Still a question on "watt per pound" rule of thumb

[trailblazer]

Hi guys,

As per subject above.

I have read several different forum columns and web published articles on the watts-per-pound issues on planes and how to determine the aircraft power train combination for each specific aircraft weight.

All of them never detail which figures to use to determine their calculations.

Is it purely battery output wattage vs. plane's total weight?

or

Is it (ESC's Amp rating*Battery voltage) vs plane total weight?


btw.. I have with me the following:

2x HET 2W-25 kV2660 87% efficiency
2x Hobbywing Pentium 60A ESC
2x HET 6904 mini fan
2x ThunderPower 3850mAh 14.8V 25C cont/50C burst (connected parallel)

Est. weight of plane: 2.5kg approx. 5.5 lbs

[edmond22]

Hi guys,

As per subject above.

I have read several different forum columns and web published articles on the watts-per-pound issues on planes and how to determine the aircraft power train combination for each specific aircraft weight.

All of them never detail which figures to use to determine their calculations.

Is it purely battery output wattage vs. plane's total weight?

or

Is it (ESC's Amp rating*Battery voltage) vs plane total weight?


btw.. I have with me the following:

2x HET 2W-25 kV2660 87% efficiency
2x Hobbywing Pentium 60A ESC
2x HET 6904 mini fan
2x ThunderPower 3850mAh 14.8V 25C cont/50C burst (connected parallel)

Est. weight of plane: 2.5kg approx. 5.5 lbs

It is a measurement tool to help you plan for your plane setup. Normally, IMO I plan for it without accounting for efficiency etc. Do the calculations, map it to the real world and see how much differing percentage in data, and go from there. From experience, you can safely take lipo cells as 3.7 volt (full is 4.2, under load is much lower ). So if you are drawing say 55 amp, you will arrive at 55amp x 14.8volt = 814 watts. You have 2 of these powerplant, you have 814 x 2 = 1628 watts or 1.62Kw. In watt per pound, you have 1628w / 5.5lbs = 296w/lbs.

Taking the w/lbs chart guide, you plane will most likely get pretty extreme performance.

150w/lb for Fair EDF performance
200w/lb for Good EDF performance
250w/lb for Great EDF performance
300w/lb (or greater) for Extreme EDF performance

For flight duration, you can google for watt hour per pound.

[trailblazer]

It is a measurement tool to help you plan for your plane setup. Normally, IMO I plan for it without accounting for efficiency etc. Do the calculations, map it to the real world and see how much differing percentage in data, and go from there. From experience, you can safely take lipo cells as 3.7 volt (full is 4.2, under load is much lower ). So if you are drawing say 55 amp, you will arrive at 55amp x 14.8volt = 814 watts. You have 2 of these powerplant, you have 814 x 2 = 1628 watts or 1.62Kw. In watt per pound, you have 1628w / 5.5lbs = 296w/lbs.

Taking the w/lbs chart guide, you plane will most likely get pretty extreme performance.

150w/lb for Fair EDF performance
200w/lb for Good EDF performance
250w/lb for Great EDF performance
300w/lb (or greater) for Extreme EDF performance

For flight duration, you can google for watt hour per pound.

woah... i knew my EDF would perform well.. but i didn't expect my set-up would make it insane.. lol

So you're taking 55Amps as in the ESC drawing 55Amps from the batteries?

[edmond22]

woah... i knew my EDF would perform well.. but i didn't expect my set-up would make it insane.. lol

So you're taking 55Amps as in the ESC drawing 55Amps from the batteries?

I assumed, did not have experience with the 2w25 on 4s. I think your motor is meant for 6s instead of 4s. I think you may want to reconsider the setup, 4s on 2w20 seems more appropriate than 2w25. Try http://www.s4a.ch/eflight/fancalc_e.htm to get a simulated result.

The result is not too promising, on 4s, you are talking about 22ish amp draw. That is about 651 watt of power total making it 118w/lbs below Fair EDF performance. This is not ideal, maiden will likely not be successful.

If you switch to 2w20, it will be about 45-50 amp draw. That makes it 1332 watt of power arriving at 242w/lbs, Great EDF performance.

Or switch to 6s on the 2w25. 52 amp draw, 2300+ watt power. 418w/lbs. This is much better. You will zip around more with less WOT and greater speed and arrive at longer flight time, more fun. You prob can see how higher cell count works to your advantage in getting more power and achieve lower or similar amp draw. Just remember to factor in the extra weight you are taking if any (normally we switch from 4s 4500mah to 6s 3000mah etc to achieve no weight penalty), the tradeoff is simply you get more power at lower amp draw allowing to use smaller mah batts but higher cell count with no weight penalty.

I think there is a possible problem with your setup, best check with someone who owns a 2w25.

[Babylon5]

I assumed, did not have experience with the 2w25 on 4s. I think your motor is meant for 6s instead of 4s. I think you may want to reconsider the setup, 4s on 2w20 seems more appropriate than 2w25. Try http://www.s4a.ch/eflight/fancalc_e.htm to get a simulated result.

The result is not too promising, on 4s, you are talking about 22ish amp draw. That is about 651 watt of power total making it 118w/lbs below Fair EDF performance. This is not ideal, maiden will likely not be successful.

If you switch to 2w20, it will be about 45-50 amp draw. That makes it 1332 watt of power arriving at 242w/lbs, Great EDF performance.

Or switch to 6s on the 2w25. 52 amp draw, 2300+ watt power. 418w/lbs. This is much better. You will zip around more with less WOT and greater speed and arrive at longer flight time, more fun. You prob can see how higher cell count works to your advantage in getting more power and achieve lower or similar amp draw. Just remember to factor in the extra weight you are taking if any (normally we switch from 4s 4500mah to 6s 3000mah etc to achieve no weight penalty), the tradeoff is simply you get more power at lower amp draw allowing to use smaller mah batts but higher cell count with no weight penalty.

I think there is a possible problem with your setup, best check with someone who owns a 2w25.

I own a 2w25 but I don't think this motor can handle over 1Kwatt of power .
At 800 watts the motor is running pretty hot without heat sink. (Sorry I did not measure the temperature as this motor is still undergoing trial testing.)

On power per pound or watts per pound is also useful to include the efficiencies factor to your electrical systems.

On EDF, the air ducting is critical and will affect the overall efficiencies of the electrical power systems.



Just my 2 cents.

[trailblazer]

I'm using two 4cell lipos in parallel... that would essentially mean 8S2P

Fancalc gave me the "Your ESC's on FIRE!" warning..

Well.. I'm stuck with my two 2W25 motors. I have no others for replacement as of now.

I'll have to set-up the entire plane first.. we'll see.. changing of battery to ESC set up is a clean, monkey-friendly exercise

[Raven]

Emm... If your running on 4s setup, you will need 70amps ESC. If your on 6s, 55-60amps will be on the safe side.

I only got 1 calculation. Point the plane nose up and test throttle. If power to weight ratio is 1<: 1 den your safe. Most plane can fly nicely on 1:1 ratio. Some deltas can fly on 3/4:1 ratio.

[trailblazer]

Oh no... that means two of my 60A ESCs might burn

[edmond22]

I'm using two 4cell lipos in parallel... that would essentially mean 8S2P

Fancalc gave me the "Your ESC's on FIRE!" warning..

Well.. I'm stuck with my two 2W25 motors. I have no others for replacement as of now.

I'll have to set-up the entire plane first.. we'll see.. changing of battery to ESC set up is a clean, monkey-friendly exercise

That is 4s2p, not 8s2p. Big difference.

if you have those motors already, you will need 6s to run it.

[edmond22]

I own a 2w25 but I don't think this motor can handle over 1Kwatt of power .
At 800 watts the motor is running pretty hot without heat sink. (Sorry I did not measure the temperature as this motor is still undergoing trial testing.)

On power per pound or watts per pound is also useful to include the efficiencies factor to your electrical systems.

On EDF, the air ducting is critical and will affect the overall efficiencies of the electrical power systems.

Just my 2 cents.

Hi Bro, I trust RC-Warbirds and Typhoon's claim of 1100 watt rating. See attachment. I have a 2w30, no heatsink (does have the ring though), using 6s and from experience can say the setup is pretty conservative, no where near pushing the Jeti Spin66 ESC or motor temp. I would think the 2w25's slightly hotter KV will be ok. I mean to each his own really, you dun think it can take more than 1kw of power while I have already zipped around above 100mph at 1kw for Typhoon motors for a long time already, zero incident.

I dun normally include efficiency calculations because IMO, they serve no real purpose other than confuse you further. Just take the max max power reachable and make sure you are within limits. So when I say 1100watt, in reality, it will be 1000++ or even lower.

[trailblazer]

Great JAVA website though...

on custom battery 4S1P 3850mAh 14.8v.. no warning.. and still ok... which means i'm still on track..

managed to plug in 1 motor + fan unit to the battery to try.. bloody loud and windy

[edmond22]

Great JAVA website though...

on custom battery 4S1P 3850mAh 14.8v.. no warning.. and still ok... which means i'm still on track..

managed to plug in 1 motor + fan unit to the battery to try.. bloody loud and windy

Measure the amp draw while you are at it. Then you can calculate your watt per pound rating and see where you are. I won't fly it on 4s seriously.

[edmond22]

Hi Bro, I trust RC-Warbirds and Typhoon's claim of 1100 watt rating. See attachment. I have a 2w30, no heatsink (does have the ring though), using 6s and from experience can say the setup is pretty conservative, no where near pushing the Jeti Spin66 ESC or motor temp. I would think the 2w25's slightly hotter KV will be ok. I mean to each his own really, you dun think it can take more than 1kw of power while I have already zipped around above 100mph at 1kw for Typhoon motors for a long time already, zero incident.

I dun normally include efficiency calculations because IMO, they serve no real purpose other than confuse you further. Just take the max max power reachable and make sure you are within limits. So when I say 1100watt, in reality, it will be 1000++ or even lower.

B5,

If you check the Highendrc site http://www.highendrc.com/index_eprod...products_id=80

They listed it even higher rating of 65 amp vs 50 as shown on RC-warbirds. Since you have the motor, have a bit more faith in it and enjoy it.

[trailblazer]

Measure the amp draw while you are at it. Then you can calculate your watt per pound rating and see where you are. I won't fly it on 4s seriously.

you use a normal multimeter?

[edmond22]

you use a normal multimeter?

I normally used a clamp meter. Sometimes will borrow the wattmeter if someone at the field has it.