HI,
I have a cruise fixed wings plane, my motor is 550w and it consumes around 10A on a 3S battery.
But I have few questions:
1- If my motor is consuming 10A on 3S, will it be 5A on 6S battery?
2- If I want to compare 2 batteries: 3s 11.1v 6000mah and 6s 22.2v 3000mah.
Which one will give more flight duration? will it be the same? if yes, then it is better to use the 6S, as it has 40% less weight.
3- If I use a 6S, will the take off will be more powerful than using a 3S?
4- What is the maximum we can use per Cell, which means, at what voltage I should do the landing before damaging the battery but taking into consideration if we want to do the longest flight ?
Elios
Replies
Thanks for your reply. I was giving an example between 3s vs 6s to make it easy for calculation but in fact the reality will be between 3s vs 5s. Our motor can handle 3s to 5s.
For now we tried 2 x 3s battery 9000mah 25c. But we are thinking to use 2x 5s battery 5250mah 25c instead.
Because we will save 300g and we will get higher torque for the take off.
Our plane has 2 configurations: 1 pusher motor or 2 puller motor. The flying weight is 2.5kg without the battery if 1 rear engine and 3kg if 2 front engines without the batteries.
We tried 3s 9000 mah with the one rear engine, The only issue that we have is the take off because when it is on the air, it was flying very easily. But we went to a hill to be able to do the take off. We were using the motor 550w 1000v with a 3 blades prop 10 x 7.
We know that it needs 2 motors for better take off but as we know that 2 motors will consume faster the battery means less flight duration.
So we are thinking to use 1 engine for less consuming and more flight duration with a 5s battery to give more torque on the take off because when it is cruising no need for power.
The Motor that we are using is : http://www.conrad.at/ce/de/product/428816/Jamara-E-Motor-Magnum-A28...
Specifications
Revolutions / v: 1000
Internal resistance: 59mOhm
No load current: 2.0 A
Max A: 37A / 60S
Shaft Diameter: 4.00 mm
Power: 550Watt
NiMH Battery: 8 ~ 14
LiPo Battery: 3 ~ 5
Propeller NiMH: 11x8
Propeller LiPo: 3xLiPo 11x6 / 10x7 4xLiPo
Hi Elios,
I can't tell motor KV from that description or from on line and that is important for determining appropriate prop and performance of plane.
As iskess says above Ecalc is a great resource for seeing various motor - battery - prop - weight performance curves but I don't know if they list your motor.
That said assuming you don't go over current the biggest (usable) prop with the lowest number of battery cells will almost always yield highest motor efficiency.
Prop disk size is king and the most efficient system uses the largest prop possible (at speeds below supersonic tip speeds anyway.
This is just a simple fact and the reason two motors seldom work as well as one is that they generally use smaller diameter props with less than the swept area of the larger single prop.
The other important fact is that all brushless motors operate more efficiently at lower voltage than higher (of course they produce less maximum thrust as well).
If you can get satisfactory performance from a 3 or 4 cell battery you would generally be able to get longer flight times from the reduced number of cells with the same weight battery as a 5 or 6 cell one.
This motor is rated at 3 to 5 cell and if you don't need the 5 cell for your takeoff requirements it will probably fly longer on a 4 or 3 cell one of higher MAH but the same weight as the 5 cell one.
Reality is you are probably going to want to try 3, 4 and 5 cell batteries and various props to see what will actually work best for you.
Very possibly 3 cell will simply not provide sufficient thrust for your minimum performance requirements.
Most important is a single motor and a prop with a large diameter.
Since you are using a rear motor I can imagine that your prop diameter is limited by interference with the rear Fuselage or strut.
If that is the case a 3 bladed prop of largest diameter that can fit could help (Master Airscrew makes some nice ones).
After you get the prop right the fewest number of series battery cells possible will give you the longest flight time for a given weight.
Best Regards,
Gary
Just a few thoughts.
Motor efficiency is always higher at lower voltage - the difference can be considerable.
If there is a chart for your motor look at G/W efficiency for both 3 cell and 6 cell.
You will probably see efficiency is 1/4 to 1/3 worse at the same thrust with 6 cell than with 3 cell.
That said, motors are all designed to be optimized for a particular size battery.
Usually 3 cell to 6 cell is too big a jump to be within the motors reasonable operating range.
Some Motors 3 cell and 4 cell are OK others 4 cell and 6 cell, but usually 3 cell to 6 cell is a real stretch.
That said, generally a motor that is optimized for a low number of cells and relatively low KV and optimized to turn a large prop will yield the greatest overall efficiency (longest flight times) on a given total battery energy content.
You do not want to operate the motor at higher than it's rated current so that is also important in selection process.
Generally a 3 cell battery that is twice the MAH of an equivalent 6S battery has roughly the same weight and your specific battery specs are not particularly reperesentative.
That said, the best motor / prop diameter operating within optimal efficiency range will pretty much always trump any normal battery weight difference.
This is true for fixed wings and for multicopters.
Finally you really want the whole thing to be optimized for your "normal" cruising speed.
Best Regards,
Gary
I've been wondering about "motors being optimized for a number of cells". I'm running a 450 kV 3 S 20" power system on a fixed wing. The motor manufacturer gives prop recommendations when running on 5 or 6 S. I have yet to see a low kV low voltage recommendation from a manufacturer. It is usually high kV and low S or low kV and high S.
Someone told me that what I'm doing is just plain wrong because the prop is too large, the motor is overloaded and running it such a low voltage is not efficient.
Thanks Gary,
Very good points. I meant to include that he needs to either look carefully at ecalc (http://www.ecalc.ch/motorcalc.php?ecalc&lang=en) or really study the specs on the different motors. I run my Skywalker on 3s with a 12x6 prop because it was the most efficient for the motor. I also enjoy the fact that all my batteries are 12v and work interchangeably with my FPV and GCS systems.
I have a cruise fixed wings plane, my motor is 550w and it consumes around 10A on a 3S battery.
But I have few questions:
1- If my motor is consuming 10A on 3S, will it be 5A on 6S battery?
*Yes
2- If I want to compare 2 batteries: 3s 11.1v 6000mah and 6s 22.2v 3000mah.
Which one will give more flight duration? will it be the same? if yes, then it is better to use the 6S, as it has 40% less weight.
*It will be the same. Some people will tell you the 6S is more efficient because the lower current means less heat in the wires, but this is negligble at the low currents you are flying at.
I'm surprised and doubtful that a 6s is 40% less weight. Check again to be sure.
3- If I use a 6S, will the take off will be more powerful than using a 3S?
* Probably. Higher voltage will usually provide more power, but it really depends on the prop and motor combination. You must find a different prop and motor combo for 3s vs 6s.
4- What is the maximum we can use per Cell, which means, at what voltage I should do the landing before damaging the battery but taking into consideration if we want to do the longest flight ?
* I use 3.5v as an absolute minimum. I usually start my approach at 3.7v. The lower you go the shorter your battery life span will be.
HI Iskess,
Thanks for your help. I did a mistake in the calculation weight. Here are correct weight with 4500mah vs 9000mah.
3s 11.1v 9000 mah total: 99,900 for a weight of 660g
6S 22.2v 4500 mah Total: 99,000 for a weight of 575g
it is around 15%.
But even with this 15% less weight only, I am getting a better take off because I have a heavy plane.
And I was planning to use 2 batteries to get more flight duration, so instead of using 2 x batteries 3S 9,000 mah, total weight will be 1320g for a 199,800. it is better to use 1 x battery of 5S 18.5v 11,000 mah, it will be 18.5v x 11,000 = 203,500 for a weight of 1075g
from maxamps www.maxamps.com/proddetail.php?prod=Lipo-11000-185-Pack&cat=362 or another good provider.
Elios