Jump to content

Motor confusion


James Humphreys 1
 Share

Recommended Posts

Posted by James Humphreys 1 on 10/11/2020 19:50:23:

Ok so the problem is this I'm building a sky 40.

Now in the manual it recommends a

axi4120/14 which is 660kv

So on the 4max site the equivalent is only 595kv but the sky 40 setup on the 4max site suggests a motor of 960kv. so I'm a little confused as to what I should use.

The axi4120/14 which is 660kv and the 4Max equivalent 595kv are for 5-8s lipos.

The 960kv is for 3-4s lipos.

Obviously the prop sizes will be different.

Cheers beer

Steve

Link to comment
Share on other sites

I was recommended to use an Axi 4120/14 to put on the front of the "Mk1" artf Ripmax Wot 4, fed by a 4000 4S LiPo - I can't remember how many watts* or the prop size (it's a few years ago) but it gave a passable imitation of a scalded cat if taking off at full throttle and then gave a good 10 minutes-worth of mixed sports flying. I hope this might be helpful.

* possibly about 900 watts.

Link to comment
Share on other sites

That Axi has a can size equal to a 5055 and is rated for 4 or 5 cell, remember that if you swap to a 35mm motor on 3S you will probably need to move the motor mount forward and add quite a bit of lead. 595 to 660kv is close enough to be an equivalent, it may need an inch less pitch at most.

Remember that at 65" span it's a lot bigger than a Wot4 so can cope with a little extra urge.

Link to comment
Share on other sites

Posted by Bob Cotsford on 11/11/2020 10:44:29:

That Axi has a can size equal to a 5055 and is rated for 4 or 5 cell, remember that if you swap to a 35mm motor on 3S you will probably need to move the motor mount forward and add quite a bit of lead. 595 to 660kv is close enough to be an equivalent, it may need an inch less pitch at most.

Remember that at 65" span it's a lot bigger than a Wot4 so can cope with a little extra urge.

Yes, but he's building it to use as a trainer - surely there's a 40mm or so motor that on 4S will provide the right compromise.

George at 4-Max has clearly spec'd the setup for use as a low-powered trainer. Ask him what he'd suggest what power-train would give him one notch more power, rather than a whole load more.

Link to comment
Share on other sites

I'm not sure what the can size has to do with it being used as a trainer Jonathon, you really don't need to load the motor to it's limit. Pick a prop to give the power level you want! My point is that a 35 or 36mm motor will be both considerably lighter and shorter than the motor that TN recommended so some lead may be needed, and the 5055 595kv motor mentioned in the first post is a close enough match. There are loads of motors that would suit, eg Propdrive 4248 or 4258 600kv, Eflite Power 46, Turnigy G46.

Has James stated he wants it as a trainer? I must have missed that.

Link to comment
Share on other sites

It's a lot easier to moderate the power of an electric set up than increase it, simply by changing propellers. Moreover a power train more than adequate for the airframe will be understressed and its extra weight reduce the need for ballast to get the right CoG.

I agree with Bob and would fit the 5055 - 595 and prop appropriately to give the performance you need. That's the motor I'm fitting in my Flair SE5a with a 4S LiPo and will probably use a 13x6.5 prop (rather than the recommended 14x7) but I'm hoping to be able to use a 12x6 to give bit more endurance. It's replacing an OS52 Surpass which had a 12x6 prop and was adequately powered.

Electric power trains are very flexible and therein lies both their advantages and confusions.

Geoff

Link to comment
Share on other sites

Jonathon, to clarify George is recommending a 3 cell setup whereas the bigger motors are for 4 cell or more. Less cells = lower voltage which needs a higher kv (1000 rpm/volt) to get the prop turning in a reasonable rev range for the sort of prop size the model needs. As an aside, it also draws a higher current for the same power output.

Link to comment
Share on other sites

I've noticed a few of George's power train recommendations on 4 Max's web site include 3 cell LiPos where I've opted for 4 cell and lower kv motors because of lower currents. I'm sure his recommendations work perfectly; il's just that I prefer the slightly higher voltages.

I keep my 3 cell systems for smaller models (like my Das Liddle Stik) but for bigger ones like the Sky 40 I'd always go for 4s LiPos.

Geoff

Link to comment
Share on other sites

Posted by Geoff S on 12/11/2020 14:35:19:

I've noticed a few of George's power train recommendations on 4 Max's web site include 3 cell LiPos where I've opted for 4 cell and lower kv motors because of lower currents. I'm sure his recommendations work perfectly; il's just that I prefer the slightly higher voltages.

I keep my 3 cell systems for smaller models (like my Das Liddle Stik) but for bigger ones like the Sky 40 I'd always go for 4s LiPos.

Agreed - if in doubt I always go for the higher voltage, lower current system as they give you more headroom to tune the performance, plus your batteries and ESCs are working less hard so tend to have longer lives.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...