ytell

Hi,

Would appreciate your view on the following:

My current setup is:

Model: Vanquish
Motor: Turnigy Aerodrive C4240-900 with rated power of 550W on 45A
ESC: 45A
Lipo pack: Rhino 3s 2350mAh 25C
Prop: 13 X 6
Measured maximum static current with current configuration: 40A

Can I safely switch to 4s pack with same Lipo spec? What would be the other expected affect (current, power) if I switch from 3s to 4s Lipo?

Thanks

Yoav

DaveL322

Yoav,

I'm not familiar with the specifications and running characteristics of the motor/ESC you have, however, if both are capable of handling the voltage of 4s, you will definitely see a relatively large increase in current (amps) and power (watts).

When you increase the voltage from 3s (maybe 10.5 volts under load) to 4s (assume 14 volts under load), that is roughly a 33% increase in volts, and you will see at least that much increase in the current - so from ~40 amps to ~53+ amps, and a similar increase in watts from ~420 watts to ~740 watts. Those numbers would be exceeding the amp/watt ratings you have given for the ESC and motor.

Regards,

Dave

speedy72vega

Ytell, one more thing. You can safely run 4S, as long as your motor and ESC are capable of running on 4S, but you will most likely need to reprop in order to keep from overdrawing your system. Changing from 3S to 4S, or 11.1v to 14.8v will increase the RPM's the motor will try to turn. Kv rating means turns per Volt, so the motor will try to turn as close to 13,320 RPM's as it can get on 4S vs. 9,990 on 3S (42-40-900Kv motor). With that increase in RPM, the power required to turn that 13X6 prop goes WAY up. You can reduce prop diameter, pitch, or both to get the current draw down to a level that's safe for your motor and ESC.
If you have a Wattmeter, you can experiment with different props on 4S and see if you can get the draw to a safe level.

ytell

My intuition tells me that if my configuration reaches maximum current (and power) with 3s, than once you switch to higher voltage, than the motor that now has 15V instead of 11V will use less AMPs to achieve the same working point (maximum power).
What's wrong with this logic?

Yoav

speedy72vega

You also have to take into consideration the rotating mass and pitch of the prop. As you increase RPM (higher Voltage), you need to reduce prop diameter or pitch to keep from overdrawing your system.

OhD

Doesn't work that way. Like the guys have said, more voltage yields more rpm, more current, more Watts. The same size motor with a lower Kv that would result in the same rpm with the 4s would produce the same Watts and lower current.

Jim O

jooNorway

Yes, it takes e.g. 47A to reach 500 Watt on 3S LiPo, and it takes 35A to reach 500 Watt on 4S. And of course you need to use different propellers to achive this.

Ed Lyerly

ytell,
As jooNorway pointed out, if you switch to 4S you will need to prop down ..... and a bunch. With your current prop, your amps will jump to about 70 and watts to around 1000.
Ed

OhD

This is true if you are willing to go to a smaller prop. I figure it would take something like an 11x5 turning about 11,700 rpm to match your current 550 watts. Try it and tell us how you like the performance of the plane with the small high rpm prop vs. the larger slower turning prop.

Jim O

ytell

I will definitely try the 4s setup.

I would expect the following from the same 550W:
- more weight from the 4s pack
- less efficiency from the smaller prop
- More flight time from the lower amps

Thanks guys,

Yoav

Velco

ytell, actually 900kV is "sweet spot" for 3S on F3A, acro, 3d applications. You would want to go 4S on a motor with 900kV only on something that needs more speed, hence a hotliner. I would suggest to stay on 3S and go to 13x10.

If you wanna go 4S then 4250-700 is the right choice with 12x8, very good combo!

2Sunny

Yoav,

Forgive me please if I am telling you something you already understand, but it seems to me you may not understand what kv or rpm/volt and power ratings on electric motors mean.



1) KV means the more voltage you put on your motor the faster it will spin and the more power it will draw if the prop is the same.

2) The power rating of 550 watts does NOT mean the motor stops at 550 watts. It means YOU must use a battery and prop that will NOT exceed 550 watts.

3) The motor WILL give you MORE THAN 550 watts if you use 4s and a 13 inch propeller.
-
4) The motor is designed for 3s and is limited to 550 watts by YOU. YOU can make it go over 550 watts, but it might burn up.

5) Your logic on voltage is correct, but I think to do what you want you need a 4s setup not a 3s pushed to 4s.

6) Higher voltage systems are more efficient than lower voltage setups for the reasons you are describing. That is why electric pattern setups use 10s which is the maximum voltage allowed by rules.


Perhaps I am stating the obvious and if so I ask your forgiveness again.



JP

ytell

Thanks for your comments, JP.

That's what I like in electric. Almost everything can be measured and quantified and yet, the complete system (the flying model) is far from trivial to master and we still have large room for personal preferences and optimisation.

Having tried a few props for my setup, I didn't find yet a prop that loads the motor to its maximum power (about 350W out of 550W). would you agree that it would be best (in terms of performance) to enlarge the prop (diameter and pitch) until it is close to (say) 500W on a static test?

Yoav

Velco

Yoav,

it is not just about the maximum power, because let's not forget that a Ferrari has 500HP and a big truck can have the same power too, but they are 2 completely different things.

But not to complicate the life too much with explanations right now let's stick to your combo. First of all if you want to push that motor to 500-550W you would need to change your ESC to 60A because, usually, a 3S bat under load gives some 10-10.5V, therefore for 550W you need 55A. As your current ESC is only 45A that would not be enough, and lets not forget that ESC is also supplying your servos and receiver with power, so has additional load.

As I have a motor with same kV 900 I have played a lot with different props so I think I can give you some good advice .

In general for F3A type of flying you need a prop that gives you good reserve of power, and this you get from higher pitch. You don't need to go much in static thrust above 1,5:1 (thrust:weight) but you want to have is as much reserve of power as possible from your cruising speed in horizontal line to be able to maintain speed in verticals. So a 13x8 or 13x10 is what you need here. Don't forget that different brands of props can give (very) different results, so my vote goes to Turnigy light electric. I think that with 13x8 you can even keep your current ESC, usually 40mm motors are more efficient than 35mm, the one I have.

At the same time 3D type of flying needs something completely opposite. A lot of static thrust and low max speed. Hence a 14x7, 15x8 would be great to give you up to 2:1 ratio, which is what you need for 3D.

The beauty of electric is that one same motor can successfully be used on many completely different application from F3A, 3D to warmliner.

And last but not least. The static measurement taken on the ground is just an indication of how good is your combo. Prop unloads a lot in the air and my guess is that with your current combo you had very good flying time but the throttle stick was at 2/3 at least all the time and the reserve of power was very low. I believe it is now more clear why.

Velco