Need help with power and thrust efficiency on a micro-hex

[Clocks]

Yep, its another newbie "audit my build please" thread. The intent is for a hexacopter on a small, light frame, that can take a simple go-pro.

I used to have a couple of bookmarks to do the efficiency calculations, but even then I don't think that the online tools I remember looking at knew how to take into account a tri-blade propeller. Reminders of where to figure out my effiency and estimate flight time would help.

The intent is to use the Hex-I configuration which I feel like offers pretty good stability.
In an ideal world, I would have about 20 minutes flight time.
Enough power to go to a hundred feet or so.
Ground speed is pretty irrelevant, frankly, but enough power that I can move around without getting super unstable.

Do I have a build worth pursuing here? Where can I be more efficient, or update to get longer flying time?

My fall-back if I am completely off track is to just wait and buy a Phantom 2 Vision+ but I like to build my own, and like the higher quality video modes that the GoPro provides over the Vision+. Especially given that I don't use an iPhone so I cant get all the nifty vision+ streaming and so on from the DJI platform.

 

[econfly]

It's hard to say without thrust and power specs on the motor/prop combination.

All I could find on the HK website is that the motors you list have a maximum power consumption of 39 watts or 5 amps (5 amps at 2s power -- 7.4 volts -- is 37 watts) . Just for the sake of argument, say the prop/motor combination can get you enough thrust to hover at one-third of that power, which would be a very loose ballpark number assuming hover around 50% throttle. That means you would need a little over 10amps to hover (6 motors * 5 amps * 1/3). Your battery is 0.95Ah, so at 0.95Ah / 10A you have a hypothetical hover time at 0.095 hours or about 5.7 minutes -- and that's only if you can really hover at 10amps and you eek out every amp in the battery (which won't happen more than once!). All of this is speculation because I don't know the thrust/power curve for the motor/prop combination you list, but my guess is that you are nowhere near a 20 minute flight time.

Put another way, to get 20 minutes of flight time out of 0.95Ah assume you draw down 90% of the battery. That's about 0.86Ah. Divide by six motors (and ignore overhead power for the Naza, etc.). You get 0.143Ah per motor. For that to last 20 minutes you would need to draw it down at a rate of about 0.43A per motor (0.143Ah / 0.43A = 0.33h = 20 minutes). That's a really minuscule amount of power -- about 3.2 watts/motor. Your weight is a little over 100g per motor and 100g of thrust over 3.2 watts is an efficiency of over 30g of thrust per watt. I don't know of anything in this thrust range that could even come close to that level of efficiency.

To make this work I would go with 3s or 4s power, a much bigger battery, and a quad setup -- but this is the reason the Phantom is the size it is.