Motor / kv questeions please

[john haz]

Hi all,

 

After some advice from Tim regarding motors, props etc, I think I have grasped most of it.   I have just changed a 38" mustang over to brushless & done all the wattmeter tests with different props & results add up ok (in my head).

 

My question is more related to the kv of a motor.  I understand what it does (rpm per volt),  but can't see any easy way to choose a kv other than a very basic, big prop smaller kv, small prop high kv.

 

 

My questions  .....  based on a 38" foam mustang P51  (my "test bed / learning about props plane").

On a 1000kv 20A motor the wattmeter shows:

160w on a 10x6

145w on a 8x3 3 blade

110w on a 8x5  

 

If I changed the motor for a 1200kv or 1500kv, what would I gain or lose ?    

 

Is there a "best" speed for different size props ?

 

I got the idea that smaller props need to spin faster, but I do have two little weedy motors, rated:

 

A2208/17 7A 1100KV Outrunner Brushless motor

 

Presumably these would only turn a small prop, which sort of screws up my theory.

 

Somebody enlighten me a bit please ??

 

 

[Romeo Whisky]

You have grasped the basics that the bigger the prop the more watts (and therefore Amps, since the battery voltage remains the same) the setup will pull.  Since the the Amps relates to the drain on the battery pack, the more amps you pull, the shorter the flight duration will be.  As you have noted, there is a limit to the power which any given motor can give, so it is not so much how big a prop a motor can turn, so much as the maximum power it can pull before it overheats and fails.  

There a few other important issues too.  The power you want to pull relates to the type of model and the type of flying you want to do, and there are some guidelines for this.  There are those who would say that you need to aim for 100 Watts per pound weight, but this is rather crude in my view.  I have seen many sets of figures for this, but I have synthesised these into my own simple little set of guidelines as follows.

Watts per lb    Performance                                                        Watts per Kg
< 35                  Not adequate                                                         < 75
35 - 45             Slow Fly or Light Park Fly Models                        75 - 100
45 - 65             Trainers, Multi-Engine                                           100 - 140
65 - 90             Sports Models, Scale Warbirds                           120 - 200
90 - 135           Advanced Aerobatics or High Speed Models    200 - 300
135 - 200        3D Models, EDF Jets                                              300 - 440
200 +               unlimited 3D Models                                               440 +

There is also the pitch of the propeller to consider as the greater the pitch (the second number of the prop size - eg. 10x5) which is rather like the gearing of a car.  High pitch (eg. 10x8) is like high gear - just what you want for a high speed model, whereas low pitch (eg 9x3) is low gear which you might need for a slower but heavier model with a high lift wing.

Then again, some models (eg. a small fast model) might do well on a fast Kv with a small prop, whereas a big powered glider or large trainer will fly better on a large prop at slower RPM (ie. Kv).

Assuming the most popular battery size to be 3S, of course while keeping an eye on the maximum power rating of the motor, you can always consider increasing or reducing the voltage by using a 4S battery pack (14.8V nominal), or reducing it by going to 2S (7.4V nominal)

Hope this helps.  RW

Edited By Romeo Whisky on 31/07/2010 09:17:40

[Reno Racer]

In my experience (30+ electrically converted models) I have always worked on 100 watts/Lb, not per Kg. Therefore:

vintage or Trainer 75-100 watts/Lb

Scale/ Sports 100-150 Watts/Lb (from gentle Aeros through to plyon racer)

Unlimited Aerobatics 150-200 Watts/lb- i.e 3D or F3A

EDF 200+ watts/Lb.

I would not be confident flying a sports model on 65 Watts/Lb. In fact, you never fly a large scale 1000hrs model on less than 100 watts/Lb, i.e roughly 200 watts/Kg.

Obviously, its better to have more power than required so that you have a reserve of power should you need to get out of a tricky situation and more importantly, that you are not flying at full throttle all the time. Generally, a larger motor flying at 1/2 throttle on the same watts, will pull less Amps than a smaller one flying at full throttle. This is due to the lower kv of the larger motor used, and hence larger prop and increase in thrust.

Before anyone dives in, of course Ohms law is NOT is question and you cannot create extra power, amps or volts from nothing, but the key is efficiency of the power system.

[Reno Racer]

Forgot to add, 

I will try and upload an Excel table I use; 1 sheet is a reference of Watts required by type of aircraft (sports, 3D, EDF etc) and by weight. The 2nd sheet covers Max amps drawn, watts and duration by Lipo size. I find this useful as a quick reference when converting I/C planes to electric.

I have also used webocalc here which allows you to play with motor Kv and prop size, but has more detail such as wingspan, wing loading etc to give you a better indication of how the aircraft will fly, stall speeds etc.

It may be of use; I'll try and upload my database, which also lists a few models. The 4-Max customers models tab is also quite a useful 'guide' . My yellow speed air is on there.

[Reno Racer]

here is full software on-line

[john haz]

What a brilliant couple of replies, thanks to you both.

 

The watts per pound/ kilo is, I guess, a bit of an individual preference based on known quantities.  

 

Reading the above, if I based my small 1 mtr wingspan warbird(s)  on about 240-280 watts per kilo, that should give me reasonable performance on the right prop ??

 

thanks again for the replies, excellent reading

[Romeo Whisky]

Like I said - it depends how you like to fly.  I prefer to fly for duration rather than raw speed, and I fly nothing bigger than a 3S pack can power.  I have seen charts giving both lower figures than mine above, and also higher.  In the end you will learn from your own experiences, and my chart above is just that - it's mine, based on my own models, flying style and experience.

 

One other factor to consider is the rating of your battery pack..   For instance a 10C rated 2000 mAh pack  is only good up to 20A (and probably pushing it at that), so the power train setup must take account of the drain on the battery for both flight duration and C rating.

[Reno Racer]

Here's my rough calcs for a Glens model CAP 232 58" which I intend to convert when it arrives on Monday. Just downloaded ver 1.5.2

[Romeo Whisky]

I think it might be worth noting that power-to-weight requirements for a model designed for electric power is likely to be rather different to converting an i/c model, which might explain Ackers preference for more grunt. 

 

Purpose-built electric models are usually built specifically to keep them lightweight - (as an example, foamies were virtually unheard of before electric flight came on the scene!) whereas i/c models are usually built to be sturdy and strong to cope with more vibration etc. and are therefore often heavier with higher wing loading.

[Tim Mackey]

Another method of calculating watts required for an ex IC model is the "double up add zero".  If it had say a .25 IC engine, double that to 50 and add 0 = 500 Watts.

Rough and ready, and doesnt work very well in a linear fashion, but then nothing in leccy flight is an exact science, and IMO nothing beats getting a few props and trying them with a wattmeter first.

[Wingman]

"Double up and add zero" - brilliant - I just love formulae like that - ideal for our kind of calcs. I'm with you Timbo, wattmeter, variety of props and a tacho is the way to go - it's too easy to get bogged down with the sums and lose sight of what the end result should be (and it's more fun!) 

[john haz]

What a brilliant tool that calculator is, .many thanks for that.

 

I did the specs for my foamie mustang on it & it seems I could run a GWS 10x6 ok, but at full throttle bench testing it's getting quite warm after 1-2 mins.

 

this is the motor spec (with 20A esc / 1300 3s lipo)

 

Model: 2409-18
Weight: 70g
Voltage: 10v (2~3 Cell lipo pack)
Idle Current: .9
Kv: 1000
Shaft: 3mm Diameter
Max efficient current: 15.5A
Max Load: 20A
Max Power Output:180W

Suggested Propeller: 8*.3.8 or 8*6 GWS Hyper Drive

 

 

Have I made a really obvious error because the calc seems to say that a 10x6 should be ok  (2 blade prop not the standard mustang 4 blade)

 

APC-TE APC-TE APC-SF GWS-HD

9.0 10.0 8.0 10.0

6.0 7.0 6.0 6.0

7419 6421 7929 7152

67.8 68.5 72.5 65.4

642.2 639.2 603.9 689.1

-47.0 -49.9 -85.3 0.0

0.87 1.01 0.82 0.91

 

 Maybe the motor is a bit low powered for a 600g warbird ??  - flight test later today, will post results (I have fitted a 9x6 hoping it gets off the mark ok & doesn't burn out the motor)

 

 

 

[Tim Mackey]

I always treat the online calculators with a hefty dose of skepticism - results can and do vary a lot.

Most motors will get warm - especially on bench testing and full throttle runs should be limited to a minute or so max.

[John Privett]

Posted by john hazeltine on 31/07/2010 13:06:02:

What a brilliant tool that calculator is, .many thanks for that.

 

I did the specs for my foamie mustang on it & it seems I could run a GWS 10x6 ok, but at full throttle bench testing it's getting quite warm after 1-2 mins.

 

this is the motor spec (with 20A esc / 1300 3s lipo)

 

Model: 2409-18
Weight: 70g
Voltage: 10v (2~3 Cell lipo pack)
Idle Current: .9
Kv: 1000
Shaft: 3mm Diameter
Max efficient current: 15.5A
Max Load: 20A
Max Power Output:180W

Suggested Propeller: 8*.3.8 or 8*6 GWS Hyper Drive

 

 

Have I made a really obvious error because the calc seems to say that a 10x6 should be ok  (2 blade prop not the standard mustang 4 blade)

 

APC-TE APC-TE APC-SF GWS-HD

9.0 10.0 8.0 10.0

6.0 7.0 6.0 6.0

7419 6421 7929 7152

67.8 68.5 72.5 65.4

642.2 639.2 603.9 689.1

-47.0 -49.9 -85.3 0.0

0.87 1.01 0.82 0.91

 

 Maybe the motor is a bit low powered for a 600g warbird ??  - flight test later today, will post results (I have fitted a 9x6 hoping it gets off the mark ok & doesn't burn out the motor)

 

 

 

 John, what current was it drawing in your bench test with the 10x6?

[john haz]

Hi John,

 

With a 10x6 the meter showed about 18A, which dropped off slightly after a few seconds (fully charged 3s 1300).  

 

Didn't manage to fly it this afternoon as it's so windy, but with the 9x6 fitted it showed a max of 15A  & if i back off to 75% throttle there still seems quite a decent pull, & showing around 11-12A    

 

Maybe later if the wind drops

 

 

[john haz]

Well, i tried the mustang tonight with that motor & I don't think it has enough power.  About a 3 min flight, it hand launched fine & fast enough for me, but after landing the motor was too hot to hold my finger on.

 

Was using the motor above & a 10x6 prop, although to keep the speed sensible & not worry about tip stalling it, I was at 70%+ throttle all the time.

 

Intend to replace it with something a bit beefier, I can always throttle back after it gets to a safe height & as mentioned here many times, get me out of (or into) trouble

 

This is the one I think I will try next  -

 

FC 2215 1200KV 25A Brushless Outrunner Motor 

Specifications:

Max Current: 24.5 A/60s
KV ( RPM): 1200 
Voltage: 11.1V 
Lipo battery: (3 cells) 
Weight: 59g 
Size: Unknown (bottom to shaft end) 
Shaft size: 3mm Thread 

 

All good fun this motor / prop / fly testing, and the redundant motors will be useful at some point I hope.

 

just wondering if to use two of these on my P-38 build, should give me plenty of power & they are cheap enough  

[Romeo Whisky]

John,   that FC2215 25A is a great little motor - I used it to power my GWS P40.

 

I guess all of us leccies have a box of motors which we hope will be used "at some point".  I added a new one only yesterday!

[john haz]

Sounds good to me, let's hope the FC works ok.

 

Thanks everybody  for all your help, the build is coming along slowly.