Mighty Mini B25 Mitchell: Red Bull Design Edition

Red Bull Mighty Mini B25 Mitchell

230 mph: Average cruise speed of the B25 Mitchell

440 mph: Approximate maximum speed of the Focke-Wulf 190 D-9.

Late 1943... 35,000 feet over the pacific theater, during the height of the second world war. Bulky, slow allied bombers are lumbering along in formation, when suddenly, enemy fire approaches. Within the blink of an eye, FW-190 A-6 fighter planes hurtle past the squadron, MG 17 rounds blazing. The graceful bombers are but sitting ducks for the high speed precision attack of the nimble single engine fighters... Fast-forward to 2018. With the gruesome war long over and the cruel demands of age-old aerial bombing long subsided, would the result be any different in a modern speed and maneuverability contest between single engine fighters and multi-engine bombers?

The legendary medium bomber we chose to challenge the historical saga, is the North American B25 Mitchell, arguably the most important and versatile medium bomber operated during the second world war. Known for its success in multiple aspects of aerial functionality including strafing, ground attack, long-range bombing, aerial refueling and reconnaissance, the Mitchell was the perfect choice to represent bombers in this tournament. The "Jack of All Raids" was commissioned once more – this time – for the 2018 Red Bull Challenge.

Some of you may have chanced upon the Mighty Mini B25 Swappable release article that was put out on the fourth of July. After all the fuss regarding the release of the plans and the build video subsided, I decided to do something with my post-release model, that I had wanted to do for a very long time: HOT-ROD IT!!!

Meet the newest mighty mini B25 swappable, RED BULL EDITION, parked along with its companion, the original release prototype with its scale scheme painted by my friend and involuntary test pilot Dominic Peluso. 

The plan was simple: stay together in formation for as long as possible while maintaining a slow, scale, speed, and pray that both ships hit the ground in one piece – each.... But destiny had other plans as we both tossed our mini Mitchells into the blue skies. The formation flight soon turned into a gloves-off race! Wing-men became ruthless competitors! Watch the video below to get a taste of how it all turned out.

Before I talk about the modifications and the power setup that tugged around the Red Bull Mini B25, let me take you on a quick journey down memory lane, about the birth of this design.

Mini B25 Design Process

The idea to design a B25 popped into my head around the same time I joined the hobby – about a year and a half ago. The Mitchell's striking nacelles, unique fuselage and H-tail captured my attention and imagination. I explored images and videos of the real Mitchell and scale models designed by various individuals and companies including Ultramicrobe (on the FT forums), FMS models and E-Flite. 

For the first few months, I did nothing but just imagine in my head, how foamboard and posterboard could be bent, folded and curved to approximate the complex curves on the B25. After viewing other designs such as the Nerdnic A10 Warthog and the FT Sea Duck, I chose to tackle the option of modeling many of the compound curves on the Mitchell using molded foamboard, by peeling off paper on one of its sides. Nevertheless, I wanted to avoid having to apply a heat gun, or use any other advanced building technique beyond the standard current FT skill-set. My design tools of choice were Autodesk Inventor® and AutoCAD®.

The first major challenge was designing the nacelles. I had two options, either to model it as a 'banana', where it would look curved from the side, but polygonal from the front, or as stacked rings, where it would look round from the front, but linearly approximated from the side.

I chose the second because it allowed for easier designing, building and integration with the rest of the wing structure. To unfold the curves, I tapped into my high school geometry and developed some simple formulae to determine the dimensions of the 2D annular section from the corresponding 3D frustum section.

Beginner's luck on the first prototype

The wing was much simpler due to the power of the "Loft" tool in Inventor. I chose to taper both the thickness and the chord of the wing so that I could avoid warping the slanted top surfaces and parametrically define them for easier conversion to plans.

The loft tool also allowed to me to elevate the middle part of each wing section to achieve the authentic gull wing (inboard dihedral and outboard anhedral). I divided the wing into three sections to simplify the building process and prevent the section plans from overlapping.

Ed from Experimental Airlines introduced the unique tubular fuselage technique to the FT community many years back, and that was perfectly suited for the B25. For the rear fuselage, I chose to taper down the radius of the curved edges to approximate the scale appearance. I began with almost fifteen formers for the entire fuselage, then trimmed down the count to around five to reduce weight.

The parts that I enjoyed most to design were the little scale details including the turrets, the canopy, the cowling scoop and the characteristic bleed-over of the nacelles on top of the wing, also known as the hourglass shape. I exploited the sheet metal features of the software to model and unfold the molded curves on these components.

The toughest challenge of the airframe design process was the nose. Since I had chosen to use a tubular fuselage, the curves on the edges of the body prevented me from creating a simple uni-axial mold similar to the Mini Guinea. However, I also couldn't simply taper the fuselage's cross-section down and then finish it off bluntly, as it would detract from the scale appearance of the rest of the plane. After some research into origami curves, I came upon a simple but powerful technique to loft from a curve to a straight line: create two lines from the corners of the line to any one point on the curve. The result will be one triangle sandwiched by two conic sections, something that is easily achievable with posterboard and formers. 

I implemented this technique on the two bottom quarter-circles and the top spline. The result was a loft from the fuselage to a simple quadrilateral, which can then be easily curved like flower petals to converge at one point. The only downfall with this method was the introduction of two crease lines on top of the nose, which is a clean compound curve on the original ship. Fortunately, the beta builders and I built the nose both with and without the crease lines and the fit was clean either way.

There were several other challenges in the design process, including learning how to properly use the sheet metal unfolding tool, dealing with tolerances associated with foamboard's thickness, channeling power wires through the wing spar, and configuring the control electronic systems to be completely removable and replaceable without damaging the airframe. It took several months of designing on the computer to achieve a model ready to be converted to plans and cut out.

Since this was a rather involved design and I was only a beginner builder and pilot, I chose to use the lasercutter at my college's design studio to cut out and test prototype iterations. The first successful flight of this design took place on December 28th of 2017, but unfortunately I failed to take a single picture or video of the flight itself. Another prototype flew and crashed later in the beginning of 2018, due to my poor piloting skills, and that's when I began to contemplate the possibility of recruiting a team of beta builders.

Leaked NTSB burst photo taken moments before first crash of mini B25!

Beta Build Team:

The FliteTest Fans facebook page was the ideal forum for me to source builders who were willing to take on the challenge. Within a few weeks of the end of January when I opened the official beta build page, many of the talented builders including Rosemary Jenkins, Jason Potosky and Ryan Baillargeon were far into the build if not finished and painted! I was astounded to witness the growth of this design in the hands of true FT experts.

Joseph Lujan

Ryan Baillargeon
Andrew Evans (3S - 2205 - 6045BN speed demon)Michael Hetrick (personal needle cutter test)

The names are endless on the list of folks who provided encouragement and useful feedback regarding the design and the plans: Jason Potosky, Minus Joo, Bruce Blackman, Charile Champion, Robert Lunsford, Minus Joo, Léopold Swertvaegher, David Wright, Jon Arthur, Bobby Maynard, and many many more wonderful community members. For several months, I had the honor of communicating my design quandaries with them and receiving suggestions for simplifying the plans.

Around this time, I also met my first and best flying buddy, Dominic Peluso. He became an extremely important part of this journey very quickly. As we wrapped up the plans and I began to shoot a simple build video with two stationary cameras, I decided to hand the final model to Dominic to paint and fly, due to his superior piloting skills. The build video itself took months to shoot, voice-over, edit, compile and publish in the midst of college pressures, but the moral support from my team at every step pushed me to keep working.

The final build video walks through the process step by step to ensure that the model is fully reproducible by a strong intermediate builder.

Probably the most important modification made to the beta model was the reversion to reduced undercamber at the wingtips. Before releasing the beta plans, I had increased the amount of FT-style wingtip undercamber, believing that it would make the airplane more docile and expand its speed envelope at the lower ends. I also included inboard flaps. Both of these choices were detrimental to the flight characteristics of the mini B25, increased its stall speed, decreased control responsiveness at slow speeds and ruined its high-speed cruising characteristics as well.

A key beta team member, Aaron Lefohn, recalled that he had the same experience on his FT Mini Cruiser, and that a retired Boeing Engineer at his field had pointed out the possiblity of turbulent airflow scattering at the wingtips due to the extreme undercamber and the blunt foamboard leading edge. We decided to reduce the undercambered portion to its original size and also seal off the exposed foamboard with a paper bleed-over and an optional barbeque skewer to provide the round leading edge.

The updates proved to be salient, as the final model soared into the morning skies in the hands of Dominic. Althought on the first flight the battery hadn't been moved forward enough, the B25 proved to have a wide flight envelope, easily cruising at low speeds only a few feet above the ground, while having no problem ripping smooth, fast lines, loops, and barrel rolls. The wing showed no signs of folding but there was a little bit of wiggling, which we attributed to my slowly developing building skills. The release article linked at the bottom of this page explores the flight characteristics of the mini Mitchell more deeply. Check out the official release video below to view compiled footage of the first three flights.

Red Bull Modifications:

As spring quarter of my third year studying Aerospace Engineering at college slowly ground to an end, I decided to kick off my summer break by building another B25!!! But this time, I personalized it using some cheap decorative tools I had around the house. I covered the cut-out pieces with gray printer paper and trimmed them so that the airframe would have an automatic dull gray basecoat. I was planning to give it a semi-scale appearance, when literally, the night I was planning the scheme, I got a tip from my friend David Haakinson about the all new Red Bull design challenge...

I completely changed my plans and decided to shoot for a racing theme, with red and black electrical tape and sharpie markers at my disposal. I focused on spreading the lines parallel to the roll axis of the airframe to provide the streak of a hot-rod, and also filled in the windows with black sharpie.

The flames and supposed "Red Bull" logos provided the finishing touches. I also added bulls eyes below, at which point my family strongly suggested that I refrain from cluttering the airframe any further with more decals.

The very last step was to upgrade my power setup to suit the demands of this racing Mitchell. I switched from the stock FliteTest A-pack (Emax MT1806 2280 KV motors and 12 amp blheli esc's) to the revolutionary DYS BE1806 2300 KV motors coupled with blheli 20 amp esc's. Given the additional current capacity, I chose to use 6040 King Kong propellers to reach the 1000 gram thrust threshold with the twin power setup.

The speed difference was clearly visible in the air as Dominic and I communicated, trying to line up in formation. Due to my beefier power setup, the Red Bull B25 had almost 35 grams on the silver Mitchell and required a higher throttle setting to cruise without losing altitude. The climb rate was insane and flying at top speed was nerve-wracking, but exhilarating nevertheless. 

From a designer's perspective, the B25 performed surprisingly well, handling various speeds well without biting, and most importantly tracked well without any bad tendencies on turns. To put the icing on the cake for this entire project, the landing was as smooth as butter. It was simply astounding to see the two flying together at our local park, after over a year of hard work by the beta team.

Every aspect of this year-long design journey has been educational and inspirational beyond words and I owe the FliteTest team and their wonderful community for its completion. I would like to extend my personal gratitude to Josh Bixler, our favorite David Windestal, Peter 'the mad scientist' Sripol, Andres 'the next mad scientist' Lu, Ben 'the jet' Harber, Nic 'speed' Lechner, and all of the other wonderful designers and builders in our family that inspired this effort. I hope to continue designing foamies and contributing whatever is in my capabilities to the FT community, and I am highly honored to be a small part in it. 

As for the future of this design, many have asked whether I well sell plans or kits of sorts. My only desire with these designs is to spread the educational and inspirational value of the hobby and encourage the younger generation (which I hope I'm still a part of) to aspire in the fields of science and engineering and to solve problems creatively using an economical platform: scratch-building. Free plans and a build guide if not a video should be available for any swappable I design in the future. I sincerely hope that this design inspires at least one person out there in our community to think outside the box, and even more people to go out there and "make a memory"!

The initial question still remains to be answered. Would this 'high performance' racer really match the nimble single engine fighters of the WWII era, in a contemporary pylon race? We'll leave that as a challenge to you, the FliteTest community. The plans for this design are available in the aforementioned release article. They are all yours. Thank you very much for taking the time to read this article. Happy flying!

Signing off - MiniacRC