Coaxial motor layouts

[roycruse]

my "free wheeling" prop was there to serve the purpose to show that the air in front of a propellor is accelerated too.

Please explain to me how a propeller breaks the laws of physics by creating a mass of air to eject out the back of a prop without "dragging" it in from the FRONT !!!!!

the load on a propeller is generated by several forces

parasitic drag from pushing its self through the air
induced drag from creating the lift
the inertia that needs to be overcome to accelerate the mass of air
and fighting against a high pressure build up behind the prop

the lower prop will help reduce the last 3 of those

i cant belive that even showing you that air is flowing fast from the front into the prop - you still believe the top prop is operating in still air

i think i give up trying to explain now - im losing the will to live.

 

[jes1111]

I'm not saying that the top prop is operating in still air - just that the velocity of the air entering it is "effectively" zero - in terms of selecting a suitable propeller pitch. What percentage increase in the pitch of the top prop are you proposing?

 

[Bartman]

just to keep things friendly here at the good old multirotorforums, let's just say that the airflow is different approaching the top props vs the bottoms. if someone wants to invest $10 they can probably buy all the motors, controllers, test equipment, and props from hobby king that they need to do a full analysis. i leave it to the masses to resolve this scientifically. personally, i see the rationale for higher pitch on the bottoms to be intuitive enough that I plan to do it. but then again, i'm building a quad that looks like someone sat on it so maybe all y'all shouldn't listen to what i have to say.

bart

 

[Droider]

By changing props as suggested here what difference are we going to experience?

Better efficiencey?
Longer flying time?
More lift?
Better flight characteristics?

OR

Is it just going to blow the flight controllers mind!

I am sure there is a perfect set up out there.. but as far as I am concerned unless it gives me significantly smoother flight ot doubles my flight time i'd rather just fly my beautiful 13x5 Xoar carbons.. just coz they look the dogs bollocks!

Oh and they also give amazing performance.. I think Geoff videoed my climb out rate which is scary..

Anyhow keep up the good work Roy.. I love a Brit on a mission ;-)

Dave

 

[roycruse]

Droider.. thankyou for putting this debate into perspective...

Your right arguing whether the top prop needs to be higher pitch or not doeasnt really matter if yopu don't gain performance of some kind.

What I'm trying to prove is that two motors working to move the same column of air as in a coaxial setup will result in said column of air moving faster.
(both into the top of the propeller disc and out the bottom)

This will require a steeper pitch prop on both top and bottom props in order to "load" up the motors the same as if they were side by side pushing twice the volume of air.

So if prop a on motor x is say 5inch pitch to pull the highest safe amount of anps through a motor to most efficiently use its capabilities. Then you may need say a 6 inch prop on the same motor if two are used in a coaxial setup as the same columb of air is moving faster.

If you don't pitch up the top prop also I'm theorising that the amp draw will be down on the top motor and effectively you are carrying dead weight as you have a motor which will never be fully utilised (ie the same as under prolong any motor)

The driving force behind my trying to prove this is that I'm trying to build the most effecient ship I can with the cheapest lightest motors I can. So having a motor capable of safely and reliably pulling 20 amps that is under propped and only ever going to pull 10amps would be a waste of weight and expense.

 

[Droider]

Morning Roy..

What about the FC?

Its on a mission to keep your craft level or at the attitude you want it at.. To do this the motors are individually controlled to do a multitude of things.. I could understand you point if the thing was just going to take off and sit in a level hover no yawing no chasing bikes/cars/Kangaroos!

My AXD3 (Y6)and all other MK setups has what is called a motor mixer table. In this table you set the motor throttle curves. Once these are set for you configuration each individual motor is performing its own job. If you go to my youtube page you will see a film of my coaxial set up from beneath to see this in action.

Another point of a coaxial set up is redundancy. Mactadpole posted a video of his X8 in a motor failure and he was able to land it OK. I wonder if he would have been able to if he had different prop sizes.

Just a few early morning thoughts.. Now off to my unit to get it ready for tomorrows MRF UK meet!

Dave

 

[thepelell]

this summer i did a quick and dirty test, trying to measure thrust in different combination.. at the time i didn't have many motor-prop combo to choose from.
from the past 2 months of coax usage i can say first hand that yes, higher pitch on lower props is good for thrust and flight handling (with multiwii board configured as quad)
When i had 2 separate power supplies, i would recharge 2000 ma for lower lipo and 1500 for top lipo (from 2200 4s lipos) props used are 10x5 xoar on top and 9x6 apc-e on bottom.
yesterday i tried 9x4.7 apc on bottom but didn't like the way it flied.
I also once tried 11x4.7SF on top and 10x5 xoar on bottom with a 3kg AUW setup, but didn't like handling, probably had some pid tuning to do.. props were making strange howling noise too.. i guess using 11"Sf prop on a 900kv motor @4s is a bit too much...

here the data i collected.. it was done it crude manner, no special instruments.. just gave it full throttle and used a kitche scale to measure thrust, so don't take them as absolute values...

 

[jes1111]

Interesting figures. If I am interpreting them correctly, the top motor on its own gives 940g thrust with 10x4.5 and the bottom motor on its own gives 825g with 10x4.5 prop. But together they only manage 1420g - against a theoretical "no loss" target of 940+825=1765g. If we assume that the the top will give the same thrust whatever is going on underneath it (which may not be the case, as Roy says), then the bottom is losing 480g thrust out of its maximum 825g - a 58% loss in efficiency! It's a shame you don't have the current consumption figures - I think one would need those in order to get a true picture of what's happening.

Did I get my figures right? (Only had one cup of coffee so far this morning!)

 

[thepelell]

i forgot, i updated the spreadsheet to see the theoritical value against real values..
here is the link
https://docs.google.com/spreadsheet...E4xclQyb2RaQzZwSl9jVlpqaDZpcEE&hl=en_US#gid=0

yes , i wish someone with proper instrumentation would do this test in a more scientific way...

 

[gtranquilla]

Some more technical data on coax but within ducting......

http://www.personal.psu.edu/faculty/c/x/cxc11/papers/Double_Ducted_Fan_0903_Detailed_Description.pdf

just to keep things friendly here at the good old multirotorforums, let's just say that the airflow is different approaching the top props vs the bottoms. if someone wants to invest $10 they can probably buy all the motors, controllers, test equipment, and props from hobby king that they need to do a full analysis. i leave it to the masses to resolve this scientifically. personally, i see the rationale for higher pitch on the bottoms to be intuitive enough that I plan to do it. but then again, i'm building a quad that looks like someone sat on it so maybe all y'all shouldn't listen to what i have to say.

bart

 

[GASSED]

I have played quite a bit with the coax system trying to achieve same flight time as single props, but not achieved, best is a 10-12% loss.

What Roy is saying, if on a single prop your best prop for your rig is lets say 13x5, a possible coax set up could be somthing like 13x5.5/13x6 top(to help with the air that is already moving because of the bottom prop sucking in air from top so increase the pitch slightly) and 13x6.5/13x7 bottom or 14x5.5/14x6 if you want to increase the bottom diam.

Hope this helps.