Motor, and prop selection question.

[Richard Baggaley]

Hi All,

I apologise profusely for asking a question straight off. I have searched extensivly but am struggling with the information at hand (and being a newb to fixed wing aircraft).

I am undertaking a scratch build of a 79" Fieseler Storch.

I have the plans and it all seems pretty straight forward, however the motor suggested (The plan is at least 15 years old) a 3:1 geared speed 600.

I want to swap this out for a brushless outrunner direct drive.

So. I believe i have selected a suitable motor at 1000Kv, 520W on 3s (11.1v).

First question is, Is this ok to run direct drive? I imagine the shaft speed is quite a bit different to a 3:1 600 brushed.

Second is what Prop size/pitch would suit to in effect gear down the motor?

I have tried Motocalc but i dont think i am using it correctly as on a 12x8 prop its telling me a flight time of 1min 15 seconds!

Any assistnace on a good set-up (even battery size!) would be incredibly useful.

Cheers

[Biggles' Elder Brother - Moderator]

Motocalc might not be that far out! It depends on how much capacity your 3s battery has?

But 520W from a 3s is a current draw of around 50amps - so lets say your using a 2200mAh battery (probably the most common size of 3s) that would give you an absolute max of about 2.5 mins on full throttle!

Regarding props as a general rule lower kV means you can swing bigger props - you can almost think of kV as a gearing system. 1000kV is low-ish, but not particularly low. For a model this size you might want to consider something more around 800kV which would allow a more scale like prop size.

BEB

[NigelH]

Hi

I'd say that geared motors are now very much in the minority.

Which motor did you choose ?

How much do you thin the model will weigh ?

Prop choice depends on motor choice and speed controller choice depends on the first two.

Battery voltage and capacity also needs to be considered.

I think that 3S probably isn't enough for your application. I suspect that the estimated short flight time is due to the pack being drained with high amps. More volts will mean less amps and more flight time.

[Reno Racer]

BEB and Nigel H have hit the nail on the head.

I would use a 4S set up. Something like the Turnigy 5045-890 kv will spin a 12x6 prop on 4S at about 50 Amps max throttle, assuming 850 watts and 15V (fully charged would be 16.8V) and a 4S 4000mah pack. You would require nothing more than a 25C pack. (Max amps is 100A for a 25C 4000mah pack, so you would be using 1/2 this at full power).

Of course your storch would fly on 1/2 throttle. Now 70% throttle is normally about 1/2 the Amps drawn at full, so assume 25A, at 1/2 throttle you would pull less than this, say 18A (obviously, you will need to connect to an Ammeter to get and accurate reading and hence duration).

This would give you 5 mins at full throttle, just over 9 mins at 70% throttle and 13 mins at 1/2 throttle.

[Frank Skilbeck]

Just for comparison my E Flite DHC Beaver 68" wingspan flies on a 700kv motor with a 12 x 6 prop and a 3200 mah 4s battery and takes off on less than 70% throttle, when the batteries were new I got an 11 min flights time, but as the batteries are now a bit tired it's down to 8 mins.

Note I changed the prop from a 13 x 6 to a 12 x 6, as it was too powerful and gave strong torque raction on take off. The motor would probably run upto a 14 x 6 prop if required.

I'll look it up later but i think max amps were around 35A (on the 12 x 6) and I used a 60A ESC.

If it used to fly on a geared speed 600 probably using heavy Nicad batteries then the above setup would actually be a significant power upgrade, 1,000 kv is out of keeping with the model you are building and something around 650 - 800kv and 4s would be fine, the lower Kv motors swinging a bigger prop which will suit the model better.

[Richard Baggaley]

Thanks for all the replies guys.

I would be safe to say 100Kv is too fast. Good. As i havent bought one yet

I though 3s as the rpm goes up with higher voltage but the assumption there forgets that i wont be at 100% throttle all the time (if ever!)

My plan was for a 3540 outrunner with 3000-3300 3s and a 45-50A ESC.

I think now i will go to a 40 size outruner 800(ish) Kv and 4s.

I am interested in the 68" DHC-2 as it was a toss up between the storch and a scratch built beaver.

I am a scale nerd and the storch offered me a STOL model with warbird scale potential. I wonder if the beaver would be an easier build??? But thats a whole other question.

I guess as both models (according to thier plans) are 1.7-1.8Kg in weight (including motors/Glow) whatever set-up i choose should work with both.

 

I am now looking at a motor similar (if not the same) as this one...

 

Edited By Richard Baggaley on 20/09/2012 14:42:43

Edited By Biggles' Elder Brother - Moderator on 20/09/2012 14:50:12

[Biggles' Elder Brother - Moderator]

Richard - a 45-50A ESC would have been much too low with a 3s set up. As I worked out above you would be drawing around 50A - that leaves no "headroom". You need to spec your ESC to be about 15-20% over tha max current - at minimum. So if you are going to draw 50A you need at least a 60A ESC and 70A would be better.

In my view with scale models larger cell counts always win. The reasons are:

1. they have a higher voltage - so that results in a lower current for the same power. For a given battery it is current vs battery-capacity that governs duration. So all things being equal a higher cell count will, generally, give you more duration for the same battery capacity.

2. The higher voltage means that to keep max revs sensible you need to drop the kV. But that actually works in your favour because lower kV motors can as has been said, swing bigger, more scale-like, props! So you win on that as well.

BEB

PS I assume the "100kV" in your post above is a typo and that you meant "1000kV"

PPS Don't worry about asking questions - its what the forum is here for!

Edited By Biggles' Elder Brother - Moderator on 20/09/2012 14:52:05

[Tim Mackey]

Most of what I would have said, has been. I would re-inforce that 4s is the better way to go, in order to keep the current down to lower levels ( its current that kills our gear not volts ).

As for the higher voltage producing higher RPM...... thats why you use a lower Kv motor on larger cell count packs

[Richard Baggaley]

whoops.

Yes i did mean 1000Kv and not 100Kv

Also my mind is working faster than my typing fingers. With a change to the motor i was speccing up a 70 possibly 80A ESC but never typed it...whoops, my bad

Thanks for the feedback and comments. I now have a much better idea of what to go for and i will indeed be going for 4s probably 3000-3500mah too

[Frank Skilbeck]

Richard, the E Flite Beaver is an ARTF (but has functioning flaps etc), I used a Turnigy motor but it's no longer listed, a 4250 700kv.

The one you've picked looks pretty good for what you want, rather than buy the batteries now, you may want to see how much nose weight you need and use this to select your battery, much better to carry electrons rather than lead.

[Garbo]

dont want to muddy the waters too much but my suggestion is a NTM 3548 800kv rated at over 800watts so will give out 600watts all day long on a 13x6 and a 4s. If the Storch ends up on the heavy side you have the flexibility to go a 14x7 and use the full 800 watts and as BEB has said a 70amp speed controller will also cover all bases.

[Richard Baggaley]

Hi All,

I just wanted to thank you all for the help so far.

I havent had time to start the build but i have on order my motor and prop (I combined postage and saved on the prop).

I have plumbed for the NTM 3548 motor with 13x6 prop. Battery e.t.c. will wait until i know all up weight.

I was thinking about an 80A ESC and wondered what Mystery/Hobbywing where like? I have a hobby wing marine in a jet boat which is pretty good but if it fails the worst i have is wet feet.

Thanks agian chaps!