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Old Nov 02, 2012, 03:54 AM
Glow 😡 no no no
Australia, SA, Evanston Park
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Originally Posted by peterreebok View Post
The g46 670 kv maxed out at 525 watts, and drawing only 30 amps. Using a 12 x 8 apc e prop. Battery was 4s 4500, but only at 40% capacity, so may be more with a full charge.
never owned a watt meter before, so had to change the connectors, and read the instructions, which are in chinglish, so there should be more data to come when I work it out. Main effort was securing the model so it didnt disappear on me!.

so, at about 5 lbs, and over 500 watts, should be more than adequate.

Just gotta watch out for those trees.
You might be better with a 12x8x3 or a 13x8 2bl, 3 watts per gram is a rough rule of thumb, in your case about 900watts.
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Old Nov 02, 2012, 07:30 PM
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Australia, VIC, Ferntree Gully
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Quote:
Originally Posted by aeromaniac View Post
You might be better with a 12x8x3 or a 13x8 2bl, 3 watts per gram is a rough rule of thumb, in your case about 900watts.
will test a 13 x 8, but will get some bigger wheels as well. Current bottom of prop is in line with top of wheel, giving ample clearance. Would be good to maintain this.
Is 900 watts for trainer duties, or for sports flying.?
Most data I have read quotes 70 watts per lb for trainer, 100 for sports flying, and 150 for advanced aerobatics. In general terms of course.
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Old Nov 03, 2012, 03:09 AM
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Quote:
Originally Posted by peterreebok View Post
will test a 13 x 8, but will get some bigger wheels as well. Current bottom of prop is in line with top of wheel, giving ample clearance. Would be good to maintain this.
Is 900 watts for trainer duties, or for sports flying.?
Most data I have read quotes 70 watts per lb for trainer, 100 for sports flying, and 150 for advanced aerobatics. In general terms of course.
A Boomerang on around 500W and a 12 or 13" prop will have enough power for pretty much anything short of actually hovering.

Trainers that size and wingloading are actually very solid aerobatic performers at anything over about 60W/lb when using decent sized, ie 12" + props, and a reasonably efficient power system, which a G46-670 on 4S is. The efficiencies, both motor and prop that the original rules of thumb were based on were much lower - nearer 50% for motors than the 75% we get out of half decent outrunners, and props now are typically bigger too.

Yet we seem to keep inflating power required to excessive levels. :confused
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Old Nov 03, 2012, 04:03 AM
Glow 😡 no no no
Australia, SA, Evanston Park
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Originally Posted by scirocco View Post
A Boomerang on around 500W and a 12 or 13" prop will have enough power for pretty much anything short of actually hovering.

Trainers that size and wingloading are actually very solid aerobatic performers at anything over about 60W/lb when using decent sized, ie 12" + props, and a reasonably efficient power system, which a G46-670 on 4S is. The efficiencies, both motor and prop that the original rules of thumb were based on were much lower - nearer 50% for motors than the 75% we get out of half decent outrunners, and props now are typically bigger too.

Yet we seem to keep inflating power required to excessive levels. :confused
Well I can only report what I have observed with golf lakes boomerang, he started off with a 12x8 but felt it lack performance and went upto the 13x8.
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Old Nov 03, 2012, 07:12 AM
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I am happy for the feedback. The deal is i take the feedback and choose what i use from this, so am grateful for others experiences.

At this point i am looking for benchmarks and rules of thumb.

Dont need to overpower this trainer, but facts are very hard to come by. Plenty of marketing info, but little hard data.

Reckon i can trust the wattmeter, using a 70% efficiency factor, gives me 70 watts per lb aprox.

Bigger wheels will never hurt, and a choice of props wont hurt either, as long as i collect the data ON the ground.
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Old Nov 03, 2012, 06:42 PM
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Quote:
Originally Posted by peterreebok View Post
I am happy for the feedback. The deal is i take the feedback and choose what i use from this, so am grateful for others experiences.

At this point i am looking for benchmarks and rules of thumb.

Dont need to overpower this trainer, but facts are very hard to come by. Plenty of marketing info, but little hard data.

Reckon i can trust the wattmeter, using a 70% efficiency factor, gives me 70 watts per lb aprox.

Bigger wheels will never hurt, and a choice of props wont hurt either, as long as i collect the data ON the ground.
Good approach!

Re the wattmeter, it is reading actual power in to the the ESC. At full throttle the ESC has minimal losses, so it's basically power getting to the motor. That power in is what all rules of thumb are based on - no need to factor it. As I said in an earlier post, the rules of thumb were developed empirically, so the efficiency of the systems being used was embedded in the rules.

But, as you realise, power in is not the same as power to the prop. It is easy to quote a big number from a wattmeter, but if half of it is going to waste heat, there's not much point.

Power out is best measured directly with a dynamometer, but they're one of the few things Hobby King doesn't seem to sell.

The next best thing is dead simple - the humble tachometer. IMO, it's just as important as the wattmeter. The wattmeter is an essential tool to ensure our systems do not happily self destruct, but can't give an accurate indication of what is coming out of the system.

If we know prop rpm, we know several important things. First, it provides a direct comparison between different power systems swing the same prop. Regardless of power in, if the prop is going faster power out is better. Second, it provides a validation of the motor simulations we run in eg eCalc. Third, it provides a baseline for thrust calculation.

There is a lot of test data on a wide range of props here: http://www.flybrushless.com/prop/search, but mostly smaller and few folding props

AeroNaut provides some detailed test data, especially for their folding props. The N-100 tables are measured prop rpm at 100W at the prop. Note that the performance charts are exponential.

Of course thrust calculations are only static and change once the model is flying, but that's why the pitch speed rules of thumb exist - if static thrust is OK and pitch speed is OK, in flight thrust will be good as well.

So for me, bottom line is power readings without corresponding prop size and rpm need to be carefully considered for validity/usefulness.
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Old Nov 04, 2012, 01:13 AM
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bought some 13 x 8 x 2 props, but arent going to try them.

Charged the battery fully so I had a proper baseline, and the 12 x 8 APC e pulls 720 watts and draws 42amps.

more than what I need, so dont see the need to change anything else.

pretty academic given those figures
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Old Nov 04, 2012, 02:22 AM
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Man that is an awful lot of power and current for Kv=670 on 4S with a 12x8.

It doesn't gel: 720W @ 42A is 4.28V per cell for a 4S pack - more than full charge.

720W early in the discharge at say 4V would require 45A - and the motor just shouldn't draw that much on a 12x8. Alternatively 42A at 4V would be 670W.

I had a look at your earlier post 525W at 30A for a 40% pack. That suggests 4.38V/cell - again way high. I wonder whether your wattmeter is a bit dodgy, either overcalculating power or under-reading current. Looking at some other motors of similar weight and Kv, I suspect your current reading is close but power is overreading.

In any case, to see either those power and current levels it looks like you got a fairly 'hot' G46, ie Kv higher than advertised, which is not too uncommon for cheaper motors.
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Old Nov 04, 2012, 03:04 AM
Glow 😡 no no no
Australia, SA, Evanston Park
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Originally Posted by scirocco View Post
Man that is an awful lot of power and current for Kv=670 on 4S with a 12x8.

It doesn't gel: 720W @ 42A is 4.28V per cell for a 4S pack - more than full charge.

720W early in the discharge at say 4V would require 45A - and the motor just shouldn't draw that much on a 12x8. Alternatively 42A at 4V would be 670W.

I had a look at your earlier post 525W at 30A for a 40% pack. That suggests 4.38V/cell - again way high. I wonder whether your wattmeter is a bit dodgy, either overcalculating power or under-reading current. Looking at some other motors of similar weight and Kv, I suspect your current reading is close but power is overreading.

In any case, to see either those power and current levels it looks like you got a fairly 'hot' G46, ie Kv higher than advertised, which is not too uncommon for cheaper motors.
This motor weighs 300grams and with the 3watts per gram rule, it should be able to go to 900watts.
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Old Nov 04, 2012, 03:50 AM
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Potentially it should cope with 900W, but it will only need to do that if you put a big enough prop on it combined with enough volts/cells.

In this case, the problem is that the current and power readings Peter is reporting are not possible at the same time, and they are both a bit higher than would be expected for a 300g Kv=670 motor on a 12x8. Not too high, just a fair bit more than predicted for an APC 12x8E. If it's a different prop, then it might be spot on.

It would be really good to know the actual prop and its rpm.

If the actual Kv is a bit higher than advertised, that's turns out to be a good thing if a 12" prop fits better than 13".
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Old Nov 04, 2012, 04:51 AM
Glow 😡 no no no
Australia, SA, Evanston Park
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Quote:
Originally Posted by scirocco View Post
Potentially it should cope with 900W, but it will only need to do that if you put a big enough prop on it combined with enough volts/cells.

In this case, the problem is that the current and power readings Peter is reporting are not possible at the same time, and they are both a bit higher than would be expected for a 300g Kv=670 motor on a 12x8. Not too high, just a fair bit more than predicted for an APC 12x8E. If it's a different prop, then it might be spot on.

It would be really good to know the actual prop and its rpm.

If the actual Kv is a bit higher than advertised, that's turns out to be a good thing if a 12" prop fits better than 13".
I only run mine on a 12x8 but it does 3blades thou
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Old Nov 07, 2012, 07:31 AM
Dave
Australia, QLD, Brisbane
Joined Apr 2012
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Recommended Standard Servo

Can someone please recommend a link for standard servos to suit this plane?
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Old Nov 07, 2012, 02:32 PM
Glow 😡 no no no
Australia, SA, Evanston Park
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Originally Posted by AscotRC View Post
Can someone please recommend a link for standard servos to suit this plane?
I was recommended this by a couple of guys who are very experienced modellers
http://www.hobbyking.com/hobbyking/s...dProduct=23995
The reviews on this one seem good so I got some for the stick 1500 which should be here this week.
http://www.hobbyking.com/hobbyking/s...dProduct=16269
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Old Nov 07, 2012, 02:51 PM
Glow 😡 no no no
Australia, SA, Evanston Park
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There has being a lot of useful information posted in this thread, the old 70watts for a trainer etc, is a guide, that might be 70watts out not in, i believe 100watts in gives ok performance, not outstanding. Always nice to have some reserve power lol.
We have throttle to control the motor power output, remembering diameter is thrust and pitch is speed, and once you model hits pitch speed your amps will drop because your motor is running at minimal load.
I used a 13x4 on a high wing trainer with a 32 sized motor before.
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Old Nov 07, 2012, 04:38 PM
Dave
Australia, QLD, Brisbane
Joined Apr 2012
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Quote:
Originally Posted by aeromaniac View Post
I was recommended this by a couple of guys who are very experienced modellers
http://www.hobbyking.com/hobbyking/s...dProduct=23995
The reviews on this one seem good so I got some for the stick 1500 which should be here this week.
http://www.hobbyking.com/hobbyking/s...dProduct=16269
So the boomer doesn't need metal gears or higher torque?
Thanks Aero
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