Thread: 500 Blackhawk
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Old 05-26-2020, 03:19 PM   #2 (permalink)
Medic33
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My HF Map location
Join Date: Jul 2006
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Quote:
Originally Posted by Thtasher View Post
Any advice is greatly appreciated.
I'm not going to give you a fish... I'm going to teach you to fish...

Generally, scale helicopters are happiest between 80-100 watts/lb power loading - on a 2 blade head. For comparison, full on 3D machines are often in the realm of 300+ W/lb. For more comparison, their full size couterparts are often much, much lower. In your case - the full size UH-60 weighs 23500lbs (MTOW) and has 2 x 1410 KW engines - which works out to 120W/lb.

So, what the heck does this have to do with pinon and motor Kv? They're the last thing to think about - because they are actually the easiest to alter.

You need to know first of all (approximately) what your machine will weigh at takeoff. For a 500 size fuse, it wouldn't be far off to estimate 10lbs, I'm guessing. Easily more. But for the sake of argument, we'll say 10lbs takeoff weight. So, at 120W/Lb, you'll need at least a 1200 W (continuous!!!, not peak) motor. But wait - that's for a 2 blade head... You're using 4 blades. Rule of thumb (that I've heard, there are other thoughts) is to add 25% for each extra blade. So we actually may need to have 180W/lb, so now we're looking for a 1800 W motor.

Motors are most effcient running near to 100%. So are ESCs. So we need to guesti-mate our desired headspeed. My 600 class Bell 412 flies very happily at 1650rpm (2 blade head). My 600 class Mi-24 at 1280rpm (5 blade head). Flying scale, we don't need high head speed. Running multi, even less. But there's no hard rule. It's what feels right. Because we don't know what our final headspeed will be - you want a gearing / Kv solution that allows your ESC to be running around 90% or so.

Lastly... (yes, there is an end to this)... How many cells are you going to run? Higher cell count = higher voltage = lower amp draw = more effciency. Great! Unfortunately, higher cell count = more weight & higher cost.

I hope that helps - and doesn't frighten you!

ed
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