Coaxial motor layouts

[Bartman]

In another thread I'm building a coaxial quad and I'd like to have four 10X7 US std. rotation (CW) props on the bottom and four non-std 10X5 props on top so all motors on top would spin one way and the bottoms would all spin the other.
Is there any theory as to why one layout is better than another? I suppose they could alternate from arm to arm but can tops and bottoms be the way I've described?
Thanks,
Bart

 

[jes1111]

I'm guessing, but the only negative I can think of is the possibility of uneven left/right yaw response due to the likely difference in thrust between top and bottom. The OpenPilot CC configs have the Y6 props this way, but the X8 alternating, but nobody could tell me why when I asked. I agree that having it the way you suggest makes prop choice easier.

 

[Bartman]

i can see that being a reason for alternating but it will be a huge PITA to buy props for it to be set up that way. i believe HoverFly has their standard X8 set up with all one way on top and all the other way on bottom....using 10x5's on top and 10x7's on bottom should have the thrust pretty close i'd think. hmmm, this will have to be a game day decision.....i have enough 10x5 props to cover all eight motors but i don't want to have the same prop top and bottom for easons that have already been discussed here.

 

[jes1111]

Cut & paste from a thread I started on the OP forum some time ago:

The physics (and people's experience) says that the best prop combo for coaxial layouts is to use a higher pitch on the bottom motor. At first I thought this dictated the same prop requirements as all other layouts: you need both CW and CCW props available in both pitches, since half will be on top and half below - resulting in a very limited choice. But recently it occurred to me that our Y6 mixer calls for all the top props to rotate one way and all the bottom props the other. I realised this actually expands our prop choices - quite usefully so.

For example, in the APC range:
14x4.7SF top and 14x8.5EP (a bit high, maybe) bottom
13x4.7SF top and 13x6.5EP bottom
12x3.8SF top and 12x6EP bottom
11x3.8SF top and 11x4.7SFP or 11x5.5EP bottom
The underlined props are the "hidden" choices, i.e. their rotational opposite is not available (or otherwise unsuitable).

This may be old news to more experienced builders, but it's an exciting realisation for me

Anybody tried any of these "hidden" combos?

 

[roycruse]

Can someone explain the physics behind this two different prop pitches or diameters on coaxial setups as by ny logic and bench tests the props should be the same.

To me the statement that the lower prop is working in moving air doesn't take in the complete picture.

 

[Bartman]

roy,
the top props are moving in what is, more or less, still air so the pitch of the prop is encountering the air head on as it's rotating. Once the air is pushed downward by the top props it has a rotational nature to it so it's moving downward and into the oncoming lower prop. That combination of motions causes it to come into the rotating lower prop at an angle that serves to effectively lower the pitch of the bottom props. To account for this you have to use a higher pitch prop on the bottom so the prop will then have the same angle of attack as it's hitting the air that the top props have. You're trying to have all the props generate about the same lift so the flight control can do its job most effectively plus you don't want to waste power while making less than optimum lift.
In other words the air from above is blowing into the path of the lower props from in front and above the oncoming lower prop blade (imagine a fan above and in front of you blowing in your face), the lower prop blade is angled up and into that wind so the angle between the wind and the pitch of the prop blade can be effectively zero for a low pitch prop. The prop has to be angled up (relative to the wind) so it can make lift so to do this you have to use a higher pitch prop. You may use a 7" pitch prop on the bottom but it will cut through the oncoming air more like a 5" pitch prop because of the direction from which the wind is coming.
Make sense?

 

[Bartman]

Cut & paste from a thread I started on the OP forum some time ago:

I see what you're saying Jes and it's similar to what is driving me to try the more simple layout as I can buy std. rotation 10x7 props for the bottom from HobbyKing for about $1.80 and they're very similar in weight and shape to the Xoar 10x5's that I already have. To go the alternating props route I'd have to buy two RH rotation 10x7's and two LH rotation 10x7's so there's four times the cost plus another wait as the order gets filled in China and shipped to me in NJ via Florida.
What's nice about the newer Mikrokopter firmware is that I can set up the motors however I'd like and change the mixer however I'd like and set the motor directions in MK tool so one flight can be one way and an hour later I can try it again the other way.
Bart

 

[jes1111]

You've referred to the "suction" that the bottom blade would exert, but in reality the effect would be negligible - the bottom blade is not pulling air down in a straight-sided vertical column, but rather pushing the air that it encounters downwards - the "void" so created is filled from every direction, not just from within the column underneath the top blade. You can verify this by putting your hand behind and in front of a spinning propeller or fan - the "pressure" you feel in the direction of the air movement is not the same behind the propeller/fan as it is front.

I believe your bench test was skewed by the tiny motor spinning a very large prop - effectively the system was "current limited", as is evidenced by the fact that your maximum thrust measured was 1000g whereas that prop should be able to produce around 3000g (with a large enough motor). Since the current was the limiting factor, the perception that consumption was equal between top and bottom would be misleading.

 

[roycruse]

current is never limited in a brushless motor setup unless your power system is unable to deliver it. it is a well known fact that the power drawn by a motor is directly related to the load you supply it with, even to the point a motor will simply draw enough amps to burn its self out if you over load it.

I disagree that the effect of the lower blade "pulling" on the top one is negligible - i will conduct further test when my larger motors arrive. Of course airflow in front of the propeller is drawn in from a wider angle but the airflow mediately in front of the prop must be at the same speed as behind the prop otherwise the laws of physics must be getting broken. (matter cannot be created or destroyed - the conservation of mass)

I would expect two motor pulling/pushing in unison would have an airflow pattern with extremely little spillage of air flow out the sides between the two props which would indicate the bottom motor is helping pull the top motor just as much as the bottom motor is "pushing" the bottom one. resulting in overall you needing steeper pitch blades on both motors in a coaxial setup when compare to using the same motor used individually.

I would love to see some proper data or wind tunnel/ airflow video that goes against this theory.

does anyone have an idea how I could measure the rpm of each prop individually - Im not sticking my hand and tacho in between both props and if I try to measure from above or below Im worried I will get readings from both propellers.

View attachment 1323

 

[jes1111]

I think practical experience supports the theory (well explained by Bart). Many coax machines fly well enough with identical props. I've even seen people putting higher pitch on the top and they still fly. Also seen bigger diameter on top, bigger diameter on bottom and every other variation. They all fly. But the only scientific, supported info I've seen (in the Power Systems forum at RCG) says 20% more pitch on the bottom is the way to go. What pitch you have on top is determined as much by availability as choice - but in general you want the lowest pitch you can find. Low pitch = high acceleration, less drag (but low top speed) - that's what we want.

 

[jes1111]

To measure rpm, maybe a strobe like we used to have in the physics lab at school (100 years ago)

 

[jes1111]

Thinking some more - using only the kit you've got, you could get a relative measurement of what each motor/prop in a pair is doing by starting with two identically-charged identical packs, running at a fixed throttle position for X minutes, then seeing what you need to put back into each pack.

 

[roycruse]

I have a comprehensive watt meter and can see exactly how many milliamp hours go through each motor - can you give me a link to the stuff on rc groups that came to the conclusion that a 20% pitch difference is required.

The only thing I can find so far is some data for a contra rotating prop system on a plane - and they have around 2% difference in pitch range of the front and rear props. which in the world of rc prop choices would mean identical props are closest to ideal.

some other interesting data Ive found suggests that contra rotating prop systems are actually around 6% more efficient than standard single props as the energy usually lost in rotation of the prop wash is re-cylcled by the contra rotating prop.

I love this hobby - always something new to learn

 

[jes1111]

I have a comprehensive watt meter and can see exactly how many milliamp hours go through each motor - can you give me a link to the stuff on rc groups that came to the conclusion that a 20% pitch difference is required.

The only thing I can find so far is some data for a contra rotating prop system on a plane - and they have around 2% difference in pitch range of the front and rear props. which in the world of rc prop choices would mean identical props are closest to ideal.

It's there somewhere - a long thread about contra-rotating motors. Somewhere in the middle some scientific bods weighed in and spelled out the theory.

some other interesting data Ive found suggests that contra rotating prop systems are actually around 6% more efficient than standard single props as the energy usually lost in rotation of the prop wash is re-cylcled by the contra rotating prop.

That sounds like the wiki article on contra-rotating props - doesn't apply at all here - that's using two props driven by the same engine. The back prop (which is locked in sync with the front) "recovers" some of the losses of the front one, resulting in a net gain in the conversion of the engine's power output to thrust.

 

[roycruse]

the wiki article says nothing about this effect only working with propellers attached to the same engine - its to do with counteracting the rotational aspect of the prop wash - in fact how are air atoms rotating in a corkscrew fashion flowing past the second prop aware of the mechanism driving the second propeller ????? the prop wash rotation is cancelled regardless of the drive mechanism of the second propeller resulting in a non rotating thrust so more of available energy is linear.

 

[Bartman]

those studies also suggest that you should have more propeller blades on the rear prop than the front in order to keep the frequencies unsync'd

roy, your drawing is incorrect, the prop on top imparts a spiral motion on the airflow, planes suffer from this as a sideward force on the vertical stabilizer causing yaw at high power settings and slow airspeeds. ignoring the spiral nature of the airflow would make any conclusions incorrect.

 

[roycruse]

And you are all ignoring the fact (and I don't know why - blind faith in someone's miss information I guess) that air enters the front of a propeller at exactly the same speed it exits the back...

THE LOWER ROTOR WILL THEREFORE HAVE A BIG EFFECT ON THE UPPER ROTOR

I just went outside to my workshop to take this video

the rig is upside down for thrust measuring reasons on a normal set of kitchen scales - so think of the upper rotor as the lower one and vice versa - they are thrusting up in the video.

There is no power to the bottom rotor in my video (equivalent of upper rotor in flight)

So how do all you non believers explain that your so called "negligible effect" is turning the un powered rotor at great speed !!!!!!!

by the way - this was only at 1/4 throttle

and remember the top propeller is the only one that has a battery connected and it is thrusting UP - this is clearly not negligible and shows that the upper rotor in flight is definitely not operating in "still air" as you are all claiming.

heres the video

 

[roycruse]

roy, your drawing is incorrect, the prop on top imparts a spiral motion on the airflow, planes suffer from this as a sideward force on the vertical stabilizer causing yaw at high power settings and slow airspeeds. ignoring the spiral nature of the airflow would make any conclusions incorrect.

Im not certain but I believe the rotational speed of the prop wash is many magnitudes slower than the rotation speed of the prop so in the distances we are talking between the two props the rotation would be a tiny fraction of a revolution and therefore can almost be ignored - as I said before I would love to see some slow-mo smoke tests of airflow through a prop to prove or dis-prove this.

 

[roycruse]

To measure rpm, maybe a strobe like we used to have in the physics lab at school (100 years ago)

the perfect solution - i just wish i had a timing strobe from a car now... although the frequency would not be anywhere near enough for a prop...

i suppose i could look for an electric circuit to do the job - in pitch black - even an led would be enough to freeze motion

 

[jes1111]

The top prop in your video is "free-wheeling" - of course it will do that. Your proposition was that the top prop would need a higher pitch than "normal" because of this effect - I'd still reckon that's not the case. What's "normal"? The fact remains that the bottom prop needs more pitch in order to be able to accelerate air that's already moving (considerably) faster than it was when the top prop encountered it - that's the physics of it