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Old Mar 04, 2009, 08:47 PM
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Joined Feb 2007
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Heli Size & Motor KV Rating

Hello Guys,

I have a question when it comes to builiding up an elctric Heli.

1)We get helis with description , 200 Size, 450 Size, 600 etc, What do they really mean....For e.g align has a 200 and 450 , theres an equvalent heli coming from Copter X, so they have something in common...

2) What should be the Motor KV that goes into a particular Heli ? Say , which heli does a 4200 KV be ideally fit for?


Best Regards,
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Old Mar 04, 2009, 10:25 PM
DVA
DVA
Chandler, AZ
Joined Jul 2004
298 Posts
1.Align heli sizes are based on motor size for the electric helis and blade length on the nitro helis. Brushed motors had 480s and other sizes this is where the 250, 450 and other motor sizes come from. But looking at blade length the Trex 450 has 325mm blades and a Trex 500 has 425 mm blades. The Trex 600E and Nitro have 600 mm blades but that is just a coinsidence with the 600e.

2. KV is a motor constant for a given rpm needed to develop one volt. When a 4100 kV motor is given one volt it will spin 4100 rpm. If it did not generate the one volt of back EMF the motor would continue to accelerate and current would be equal to the volage devided by the motor resistance. As a motor is loaded there is less back EMF and more current flows through the motor. There are many kV motors that work for a given size heli. KV selection depends on cell count, gear ratio, and desired head speed for starters.
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Old Mar 04, 2009, 10:39 PM
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ozace's Avatar
Melbourne , Australia
Joined Jun 2002
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Quote:
Originally Posted by MONCY777
Hello Guys,

I have a question when it comes to builiding up an elctric Heli.

1)We get helis with description , 200 Size, 450 Size, 600 etc, What do they really mean....For e.g align has a 200 and 450 , theres an equvalent heli coming from Copter X, so they have something in common...
The zap 400/zoom 400 stated the naming of mini helis as they ran brushed speed 400 motors. Align played the marketing game by going bigger (in number) and started the silly names we now have. The only way to compare helis is with blade size and the numbers in the names of the Asian helicopters means nothing. The Logo series uses the blade size so if you compare a 500 logo to a trex 500 they are not in the same class. Most 450 size helicopters are really 325mm bladed machines, the 500 con is really 425mm.
Quote:
Originally Posted by MONCY777
2) What should be the Motor KV that goes into a particular Heli ? Say , which heli does a 4200 KV be ideally fit for?
The KV of a motor used is based on the available gearing for a particular machine and the headspeed required. These days most helis run lipo battery packs with 11.1v starting voltages and i cannot think of any current machines that would run such a high kv motor. In the old days with nicad and nimh cells 4200kv would have been used on hornets and JR voyager E's.
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Old Mar 05, 2009, 06:40 PM
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If I had a 4200 KV brushless motor , what Heli could I fly it in ? How do I decide on the head speed etc ? This excersie will help me work out the math when I try to assemble a Heli from parts.
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Old Mar 05, 2009, 07:48 PM
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I run a 4200 kv with a 11.1 lipo 3s 30C on my walkera 60B, loads of power,
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Old Mar 05, 2009, 09:15 PM
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osterizer's Avatar
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Joined Jan 2007
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Moncy777, the smaller the heli, usually the higher target head speed. What kV motor you want to use is controlled by the voltage of the pack, the gear ratio, and your target head speed.

The motor pinion gear drives a main (spur) gear on most model helis. 325mm helis generally have a 150 or 140 tooth spur gear, and a selection of pinions from 9 to as many as 15 or 16 teeth. If you're using a 12 tooth pinion and a 150 tooth spur (say, on a TRex or Mini Titan), then the main drive ration is 12.5:1 (for every 12.5 turns of the motor, the spur rotates one turn).

The natural speed of the motor is the kV times the voltage of the battery pack. A 3S pack, for example, is 12.6V fully charged, but will run about 11-11.5V in flight. With a 3300 KV motor, on 11.5V, it will want to spin to 37,950 RPM- pretty quick. In practice, with the drag of the drive system and blades, and other loads opposing the motor, it will rarely turn more than about 80-90% of its no-load speed, so you're more likely to see something like 30-32,000 RPM.

Since that's how fast the motor's turning, we can multiply it by the gear ratio (divide by 12.5 in the example above) and come up with a head speed of about 2500 to 2700 RPM.

Short version: (((motor kV * pack running V) * pinion teeth) / spur teeth) * 0.8 => approximate head speed.

When you're looking at what motor and pinion you want to put in a heli, you balance those numbers to come up with a likely setup. Then based on the power you want from the helicopter, you pick the right motor based on its operating voltage, current capacity and so on.
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Old Mar 15, 2009, 02:14 AM
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Dear osterizer !
Wow ! that was a great explanation. Thanks for this info .
I now have a much clearer understading of this matter .
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Old Mar 15, 2009, 04:00 PM
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Thanks . Glad it helped.
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Old Mar 15, 2009, 04:07 PM
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There is a little bit more to it than that... for example, I have a 3400kV motor in my Trex 450, and I have a 3400kV motor in my 250... do you think the 250 motor will fly the Trex 450? No, it can't - it doesn't have enough torque to turn the blades of the bigger heli, and it can't handle the watts required... even on the same voltage. So, we need to know more about your motor than just the kV - mostly we need to know the wattage limits.
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Old Mar 15, 2009, 05:10 PM
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Of course, Jas- that was the point of the last sentence . A 200W motor would throw the 250 around no problem, but would be straining hard in a 325, and molten slag in a 550.
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Old Mar 15, 2009, 05:39 PM
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Quote:
Originally Posted by osterizer
Of course, Jas- that was the point of the last sentence . A 200W motor would throw the 250 around no problem, but would be straining hard in a 325, and molten slag in a 550.
Like this???
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Old Mar 15, 2009, 05:41 PM
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Doh! Yeah, somthing like that..
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Old Mar 15, 2009, 08:47 PM
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San Carlos, California, United States
Joined May 2002
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The motor selection guide of the EHBG v17 may help:

http://www.swashplate.co.uk/ehbg-v17/ch30s02.html

Toshi
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Old Mar 17, 2009, 09:37 AM
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Bradenton, FL
Joined Jan 2007
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"molten slag"...I like that

Still wondering if I can burn up my Scorpion HK4025 so I can upgrade to an HK4035, but no luck yet, even with over 3000 watt peaks.
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Old Mar 17, 2009, 08:16 PM
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Maryland, USA
Joined Jan 2007
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I haven't been able to abuse mine past the low 2000's .
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