Mar 04, 2009, 08:47 PM Registered User Joined Feb 2007 21 Posts Question 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,
 Mar 04, 2009, 10:25 PM DVA Chandler, AZ Joined Jul 2004 303 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.
Mar 04, 2009, 10:39 PM
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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.
 Mar 05, 2009, 06:40 PM Registered User Joined Feb 2007 21 Posts 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.
 Mar 05, 2009, 07:48 PM Registered User Joined Jan 2009 82 Posts I run a 4200 kv with a 11.1 lipo 3s 30C on my walkera 60B, loads of power,
 Mar 15, 2009, 02:14 AM Registered User Joined Feb 2007 21 Posts Dear osterizer ! Wow ! that was a great explanation. Thanks for this info . I now have a much clearer understading of this matter .
 Mar 15, 2009, 04:00 PM Psurvival = 1/ThArrival Maryland, USA Joined Jan 2007 8,577 Posts Thanks . Glad it helped.
 Mar 15, 2009, 04:07 PM Rocket Programmer United States, AZ, Mesa Joined Jul 2007 26,043 Posts 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.
 Mar 15, 2009, 05:10 PM Psurvival = 1/ThArrival Maryland, USA Joined Jan 2007 8,577 Posts 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.
Mar 15, 2009, 05:39 PM
Rocket Programmer
United States, AZ, Mesa
Joined Jul 2007
26,043 Posts
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.
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