Same motor with different kv

[rcaddict]

what is the point of having the same motor with different kv - eg the turnigy g46 easymatch has 3 models 420kv, 550kv and 670kv. i dont see the point as surely you would pick the highest kv so you can use the smallest prop - or am i missing something

Edited By oldgit on 07/12/2018 10:14:42

[Bob Cotsford]

I'd try to pick the lowest kv for the number of cells I want to use so I could use the biggest prop! I think the different kv are available so that the best fit can be picked for the number of cells you have and the prop sizes your airframe can accomodate. Lower kv = bigger prop or more cells with a smaller prop, higher kv = less cells or smaller prop.

Edited By Bob Cotsford on 07/12/2018 09:55:28

[trevor wood 2]

Except for those occasions when you need fit a large prop to a scale model, or have built a large 2channel retro design for floating around the sky on warm summer evenings, or you want to fit a 4S lipo in place of a 3S in order to reduce the current draw. There will probably be other responses giving more reasons.

Bob beat me to it!

 

Edited By trevor wood 2 on 07/12/2018 10:01:22

[Nigel R]

Sometimes you want a small prop.

Sometimes you want a big prop.

A selection of kv ratings provide that choice.

And, higher kv often handle more current, being fewer turns of thicker wire, but at lower volts.

Again, all about choices.

[Bob Cotsford]

There should be a number of electric power primers in the beginners section that will help with some basic principles, iirc they were by Tim Mackey and BEB.

[Piers Bowlan]

My take on it, is that if you want an aerobatic model with a good vertical performance you will need a bigger prop with a fairly fine pitch turning more slowly, to provide the best efficiency (a large draft of air moving relatively slowly). Hence a low kv motor will be best as it will have lots of torque. e.g. A 3D or an indoor model. On the other hand if you want to build a pylon racer, hot-liner or any quick flying model a small, coarser-pitch propeller will be more appropriate. Therefore a higher Kv motor will be more suitable for that application as you want lots of revs. e.g. EDF. As for which kv in particular is best, perhaps one of the motor calc. applications will provide some numerical answers.

Edited By Piers Bowlan on 07/12/2018 10:43:42

[Nigel R]

Piers, not necessarily, I run a high pitch prop on an aerobat, it has a turn of speed and vertical acceleration, all down to total airframe weight; with a light enough airframe you can 3d an EDF.

[Simon Chaddock]

oldgit

Perhaps if we ask the question "Which is more efficient? A small prop running at high rpm or a larger, coarser pitch prop running at lower rpm but giving the same pitch speed"

Within limits the answer is the second case.

In general with the same battery voltage a lower kV motor turning a bigger prop will use less Amps to create the same level of thrust, but it won't necessarily sound like it is!

[Piers Bowlan]

Posted by Nigel R on 07/12/2018 11:23:26:

Piers, not necessarily, I run a high pitch prop on an aerobat, it has a turn of speed and vertical acceleration, all down to total airframe weight; with a light enough airframe you can 3d an EDF.

 

You are right of course Nigel, any high powered slippery model will be able to do aerobatics well, converting speed into height and height into speed. Alternatively it will be able to climb OOS as long as the thrust is equal to, or greater than, it's weight. Same for hovering an EDF. But you will be able to do it for a lot longer with an efficient set up which means moving a large volume of air slowly, relatively speaking, which usually requires a large, fine-pitched prop and low kv motor. How large a prop with what pitch and how low the kv motor, also how large the battery with how many cells? Too many variables,  so one can only generalise. The main thing, is to get a set up that works for you and brings a smile to your face . 

Never was a truer word said when you pointed out that lighter (loaded) aeroplanes fly better than heavy ones .

Edited By Piers Bowlan on 07/12/2018 14:41:10

[Bob Cotsford]

Yes, it does depend on the airframe and how it is to be flown.

Curare - 5055 430kv 6S 12*12 prop - an airframe that likes speed!

Bolero - 5055 580kv 6S 13*6.5 prop - a slow speed design, a 12" pitch would lead to all sorts of flutter and even the 6.5" pitch is bit much. 14*5 would be good to try.

Provost - 5055 430kv 6S 15*8(?) prop - big, bulky airframe, no need for speed but a real need for thrust.