FT-Style Mighty Mini B25 Mitchell Build

by MiniacRC | July 4, 2018 | (20) Posted in How To

FT-style Mighty Mini B25 Mitchell Swappable    

    The Mitchell has significant history behind it and designing a scale model, especially at this size, was a daunting task. The B25 isn’t hailed as the “Jack of All Raids” for no reason, and I wanted this small foam board design to reflect that versatility in its flight envelope. After about a year and a half of deliberation, design, prototyping and beta-building, today marks the official release of my first ever original FT-style swappable design! If you haven't already, check out the release video below.

    I’ve only been in the hobby for less than a couple of years, but I’ve had a passion for aerospace design and aviation as far back as I can remember. I bit the bullet to commit to this design around the middle of last year. The design process has been long and difficult with several occasions when I was very close to quitting completely. The main source of inspiration that motivated me to get back to the design table was FliteTest and their wonderful community, especially the wonderful beta builders, some of who are recognized at the end of build video. Words cannot do justice to the commitment and passion they showed in pushing this design to success by chiseling away at its weak points.
    As always, there are free plans and a full build video that I prepared over the course of several months to assist potential builders in the processes necessary to assemble this plane. The Mini Mitchell is a standard mini swappable built out of less than two sheets of foam board and one 11”x14” sheet of poster board, all from Dollar Tree. The B25 is a great performer but a strong intermediate build and flyer. Read on to find out whether this could be your next build!

Plans:

B25 Release AIO Plans.pdf

Specs and Materials:

Center of Gravity: 1.25 inches from leading edge at wing root
Control Surface Throws: 10 – 14˚ deflection, 30% expo all around
Wingspan: 32 in. (813 mm)
Motors: Two 1806 size 2280 – 2400 kv miniquad motors (Twin Engine A-pack)
Prop: 6*3 two or three-blade
ESC: 12 – 20 amp
Battery: 3S 800 – 1200 mAh
Servos: 3 servos for 4-channel control ; 2 additional servos for cargo bay (5 - 9 gram servos)
Power Y-harness: ≥ 6 on each side

Accessories: BBQ skewers and Craft knife

Build Process

    This FT-style build video walks you through virtually every step in the procedure to assemble this ship and pack in the appropriate electronics. As this is an intermediate to advanced build, the process has been divided into several components for clarity. I highly suggest that builders, as with all other FT build videos, please watch or at least skim through this video once before beginning the build process.

https://sketchfab.com/models/3c7fee4e527345c7838ccb572e09f103

    A digital 3D model of this design is provided in the link above, to help builders better understand the structure of the airframe.


Scale Design

    This B25 has been designed with attention to scale proportions and details. The design is based off of original blueprints of the Mitchell obtained online.
    The tapered wing sports an authentic gull wing with inboard dihedral and outboard anhedral. The box spar inside along with doublers at the joint between wing sections provides tremendous strength to sustain high speeds and loading. The presence of the spar also eliminates the need for any dihedral gauge in constructing the wing. Located under the wing, the nacelles of this plane are built with the Rounded-Approximation-Method (RAM), using several molded rings of foam board to approximate the compound curve. The thin rings both provide a scale appearance and make the molding process very easy.
Functional Cargo Bay
    This design features a unique mechanism that can add scale to the mighty mini experience! The Mini Mitchell has an optional functional cargo bay that can be operated by two stock Emax 5-gram servos.
    The mechanism is immensely robust in carrying weight even during aerobatic flight. Servo speed can even be adjusted to provide a scale appearance. Note: Please do not use this mechanism to carry items that may harm the plane or people/animals below. The cargo bay is perfect for backyard target challenges with small Nerf darts or even the mock shells provided on the plans.

Flying Characteristics
    My flying buddy Dominic Peluso painted and flew the final pre-release prototype of the Mini Mitchell and provided some pointers for pilots out there. Despite a couple of years of experience flying RC planes, this was in fact his first ever twin engine model. However, he got the hang of this ship within three successful flights, which are compiled in the release video.
    As with the FT Mini Cruiser, the elevator on the Mini B25 will have generous control with minimal deflection. Use lots of expo to filter out the nervous finger-jittering that is bound to occur on the first few flights. The ailerons will behave similarly but the plane will respond in a more scale manner, with a slight delay in the order of half-a-second. Dominic warned not to panic and provide too much aileron input during this delay period as it may cause the B25 to over-roll into an inverted orientation. As long as you plan your turns and take care to use the correct ratio of speed, aileron and elevator, the B25 will turn solidly.
    Lacking a function rudder, the key to scale flight with this bird is differential thrust. Again, the rates must be set relatively low to avoid entering a descending spiral, but when combined with elevator and aileron, differential thrust will provide a much smoother coordinated turn, pulling the leading wing around.  
    The B25 has more than enough power to go vertical on the stock A-pack with a 3-cell battery, but the plane shines in slower flight with scale low passes. It resents snappy aerobatics and prefers long smooth lines, whether slow or fast.
    This is definitely not a beginner plane to build or fly. However, if built, balanced and dialed in correctly, in the hands of a strong intermediate pilot, the Mini Mitchell can be a wonderful addition to your fleet of scale mini warbirds. I flew my FT mini mustang (post-crash pic below) with Dominic’s silver Mitchell (above) and the match-up was glorious to witness. I hope that this design becomes a solid performer at your field and adorns your hangar for years to come.

    Words aren’t enough to thank FliteTest for the encouragement and support they provided through their show and their community for me to pursue this design in an economic and educational manner. Thank you for reading this article and happy flying!!!

COMMENTS

Michael763 on July 5, 2018
Great article, video and plans!
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MiniacRC on July 5, 2018
Thank you very much sir!
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Jackson T on July 6, 2018
That is one beautiful plane!
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MiniacRC on July 6, 2018
Thanks!
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Kunzie on July 6, 2018
Very nicely done! The B25 has always been one of my favorite birds. Thank you.
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MiniacRC on July 6, 2018
Thank you very much!
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wickedwindsor351w on July 6, 2018
Nice Work! Enjoyed your article and video very much. Definitely going on my build list!
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MiniacRC on July 6, 2018
Awesome thanks!
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Nathaniel on July 6, 2018
Very nice plane!!
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MiniacRC on July 6, 2018
Thanks!
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bruceblackman on July 6, 2018
The only thing left is for Sankaran's effort be acknowledge in one of the weekly YouTube videos. His interest in sharing, creativity, skill & tenacity needs to be acknowledge as a "Job Well Done"!
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MiniacRC on July 6, 2018
Thank you very much Bruce! The beta builders also must be recognized for their effort and motivation!
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dwnhlbikr on July 7, 2018
This is such an awesome plane that has amazing scale features. Guys in my club love mine.

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kilroy07 on July 7, 2018
Well, you've set the bar pretty high... Amazing work (both in the design itself and the build video!) My ONLY beef, is that it's a BOMB bay, not a cargo bay... LOL

Seriously Great Job! This is definitely going on my build list.
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MiniacRC on July 11, 2018
Thank you very much for the kind words!
True, I was trying to go full-on PC and non-controversial with the "cargo bay". Took a good month to find a suitable synonym. Please let us know how the build goes if you get to it! :)
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tomh on July 10, 2018
wow!! I can appreciate the tremendous amount of work that you out into this project, and the resulting plans and video are as professional as any flitetest production. I can see building this in the off flying season..
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MiniacRC on July 11, 2018
Thank you so much. I tried as best as I could to emulate their production value and I'm so glad it came out well! Best!
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Beavis on July 15, 2018
Fantastic! The build is great, the video is superb! One thing I love is that you chose a mini scale, which comes in just about the right size on a twin engine bomber - not too big, not too small.

So... how important is differential thrust to overall flight? Would it be a nightmare without it? I'm thinking some rudder would compensate for no differential. I fly with a DXe, so program capability is more limited. I ask this because this is now on my build list!
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MiniacRC on July 18, 2018
Thanks for your comment. :)
I flew my b25 this Sunday and mainly flew bank and yank with maybe a slight touch of differential for its first flight. I was largely using bank and yank it was perfectly fine. I am preparing footage that I'll post soon. I can say that this will fly without differential thrust, but you need to carry a little more throttle.
Those very slow passes and turns you see on the video were done by my friend using differential thrust. He said that when the plane is flown slower to emulate scale flight, differential helps you bring your wing around because you don't want to bank this heavy warbird at such a slow speed. Even a little rudder should work fine because he only touches the differential slightly to yaw the plane.
So if you are going to go completely without any yaw control I'd recommend flying above 75 percent throttle on turns. For straight passes you can still slow down just fine.
Again, thanks for your comments! I really appreciate them.

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MiniacRC on January 13, 2019
So update!
It turned out the fault was more our lack of experience. A couple flights with this planes and then I started to be able to slow it down to a crawl, even on bank and yank turns. It's just an intermediate flyer. Differential thrust is not a must, but does add obviously powerful yaw authority if you like.
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erad on January 4, 2019
Really beautifully designed Model and very scale. Congrats. Encourages me to do another design that spins in my head for quiet a while now: Polikarpov.
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MiniacRC on January 13, 2019
Thanks so much!
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MiniacRC on March 4, 2019
And go for that design!!!!
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808aerosquadron on February 12, 2019
Really well done. Thank you for sharing.
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MiniacRC on March 4, 2019
Thanks!
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Kletus on April 1, 2019
I just discovered this plan today! Im 65 and have given up on nitro/gas. Sold everything and am starting over in foam board. Been scratch building since I was 16 but I've found that this stuff ain't nuthin' like balsa/ply, lol. I'll get the hang of it and then build this beautiful bird! Thanks!
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MiniacRC on April 8, 2019
Kletus, glad you've hopped on the foam train sir!
Alot of the build techniques will come easily to you since you have a background in balsa. Yeah I'd definitely start with some simpler FT planes since this one can be taxing as a first plane. Hope you enjoy your journey in foamies!
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FT-Style Mighty Mini B25 Mitchell Build