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Old Mar 13, 2012, 10:49 AM
Hexapilot
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@Don, thank you for this good explanation.
Now in this case we are talking about multi rotor platforms and one of the reasons to use 6 or 8 motors is to have additional safety and redundancy.
On the other hand using a coax setup would reduce to some extend the size and the weight of the copter. In any case we need to use minimum 6 or 8 motors, it is only a question of not loosing efficiency and flight time with a coaxial setup.
Usually this type of copter will be used for aerial photography and video, so most of the time the motor will run at hoover speed.
Until now most of the copters with coaxial design out there are using two motors back to back, where the propellers are mounted too far away from each other.
I don't believe this could be very efficient. Also most of them proposing to use the same prop size but increase the speed of the bottom prop.
What is your opinion on this and what would you recommend?
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Old Mar 13, 2012, 11:03 AM
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Increasing the separation between props means that some of the swirl will dissipate due to friction with the surounding free stream before it gets to the aft prop. This reduces the amount of swirl energy that can be recovered, and also limits the amount of power the aft prop can absorb before it starts imparting swirl of its own the other way. Also, contraction of the slipstream means the aft prop should be smaller than if the separation was less, and propeller efficiency is very sensitive to prop diameter.

OTOH, getting them too close can result in some aerodynamic interference between the props.

In an engine-out situation, the dead prop will windmill, hurting the efficiency of the remaining prop in the set. It would be better to have them separate, so no interference, and larger total effective disc area, which also helps efficiency.

Running the aft prop faster increases the local airspeeds over the blade surfaces, which increases the profile losses. Since these are the dominant factor on our low-disc-loading props in the first place, that's bad.
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Old Mar 13, 2012, 12:44 PM
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Good thread. I am planning a heavy lift Y6 now and am looking at motors, props etc. In my opinion I don't mind losing a little efficiency to have a safer platform. Losing a motor and still having control and being able to land is a big plus.

Also, the different pitch/speed props on the top and bottom is interesting but I don't think it is viable. the yaw is controlled by torque differential between the top and bottom props. Having different loading/rpm between the 2 will likely cause issues. I can see one set of motors getting much hotter, running a lot less efficiently.

I would like to see a test of 4 scenarios. (amp draw, hover time etc with all the same equipment)

1. Conventional 6 arm hexa
2. Tricopter (obviously with 3 less motors but all else the same)
3. Y6 with "standard" distance between props (4-6")
4. Y6 with 12" distance between props.

I am guessing the differences would only equal out to seconds, maybe a minute, differences in flight time?
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Old Mar 13, 2012, 01:09 PM
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Quote:
Originally Posted by machina View Post
...Take a look. A non-contra rotatingmark XIX spitfire ...
I think you did not push the rudder yourself, piloting a powerful single engine, with large control surface.
Try full throttle on an acrobatic plane of today, and you will understand!

Like the P51 mustang, contra-rotative was a solution to cancel the dangerous couple.
But contra-rotative, which is not coaxial propellers, meant a complicated machinery, increasing the maintenance aspect. So, this disadvantages kill this design
The RAF was focusing on jet propulsion, early at the end of WWII, and propeller fighters weren't in the foreground anymore.

Before the upcoming reign of jet engine, we can find numbrous applications of contra-rotative design.

The point is that 2 propellers, close enough, cancel the tangential thrust, and thus, is more efficient.

Few RC enthusiasts can afford to use CFD to visualize flow, and there is a lot of conveyed design beliefs.
The usual coax opposite motor mounting has really no advantage, except of losing 25% of thrust.

@hexacop
Some messages above, "midiman007" will experiment coax propellers on the test bench. He should try to vary the propeller distance.
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Old Mar 13, 2012, 02:37 PM
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Can I throw a different angle in here.....

I have a Y6 of sorts, but its configured as a standard tricopter in the MWC firmware. (Each arm has two ESCs whose control input is Y'd to a single controller output.)

I've put APC 17x4W props on the top which give me TONS of lift. I first flew the machine as a tricopter with only top motors. It was unstable because the 17" props couldn't adjust speed fast enough to keep the thing stable. They had too much inertia.

I added the bottom motors spinning APC 12x6EP props. Now it flies perfectly stable.

So my conclusion is.....

I can get more lift out of my machine with this coaxial configuration then I could if it was a flat hexa because I can use mis-matched props top/bottom. Where as if I used these large props on a flat hexa it wouldn't be stable, so I'd have to use smaller props and not get as much lift.

Thoughts?
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Old Mar 13, 2012, 02:45 PM
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Quote:
Originally Posted by techspy View Post
Good thread. I am planning a heavy lift Y6 now and am looking at motors, props etc. In my opinion I don't mind losing a little efficiency to have a safer platform. Losing a motor and still having control and being able to land is a big plus.
It isn't necessary to have contra-rotating props to have the redundancy benefits. As long as the pairs of props are near each other, but not actually co-axial, you will have the redundancy benefits, and better efficiency, whether both are running, or with one dead.

Quote:
Also, the different pitch/speed props on the top and bottom is interesting but I don't think it is viable. the yaw is controlled by torque differential between the top and bottom props. Having different loading/rpm between the 2 will likely cause issues. I can see one set of motors getting much hotter, running a lot less efficiently.
You're missing the point. If you have them co-axial, and you DON'T have the pitch greater on the aft one, then you will not get equal loading on the two, nor equal torque. The inflow to the aft prop is different than on the forward prop (because of the induced flow of the forward prop), so using the same pitch for the two props results in a difference in their operating torques, powers and RPM's.

Yes, running the aft one faster is another way to accomplish equal torque (and therefore cancel out yaw), but power is torque times RPM, so the aft one will pull more power than the forward one. Of course a lot of that extra power will go into the aft prop's profile losses, because of the higher local airspeeds on the blades.

The real problem with all of that is that the difference in pitch required depends on the power setting and altitude. If you are using fixed pitch props, then there will only be a single throttle setting where the two props will be matched. At all other throttle settings you will have some mismatch and some efficiency loss. To get around this problem requires variable pitch props. One approach would be to run the forward one in fixed-pitch mode, use the variable pitch function of the aft prop for yaw control, and run the same power and RPM on both. That way, whenever the plane was in trim on yaw, the two rotors would be matched.

Altogether, this all still makes a pretty decent case for skipping the complexities and potential efficiency loss of contra-rotation altogether, and just putting the two props near each other, but separate.
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Old Mar 13, 2012, 03:39 PM
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Don, your posts have been incredibly helpful. Thanks.
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Old Mar 13, 2012, 04:19 PM
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Quote:
Originally Posted by Don Stackhouse View Post
... The real problem with all of that is that the difference in pitch required depends on the power setting and altitude. If you are using fixed pitch props, then there will only be a single throttle setting where the two props will be matched. At all other throttle settings you will have some mismatch and some efficiency loss. To get around this problem requires variable pitch props. One approach would be to run the forward one in fixed-pitch mode, use the variable pitch function of the aft prop for yaw control, and run the same power and RPM on both. That way, whenever the plane was in trim on yaw, the two rotors would be matched...
Very interesting input.
Actually, variable pitch prop are not necessary.
To maintain efficiency on a range of RPM, the 2 propellers must spin to cancel the tangential thrusts.
To do this, the RPM of lower stage is variable, and depends on the upper stage RPM.
This simplifies design with constant pitch props.

Of course, this condition means to also have proper distance, proper choice of propeller.
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Old Mar 13, 2012, 04:47 PM
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No, running a higher RPM results in increased profile losses (which dominate in our applications), and therefore reduced efficiency. That means that the efficiency of the variable RPM lower prop will be reduced whenever it has to run at increased RPM. You can get good efficiency at the design point, but will lose some efficiency at off-design operating points.

Although a variable pitch prop also loses some efficiency at off-design operating points (because the twist is no longer optimum), that loss is less than the increased profile losses from running at higher RPM's.
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Old Mar 14, 2012, 03:52 AM
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Quote:
Originally Posted by Don Stackhouse View Post
No, running a higher RPM results in increased profile losses (which dominate in our applications), and therefore reduced efficiency. That means that the efficiency of the variable RPM lower prop will be reduced whenever it has to run at increased RPM. You can get good efficiency at the design point, but will lose some efficiency at off-design operating points.

Although a variable pitch prop also loses some efficiency at off-design operating points (because the twist is no longer optimum), that loss is less than the increased profile losses from running at higher RPM's.
Being on the experimental side, to sustain any theory or design principle, the statement you expose here is difficult to discuss without experimenting
on the test bench, and measuring.
My duct and shroud project is a good opportunity to study coaxing on the test bench.
This needs time and I will publish my precise tests, using several flavor of motors and propellers.
The hovering state matches a thrust range of values, and the practical goal is to obtain an equal, or bigger value of thrust in this region.

Also, open propeller coaxing performances could be at a lesser level of interest compared to ducted airflow, removing the blade tip vortex.

Let's keep this very interesting topic on hold, needing experiment.

By the way, apart Bigger Motor, Bigger Proppeler and Bigger Lipo, is there any new interesting ideas to increase the weight to thrust ratio?
I don't think so.
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Old May 06, 2012, 06:19 AM
Hexapilot
Germany, BY, Munich
Joined Mar 2012
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I've finished my tests with AHM-36-6 coax motors and finally build my new CX8 copter.
My goal was to find the most efficient propeller combination at the preferred thrust of 1kg per coax motor.
In the end I was not able to measure an increase in efficiency but the current performance is not bad.
Some of you may have followed the thread here:
http://forum.mikrokopter.de/topic-32866-2.html
where I've posted some test results.
Finally I have decided to use a smaller propeller 12x6 on top and 13x6.5 for the lower propeller.



The carbon frame is made by http://www.kopter4u.eu
All electronics from Mikrokopter
Motors 4x AHM 36-6 coax (alpha versions)
Battery 5s 8000mAH
The flight time is around 19 min with this very light configuration of around 3kg
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Old May 06, 2012, 07:51 AM
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How come you decided on having the smaller diameter prop on top?

I guess you could optimize it even more by tuning it upside down, so the prop-wash won't hit the arm.
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Old May 06, 2012, 08:25 AM
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More pitch on the bottom prop is good. A lot more (and in this case, an inch is a lot!) diameter on the bottom is bad, perhaps very bad.

The reason for less diameter on the bottom is because the first prop increases the speed of the flow as a natural result of making thrust. In accordance with Bernoulli, when the speed increases, the diameter of the "stream tube" (the boundaries of the flow path of the air flowing through the prop) decreases. Making the lower prop smaller keeps its tips aligned with the boundaries of the stream tube.

If you get this wrong, portions of the tip region are working in different airflow than the portions right next to them, resulting in distorted flow and loading, and a loss of propeller efficiency.
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Old May 06, 2012, 09:18 AM
Hexapilot
Germany, BY, Munich
Joined Mar 2012
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Propeller comparison:
first is always the top propeller the second is the lower propeller
XOAR 12x6 (top) + 13x6.5 (bottom)


If you compare the data from the chart above you can see why I choose 12x6 on top and 13x6.5 on the bottom.
Here my learnings from the various tests I made:
1. the motor performs best with 13' propellers
2. due to the long motor shaft the motor is very sensitive for vibrations, the shaft for the upper propeller is 4mm only and as larger the propeller as harder it is to run the motor without or low vibrations, therefore 12' on top.
3. There is no doubt about the need of higher pitch of the lower propeller but if you compare the chart data above you can see that any other combination where the lower propeller was smaller than 13' is less effective.
4. Initially I thought I could run both motors at same or very similar wattage, but actually that is not an option. The lower (propeller) motor needs to work harder than the upper one.
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Old May 06, 2012, 01:25 PM
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hexacop, did you make tables where you tried different speeds on the top and bottom prop? could be interesting to see.
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