Bigger propellers on octocopter at the same size of frame

Hi

I have quite big octocopter with flat frame cinenestar type with 16" propellers and t-motor u7 420kv motors. It has 120 cm frame size. I would like to change the propellers in it to 18" to have longer flight time. I can use longer booms but the copter will be too big for my car so I am wondering about using the same booms but 4 motors up and 4 motors down like in the picture in attachment. Is that setting is as efficient like flat octo or I will lost same flight time like on coax setup ?? What kind of mixer I should use for such setting ? I am using Mikrokopter FC but I think that will be similar in APM or Pixhawk.

regards

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You shouldn't need any kind of mixer as long as the Mikrokopter FC can be configured for an octocopter. As long as the motors spin the right direction relative to the frame, what difference would it make if the motors where pointing up or down ? I have heard that a partial overlap can increase efficiency a little bit. The key is the motor spin and the props installed correctly as per the documentation of your flight controller. Also, after changing the arm dynamics, that means if you change the arm length, or the arm end mass, you will change the resonant frequency of the arm. You may also change the RPM with different prop/motor combinations. If the RPM *2 or the aerodynamic excitation frequency approaches the arm resonance frequency or harmonic you can get a lot of z axis vibration. This is not good. I don't know about Mikrocopter, but with Pixhawk you have all the data logging and tools to analyze your vibrations. Consult the motor charts and use the props specified, not kinda close. I had cheap 15-5 props because I didn't want to spend extra for T-motor props (14-4.8) that was specified. When I invested in the good props I gained 18 - 20 % efficiency improvement. I sound like a salesman, but I am just a happy flier :)

Going from  16" to 18"  depends also if you are actually with 4S or 6S batteries .

Finally you will get a 10% more thrust if T-Motor chart correspond to reality but the propeller weights more so it will less than 10% and has more inertia so if it a good choice  depends much if you are already flying close to motor limit or not.

No need for any mixer, just put as in photo above three motors upside down.

Thanks for answers. I did calculation in ecalc.ch and checked the documentation of the motors so I know that on such configuration on flat octo if I will change propellers from 16: to 18" I will get extra 1 minute of flight time. My problem are this upside down motors they will overlap a little bit with normal up motors so this overleaping will increase the efficiency ?

If I will just turn motor upside down it will rotate in different direction so copter wont fly correctly. I have to change also direction of restoration of motor in Flight Controll.

The overlapping will cause no problems but I doubt it will increase efficiency .

You can change by software the motor direction if you have black MK Esc or last version MK Esc,  if you have the red MK Esc it is not possible and you have to switch two motor cables that arrives to Esc to change the rotation  direction.

What is your AUW, payload and Flight Time ?

Octocopter weights about 13kg with camera and gimbal and batteries (2x10 000). It fly around 13 minutes.

I have done some maths , not with E-calc,  you should hover at 91,6A with a 6S setup .

Are you using standard 20-30C Lipo batteries  with 3000 g of weight ?

yes

hover is 70-80A

In that case I suggest to switch to a different type of batteries , Sony Lion high C discharge , so you can go up to 20 minutes. 

Yes but that's bigger weight and much bigger price for batteries I think ??? Are they better than tattu ? Do you have any link ? Can I have 12000 or 15000 batt in the same weight as tattu lipo ?

Guess that 4 pair of good 18" propellers as T-motor are not cheap.

Good Luck for your project !

Why not do some testing with the original prop in the original configuration versus the "up/down" configuration, everything else being the same? Not only will you have your answer, but come back here and let us know what the results were. Nothing better than real world results versus theory.

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