Prop/motor sizing help

[Steve_Wright]

Tried trowing in some numbers to some of the online calculators and they are throwing errors/confusing the hell out of me so throwing this out to the ones in the know.

 

My Pichler Bronco twin has the following specs

 

The motor they list seems down on power according to the online calculators (they only list their own motor rather than requirements)

 

I am new to electric stuff and need assistance on sizing the motors, will be using 5000ah 4s packs (1 per motor). interested in STOL rather than outright speed. With the above motor the manual  state a 13x6.5 prop, while some googling throws up a 12x8" prop being used with an unknown motor and another a 16x7

 

Any suggestions on motors and a start point for a prop? I was initially going to get a pair of Turnigy G46's but they seem a bit lardy at 300g compared to the above...

 

 

cheers

 

 

[Allan Bennett]

The estimated flying weight of 6900g (just over 15lbs) suggests that any of those motors would fly it with, obviously, the one you've highlighted giving the most performance.  2 x 850 watts is about 112 watts per pound, which is in the oft-recommended range of 75 - 150 watts per pound.  The Turnigy G46 looks similar to the one in your list though, as you say, it's heavier.  That's not necessarily a bad thing for it dissipates the heat better.

 

One thing that may be confusing you is the range of props quoted:  Usually the manufactureres quote a range of battery cell-counts that the motor is suited for, and then a range of prop sizes.  Generally speaking the largest quoted prop will be for the lowest quoted cell-count, in order to keep the amps within the motor's limit.  But, being new to electrics, one thing that can save you a load of heartache is a wattmeter.  You plug it in line with the battery and get a reading of the watts, amps, and volts so you can check out different props with your motor and avoid overloading it and frying it and the speed controller.

[Steve_Wright]

Sorted on the watt meter front, I am a bit worried when I stick the figures into ecalc for the g46 to get a sensible pitch speed I am getting a bit high on amps for the motor

[Allan Bennett]

I've never used eCalc (too many variables for my brain!), but I have read/heard that any prop/motor calculator should only be used as the starting point for prop selection.  The final test is the wattmeter and either a thrust stand or a 'feel' for the model's performance.  As an example, 12x6 and 11x7 props give almost exactly the same watts with one of my setups, but 12x6, because of its lesser pitch, gives more thrust and better acceleration for take-off.

 

I hope someone with better knowledge will be able to help you with more-specific information than my generalisations ?

[Steve_Wright]

Thinking of going with a pair of 4250/06 800KV Tornado Thumper V3's motor wise now, lighter than the G46 and more watts/kV, will just have to play around with the prop sizes. Digging around in the box I seem to have a few 13, 14 & 15" props of various pitches so will have a play.

 

 

Edited July 27, 2021 by Steve_Wright