What size ESC ?

[cagey]

I have a 50 /60 A, 600 Kv motor which I intend to use in a 5.5 lb warbird. My thinking is that I should use a 60 / 75A ESC with this to give a 20% safety margin. Is this right ? It's all a bit of a mystery to me sometimes and to date haven't found a satisfactory analysis of the formula for correct motor / ESC / A/C that explains how the values of each component relate to each other. For instance if I used a 40A ESC for a 50A motor, would the ESC burn out, or would the motor just not run satisfactorily ? Any help would be gratefully received peeps.

[John Hickson]

You need some margin - 40A motor, 50A ESC for example. The motor will pull all it can from the ESC & battery. Going for a 30A motor + 30A ESC leaves no room for error.

You should consider a wattage meter if your going to experiment with different combo's, then you will know if your going to blow your ESC or flatten your pack shortly after take off

[Steve Hargreaves - Moderator]

Well.....you match the ESC to the max current the system will pull....personally I always add a 10% or so safety margin too....

Just because a motor is rated at 50A doesn't mean it will pull 50A...that depends on the battery & prop used....

Lets take your model.....lets say you need 600 watts to fly it nicely...on a 3S battery this will result in about 60A max current ...600watts divided by 10 volts equals 60A (a 3S battery is around 10V under load. Or rather thats what I use as it makes the maths easier!!)...so I would use a 70A ESC....

On a 4S battery we would get around 13volts under load so 600watts divided by 13 equals 46A so I would use a 50A ESC...maybe a 60A unit.....

The only formulae you need come from Ohms law......

I would also suggest that your motor might not be best suited to a 5.5lb warbird.....600kv is quite low & would need quite a high voltage to get the right revs out of it. (5 or 6S maybe) Or else you would need a huge prop to get it working well on a 4S set up.....

What battery do you intend using??

[Olly P]

Cagey,

The first thing to look at is expected AUW of the aircraft. This then gives you a guide for wattage.

Once you have the wattage you can calculate the current (Amps) for different batteries.

Regards what the motor will draw - it will always try to spin the prop at the speed defined by the ESC. The prop on it determines how much current it will take to spin this - a small, low pitch prop may spin at the appropriate speed at very low amps, but a coarse pitched one may need loads of power.

Have a look in the begineers to electronics threads for more info, or feel free to ask!

Olly

[cagey]

Thanks for the info guys.

Steve, the set up has a 3900 Mah 4s lipo driving the motor and a 2100 2s Life r/x pack. I'm hoping the 600 Kv motor will turn a 4 blade 13 x 6 prop with a bit more grunt than the original 500Kv motor, which was to be honest pokey enough but I'm of the impression that you don't have to use all the available ergs at your disposal and its better to have too much and not use it rather than not enough and overload things. Perhaps I shall experiment with different size props, like 14 x 8 again 4 blade. ?

[Steve Hargreaves - Moderator]

OK...a 4 blade 13 x 6 IS a big prop......be interesting to see how you get on with that Cagey......

Don't forget to apply the wattmeter at regular intervals.....

[Erfolg]

Will you get the speed from the model with a 6" pitch if the Kv is 600kv.

I would guess you will do well to get 5000 revs, which is only 28 mph without slip, could only be 20 mph?

[Cyclicscooby]

For the extra £5, i'd just get an 80A... It wont have as much overheating issues, and even if the motor goes pop cuz it's been over-proped, it shouldn't take the Esc too... !?!

HTH's

Luv

Chrisie.. xx

[cagey]

This gets more intriguing with each post. You see, the original set up had a 500 Kv motor with a 50A ESC as supplied stock. This was successfully flown with a 3600 and 3900 Mah 4 cell lipo. Which was the recommended set up that David Ashby used in the review model back in 2010. Mine though after several very satisfactory sessions had a total loss of radio contact and suffered the indignity of a sudden arrival. Since rebuilding and strengthening all the weak points, I have decided to up the power of the motor to 600 Kv with a 60A ESC to compensate for the now heavier A/C, also to incorporate a 2100 mah LiFe R/X pack and Ubec the ESC by removing the + lead. I intend to use the same prop and spinner as well, as this flew the A/C well enough at around 50 mph by the way Erfolg. So ? This should power the motor well enough and prevent any LVC problems, which I've put it's last prang down to.

If anyone can see a problem with this, feel free to have a poke at it because it's all guess work at this end Ho Hum !

Regards all Cagey. HTH's = ?

[Erfolg]

I assume that you were using something above a 3s Lipo? Ah yes I see now a 4s. Although I am still at a loss of how it is done.If you take the highest figures of 4.2v per cell @ 4s = 16.8v. That seems to indicate that the highest revs would be 8400.

using this figure 8400 * 6/ 1056 =47mph.

If I make an assumption that volts drop, quite quickly to say 12v (as it is a nice number) the max revs becomes 6000

again 6000 * 6 /1056 = 34 mph

All these numbers assume a 100% efficient propeller, and drag forces do not happen, etc.

At the end of the day, the model flew, you were happy. I just do not see how it flew so fast.

I do not doubt what you say, or suggest that the selection was wrong. I just do not understand, then again, that would not be anything new.

My own prejudices, turn me to motors of about 1000kv or slightly more, again working with a 6" pitch. I am looking for about 10,000 revs, ( I use 3s) which again is 10000 * 6 /1056 = 56 miles per hour (assuming 10v), although in practise will be much less.

ps the 1056 comes from = inches *60 (to get inches per hour)/12 to get feet, then divide by 3 to get yards, then divide by 1760 to get miles. Or 12 *3 *1760/60 =1056

I am not trying to say you are wrong, or cause offence.

Edited By Erfolg on 07/08/2012 00:42:00

[DB]

HTH = Hope this helps

DB

[cagey]

Hi Erfolg, No offence taken old son. Love the maths,did a not too dissimilar set of calcs myself with the original FMS 57" span foamie P51d, before I read David Ashby's review, and lust overcame the figures so I bought.

To be honest I was astounded to have it leap off the ground within ten feet and climb like a scalded cat to 50 plus feet effortlessly at around 90% power, no kidding buddy I was more than happy and quite surprised with it's performance on a 500 Kv motor and 3600 4s lipo. Frankly I had a hell of a job keeping it down to a sensible height.The AUW then was 4 lbs 12 oz . Since then there has been quite a substantial strengthening of weak points plus a fair amount of epoxy and ply added in the repair, with a consequent AUW of close to 5.2 lbs, not sure yet as I've yet to do a final weigh.

The V2 of that machine has been out for a while now, it has a 600 Kv motor with a 50A ESC and drives a 4 blade 14 x 8 prop, one of which I may try if the 13 x 6 doesn't have the grunt.

I'm prepared for it all turn to a bag of worms as I agree with you regarding the figures,but anyone who has had one will tell you how well they fly on the stock set up.Again our own David Ashby was very complimentary of it in the review, look it up in the digital archives, July 2010, page 82 I think. A mystery there huh ?

Final shot here. RCGroups had quite a following on this machine when it came out and a lot of the owners did as I'm doing, upped the 500 to 600 Kv and used a 3900 4s lipo with acceptable flight improvements. All my fingers and toes are crossed though. Cheers bud.

[Erfolg]

I have thought for some time that models often fly a lot slower than is commonly thought, particularly small electrics (which I fly).

I vaguely remember when studying that small birds flew at about 30 mph. I have occasionally thought of getting my old notes and text books out of the loft. My wife would not like that and are probably now totally encrusted with dust and mould. I seem to remember the calcs had something to do with L/D, weight of the bird (which i guess are quite light) and drag etc. It must have been when gliding.

At first I was attracted to low Kv as they were reputed to turn big props, which are more efficient as we know, Then I realised after some tests, that the low speed limited top speed. I then started to think about armature dia and length, what impact it had on torque and hence propeller diameters possible. I am still at this point, wondering.

When selecting a motor these days I consider, Kv, recommended propeller and max amps. Yet still feel i am a long way from identifying the perfect motor. When graphs were available, I found that the max efficiency was invariably on a propeller size smaller than I would like. Now you do seem to see graphs at all.

[cagey]

Well sir, when you do hit on that magic formulae I wouldn't mind betting a lot of the RTF kit out there is woefully short of the mark. But does it not strike you as odd that the figures fall short of the ideal with the FMS 1400 series warbirds ? All of which have most satisfactory performances on low Kv motors and using fairly large props. The FW 190, Spitfire and P51 all use largish multi bladers, 4s lipos and 500 or 600 Kv donkeys. To me, it's the tried and tested that proves the point. Big chunky motors, low (ish) Kv, big props, big batteries give big results. Maybe there's another factor that eludes me, but if my new set up works I'll be more than happy with leaving it at that. Hopefully !

[Erfolg]

I guess we modellers are similar to fishermen, where with them it is the size of the one that got away, with us it is the speed that models fly. Frequently suggesting speeds of 80-100 mph.

The physics of the Kv of motors and propeller slip, suggests that many models are doing well to reach 50 mph. My own measurements and calcs suggest that the actual revs are 80% of stated Kv being typical. Propellers from a peice in Model World suggests that propeller slip is about 5-10% of the pitch.

For those of us who use 3s Lipos, then a 500 kv could be marginal as the best speed would would be in the 30mph region, flat out. That could be why approx 1000 -1200 Kv seems popular, providing speeds in the 50-60 mph flat out.

I am now curious at what speed typical small models do stall?

[John Cole]

It's not the prop SIZE that might not give enough airspeed, it's the PITCH.

[Erfolg]

John, you are so correct, but isn't pitch part of the sizing? I thought I had used pitch.

I think the other point some may be making is that dia has an effect on prop efficiency and that multi bladers are a compromise on dia and using the torque.

If it is me that is at fault with sloppy wording, I am sorry and will try harder

Edited By Erfolg on 09/08/2012 11:51:37