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Old 11-28-2016, 10:20 PM   #1
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Default 180 CFX Payload Question

I'm kinda interested in using a small cam on my 180 CFX like a RunCam 2 or mobius 2 but preliminary research disagrees with my math. these cameras would add about 50 g to the heli's 200 g but I've seen videos online where a 200 SRX can barely lift the 50 g mobius so what chance does a 180 have?

on the other hand, knowing I draw about 6 A during flight at 50% throttle (5.55 V) and assuming an electrical+drive train efficiency of 80% and a rotor disk efficiency of 70%, I get a max hover thrust of 450 g, which is close to a T2W of 2 for the combined 250 g TOW. If I step that up to the max power from a 20 C 450 mAh lipo with 35 mOhm internal resistance (its much lower than that i'm just assuming the worst) i get 97.2 W and a respectable 3 min of air time but a hover weight of 922 g or T2W of 3.6. These number seem to make sense since without the camera it means the 180 CFX can pull 4.9 Gs which seems like what its doing in full stunt mode.

Coming from a very technical/theoretical background, I'm worried I'm missing something but, on the other hand, there are a lot of incompetent people on the internet.

Thanks for any help
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Old 11-29-2016, 01:19 PM   #2
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Here is a thread I've been following.

There may be information of interest for you also?

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Old 11-30-2016, 01:29 PM   #3
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Generally speaking these machines burn about 80W/lbf thrust. That can vary with headspeed and gearing- and with many other considerations. But that is a fairly typical value. It is consistent with your hover values (assuming the voltage is correct- not sure how you came to that), and so seeing 90W at 450g thrust is plausible. Your rotor is no where near 70% efficient in hover. At the a 200g thrust level, your Ct is at about .0027. For efficient rotors (with some twist, proper airfoils, etc), your figure of merit (which is just ideal power / actual power) is probably closer to about 0.5.

What isn’t clear is if the power system can deliver that output, and if the rotor’s blade loading remains low enough to be useful. The later should not be a problem assuming stock RPM (4200?), where your blade loading is only at about 0.03, and normally stall is not seen in hover until closer to 0.15 for reasonably good rotors. That margin drops when you start adding a lateral flow component, since retreating blade stall becomes more and more a problem etc.

I don’t think you will find much difficulty in a 50g addition to AUW from a thrust perspective. But there are other considerations- especially as regards CG and moments of inertia, where you may find some weird resonance/instability in the control loop, or significant drag in FF, etc.

Is there any reason you have not just tried to fly with an equivalent-mass ballast?

Good luck with the effort-

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