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09-30-2014, 09:40 AM | #1 (permalink) |
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Join Date: Oct 2010
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Making sure I get the numbers right
I am building a scale 500-size Apache AH-64 (the RC Aerodyne fuselage) and will be using a 4-blade rotor head. I'm getting to the point in the build where I will be assembling the power system and need to make sure I've got the numbers right for the headspeed.
From what I've read here it looks like guys are running 1700 RPM for the headspeed. Is that correct? My power system includes the following:
Do those numbers sound good to you guys who have more scale experience than I?
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Scott KBDD Team Kasama Dune 600 |
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09-30-2014, 11:49 AM | #2 (permalink) |
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Join Date: May 2011
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The formula for calculating RPM is:
Kv of motor * volts * 0.9 (90% motor efficiency) / (main gear tooth count / pinion tooth cout) I'm sitting in a waiting room using my phone to brouse so I can't run the numbers for you but you can do that on your own.... I run 1600 on my 500 size seahawk.. flies OK... Gh |
09-30-2014, 04:03 PM | #3 (permalink) |
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Join Date: Jul 2013
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What do you use for volts (per cell)?
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09-30-2014, 04:23 PM | #4 (permalink) |
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Join Date: May 2012
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You would use all 6 cells on a 6s battery. A fully charged 6s battery would be at 25.2 volts. Two 6s batteries for a 12s system, connected in series would be at 50.4 volts fully charged.
Using fully charged batteries in the head speed calculation would give you an idea of the head speed at lift off. Volts as well as the head speed would drop as the flight continues....
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Velos 880/Sab Urukay, KSE, Carbon Speed/Mikado Uvular/RCA Blackshark/Chaos 500 Evo, 550, 600, 700/Trex 500, 600, 600 Pro, 700/Roban 470 Cobra, 700 Apache, Cobra, 429, Jayhawk, 800 Airwolf/TFM 800 Blue Thunder, Seahawk, Jayhawk/ DX8 DX18QQ |
09-30-2014, 04:23 PM | #5 (permalink) |
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Join Date: Oct 2010
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A standard LiPO is 3.7 volts/cell. So a 6S would be 22.2 volts.
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Scott KBDD Team Kasama Dune 600 |
09-30-2014, 04:51 PM | #6 (permalink) |
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Join Date: May 2012
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Note that I said a fully charged battery, as in fresh off the charger...
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Velos 880/Sab Urukay, KSE, Carbon Speed/Mikado Uvular/RCA Blackshark/Chaos 500 Evo, 550, 600, 700/Trex 500, 600, 600 Pro, 700/Roban 470 Cobra, 700 Apache, Cobra, 429, Jayhawk, 800 Airwolf/TFM 800 Blue Thunder, Seahawk, Jayhawk/ DX8 DX18QQ |
09-30-2014, 06:16 PM | #7 (permalink) |
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Join Date: Jul 2013
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I asked the question because everyone seems to have a different opinion. It seems to me you would use an average ( like 3.9 per cell). I know some people use 4.2, but that'smax voltage that does not last long. I know some use 3.7, but that seems low to me. Many of my flights end with a voltage of around 3.7. Wouldn't an average be better?
I'm not trying to start a war here.........I'm new to electrics and trying to learn. |
10-01-2014, 04:23 AM | #8 (permalink) |
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Join Date: Apr 2008
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IMO 1700rpm may be a touch high for a 4 blade 500. I run my 2x 465mm UH-1N at 1700rpm, but 1700rpm is probably a good place to start.
RPM = 900 x 22.2 x 0.9 x 12T / 121T = 1783rpm at 100% towards the end of the flight. You would govern to around 95%, so around 1695rpm, which should be perfect. You could drop to around 90% if you want, giving you 1605rpm, which may be better. So, yes, I agree that a 12T should work just fine. The reason to use 3.7V / cell is that you really should have zero interest in what headpeed you will get in the first few milliseconds of flight, especially if you are governing. You want to know where it will be at the end of the flight, and set your governor accordingly to ensure that you can maintain a nice constant speed throughout the flight. Colin
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