Thick as Mince

[SIMON CRAGG]

I am thick as mince when it comes to electric models. I have several, which all work well on their stock set up. I have got a watt meter, but it might as well be a cabbage for the amount of useful info i get out of it. Anyway, I have just bought an HK "Skipper" in their sale which as stock comes with a 40amp esc, 6x4 bendy prop and runs on 1300mah 3s packs. I have got some 5.5 x 4.5 Apc props which I would like to try. Before I destroy what I have got already, can somebody advise in detail the testing procedure with the watt meter please?. (can it be kept basic as I am to electrics what Fred Astaire is to being a brick layer!).

[MattyB]

Fully charge the battery. Mount the prop onto the motor. Secure the model safely so it isn't going anywhere. Put the wattmeter in series between the battery and the ESC. You may need to solder on different connectors to the wattmeter or use adaptors to do this.

Ramp up the power slowly, watching the watt meter carefully. If it gets 4-5A of the ESC rating, return the throttle to zero and try a smaller and/or less pitchy prop. If you get to full throttle and the amps pulled is still comfortably less than the ESC rating note down the watts and amps, then close the throttle. That prop is safe to use with that ESC/motor/battery combo. Done!

Ps - Unless HK have been extremely aggressive in their choice of ESC for the stock powertrain you should be fine with a 5.5x4.5.

Edited By MattyB on 24/03/2018 22:39:57

[Former Member]

[This posting has been removed]

[SIMON CRAGG]

Posted by MattyB on 24/03/2018 22:37:15:

Fully charge the battery. Mount the prop onto the motor. Secure the model safely so it isn't going anywhere. Put the wattmeter in series between the battery and the ESC. You may need to solder on different connectors to the wattmeter or use adaptors to do this.

Ramp up the power slowly, watching the watt meter carefully. If it gets 4-5A of the ESC rating, return the throttle to zero and try a smaller and/or less pitchy prop. If you get to full throttle and the amps pulled is still comfortably less than the ESC rating note down the watts and amps, then close the throttle. That prop is safe to use with that ESC/motor/battery combo. Done!

Ps - Unless HK have been extremely aggressive in their choice of ESC for the stock powertrain you should be fine with a 5.5x4.5.

Edited By MattyB on 24/03/2018 22:39:57

Thanks, I'll give this a go and put the results on here. As the next poster has said, straight out of the box works. But its always good to know watts really going on IMHO.

[Chris Walby]

Tom, nothing wrong with following instructions, you only need to understand or know more if to change things from standard (funny how manufactures spend considerable sums to produce something only for people to "improve" it by modifying it).

MattyB's post is spot on although some people may find "slowly", "comfortably" and "extremely aggressive" does not tell someone without knowledge much at all.

Perhaps "slowly" over 15 seconds, "comfortably" 15% and "extremely aggressively" less than 5% margin might be a bit more defined, but that's only my opinion and others will have theirs... There is a view if you are not on the edge then you are taking up too much space, but this rarely ends well!

Tin hat on!

[Dave Hopkin]

To help turn the watt meter into something less cabbage like.....

In simple terms there are two variable components affecting how an electric set up works

Voltage - Your Lipo , for safety's sake lets take a fully charged 3S lipo as 13v volts

Loading - Your prop the bigger/more pitcher the more the load

Now a brushless motor will TRY and reach its maximum RPM no matter what the loading is, so as voltage in a set up is fixed the only other thing that can change is current

So as you load up a motor the more current it will draw (eventually reaching magic smoke) Wattage is simply found by multiplying Volts by Amps

Hence Matty's instructions to open the throttle gradually watching the Wattage (hence current rise) backing off if the current reaches close to the ESC rating

If you can switch your meter to display Amps its actually more useful for this secnario

1) At full throttle if the Amps you are pulling is greater than the ESC current rating you need to change the prop for less load OR put a bigger sized ESC in the system OR use a lower voltage battery

2) If at full throttle the current plus 15% is less than the ESC rating its a green light

3) If the current draw is close to the ESC play safe and treat it as (1)

[MattyB]

Posted by Chris Walby on 25/03/2018 07:40:48:

...MattyB's post is spot on although some people may find "slowly", "comfortably" and "extremely aggressive" does not tell someone without knowledge much at all.

Perhaps "slowly" over 15 seconds, "comfortably" 15% and "extremely aggressively" less than 5% margin might be a bit more defined, but that's only my opinion and others will have theirs... There is a view if you are not on the edge then you are taking up too much space, but this rarely ends well!

Tin hat on!

I agree with all those figures you have added, but didn't include them as, well... let's be honest, using a wattmeter is not a precise art! The only thing that really matters is the current drawn - as long as that is within the maximum ratings of your motor and ESC all should be well. How far inside those limits you choose to run is really up to the individual - 5A or 10% (whichever is smaller) works for me for small/med sized (up to 6S) models, but different people may prefer to run with more or less margin.

Edited By MattyB on 26/03/2018 12:14:13

[MattyB]

Posted by Tom Sharp 2 on 24/03/2018 23:10:16:

I have been flying electrics for 20 years, never used a wattmeter, to the best of my knowledge I've never even seen one. I just follow the instructions on the box, am I doing it all wrong?

I have seen RTF models from come of manufacturers where the powertrain components were not all well chosen and the ESC gives out in short order, possibly because they changed the prop between the pre-prod and production models without a further test. I have also seen plenty of motors in particular whose ratings on paper are extremely optimistic - for cheap motors used in sport prop models I never go beyond the 3W/gram rule of thumb, but I've seen motors on HK and Banggood that are specced at 6-7W/gram - they are in fantasyland!

For these reasons I always take a few seconds to check a new powertrain with a wattmeter to avoid a Murphy's law moment.

Edited By MattyB on 26/03/2018 12:20:56