Checking in because Like RichardGee, I too have bought the UMS 9-115 baby 9-cylinder radial engine.
My plans are for sticking it in this Flex 70cc Mamba, which I picked it up 2nd hand from my pal (cbear) of Jacksonville, FL the other day (but in full disclosure, I'm actually the third owner). Anyway, while this is nominally a 3D-type model, my intentions are rather more benign because I'm more of a sport/scale type pilot. And note; believe it or not, this is actually a scale model.
70cc Flex Innovations Mamaba is a stand way off scale model of the Pitts Model 12 equipped with the Russian Vedeneyev M14P/PF 9-cylinder engine. In this photo equipped with a StinGR85 rendering it ballistic!
So why the Mamba? Honestly, principally for want of something in which I could quickly install the engine to fly and enjoy the sound. I thought I had a line on a Bates Bearcat in 1/5th scale but because I'm stupid, I actually never did. Basically, I never had a chance of this deal materializing because it had long been sold before I ever reached out to the seller. What happened, and proof I have feet of clay is this; when
I saw the sale listing I only noticed a message to the seller dated late September (2025) and totally overlooked the fact the model had been listed late December 2023 . . . so I wasn't a day late and a dollar short but more like two years late - sigh.
So the Mamba it is and the first order of business is mounting the engine, sorting the cooling baffles and exit air situation, and then the biggest deal of all, figuring out how to get the model to balance. This, because the engine weighs in at about twice as much as the DA70 that had once been in residence. Only after I have all that sorted, will I then unbolt it from the airframe, install it on the test stand, and run some fuel through it whilst becoming familiar with its handling.
Note; compounding my Bearcat error, before making the Mamba acquisition I'd entertained hopes this gorgeous Zirolli 1/5th scale Stearman would work out. Sadly, it didn't either - but - this time I ended up with this lovely example of the standard primary trainer for getting young pilots ready to fight WWII.
Gorgeous Zirolli PT-17 (5:1) by master builder John Labell of Ocala, FL - it's one of those model which, the closer you look the more nifty details you see!
Unfortunately, once I laid eyes on John's Stearman (77 inch wingspan) I immediately realized there was no way in heck 'that' would work out. However, after putting him to the bother of driving to meet up with me to make the transaction, there was no way in heck I was backing out, so I now also have this new to me biplane!
Anyway, while the 5:1 Stearman is physically a rather nice match because the Wiliams Bros on the nose at 9-3/4" diameter vs 10-1/4" for the UMS 9-115 means the baby 9-cylinder is near perfectly scaled for the model, there's no question in my mind the mass of the engine is just too much for this model. And in this vein, it's my opinion the far better match for anyone envisioning a Stearman is the Zirolli 1/4 scale version at 87 inch wingspan, instead.
For the curious, this is how the two models compare. And please overlook the sorry state of affairs (the mess) in my shop. I've let things get out of hand and need to speed a day cleaning and picking up.
At 1/5th scale, the Zirolli Boeing PT-17 dressed in the bright yellow and blue of the WWII Kaydet color scheme is dwarfed by the Flex 70cc Mamaba
And yet, all that said, while the Mamba will receive the engine initially, what I 'really' hope for this engine is the same model in which Richard has his 9-115 installed, a 5:1 Jerry Bates Bearcat (spanning 86 inches).
Masterfully example by Richard Geertson of the Jerry Bates Bearcat at 5:1 equipped with Sierra retracts and powered by the UMS 9-115 radial engine
Maybe some day one will come my way because as of right now, there's no chance I can make the time for building one myself. Not with my work schedule before I retire (and worse, retiring is looking further and further down the road every time I raise my head and peer into the future).
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John