Big Bertha
By crashbamdicoot on Apr 16, 2019
"Big Bertha" was born out of two desires: To tear the sky in half like a sheet on 6s, and to cruise for long range flights. This I hope I have achieved using a frame large enough to support both 7 inch and 5 inch props, and picking motors with a kv relevant to each battery for those props. She's not quite finished (GPS incoming, vTX antenna will be mounted on top of the battery along with it) but I want to jump the gun and post here anyway. Love me! LOVE ME!!
The big frame gives plenty of room for the 7 inch two blade (green) props, the 1750 kv motors spin those at 4s quite nicely. On 5 inch three blades with 6s batteries there's even more room to not worry about clipping a strap or wire: Swapping a battery takes seconds.
The motors are soft-mounted (thanks JohnnyFPV!) using three layers of vinyl tape, I thread-lockered all the screws with low-temp threadlocker, the stickers are brain3d but the camera mount is from iFlight, I just put those on because I thought they were cool. The battery grip is from iFlight, but I rubberized it with spray-on insulating tape before affixing it to the top-plate. The racewire is soldered into place and then I've used grip tape from a hardware store to give a rough-and-ready appeal to the arms. Not as pretty as using that flexible tube stuff all up the wires to the ESC but I'm sure I'll be thanking myself when it comes time to fix a motor in the field. I'm using 1000 microFarads of power smoothing attached to the XT60 connector via WaFL's XT60 "cap cap".
I use all no-lead solder and an iron with a ton of power and temperature control.
I have only done gentle hover tests so far so I don't have video or flight times for you yet, but I'll check back in with those once I've got them.
The spec I've got listed here shows the 6s setup. For the 4s, I'm using a turnigy nano-tech 2200mah battery from hobbyking, and HQ Prop 7x4.5 V1S 7'' Dual Blade Props in green.
This thing is smooth... REALLY smooth. Hope it's got the performance / efficiency on 6s / 4s.
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I wonder why do you fly 5" props on such frame. 5" is easier to tune for sure.
I found 6" a sweet spot for 1750kv on 5s. It is ofcourse better on 6s but I got bit more vibes in HD.
You can also run 7" on 5s or even 6s on this kv but it's hard to get rid of oscilations.
I havent had a chance to tune this one yet although the hover tests and LOS on both prop/battery combinations was extremely smooth, so I can't answer your question for this quad yet. In the past I've dealt with oscillations with mechanical rather than software fixes (except high D osscilations of course!). I havent balanced these motors yet but it'll be my first port of call if it's untunable.
Forgot to say that I conformal coated all the boards except for the ESC (the heat sink is in the way of being able to do that) after masking all the moving parts and open apertures, connectors etc. Also that the stack JUST fits underneath the top-plate, and that the vTX had to be spaced off the f7 IMU soft-mounted standoff-board (factory soft-mounted) using four very thin nylon spacers, which meant that it was about .75mm clear and turns out that is enough.
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