50% Scale B-25 Mitchell Bomber
Another Museum Project
By Vince Homer
I was looking over topics on the “Homebuilt Aircraft Forum” when I ran across one about which WW II aircraft would make good, scaled aircraft projects. In this one, the B-25 was mentioned as a possibility. Having already built a 75% P-39 and a 55% ME-262, I decided to take up the challenge. Using a set of plans for a giant scale RC B-25 and some anthropomorphic templates I determined that 50% would be a good scale size. The real B-25 has a 67 foot wingspan, so even at 50% it would be a good sized aircraft. So, the project was set: build a mock-up of the two place, tandem cockpit of a 50% B-25 Mitchell medium bomber.
The RC plans I had were on a CD so I imported them into a CAD program that I use called Draftsight. The plans are then traced to produce drawings that can be used in any scale. The bulkheads were printed in “tiled” format meaning each bulkhead was printed on several 11 x 17 sheets and taped together. The bulkheads were made from double wall corrugated sheet I bought from the Albany Box Company in 4 x 8 sheets. It’s 1/4 the price of ¼” plywood and can be cut with a box cutter, a sabersaw with a knife blade or a bandsaw. 1/4” plywood doublers are used at stress points and the corrugated is painted with polyurethane varnish for water proofing. I use Gorilla polyurethane glue and spritz a little water on it to hasten the reaction for all glue joints.
The bulkheads are mounted on 1 x 2 furring strip spines and longerons resulting in a very strong fuselage. A simplified cockpit interior is installed using a cockpit ergonomics diagram I found on the internet. One change that was required was to make the nose gear rotate when retracted. In the full size B-25 the nose gear retracts straight up under the pilot’s seats, but there’s not enough fuselage height to do that at 50% so I made it rotate flat like a P-40. To get the right size nose wheel I made one from an inner tube wrapped with friction tape and spray coated with Rustoleum black water seal. The wheel rim is plywood and corrugated and the gear leg is PVC water pipe filled with “Rock Hard” water putty for strength. The landing gear on the P-39 is made the same way only I used wheelbarrow tires for mains.
Two throttle/prop/mixture quadrants were made along with yokes, rudder pedals and an instrument panel. For the steam gauge “Six Pack” I used drink coasters. The seats are semi-reproductions made from ¼” plywood. These minimal cockpit details are made to check the ergonomics of the set-up. An extra section of swing open canopy was provided for the passenger. The glazing of the passenger canopy is silvered to look like aluminum.
How to get in and out of your scaled aircraft may pose a problem. Entrance to a full size B-25 is through a hatch on the belly or through an aft fuselage door. Neither will work in a reduced scale version making it necessary to open the canopy. I chose a throw-over style to allow for a second canopy for the passenger. The glazing on the passenger canopy is covered with aluminized film to make it look more like aluminum fuselage since there’s no windows in that location in a full size B-25.
Aft support legs from 1” steel square tubing with large diameter castors were added to support the aft end of the cockpit as well as removable from front support legs made from ½” EMT conduit. The front supports stabilize the structure allowing persons to get into the cockpit and to allow the nose gear to be retracted.
The mock-up was skinned on one side with construction cardboard (available at Home Depot in rolls) and painted. The frame was painted with “Dash of Dill” green interior house paint to look like zinc chromate and the skin with Rustoleum aluminum.
There you have it! A fun, three week project resulting in a perfectly useless, ½ scale B-25 Mitchell bomber mock-up that you can sit in and make motor noises. I’ll make it even better with a CD I have of round engine sounds including a B-25 start-up.
If one had any inclination to design and build an aircraft, building a cockpit mock-up would be a very useful exercise. The 50% B-25 and the 75% P-39 showed the size and layout to be acceptable, but the 55% Me-262 was too small to be comfortable or safe. I could get in it, but could never get out in an emergency nor fly it for any length of time. The ME-262 should have been at least 65% scale. The first step in the process is to compile a table of “What Ifs” as shown below. Using this table, a set of suitable RC plans, the cockpit ergonomics diagram and anthromen templates the scale of your project may be chosen.
The values for weight, engine horsepower and speeds depend on the choices of wing loading and power loading used. For example; wing loading can vary from 4 lbs/ft2 for WW I replica biplanes to 50 lbs/ft2 for a full size, max gross B-25. To make the “What If” table one has to decide what you want the aircraft to fly like and what level of pilot skills will be required to fly it. Some examples are shown below:
(1) Increased dimensions from “fat” fuselage. A usable cockpit can be made in a smaller scale version by making the cockpit area a little wider and taller than scale. No one will notice small increases in size.
(2) V-Twin near stock engines with redrives or electric with 30 – 45 minute duration.
(3) V-Twin with improvements with redrives or electric with 60 minutes duration.
(4) Verner radials or Hirth or electric with 60 minutes duration.
(5) SBS or tandem if the W&B could be accommodated.
(6) Possibly a two place tandem with a bit more HP.
(7) Based on 12-15 lb/ft2 of wing loading (W. L.) and 10 lb/HP power loading (P. L.). Higher W. L. and P. L. may be used for higher performance aircraft. This will result in higher stall/landing speeds, increased ROC and higher top speeds all of which may increase the piloting skills required.
(8) Hybrid based on 20 – 40% extra TO power from parallel electric motor(s). In truth, this would be a complicated enough project using just engines. Adding electric hybrid adds another layer of effort requiring additional expertise and cost.