I was flying with an APM2 equipped X525 quad frame but there were a couple of thing I didn't like, namely:
if using a GoPro or other wide angle camera the props would show in the far corners of the video.
Also, orientation at a distance was difficult even with two bright neon-orange legs.
Thirdly, it didn't fold as compactly as I would like for transportation.
Lastly, it looked like all the other X525 framed quads.
So after seeing an H-frame quad on the 'net somewhere I set out to build my own. The first frame was from 3mm ply and balsa using the aluminium arms and feet from the X525 but stiffness was a major issue with motors 1 & 2 sitting higher and at a slight angle while in flight. This caused some strange yaw problems... well, there wasn't any at times, the motor angles cancelling out their yaw effect. I tried covering the ply with very light carbon tissue and epoxy and even some carbon tow braces but could never get rid of the twist between the front and rear arms. A fumbled toggle switch at the wrong moment caused a crash and showed another reason why ply is a bad idea, it breaks very easily!
Next was to find something really stiff and strong, either fiberglass sheeting or carbon, prices were a bit unreal both locally and online, while some suppliers showed no interest at all in selling me any (I'm still waiting for some replies).
I eventually found some 1.5mm woven carbon sheet at Hobbyking but it was only 300mm x 100mm.
The 100mm was fine but I needed 600mm (23") long. This is when an engineering friend came to the rescue (HUGE thanks again, Wladek!), he said I could join 2 sheets mechanically with fiberglass sheet and rivets! So 4 carbon sheets were ordered and some plans drawn up in CAD.
Wladek has some nice machines at his work so we were able to do some very accurate drilling, milling and cutting. 12mm aluminium square tube was sourced locally and milled into a C-channel, saving 1/4 of the weight. Two 0.5mm plastic strips were used to make up the extra 1mm thickness (the X525 arms are 13mm). The carbon is riveted with 3208 rivets to the c-channel.
At first we used the carbon in it's original rectangular shape but we have since cut down the sheet to its present form saving 1/3rd of the weight. If I had to do it again I'd use 1.0mm sheet rather than the 1.5mm saving another 1/3rd. Also initially the ESC's were inside the frame but after many unexplained "ins" and other problems in which I started to suspect an overheating ESC, I have now moved them out into the airflow under the props and all seems well.
Anyway, it flies really well, looks great in the air and fulfils all my requirements, especially no props in the video! (See here). Final weight is 1279g (45oz) dry, with a 3 cell 5000mAh LiPo adding 353g (12oz) totalling 1632g (57oz), about 250g (8oz) heavier than the X525 with the same hardware. Flight time is about 13 minutes. Motors are 20-22L's, ESC's are Super Simple 20A, batts are Zippy Compacts (3S 5000mAh), props are 11x4.7 APC SF.
More construction photos here: Picasa Album
link to CAD file>H-Frame2.dwg