Birdbrain RC Fortress 5

By CarbonRain on Mar 09, 2020

15  626  4

Trying something a little different, if I like it I'll probably end up running it as my race rig for the 2020 season.

My SKEW_R design worked pretty well last year, taking me all the way to the British Championship, but I did see a higher than optimal failure rate on the cross braces. Given the distance between motors, it's no huge surprise, but still annoying to have to keep making more only to destroy an hour's work in 4 packs on race day.

Thinking about the durability issue, I was thinking about how the SKEW_R uses 3 different thicknesses of carbon which adds a fair bit onto the manufacturing time, and thought, what if I could design a frame that only uses one thickness? I'd been tempted to try designing a box frame in the distant past, but wildly inconsistent thickness in my carbon sheets put me off (there's no benefit if I have to spend hours hand filing everything to be a snug fit!) Fortunately, moving to a new carbon supplier has much improved the tolerance on my sheet stock, and I thought it was now worth a shot.

Looking at my past race quads, I ended up thinking of my first "proper" race build, a GepRC Hawk. It flew great, making me think there might be something to the whole raised back motor idea (supposedly it has the benefits of clean air to the rear motors without needing to be a true stretch X), so I decided to try that too, tweaking the arms such that the rear motors are a bit over 16mm higher than the front. Once cut, I was delighted to find all the parts fitted together perfectly in a 3D jigsaw, with no slop to speak of. The weight, by pure fluke, comes in at exactly the same as the SKEW_R, 100g including 3D prints, however once screwed together, the frame is by far the most rigid I've made - hopefully that translates into toughness on the track.

As per usual, I ended up making the working area inside the quad extremely tight - everything does fit, but it's a pretty big effort to squish it in to 25mm stack height when the arms use the front and back 18mm of the body! I opted to put everything in the stack, and machined up a little carbon plate to fasten the XM+ side by side with the Matek VTX. A Hobbywing 60A stack provides the brains and brawn, and 2208 Xings looked great so I picked up a set to test. The Foxeer Predator has been a fave of mine for the past year, and when I saw Foxeer had just released a micro version with a full case and M12 lens I had to pick up a couple. Gemfan 51466 props and a Menace Thrasher round out the components, along with some Tinys LEDs Racelites on the back side of the front arms to make the quad a little more visible for any indoor events.



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Nabla   Mar 17, 2020  

What is the weight of the frame?

butcher51   Mar 10, 2020  

Sweeet frame :) It will be available somewhere maybe?

sal.mars44K.C.   Mar 09, 2020  

cool i dig this , have u bashed it yet.?
seems stout!!

CarbonRain   Mar 10, 2020 

Thanks! Weather here hasn't been fantastic unfortunately - I did get a maiden in on a very windy day and it seemed fine, but in a couple of weeks I have an overnight fun fly at an indoor gokarting track, that should put it through its paces!

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