of plastic hose which prevents hang ups - and there is a lot of lead weight added to the nose so it falls camera-up)

The tail is a bit tricky, because the B25 has a distinctive double rudder. To make this, cut out two rudders. They are mirror images, so cut the hinge and mortises on opposite sides (and leave the paper on the outside of the mortise so it hides the tenon from the horizontal stabilizer). Cut the horizontal stabilizer as shown, and attach the two rudders. To be honest, having now flown it I have come to the conclusions that this plane does not need rudders, so if you want, skip making them hinged and skip the complex problem of how to drive them. But if you DO want rudders, and like making things difficult for yourself, what I did was mount a single servo in the bottom of the horizontal stabilizer so just the double arm poked up. Then I cut notches in the vertical stablizers, just forward of each rudder, and inserted control horns in the foam of the rudders so they emerged into these notches (see pictures - its hard to explain). These were plastic strips from a hotel key card with a hole, inserted about 1.5cm into the rudder. Then two thin pushrods were inserted into thin flexible tube which arced to the servo arms and were held down with some glue and a couple small zip ties. The result of this is that the two rudders move together in the same direction. I tried a few variations of this, and ended up throwing them away, but this one seems to work. In retrospect, I would not bother with movable rudders.

Rudder Assembly

Rudder Hinge

The elevator is driven by a servo in the side of the fuselage, as usual. The trick here is to get both elevators to move together because they are separated by a bit of fuselage. I made a piece of wire to connect them and glued/taped it to the top, and this seems to work okay. My tail is held down with an elastic (bike inner tube, great elastics) at the back and a tab under the fuselage at the front so its removable, but again it would be simpler just to glue it down. 

By now you probably noticed the plane has no nose. This is because the foam board sheets are not long enough to keep the proportion right. To solve this, and at the same time make a  hatch to access the battery, you have to add a nose piece. This is a 3” extension of the fuselage lines that fits to the main fuselage using an inside tight-fitting coupler, basically a 3” box that is just smaller than the two pieces to be joined. Glue this inside the nose extension and this allows the nose to slid on and off (and put a skewer in to hold it, though it seems unlikely to ever fall off). The nose piece is a box just like the fuselage, and I cut the bottom at the same angle as the fuselage, so the lines are just extended.

Once you have the nose box built, install two formers on the top that continue the line of the fuselage top forward of the cockpit. I simply made them about right, then put the nose on and cut them down to match the angle on the rest of the fuselage. Now cover the entire forward section with one piece of paperboard, gluing it down on all four forward formers (attaching the nose to the rest of the fuselage temporarily), and taping down the sides, as usual. Once this has dried, take a sharp blade and carefully cut the paperboard between the fuselage and nose section to re-separate the nose. This ensures your two pieces form a perfectly consistent line from the cockpit to the nose, and looks very clean. 

The rest is optional decorations. I made cockpit canopy, top gun turret, rear gun turret, and nose-glass out of old pieces of clamshell packaging and black duct tape. I used coloured tape for the rudders and wingtips for visibility, and went with a British paint scheme with the black and white stripes on olive, and varnished and painted it. 

Tempting, but so many people gave me grief about the stickers on my Messerschmitt not being authentic...

Tail Turret - bit of garbage plastic

Front Canopies - more bits of garbage plastid (the main one and top of nose are given in the plans)

Battery hatch - I use a 2200mah 3S, which is velcroed onto the floor of the nose

I balanced the plane assuming a CG at the wing spar, and this worked well. The plane glides okay at speed, but does tip stall if you are underpowered. I saw this as I glided for landings, especially once with a tail wind. I now land under low power until the last second, then power down (see movie). Other than that it flies great, and since it is big you can get up pretty high for great bomb dropping. I would seriously consider skipping the working rudder on this build, its a lot of work and it does not appear to need it to fly well. Looks nice in the air - here are some stills from a movie during a nice early morning flight. 

Now I am asking myself, should I make plans for this? I actually hate making electronic files for plans and don’t think I do a great job because I hate doing it. But would be nice to have plans if people really wanted to make the plane, right? Unfortunately I can’t tell from previous posts I have done whether people are actually making them or just reading the post. All my friends told me I had to make plans, so I did, but I would ask one favour - if you actually do make this plane, can you please leave a comment saying so? I would appreciate it in case I am wasting my time with plans. Thanks.

BUILD PLANS:

Canopies

Formers

Fuselage

Horizontal Stabilizer (re-uploaded)

Vert Stab

Nose Coupler

Nose

Nacell

Nacell Covers

Powerpod

wing