What motor/prop combo

[andy watson]

OK, I am sort of planning my next build- just messing around with ideas more than anything really. It's going to be an electric Me163b Komet- about 50" span from the BT plans. Expected weight is in the 2kg region- maybe up to 2.5. By my reckoning that means I am looking at about 750W of performance.

So far easy enough- especially as I know I want to try use 4S batteries as well. But as we all know the 163 has a tiny (non propulsion) prop on the front, and I can't power this (entirely) by rockets (yes, I have a plan). I think there is a definite loss on the flightlines when you see one of these with a big prop on the front.

So my question is- how do I get 750W from a small prop? 4S or 8S batteries are best. And by small, I mean the smaller the better. I am thinking along pylon racer type lines- very high rpm.

[Biggles' Elder Brother - Moderator]

Well three things use up the power:

1. the diameter
2. the pitch
3. the rpm.

You don't want the diameter - that just leaves the other two! The problem is the pitch you'd need would be massive - and at lower flight speeds (like below about 100mph! ) would probably be completely stalled!

How about removing the fan from say a 90mm EDF and doing some tests on that out of its duct? It would be very inefficient with a lot of radial flow - but you might learn something useful - then again you might not!

BEB

PS Obviously you'd need a very high kV in-runner - again like the ones used on bigger EDF set ups.

Edited By Biggles' Elder Brother - Moderator on 22/04/2013 15:52:35

[Steve Hargreaves - Moderator]

The problem you will have Andy is finding a motor powerful enough for your application but with a high enough kv to make a small prob viable.

FYI I have a 2700kv motor that churns out 230watts off a 3S battery fitted with a 5.5x4.5 APC prop.....maybe not a huge amount of help but might be indicative of what is required.

Maybe if you looked at helicopter motors you might find one with a high enough kv capable of taking 750watts.

My gut feeling (& it really is no more than that) is that maybe an 8" prop spinning at high teens of rpm will get you towards 750watts..... so if we assume around 13 volts off a 4S battery then a kv of around the 1500-1600 mark would seem about right. Maybe put some numbers into motocalc & see what drops out....

Good luck....

[Steve Hargreaves - Moderator]

This might be interesting......55A & 1500kv....

I suspect a good bit of experimenting will be needed...

[Steve Hargreaves - Moderator]

Or this......1600kv around 75A max

From the comments section "Currently running a 4s batt, 8x6 prop, pulling 77amps @ 998 watts. "

[andy watson]

Cheers Steve,

I think I have looked at those motors before- but was a bit put off by the fact one is described as a Heli motor, the other (in the comments) better for cars..........

Why can't HK put some search parameters into their stock listings!!!

I would like to get below 8" if possible........ I would do it at the expense of some power (must remember to build light.........)

[Steve Hargreaves - Moderator]

An 8" prop is pretty small on a 50" model Andy.....

Going off the above figures with an 8x6 pulling 77A maybe a 7x6 will get you closer to 750watts.

Do be careful you don't over speed the prop though....there is a formula of rpm x diameter not to exceed a certain number I think but can't quite recall it...

[andy watson]

What about:

**LINK**

Seems inrunners (which I think are best suited for me) are a bit few and far between above about 500W

I think the number you are grasping for is that the tip of the prop stays subsonic.  I will check the maths but I think that becomes less of a problem as you reduce prop size.

Edited By andy watson on 22/04/2013 23:45:38

[andy watson]

And I am hoping for about 4" diameter...........maybe 5 at a push!

[Steve Hargreaves - Moderator]

Blimey Andy you'll need to spin that pretty fast to generate 750watts.....the motor I referenced in my first post turns at something like 19,000 rpm & generates 230watts off a 5.5 inch prop.....so I guess you'll be looking at somewhere approaching 40k rpm.

The lekky pylon models get to those sort of revs so maybe tracking one of those down is the best way forward.....

Good luck & let us know how you get on.....

[andy watson]

The link I posted for the HET typhoon 2W25 should (according to the test data on the site) turn a 4.1"*4.1" prop at 38k rpm on 4S for 568W, or probably better for me- a 4.7"*4.7" prop @ 34k rpm and 850W.

It looks like edf motors are perfectly capable of running these small props.

I do wonder what the performance will be like though? Very fast with little thrust or lots of thrust with low speed. I would guess the former since this use of tiny props seems to be used in racers, but I am thinking it would be at one end of the performance scale, or the other.