The Watt Meter mystery......

[Robert Cracknell]

Guys

 

Redundancy, health issues for both my partner and I and an enforced Covid isolation has curtailed my ability to get out there on to the flying field. Not choosing to be idle I have been spending some time reading up on and exploring the options of electric flight for when I do return to the fold. To this end I have found the forum to be invaluable and you are all to be commended for an encyclopaedic font of experience and knowledge.

 

I have been reading up on Li-Po’s, the types of motors, BEC’s and ESC’s and feel that I have sufficient knowledge to take the plunge with a basic model. Before my particular hiatus I was an IC flyer for a number of years.

 

There is one thing that I still need to fathom however – the Watt Meter. I know what it will indicate in terms of voltage, current, amp hours and watts but the mystery to me is how do I apply this information to the battery/motor/prop set up in my chosen model. I know for example that changing prop with give different watt readings but what is the right one?

 

Do you aim for a wattage figure based on the model weight? I have heard a figure of 100 watts/lb mentioned. Do you aim to prop for low current consumption to give longer flight times or the opposite to give out and out performance? Do you have to use it in conjunction with a tachometer and some method of measuring thrust? Is there a formula that would give an estimated thrust for a prop of x diameter of y pitch running at z rev/min?

 

I have purchased the Amazon book ‘RC Ground School – the beginners guide to flying electric R/C Airplanes (note the spelling, yes, a US book) by Jim Mohan

 

In it the author explains in detail what each component is but does not fully explain the hows of using them.

 

If anyone could recommend a good publication/article that will de mystify this area of electric flight I would be most grateful,

 

Any and all help greatly appreciated…..

 

Rob

[Paul De Tourtoulon]

It's in French but as you are studying at the moment 'maybe' you could translate it .

https://jivaro-models.org/formules_magiques/page_formules_magiques.html

[Shaun Walsh]

A watt meter is extremely useful when setting up a new model, making power train mods or trouble shooting.

When considering what components are needed for a new model (assuming it's not a PNP or ARTF) I go through the following steps:

How much performance do I want? I started with a Riot, 28 Amps on a 3S battery weight 3 Lbs, approx 100 W/Lb, performance perfectly acceptable for trainer/second model would be fine for A certificate and a bit more. I also have a Hummer, 18 Amps on a 3S battery in a model weighing 1 Lb, a nominal 200 W/Lb, unlimited verticals and 3D performance.

Next select a suitable motor based upon supplier's specs so if I want say 150W/Lb in a 2 Lb model with a 3S battery that's 300W/11.1=27A so I would be looking for a motor capable of taking around 30A continuous without burning out. The other motor variable to consider is Kv, how many rpm per volt the motor will give. To select the correct Kv rating you need to know approx what size prop you want to turn, Increase pitch or diameter reduce Kv for similar current draw.

Based on the motor Amps select a suitable ESC, for a 30A rated motor I would use a 40A rated ESC to give a little headroom and reduce the risk of overstressing the ESC.

Put it all together in a model and this is where the watt meter comes into its own. If you have got all your calculations right the watt meter will show 27-30 Amps at full throttle, all the powertrain components will be happy and you have extracted the maximum power from your chosen motor/prop combination. If the meter goes over 30A then you need a smaller prop, either pitch or diameter.

I always set up a model to extract the maximum possible power from the motor, why would't you, you can always throttle down.

I also use eCalc to look at the effects of changing motor/prop combinations without having to buy and test the components, it is very useful.

[Shaun Walsh]

"Is there a formula that would give an estimated thrust for a prop of x diameter of y pitch running at z rev/min?"

Yes, propCalc, part of the eCalc suite.

[Paul De Tourtoulon]

There is this,

https://rcplanes.online/calc_thrust.htm

 

 still in the middle ages in Lbs !.

[Piers Bowlan]

To my simple way of thinking (!) you start with the weight and type of model. A glider might fly with 75W per lb, (sorry Paul!) a sport model 100w while something aerobatic 150W+. A pylon racer or EDF 200W? 
Then, how fast will it fly and how many cells will the battery have. As a generalisation a fun fly or indoor model needs a higher torque motor to turn a bigger prop, say about 900kv for a THREE CELL battery. A sport model maybe 1000kv to 1200kv. A pylon racer or hot liner 1500kv -2000kv. EDF 3500kv? However, as you increase the voltage (number of cells) you will need proportionally a lower Kv motor for the same rpm. 

Faster models generally need smaller, courser pitch props that turn at a higher rpm, which is why a higher kv is appropriate (less torque) for these models. To put flesh on all of this, a program like eCalc could be invaluable and save spending money on unsuitable motors. Alternatively phone George Worley at 4Max who will advise on a suitable motor for your model. Much simpler!

All motors should have a spec. sheet giving the max continuous power (in Watts) the motor is rated for. As Shaun says it is logical to prop the motor to provide max continuous power but no more, which is where your Watt meter is so useful.

That’s my 2p worth at any rate. 
Now I will sit back and prepare to be shot down in flames 🔥!

[Engine Doctor]

2 hours ago, Paul De Tourtoulon said:

It's in French but as you are studying at the moment 'maybe' you could translate it .

https://jivaro-models.org/formules_magiques/page_formules_magiques.html

Yes search translate on Google . 😉

[Engine Doctor]

1 hour ago, Piers Bowlan said:

To my simple way of thinking (!) you start with the weight and type of model. A glider might fly with 75W per lb, (sorry Paul!) a sport model 100w while something aerobatic 150W+. A pylon racer or EDF 200W? 
Then, how fast will it fly and how many cells will the battery have. As a generalisation a fun fly or indoor model needs a higher torque motor to turn a bigger prop, say about 900kv for a THREE CELL battery. A sport model maybe 1000kv to 1200kv. A pylon racer or hot liner 1500kv -2000kv. EDF 3500kv? However, as you increase the voltage (number of cells) you will need proportionally a lower Kv motor for the same rpm. 

Faster models generally need smaller, courser pitch props that turn at a higher rpm, which is why a higher kv is appropriate (less torque) for these models. To put flesh on all of this, a program like eCalc could be invaluable and save spending money on unsuitable motors. Alternatively phone George Worley at 4Max who will advise on a suitable motor for your model. Much simpler!

All motors should have a spec. sheet giving the max continuous power (in Watts) the motor is rated for. As Shaun says it is logical to prop the motor to provide max continuous power but no more, which is where your Watt meter is so useful.

That’s my 2p worth at any rate. 
Now I will sit back and prepare to be shot down in flames 🔥!

No need for being shot down in flames  that's a good explanation for starters for a layman 🧯!! Obviously a rivet counter will try to complicate things but your post is a good starting point.

[MattyB]

I think you've got good advice in this thread, with one possible exception (see below). For me the wattmeter is just a final check - if I am choosing powertrain components I will do that based on eCalc so I know I am in the right ballpark in terms of ESC size, motor Kv and weight, prop size/pitch and estimated max power. Once everything is on the bench (or installed in the model if I'm confident) the wattmeter will just be used to verify no components are operating outside thier maximum ratings in terms of current draw, and the prop changed if it is.

 

That's really it, I don't use my wattmeter for anything else other than the odd on load battery check if I think a pack may be on the way out.

 

PS...

 

2 hours ago, Shaun Walsh said:

I always set up a model to extract the maximum possible power from the motor, why wouldn't you, you can always throttle down.

 

This is an OK approach in general, but there are a few exceptions to your rule...

• ESCs are actually doing their least amount of work (switching) at full power. As a result they can be overheated and have their life shortened by operating for long periods at unchanging, lower power settings, especially if they are not so well cooled at those slower speeds. Whilst modern ESCs are undoubtedly more resilient to this than their predecessors, you should still never limit full throttle by reducing the "servo throw" that the throttle channel sees - always prop the powertrain so that max power is at 100% servo throw, where the ESC itself is doing least work and generating least heat.
• Some short nosed (normally vintage) models converted from IC will benefit by having a larger, heavier motor to balance them. However, tune that powertrain for maximum power and the structure may not be up to it! Far better to prop down in this instance, especially as much of the flight it will be chugging around on low power settings anyway (see above) 

Edited September 12, 2022 by MattyB

[MattyB]

2 hours ago, Piers Bowlan said:

All motors should have a spec. sheet giving the max continuous power (in Watts) the motor is rated for.

 

One small addition re: this... Yes most motors have a spec sheet, but for the cheaper ones which most of us use those stats are often wildly inaccurate (Hobbyking, I'm looking at you... 🤔). Relying on those numbers can sometimes result in the escape of magic smoke!

 

A good rule of thumb for cheap motors used in traditional tractor or pusher prop configuration (not EDFs) is 3W/g - i.e. a 50g motor should not deliver more than 150W continuous power. This is relatively conservative rule (you can "burst" motors like this up to 200-225W pretty safely), but should result in your powertrains operating safely and reliably in the long run.

 

Edited September 12, 2022 by MattyB

[Paul De Tourtoulon]

A watt-meter is an essential piece of equipment, as said HK gear is overrated, 250w motors giving 180w if you are lucky, 40a controllers   maybe 25a,

and it can give a rough idea of whether your prop is too small or big ( diameter and pitch ) also if your battery is really delivering it's "C's" ( 20c/40c/60c )

I have had quite a few surprises with battery's.

 

 I can translate Jivaro's ratings but will you lot in Blighty understand what a Kilo is ?.😅

[Shaun Walsh]

I can see that scale models with short noses need more weight up front but isn't lead cheaper than motor? 

[Martin Harris - Moderator]

4 minutes ago, Paul De Tourtoulon said:

 

 I can translate Jivaro's ratings but will you lot in Blighty understand what a Kilo is ?.😅

No problem with Kilos - you just need to multiply them by 2.2 to get the real weight. 

[Simon Chaddock]

Robert

Perhaps the best way to look at a Watt meter is as a checking tool.

You can work out by the methods described above to select a particular motor/prop/ESC/battery combo that should do the job you desire. Of course the safest way is to simply use a set up recommended by someone who has already done it! 

The problem starting from a blank sheet is 'electrics' are relatively heavy so you have to work them relatively hard to even fly at all. A Watt meter is the ideal tool to check what should be happening actually is.

The biggest advantage is that in the first instance you can apply the motor power progressively so if you can see that something, most likely the amps, is approaching a specified maximum you can stop! Remember a motor and a battery can survive a considerable short period (10 seconds) over load but the sophisticated electronics in an ESC much less so.

Obviously the higher the performance you are seeking the closer to the components limits you will have to be.

 

Watt meters are not that expensive so avoiding wrecking just one ESC, motor or LiPo can go a long way to paying for its self.      

[Robert Cracknell]

Thanks all, the fog is beginning to clear, and things look more defined now but keep the tips coming, it all helps the homework...!!

[MattyB]

3 hours ago, Shaun Walsh said:

I can see that scale models with short noses need more weight up front but isn't lead cheaper than motor? 

 

It is, but depending on the model there isn't always space to fit it in securely, certainly not as far forward as the motor. One size bigger motor is not generally that much more expensive (not if you are using generic Chinese jobs anyway), so it can be simpler just to go bigger on that - it also gives you more proom to increase the thrust if performance were lacking for any reason.

[Piers Bowlan]

3 hours ago, Shaun Walsh said:

I can see that scale models with short noses need more weight up front but isn't lead cheaper than motor? 

Lead is also cheaper than a battery placed well forward, perhaps beneath the motor of a short nosed model (eg Sopwith Camel). But the lead won’t give you a longer flight time 🤣

[Paul De Tourtoulon]

3 minutes ago, Piers Bowlan said:

Lead is also cheaper than a battery placed well forward, perhaps beneath the motor of a short nosed model (eg Sopwith Camel). But the lead won’t give you a longer flight time 🤣

Or a fiery death,,,😅

Edited September 12, 2022 by Paul De Tourtoulon

[Piers Bowlan]

😬