New Build! X8 top bottom Props Recommendation

Hi All

I am planning to build X8 coaxial copter setup but i am not sure about top bottom propellers configuration ,
i have read three types ;

  1. Top / Bottom Propellers with same diameter and pitch.
  2. Bottom Propellers smaller diameter and higher pitch than top
  3. Vice Versa of Config # 02.

I am using Pixhawk 2.1 with Arducopter 3.5.2.
Motors etc to decide… later

What is the propeller configuration for X8 that Arducopter supports??
ANYONE with gentle advise is greatly appreciated .

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wondered myself, I just read a thread where someone used the esc software with gains on the lower motors
Flying 14s all the way around on my 630kv motors. Wondered if putting larger props on the bottom would actually do anything

I have read threads guys building X8 copters So hoping to get some feedback from them OR anyone with kind information.

There are many opinions on this on the internet. I think solution no.2 applies to 2 props sharing the output of a single motor. Since both props would run at the same rpms, the second prop would not be able to add much to the already moving air from the first prop, if it were at the same pitch. It should be smaller in diameter, because the air column from the first prop tapers in. With the same diameter the prop would run in two different “types” of air.
For your X8 copter, I would suggest making the bottom props a little bit larger in diameter. The bottom motor should be under lighter load, because it is windmilling in the airstream of the top motor. Do a hover test with different setups and meassure the motor temps afterwards.

The x8 flies like a dream. I used the same size props and built in small dihedral.
I did not notice the efficiency drop. it felt absolutely locked in and handling wind like a pro. 15x5 props , 360kv motors, 6 cell, 4.5 kg auw, for video I am using raspberry pi running ez-wifibroadcast.

Thanks a lot , i would use U8 170 Kv motors with CF 28" Propellers .

Still i would use the same props for top and bottom??

My very first x8 I built few years ago had an motor burn out while flying . smoke coming out but no control lost at all. Make your bottom props one size smaller and increase the pitch one size up. This will give you about 5% better performance. On my latest copter I used same size props. They compare the x8 to an normal octo , but keep in mind that you need 4 more arms and props need to smaller . The other improvement I made was, I used an magnetometer breakout board and installed it on landing gear. no need for gps mast.

I did a lot of testing on my Y6 with different props for upper and lower motors. Efficiency just depends on prop size, but for better yaw control it is a good idea to use props with higher pitch for lower motors.

Hubertus

I’ve always used the same props top and bottom simply because I’ve not been able to find any matching high quality carbon props with different amounts of pitch (I’m currently running T-motor 17 x 5.8" props)

After a long flight, my bottom motors are always stone cold, but the upper motors fairly warm to the touch so there is a huge imbalance in loading going on in my setup.

I remember reading somewhere (can’t find it now) that someone had an X8 with a custom motor mix which sent 10% more power to the lower motors to balance out motor loading when using identical propellers top and bottom. This may well have been on a DJI controller though.

@rmackay9 I wonder if an option to set a per-motor thrust multiplier value (eg between 1 and 1.2) to have the motor mixer send up to 20% additional power to the bottom motors would be viable?

Maybe this would cause problems with flight dynamics but if not it could be a great feature for coaxial rigs.

***Edit - it was a DJI controller: https://www.dronevibes.com/forums/threads/hl-x-flight-times. Of course to prevent output saturation of bottom motors, we would instead need to reduce top motor output rather than increase bottom motor output.

Some data: Influence of propeller configuration on propulsion system efficiency of multi-rotor Unmanned Aerial Vehicles
Edit:
https://www.youtube.com/watch?v=tFJyE3Uns3o

4 Likes

Thank you Markus.

A really good read.And as I have a DYS D800 waiting to be rigged very timely.

My research and experience:
https://www.rcgroups.com/forums/showthread.php?2475611-Evolution-of-an-RC-enthusiast-in-search-of-reliability-ZMR-250-X8-Coaxial#post32362210

Roberts presentation at Dronecon:
https://www.youtube.com/watch?v=HywcbNHroQg

3 Likes

This is all excellent stuff.

I bought the D800 for a steal as a scratch and dent from HK (photographed for the webite and reboxed ).It’s been stuck in a case since as the hexacopters get finished off (ish).It will be fun to get it out and give it a go.Not quite the sky crane I once imagined it to be it will still have a bit of grunt to spare.Maybe I can use it for mowing the lawn.

My Setup in X8 would be :

Top U8-135 Kv motor with CF 29x9.5 prop
Bottom U8-170 Kv motor with CF 28x9.2 prop.

I would appreciate comments about this Setup.

So does Ardupilot not offer a way to blend the motors? I have castle creation escs and the new firmware has an adjustment to set for more efficiency or more performance on an esc. I wonder if setting my lowers to more performance and leaving the tops on medium or efficient would be the same as blending so the bottom motors would work as hard as the top

The old U8 don’t work well at 6s and 28’’ at 8s are a little bit bigger for me

Theoretically you would want the faster higher pitch on the bottom but you may not be able to find the opposite 29x9.2 on top 28x9.5 on bottom. The bottom needs to go faster and you can do that multiple ways. So you don’t have to have higher kv and higher pitch.

I am on a similar project, and first thing was go to ecalc and try some configurations to find weight and others parameters, I think most important is battery voltage, for U8, 6-12s and flight duration in amps.
Seeing old u8 chart (new series are here) and some imputs, the main problem with large props (and heavy) are lazy behaviour and windy conditions, this means forget 6S, so go to 8-10S and this means forget 29-30 props. 28-24" propellers is the max diameter for my understanding, choice by voltage 8-10S and kv motor.
So kv100 and 10S 28-26 prop, kv135 and 8s 28-26 prop, and kv170 8s 26-24 prop I guess will fine.

About top-bottom configuration, I read a lot and seems with large props and low rpm in a efficiency config affects less the effect on bottom motors but whatever cofiguration, the bottom motors are gonna eat more power, so focus in good motor-prop in charts. Last thought is with same prop-motor combination, you can rotate motors bottom-top for compensate the extra work.

I’m waiting for some test of the new 190kv at 6s, perhaps can work… the problem at 8-10s are the batteries, the standard for big copters are the 6-12s

Interesting test https://www.youtube.com/watch?v=Mt0owkbw2jk
If you contact them they build what you want…http://www.gensace.de, well, you should buy some units I guess