tonywayne

I have a gws pico tiger moth, motor is a Westport 25 turn cd-rom motor 22.7mm stator weighs 18grms. With a 3cell 500mah and 8x4 prop it will fly about 25+ min. With a 2cell 800 mah same weight as the 3 cell it takes almost full throttle to stay up and only flys about 15 min. What would be a better prop for the 2 cell to get better performance flight times? and not only prop dimentions but a good brand too? Thank you

Woodsy

if it's performing good on the 3s why are you lookng at changing to 2s?

Dr Kiwi

Exactly what Woodsy said!

I am having a problem with the math. With the 3s 500 you are averaging about 1.2A/13W... but with the 2s 800 you are using 3.2A/23.7W... why does the 2s scenario need almost twice the power for the same performance from the aircraft? From my test data, I figure 13W on 3s will get you perhaps 6oz thrust at ~20mph... 24W on 2s will be getting you 8oz at higher pitch speed (25mph). The only way you can draw <2A on 2s (to get longer flight time) is to put a 7x3.5 GWS HD on it - but with that prop at <13W you'll only get 4oz thrust!

downunderdog

Can someone correct me if I'm wrong.... but that sounds like a battery problem to me. With everything else equivalent (same prop, same weight, same airplane/motor), and flying around at "low thrust" levels, shouldn't a 2 cell 800 mah fly 60% longer than a 3 cell 500 mah (800mah vs. 500mah)? At low thrust levels (slow cruise), you're only pushing say 6 volts to the motor regardless of whether you have a 8.4 or 11.1 volt battery supplying power. With 6 volts, the motor draws a particular current and provides a particular thrust, again independent of what is "behind" the 6 volts (8.4 or 11.1 volt supply to the ESC). So, in order to cruise at slow speeds the current through and voltage across the motor should be the same. Only when you push the 3-cell equipped motor over 8.4 volts will you see a difference - higher voltage being supplied to the motor causes it to draw more current. So a 2 cell should never draw more current than a 3 cell, and a 3 cell only draws more when you set higher thrust levels.

Do I have something incorrect in this assumption?

tonywayne

'''Do I have something incorrect in this assumption? ''' Yes i do. C rating. the 2 cell is lower c rating. would that not alow me to provide more rpm to the motor?

downunderdog

I thought that voltage defines the rpm of the motor (i.e. the kv or rpm/volt rating of the motors), and current adjusts to the load - i.e. a high pitch or high diameter prop causes the current to increase for a given amount of voltage. But at a constant speed and weight with the same motor and prop, the voltage and current should be equivalent regardless of which battery is in use because the prop is the same and producing the same amount of thrust (which is the thrust that is required for the given speed and angle of attack/weight) at the same voltage and current.

C rating x capacity gives you maximum current capability of the battery, so having a lower c rating on a 800mah battery compared to a higher rating on a 500mah battery would tend to equal out the max currents from either (depending on the actual ratings, but 800mah x 15 c = 12 amps vs. 500mah x 20 c = 10 amps...). As c rating defines the max current of the battery, I think it only comes into play if you're approaching or exceeding it... other than the fact that batteries become less efficient as you approach their maximum current (i.e. a battery will last more than twice as long a 8 amps than it will at 16 amps, even though you draw the same theoretical number of mah's out of them in the two time periods). But I wouldn't think that that would have enough of an effect to cause an 800 mah battery to last less than 60 percent as long as a smaller capacity 500mah battery does.

tonywayne

So let me see if I have this correct. Say for me to stay aloft the longest the propeller is spinning at a constant rpm volt, say I fly constantly at 4000rpm and the pack puts out 4 volts to sustain that rpm regardless if it’s a 2c or 3c pack? So actually the 2c 800mah should fly longer since its more mah and same weight? Say I never go above 4000rpm and that only uses 4 volts, the extra 3.4 volts in a 2c would just be dead weight unless a need a punch? Or if I use a 3c just to maintain that constant 4000 rpm at 4 volts the extra 7.1 volts would be dead weight right? So if a 2c can produce enough rpm it should always give more duration over a 3c of = weight and more cap?

downunderdog

That's a good question... hopefully someone who has experience using different packs on a given airfram/motor/prop combination might be able to weigh in - I've only used 3 cells in my planes (and NiMh) and 2 cells in a coax helicopter, so I'm only talking theory when it comes to swapping a 2S for a 3S.

I do have some questions for you, though -

What are you using to let you know when it's time to stop and charge the battery?
What are your voltage cutoff points for each battery?
Do you manually set them (is there a 2 cell / 3 cell switch), or does the ESC detect what voltage you started at and then cut off appropriately based on that?
Do you have a charger that tells you how many mah you are putting into the battery when you charge it? If yes, how much is going into each of them (the 2S 800mah and the 3S 500mah)?
If the charger doesn't have a readout that tells you how much it's put in, how long does it take each battery to charge after it's been run down fully?

It might be as simple as something like the LVC cutoff point for your 2 cell not being correct, which causes the plane to think the battery is dead when really it still has some capacity left.

tonywayne

Im using a timer but I land as soon as I detect power loss. ESC has no programming for cut off. I use the triton 2 and it tells you every thing. I only flew the 2c once but did not pay attention to the charger before I removed the pack. I will have to try some more flights on another 2c to see if its just the pack. I have read that if you stop flying as soon as you detect power loss the packs will be safe. Is this posible or will you be getting to close to the min voltage at the point of power loss? I have never checked the voltage after a discharge but triton2 is not suppose to initiate a charge to an over discharged pack. I also use an inline balancer with every charge. I am new to lipo, so im learning. Thanks for all the replies.

Dr Kiwi

I think you have a fundamental misunderstanding. The voltage from a battery is essentially a constant (lets say 7.0v under load for 2s, 10.5v under load for 3s), no matter what rpm your motor is spinning at. As you open the throttle to speed up a motor, the amp draw increases (do this with a Wattmeter in-line and you'll see that the voltage does not change very much - yes, it will drop a fraction as load increases, and as the pack is drained). What matters for flight duration is how many amps you are drawing in relation to the capacity of the pack - and for a particular flight speed, that will depend on the total power input (watts).

If you have a 2s pack, call that 7v... you may need 10A (thus 70W) to maintain level flight. Use a 3s pack (say 10.5v).. you'll now need only 6.7A (the same 70W) to maintain level flight.

Thus you will drain a 2s pack faster than a 3s pack, simply because (for a given power input) the amp draw on 2s has to be higher than it is on 3s.

downunderdog

Maybe I do have a fundamental misunderstanding... Maybe it's with how the ESC provides power to the motor. Motors are rated in RPM/volt, so I figured that if you were to measure the output of the ESC, the voltage would vary depending upon throttle position, up to a maximum at full throttle that is just below the voltage of the battery supply.

Can you clarify this:

If you measure the voltage input from the ESC to the motor (i.e put the positive probe between the ESC and the motor and the negative probe to the battery negative terminal), would it be be a constant which is equivalent to the battery voltage at all engine speeds?

Edit: Ah, I thought about it for a while, and I guess the ESC acts like a step down transformer, so a battery putting out 1 amp at 12 volts could drive a motor with 2 amps at 6 volts. Makes sense, I guess... But then a 2S 800mah still should last longer than an 3s 500mah (which should last the equivalent of a 2s 750mah, if the transformer thing is correct). Before thinking about it, I guess I was just thinking that the ESC controlled the voltage to the motor with something like a resistor, but that doesn't really make sense from an engineering/efficiency standpoint. Sorry for my misunderstanding/lack of thinking.