New FT MiG 3 build, going from 3s to 4s question

[Tryb]

Hey everyone!

Just got into flying airplanes and loving it! I flew helis a few years back but adulting happened and didn't have time for it. Found this site and fell in love with the foam builds so I'm making the time for this.

I just did a scratch build of the FT MiG 3 and first flight went well. It was really gusty out today so I didn't really get a good feel for how smooth she flies so hopefully tomorrow will be better.

I am interested in upping my battery from a 3s to a 4s since that's what was recommended in the video, but I had a couple of questions. Right now I'm flying a 2200mah battery and I was getting about 7 min flight times on my Bloody Barron before I brought her in. I imagine that's what I would get on my MiG if I had nice conditions to fly it in.

In the vid he uses a 4s 1600mah battery. Are mah between 3s and 4s comparable? If I go from 2200 3s to 1600 4s should I expect much shorter flight times?

Also I noticed he's using a 70c battery. Is that necessary to have such a high C rating if I'm flying with the C pack?

 

[Jetcrafter_2000]

If you have a 2200 mah 3s battery and a 2200 mah 4s battery you´ll have the same flighttime with a bit more power.
The mah´s are the capacity of the battery and the voltage ( 3s = 11,1V / 4s = 14,8V ) is how much power your motors will have.
So if you go from 2200 3s to 1600 4s you´ll have a shorter flitetime but probably the same weight.

Good Luck

Jetcrafter2000

 

[Tryb]

That's what I figured but wasn't sure how the increase in voltage affected how much power the motor draws. I'm a bit ignorant when it comes to electric power systems, I know just enough to not fry myself. In regards to the C rating of the battery, If I've done well with a 30c 3s would a 30c 4s be adequate?

 

[Hai-Lee]

An aircraft has a specific power requirement, thrust, to maintain flight. This requirement is due to the drag due to all sources and it varies almost exponentially with increasing airspeed. So if you fly at a set speed regardless of the battery then the power used will be nominally the same.

Other losses especially the Idle current of the ESC are normally relatively fixed and independent of battery voltage. So losses due to such things drain the battery at a set current rate.

To put the story together, When you increase the battery voltage you will normally have more motor power and thrust available to use and if you are using a smaller capacity to keep the weight the same then flying faster will decrease your flight time, the decrease in the battery capacity will reduce your flight time, the electrical losses will still chew through you battery charge at the same rate, and even if you fly at the same speed as you did using the lower voltage battery the flight time will be less.

If you keep the capacity the same when you fit the higher voltage battery you will still tend to find that the flight times are reduced because of increased drag, (due to increased weight and a tendency to fly faster). All other losses will be roughly the same!

If you do not change your prop when you change the battery voltage then other losses due to propeller mismatch can really chew through the battery charge. It gets a bit messy.

Where I want the same flight time and to run a higher voltage battery, (ie 3S to 4S), I try to increase the battery capacity by around 30%. If that is too much weight for the plane then I know to expect shorter flight times with higher voltage batteries.

Have fun!

 

[Hai-Lee]

As you increase the voltage you need to ensure that the ESC and the Motor can handle the increased voltage and current. As the motor will be trying to spin faster with the higher voltage you will need to change the propeller diameter or pitch to ensure that the motor is not "Over Prop'ed". Over prop'ed motors tend to burn out.

Check the motor/ESC/Prop combination to see what the current draw will be and ensure that the battery can supply the required current. You may discover that you require a higher "C" rating for the higher voltage battery.

Have fun!

 

[Tryb]

Awesome explanation! thanks!

Now the question is, will the mig 3 carry a 4s 3000mah battery??? That's nearly twice the weight of my 3s 2200. That seems a bit excessive. For the same price I could get 2 batteries around the 1500 range and just switch out batteries and fly a lighter plane and get the same air time.

Decisions decisions.

 

[Tryb]

Yes, I have a 9x4.5 prop to put on it and I have the electronics from the C pack they sell on FT store site and they do mention that a 4s will work. I just wasn't real sure how going from a 3s to a 4s changed things but you've cleared that up. Thank you.

 

[nerdnic]

Awesome explanation! thanks!

Now the question is, will the mig 3 carry a 4s 3000mah battery??? That's nearly twice the weight of my 3s 2200. That seems a bit excessive. For the same price I could get 2 batteries around the 1500 range and just switch out batteries and fly a lighter plane and get the same air time.

Decisions decisions.

Yes it will, I have run a 4000mah 4s in my nnMiG, but you have to ditch the long FT pod. A battery this size will fit in the fuse but will be too tall to sit in the pod. Also it will sit pretty far back over the CG so it might be a challenge to get lined up and secured.