General 44.4v's back down to 22.2v's

[Smoggie]

Dennis,

I just skimmed the post but all seems sensible enough... However if you want to run 1900RPM then you dont need to change anything. 1900RPM is within the operating range of a standard 12S setup. I get 7 minutes out of my Trex 700 as standard.

Reducing cell count doesnt in itself give longer flight times, what gives the longer times is reducing headspeed. There are a number or ways to do that other than lower voltage batteries.

If you went down to 6S on the standard motor and gearing then with the added weight of a scale body you would probably struggle to get off the ground. Also if running genuinely low RPM (like 1600 and less) you would want to change the tail ratio as mentioned in your post, but it's not a simple job, you need to slot holes in the frame to fit the 21t gear and stiull have the mesh correctly adjusted.

For moderately low headspeed (1600-1900RPM) you could look at using the Align 800MX 440Kv motor.

 

[holtneil]

Hay Steve your heli list is getting a long as some posts lol
Is my goblin running that sort of headspeed I know its low I get about 9 minutes but stop at 8 as I get bored lol

 

[Tony]

2100 to 1900 is a "slight reduction"? On a 700-800, that is a HUGE difference.

I stand by what I already stated though. Ohm's law is Ohm's law, the lower the voltage, the higher the amperage.

 

[Eggy 71]

Ohm's law does apply but not quite that simply. What is being proposed is reducing head speed by voltage alone. If different gearing or motors are used then the argument totally changes. Take for example some numbers that make the math easy. say a 14s heli has 1000 rpm head speed at 100% throttle which is 100A. Now install a 12s setup into the same heli. 100% throttle is still 100A but the motor only turns 860 rpm (12/14 or 86%)because of reduced initial voltage. The extra flight time comes from the larger mah of the battery. Going from 10s to 8s would drop headspeed to 80%.
Now nothing is ever quite that simple. Amps, volts and power factor (PF) all effect each other. Amperage isn't lineal with the throttle position. Loading up the head causes issues. The head will require more amperage or reduce the head speed even more because the amperage is maxed and the effective voltage drops even more. It's that drop that causes problems. The more cells, the better it can handle the drop.
This all is factored in designing the heli. The power required to load the head is what drives the voltage to be used. If you aren't loading the head very much you don't need all that voltage and can utilize the extra weight as mah.
The 'sweet spot' where the heli performs the best should be where it is designed. You might get a little more time by reducing voltage but it won't take much before it will start effecting performance. Easy flying would be ok but quick starts ,stops , turns, and lift will have the heli performing like a 'flying hippo'.

 

[Tony]

The basis I'm looking at is given the same wattage, the amperage will always go up when the voltage goes down.

 

[Eggy 71]

Amps at some point will max out. When it does then the voltage drops and or the wattage drops. That's what happens under loading. It's like filling a swimming pool with a garden hose or a fire hose. Both will fill the pool but one takes way longer than the other. It's the length of time that shows up in performance.

 

[Tony]

Current is always directly proportional to voltage. There is a max and that max is when the conductor fails. Otherwise, amperage will keep climbing as voltage drops given the same wattage.

 

[Eggy 71]

Correct, but as amps increase so does heat. The heat is draws energy from voltage because it can't from amps. Current in equals current out. That means effective voltage goes down and so does effective watts. Not all the power is useful and what you peg at a set wattage actually lowers. Ohms law is the basic theory but it's more complicated than that. This is why voltage lowers under load.

 

[Eggy 71]

My apologies as the thread is a little sidetracked. In the end if Dennis were to lower the cel count amps wouldn't go up proportionally as the system is not requiring as high a wattage. The work will only be 860 rpm instead of 1000 rpm and thus use not as much work is done in the same time frame.

 

[Smoggie]

The basis I'm looking at is given the same wattage, the amperage will always go up when the voltage goes down.

That's true tony, but if all you do is remove a 12S mattery and fit a 6S battery (making no other changes) then power ('wattage') doesn't stay the same. The lower voltage means that the rotor RPM is lower so less power is needed to spin the blades.

 

[Dennis H]

All are Great and Interesting responses, I appreciate it.

The answer I am looking for is to gain more flight time considering I am not a hard core 3D flier.

Do I change the gearing, change the motor, drop to 10 cells, what is my simple option to gain 1 or 2 minutes of flight time?

I am willing to try anything,
Dennis

 

[Smoggie]

Dennis,

It depends what stuff you already have. If you already have batteries then it's probably cheaper to buy a lower Kv motor. Changing gearing is a good option on many helis but the Trex 700 with it's fixed motor positions doesnt give much option on that front.

If I were doing a moderately low RPM setup I'd just stick in a lower Kv motor, around 450Kv would be ideal. That's precisely what Align themselves use in their F3C version of the 700.

 

[Dennis H]

Dennis,

It depends what stuff you already have. If you already have batteries then it's probably cheaper to buy a lower Kv motor. Changing gearing is a good option on many helis but the Trex 700 with it's fixed motor positions doesnt give much option on that front.

If I were doing a moderately low RPM setup I'd just stick in a lower Kv motor, around 450Kv would be ideal. That's precisely what Align themselves use in their F3C version of the 700.

OK, i haven't done anything yet, I will start off with the motor.

I have been reviewing everybody's opinion before moving forward.

I will check into the F3C 700 motor out. How about main rotor blades?

This 700 will probably end up a scale before long. It might stay just like it is. So I am need to make sure I replace the components that are compatible with a Scale Body and a few extra pounds. I have an A109 body just waiting for the mechanics to be installed.

I will also be able to run slightly "larger" batteries in the Scale, like two 8000mAh's vs. 5000mAh's.

Thanks for your help,
Dennis