V-22 Osprey Project - Selecting the right battery

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Recently, I've been able to successfully hover the V-22 around the yard and start to tune some of the control parameters.  However, each of these flights is not very long and it has me concerned about battery selection and flight times.  The stock 450-sized helicopters recommend a 3S 2200mAh Lipo battery for an estimated flight time of 6 mins.  I started testing with two of these in parallel to make a 3S 4400mAh source for the Osprey.

There's no beating around the bush, this machine is heavy.  Weighing in at 2lbs, 9.75oz (1186g) without batteries, it's a beast.  I've tried to be conscious of weight throughout the build, but with so much structure required for rigidity, it's hard to skimp out on materials knowing that any failure would be catastrophic.

So what's the right mix of battery size to maximize flight time?  I had on-hand pairs of 3S 2200mAh, 4S 2200 mAh, and 4S 3000mAh.  To round out the selection, I ordered a pair of 3S 3300mAh batteries for testing.  I set a couple of timers in my Taranis at 2 minutes and 4 minutes, and with a full battery to start, I took off and hovered around gently until the 2 minutes elapsed.  I landed as close to the mark as I could and then recorded the battery level.  I took off again and hovered for another 2 minutes and landed again as close to the 4-minute mark as possible.  I then took the battery reading and noted it down.  Using Google Sheets, I graphed these 3 data points per battery and enabled the trendline to estimate when the battery voltage would drop to a discharged state.

Somewhat surprising, the 3S 3300mAh battery lasted the longest.  I had expected more life out of the 4S packs, but they didn't necessarily perform as well.  I also compared the expected run-time to the weight of the battery pack to get a "flight time per gram" efficiency number as shown below.

Battery Estimated Flight (s) Weight (g) Flight / weight ratio
Turnigy 3S 2200mAh 25-35C 372 337 1.104
Turnigy Graphene 4S 2200mAh 65C 432 559 0.773
HRB 3S 3300mAh 60C 522 529 1.549
Zippy 4S 3000mAh 20C 489 660 1.451

As with any aircraft, the more weight you carry makes the airframe and motors work harder.  From my selection of batteries that new 3S 3300mAh looks to be the best bang for the buck balancing runtime and weight.

I also redesigned the wing retention mechanism forgoing the rubberbands in favor of a couple of nylon bolts.  I've also been redoing the foamboard fuselage.  I should have an update soon with new versions of those.





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