Battery and prop sizing for second hand model

[John Bisset]

Hello all.

I have a question relating about a model I bought second hand some time ago. I bought it cheap with little information, mostly for the servos and ESC etc. However I’d like to try flying it before I do any ‘reduce to produce’.

The motor is noted as being ‘500kvA’. The only other notation is ’42 x 600’ written by a previous owner on the engine bulkhead. The ESC is rated at 80 amp, made by RC Smart, an RS-SET-80.

The prop which cam fitted is a 12 x 06. Total flying weight of the machine, which is a 6ft span pseudo Cub look alike – construction similar to the machine by Black Horse from what I have seen online – will be around 5 ½ to 6lbs.

I am intending to use either a 4 cell Lipo or a 5 cell Lipo to power this. I don’t need high power to weight ratios; my model flying is fairly sedate, more scale style than wild aerobatics, so a minimum of 70 watts per pound is sufficient. It seems as though a 4 cell Lipo will suffice, but I’m not sure about prop sizing. Suck it and see is one option of course!

Any comments or suggestions?

[Shaun Walsh]

The Black Horse Cub seems to require a motor in the range of 1000 to 1800 watts but is a bit heavier.

I think that the previous owner may have added and extra 0 and that the motor is a 4260 500Kv in which case prop sizes range from 13x6 to 15x8 for 7S to 3S batteries.

See **LINK**.

Try a 14x6 or 14x8 with the different batteries and measure the wattage and thrust for the optimum solution.

Edited By Shaun Walsh on 30/07/2019 14:27:21

[John Bisset]

Thank you. Sound as though the existing prop may have been undersized for this combination.

Why does prop size go up ads the supplied battery voltage goes down? Or am I mis-reading your comments saying a 7S battery would use a 13x6 and a 3S battery would use a 15x8 prop?

[Shaun Walsh]

500Kv is the motor rating in terms of rpm per volt at WOT so a 3S battery at approx 12 volts will give 6000 rpm and a 7S battery at approx 25 volts will give 12500 rpm. If you try to swing too large a prop at too high rpm you stand a chance of exceeding the current rating of either the motor or the ESC which is not good. That's why I suggested getting a wattmeter and testing prop sizes battery type and thrust generated. Putting a smaller prop with the 7S battery will reduce the current drawn compared to the larger prop. The current drawn at full throttle should not exceed the capacity of the motor and ideally there should be some headroom on the ESC so if the motor is rated at 60 Amps you should use an ESC rated to 80 Amps just to add a safety margin.

I am in the process of uprating an ESC in one of my models as the uprated motor I fitted draws 37 Amps through a 40 Amp esc and it gets rather warm if I use full throttle too much, a 50 amp ESC is on the way.

[John Bisset]

Thanks Shaun, that make sense. I have been wondering about a wattmeter for a while.

[PatMc]

Posted by Shaun Walsh on 30/07/2019 18:16:29:

500Kv is the motor rating in terms of rpm per volt at WOT so a 3S battery at approx 12 volts will give 6000 rpm and a 7S battery at approx 25 volts will give 12500 rpm.

To be exact, the above rpm figures are with no load, any prop fitted will reduce the rpm.

As a rule of thumb if the prop's rpm = 80% (or above) of the applied voltage x kv at WOT then the current will be under the motor's limit.

[John Bisset]

Shaun, Pat -

A silly question ; what does the acronym WOT mean here? I am being immensely thick today!

[Cuban8]

Wide Open Throttle.

[simon barr]

Wide Open Throttle.

No such thing as a silly question, they are just questions to which you don't yet know the answer.

[John Bisset]

Thanks both - not a TLA I'd come across before! And boy, my profession is full of those darn acronyms !