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Where are the Watts?

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SIMON CRAGG29/05/2020 20:50:44
575 forum posts
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Posted by Richard Clark 2 on 29/05/2020 20:01:23:
Posted by SIMON CRAGG on 29/05/2020 19:36:43:
Posted by Alan Gorham_ on 29/05/2020 13:35:22:

There is an easy way. Watts are not the limiting factor, Current is.

So just prop your motor to stay within the max current limit.

Sorry, but most of the comments make no sense to me, apart from the one above.

Cheers anyway!

100 watts per pound weight of plane including battery is adequate for most propeller.planes.

If they don't give you watts in the motor spec just multiply the max current in amps by the voltage (roughly 4 volts per cell) and you have the watts.

Edited By Richard Clark 2 on 29/05/2020 20:02:41

Thanks Richard thats exactly how I have been doing it. Has worked a treat!. Latest motor is rated at 42A and am running on a 4 cell lipo, so its 625w which is more than enough for the old skool low winger that weighs just 5 pounds.

Geoff S29/05/2020 21:42:09
3666 forum posts
26 photos

100 watts/lb has been a rule of thumb since the days of NiCads and brushed can motors which were notably less efficient than current power trains. My latest creation (a lockdown Liddle Stik) used stuff I had available. It weighs 3lbs ready to fly and uses a motor which draws 25 amps from a 3S 2200 LiPo pack for a power consumption of about 270 watts. It drives a 10x6 APCe prop at 9500 rpm). It's aerobatic on that and adequately powered for me though I'm sure there are pilots who demand more. It takes off in just a very few metres.

IIRC my Ballerina has about 80 watts/lb and also flies well on that and probably ast Peter Miller intended.

As everyone has said, it's the current you have to watch, the watts follow from the number of cells (ie voltage) after that it's the mechanical structure (ie weight roughly - 3 watts/gram). In a perfect world with copper as a super conductor at normal tempertures it wouldn't be a problem. Even in the real world you could extract infinite power from a motor for an infintessimally short time without damage as there is always a time factor for how long you drive a power train at maximum - but we're usually looking for the continuous capability.

Geoff

leccyflyer29/05/2020 22:48:48
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1479 forum posts
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Geoff - although the 100w/lb rule of thumb was indeed derived at the time when most folks were using rather inefficient can motors, it was actually based on the higher quality, more efficient brushed motors, such as Astro, Aveox and the like.The motors were not much less efficient than budget brushless motors.

Peter Christy29/05/2020 23:03:10
1822 forum posts
Posted by PatMc on 29/05/2020 19:35:09:

Pete, you're also highlighting why measured static thrust is pretty well useless as a measure of how a prop driven fixed wing aircraft will perform in any other respect but vertically.

Well, sort of! Actually, helicopters use less power once in forward flight! Its hovering that requires the most power (I'm excluding aerobatics here, talking about scale and sport flying!).

At model sizes, helis are more efficient because they are swinging a big "prop", slowly! In the hover, the blades are running in their own turbulence, which decreases efficiency somewhat, but is still better than a fixed wing prop size can manage. Once in forward flight, the rotors are running in relatively undisturbed air, with a resulting improvement in efficiency. In forward flight, you can throttle back quite a long way and still maintain height.

There probably is an equivalent "watts per pound" formula for helis, but the "fudge factor" will be quite a bit different, and no, I can't be bothered to work it out!

But having a wattage figure for the motor makes it easier to work out in your head what will be suitable for a given heli! (I hate mental arithmetic!)

wink

--

Pete

Peter Jenkins29/05/2020 23:38:53
1602 forum posts
266 photos

Simon, you didn't say in the OP what sort of aircraft you wanted to power. You also need to consider what size of prop you wish to use e.g. a fat fuselage might warrant a larger diameter prop. Also, the motor's kv has a bearing here. In rough terms, a lower kv will mean you will want to use a larger prop at a lower rpm. You can play all sorts of tunes with no of cells, kv, amps and prop diameter and pitch.

If you want to power a 2 mtr class aerobatic aircraft, it will have a 10 Cell pack, and if using an outrunner motor it will have a kv of around 280 and swing a 21 x 13 prop. That set up will generate around 2,500 to 2,800 watts for an 11 lb aircraft so around 250 watts / lb. That is a long way from the 100 watts / lb being used as a yardstick above.

Another example. I had a 70 size aerobat with a motor of 425 kv for a 6S pack. I wanted to use a 5S pack and because of the reduced rpm through using a 5S pack I needed to run a large diameter and high pitch prop to get the power I needed while staying within the motor's current limitation. I ended up with a 17x10 prop. I fitted a 580 kv motor and had to reduce the prop size to 14 x 10. Both motors produced around 1250 watts but in a very different way.

So, in summary, what sort of power do you want for a given weight (gentle flying (80 watts/lb) or proper aerobatics (250 watts/lb). Is ground clearance a problem - use a high kv motor with smaller prop. Is there a large cowl - low kv motor with large prop. Which sort of pack are you standardising on? Adjust kv to suit appropriate prop.

I hope that helps.

Geoff S29/05/2020 23:55:27
3666 forum posts
26 photos

Peter Jenkins: There is a number of F3a pilots in our club, all withe elctric power. When they forst appeared I was ery interested to learn what the current draw and power they were using. None had a clue. They certainly used 10s LiPos (2x5S in series) but none bothered to check the current draw because they were using the established (expensive) set ups everyone else used. Considering how carefully they set up the airframes, this surprised me.

Your reference to power/weight ratios as high as 250 watts/lb is the first time I've heard a figure. It surpises me slighly but then, my aerobatic manouevres are rather modest so I'm easily satisfied

Geoff

OZ e flyer30/05/2020 01:42:42
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152 forum posts
28 photos

Simon I think there could be a simple solution to your problem. Work from your knowns and make sure it fits inside the maximums recommended.

for example;

You know the weight of your plane and the watts per pound formula to suit the style of flight you want to achieve.

you then know how many watts you’re looking for so that’s a known, let’s call it 400watts.

you know you know how many volts you’re going to have because of battery size (3s = 11.1 etc)

you know watts = volts times amps so 11.1volts times 36amps = 399.6watts

does that fall within the parameters stated on the motor data? If the answer is yes then you have a suitable motor. Select an ESC rated generously more than 36amps (50 maybe) then select and fit a desirable prop.
Using a meter, measure the amps being drawn during a static bench test.

if it looks like you are going to draw more than 36 amps at full throttle then fit a smaller prop and try again. A little trial and error until you have your desired pitch and size prop and I reckon you’re good to go.

I always add about 10% in to my calculations and I believe it’s always better to select slightly larger than you calculate you’ll need if it’s practical. It’s how I’ve always worked with my setups and I’ve never had an issue.

best of luck.

Richard Clark 230/05/2020 06:29:03
292 forum posts
Posted by leccyflyer on 29/05/2020 22:48:48:

Geoff - although the 100w/lb rule of thumb was indeed derived at the time when most folks were using rather inefficient can motors, it was actually based on the higher quality, more efficient brushed motors, such as Astro, Aveox and the like.The motors were not much less efficient than budget brushless motors.

The history of our model plane motors is interesting.

It all started with the successful 1940's 'round the pole' efforts by 'Aeromodeller' staff with a Miles Magister and their later 1940's rtp EDF DH Vampire. Then Colonel Taplin (later of 'Taplin twin' fame) with a single channel ED Radio Queen, an American Emerson motor and Venner silver-zinc rechargeable batteries.

Then Fred Militky of Graupner and his small Silentius free-flight model using a Graupner labelled geared T05 motor sourced from an 8mm movie camera and non-rechargeable water activated batteries similar to the ones in some Polaroid cameras. I had one of those but lost it in a flyaway.

Then of course we had the 'can' motors. Mostly made by Mabuchi as 'toy' motors. Available in about four different sizes and a variety of windings. Sold under a variety of model plane names. Exceptions were the Ripmax 'Bullet' and the high-quality Astro Flight samarium-cobalt magnet motors. My first electric rc effort was a Vic Smeed Chatterbox with a geared Graupner (Mabuchi). Speed 400 and a stack of nicads.

Then along came Aveox!

An American 'startup' with a range of brushless!!! motors and ESC's to match. The very first 'consumer' brushless motors and of very high quality. (Aveox have never made brushed motors.) MaxCim, also American, was a later rival but folded quickly.

They were very expensive (hundreds of dollars with the ESC) but I couldn't resist. I geared it down with a couple of thick brass gears and put it in a Flair SE5, still with nicads of course. This was somewhen in the 1980's.When the first Lipo's appeared (from Thunder Power) I used those and it's still flying with the Aveox motor and ESC some 30 years later. Only my Astro Hog (now with an OS 91 glow rather than the original K&B 35). is older.

Aveox gave up on model plane motors by about 1990 and their official history, which starts in 1992, doesn't mention them at all. It's all military and aerospace now, Large UAV motors, , lots of 'stepper' motors, and they made the motors in the Space Shuttle robot arms.

The best motors and ESC's today are probably Kontronik but most British aeromodellers are too tight-fisted to buy them.

SIMON CRAGG30/05/2020 06:48:09
575 forum posts
5 photos
Posted by OZ e flyer on 30/05/2020 01:42:42:

Simon I think there could be a simple solution to your problem. Work from your knowns and make sure it fits inside the maximums recommended.

for example;

You know the weight of your plane and the watts per pound formula to suit the style of flight you want to achieve.

you then know how many watts you’re looking for so that’s a known, let’s call it 400watts.

you know you know how many volts you’re going to have because of battery size (3s = 11.1 etc)

you know watts = volts times amps so 11.1volts times 36amps = 399.6watts

does that fall within the parameters stated on the motor data? If the answer is yes then you have a suitable motor. Select an ESC rated generously more than 36amps (50 maybe) then select and fit a desirable prop.
Using a meter, measure the amps being drawn during a static bench test.

if it looks like you are going to draw more than 36 amps at full throttle then fit a smaller prop and try again. A little trial and error until you have your desired pitch and size prop and I reckon you’re good to go.

I always add about 10% in to my calculations and I believe it’s always better to select slightly larger than you calculate you’ll need if it’s practical. It’s how I’ve always worked with my setups and I’ve never had an issue.

best of luck.

Many thanks, thats exactly what I have been doing so far.

Very clear explanation as well.

The stumbling block is "parameters stated on the motor data", which was where this thread started.

One great tool I use is "e.calc", brilliant programme where you can play around with all sorts of combinations, hours of fun.

I am getting there with converting ic to electric, and find it very rewarding.

Trouble is, after 50 yrs of ic flying it takes a lot of "getting your head round".

It looks like I have been a bit to obsessed with Watts, and not Amps!

Thanks to everybody!

Richard Clark 230/05/2020 07:25:32
292 forum posts
Posted by SIMON CRAGG on 30/05/2020 06:48:09:

Thanks to everybody!

I don't think you really needed any help at all. Just as I only use a wattmeter to check my wild guesses, we are only a check

Edited By Richard Clark 2 on 30/05/2020 07:26:16

leccyflyer30/05/2020 08:32:05
avatar
1479 forum posts
324 photos
Posted by Richard Clark 2 on 30/05/2020 06:29:03:

An American 'startup' with a range of brushless!!! motors and ESC's to match. The very first 'consumer' brushless motors and of very high quality. (Aveox have never made brushed motors.) MaxCim, also American, was a later rival but folded quickly.

They were very expensive (hundreds of dollars with the ESC) but I couldn't resist. I geared it down with a couple of thick brass gears and put it in a Flair SE5, still with nicads of course. This was somewhen in the 1980's.When the first Lipo's appeared (from Thunder Power) I used those and it's still flying with the Aveox motor and ESC some 30 years later. Only my Astro Hog (now with an OS 91 glow rather than the original K&B 35). is older.

Aveox gave up on model plane motors by about 1990 and their official history, which starts in 1992, doesn't mention them at all. It's all military and aerospace now, Large UAV motors, , lots of 'stepper' motors, and they made the motors in the Space Shuttle robot arms.

The best motors and ESC's today are probably Kontronik but most British aeromodellers are too tight-fisted to buy them.

You are correct, Aveox didn't make brushed motors, though they did make brushed controllers. It's the Astro Cobalt and similar motors that the 100w/lb rule of thumb was based on. Other quality brushed motors were similar quality - like the Plettenberg Ultra, Mega R7 and the like. Those were much better than the typical can motors, but much more expensive.

There were less pricey alternatives available - some of the Kyosho buggy motors with removable brushes and adjustable timing gave a useful hop up in power levels without breaking the bank. They also benefitted greatly from optimising the timing, fitting better bearings etc and allowed decent perfomance in the days before brushless motors and lipos were widely available.

As I'm sure you know and contrary to modern perceptions, it was definitely possible to get decent electric flight performance back in the day with Nicds and brushed motors. it just took a bit more care and a bit more work. I've just been recalling this morning about the electric fly-ins in the early part of the millenium, before lipos, and we had a lot of fun and saw some very good performance from well thought out setups and even some cheap and cheerful ones.

Richard Clark 230/05/2020 09:29:15
292 forum posts
Posted by leccyflyer on 30/05/2020 08:32:05:

 

You are correct, Aveox didn't make brushed motors, though they did make brushed controllers. It's the Astro Cobalt and similar motors that the 100w/lb rule of thumb was based on. Other quality brushed motors were similar quality - like the Plettenberg Ultra, Mega R7 and the like. Those were much better than the typical can motors, but much more expensive.

There were less pricey alternatives available - some of the Kyosho buggy motors with removable brushes and adjustable timing gave a useful hop up in power levels without breaking the bank. They also benefitted greatly from optimising the timing, fitting better bearings etc and allowed decent perfomance in the days before brushless motors and lipos were widely available.

As I'm sure you know and contrary to modern perceptions, it was definitely possible to get decent electric flight performance back in the day with Nicds and brushed motors. it just took a bit more care and a bit more work. I've just been recalling this morning about the electric fly-ins in the early part of the millenium, before lipos, and we had a lot of fun and saw some very good performance from well thought out setups and even some cheap and cheerful ones.

Yes . We had a lot of fun in the 'old days'. I started electrics because they were 'quite difficult' at the time. As soon as something gets more commonplace I tend to lose interest. Though I still build electrics up to about 50 inch span, glows above that. I am tempted by petrol but most of the engines are a bit big for the size of planes I like. I've got a 'Warbirds' Spitfire Mk9 kit coming in the next few days. It will replace the slightly larger, hopelessly inaccurate and grossly overweight 60-90 size TopFlite one I have - even the model shop owner warned me it was very heavy.

I once tried am un-named 480 size samarium-cobalt/ball-raced/adjustable timing brushed motor from J Perkins. It was hopeless. The Astro ones were fine but the expensive s-c magnets are no advantage over neodymium ones, brushed or brushless. They take a much higher temperature than neodymium without losing magnetism but that hot the motor will be in its very inefficient range anyway. BTW Astro is still gong

After a lot of fooling around with expensive ply/balsa or composite 90mm EDF kits , for the last couple of years I have concentrated on 'fun for the money' and regular planes plus a couple of 'semi-hotliners' beat them any day

Edited By Richard Clark 2 on 30/05/2020 09:29:58

leccyflyer30/05/2020 09:44:14
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1479 forum posts
324 photos

Great minds . I've also got one of Richard's Spitfire's on the way next week too.

I put one of the 14 turn Irvine Cobalts in my Zagi years ago and the power hop up was great, but for only a short time, they ran very hot and didn't last very long -ending their lives with the silver ring of death. They did come in a lovely little wooden coffin though. I found the Permax 450 to be an acceptable, robust increase in power over the 6v sp400 motor that she started with. The nominally uprated 480 sized motors with better brush gearalso gave a hop up, but some of them were thirsty beasts.

The distributors did a poor job in those days with their can motor based power trains. They would match an 8.4v motor with a 7 cell nicd pack and expect that to work, which was never going to work. It didn't take us long to discover that you had to run those can motors at higher voltages than their nominal rating and just live with the short lifetime.

Dickw30/05/2020 10:40:56
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723 forum posts
99 photos
Posted by Richard Clark 2 on 30/05/2020 06:29:03:..................................

The best motors and ESC's today are probably Kontronik but most British aeromodellers are too tight-fisted to buy them.

I have to agree with that. I have used quite a selection of Kontronik gear over the years and still do. I still have an early "hall sensor" motor ESC combo from their early brushless days.

Great company ton deal with as well.

Dick

Dickw30/05/2020 10:50:04
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723 forum posts
99 photos
Posted by SIMON CRAGG on 30/05/2020 06:48:09:.........................

The stumbling block is "parameters stated on the motor data", which was where this thread started.

........................................................

It looks like I have been a bit to obsessed with Watts, and not Amps!

Yes, thinking about watts is less useful than thinking amps, and don't forget that each different "kv" version of the same motor will have a different amp rating even if the watt rating remains the same. As an example, I am currently using a particular geared motor available in three diferent windings (kv). All three are rated at 1400 watts, but the current ratings are:-

2100kv - 120 amps for 5 secs

2700kv - 180 amps for 5 secs

4100kv - 240 amps for 5 secs

But refering back to something I said earlier - if you want a rough idea of the watts a motor can safely handle, just multply the weight in grams by three - another rule of thumb that isn't perfect but has its uses.

Dick

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