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Old Apr 30, 2012, 07:09 PM
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Two motors off single esc weird configuration..

Flyer prop question.
I've never run two motors off a single esc for sync, so first off do I double up the power of the esc?
OK now for the weird bit.
Depending how you mount them can have a dramatic effect on thrust.

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With both motors mounted fairly close together as in the pics, with the motors mounted flat the combined thrust is one motor thrust times two.
100% + 100% = 200%

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With each motor facing out by 45º (total 90º) total thrust is again twice each motor's thrust combined.
100% + 100% = 200%

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However with the two motors mounted at 30º (total 60º) then the combination of the thrust coming together gives half as much thrust again.
100% + 100% = 300%

OK something is happening beyond my ken here and I'd love a (layman's) explanation.
Is this just really efficient and we get 50% more thrust, or is there extra strain, or over reving on the motors and/or greater current draw.

Some trick of physics?

have a look at this, where Rimshotcopter demonstrates this in action.

http://www.youtube.com/watch?feature...&v=0Jy-fUCvzZw

If this is right then the quad I'm building is going to become an octo with two motors opposed at 60 degrees on each arm, that flies on quad software, and gets the thrust of a douzainecopter.
I hereby claim the word "douzainecopter" for Her Majesty, Bess II
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Old Apr 30, 2012, 07:51 PM
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The test is worthless without current draw/wattage numbers
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Old May 01, 2012, 04:05 AM
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Originally Posted by NACA0012 View Post
The test is worthless without current draw/wattage numbers
Not really worthless it shows something is happening as the maximum thrust increases, but draw/wattage are certainly relevant and I'd like to know what they are.
He's using the same cell on all 3 tests.

I'd love to see this done in a lab with smoke streams, on a proper thrust gauge (rather then scales) and in a clean air-stream rather then on a set of desk scales, and with different pitch props, as this could be cancelling a cavitation unique to the props he used.
Even so the simple experiment shows there is something going on that could stand some investigation.

Unless it's already well investigated and I'm just waiting for the right RC boffin to explain it?
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Old May 01, 2012, 04:41 AM
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The results are meaningless because of the effect of the propwash on the scales and motor mount.
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Old May 01, 2012, 08:41 AM
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Quote:
Originally Posted by docinfrance View Post
Attachment 4826578

With each motor facing out by 45º (total 90º) total thrust is again twice each motor's thrust combined.
100% + 100% = 200%
...
Not 200% but 100 x sin (45°) = 141%.


That will teach you not to do math at 2am


Quote:
Originally Posted by peterangus View Post
The results are meaningless because of the effect of the propwash on the scales and motor mount.
Yep. Reverse motor direction to reverse prop wash. Don't forget to reverse the prop too.


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Old May 01, 2012, 11:02 AM
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The video clearly demonstrates the flaws in this demonstration.

One problem is that the original one motor 0,32 kGm reading is likely incorrect.
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Old May 01, 2012, 11:10 AM
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Originally Posted by peterangus View Post
The results are meaningless because of the effect of the propwash on the scales and motor mount.
The results are certainly inaccurate, but the propwash doesn't swing it for me.
In the single prop test he gets 32g with the propwash directly down onto the scale.
In 90º I'd expect more propwash to spill over the scale, but the result is 64g. Double that of a single prop under the same conditions.
At 60º the lift is greater under the same basic conditions.

However inaccurate the methodology there is something going on which warrants further investigation.

I'm just wondering if it's something that people have experience with or a scientific knowledge of.

If not I can see me having to built a better experiment which will take time, and I don't want to replicate other people's work if the information is already out there.
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Old May 01, 2012, 11:46 AM
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Originally Posted by Ron van Sommeren View Post
Not 200% but 100 x sin (45°) = 141%.


That will teach you not to do math at 2am


Yep. Reverse motor direction to reverse prop wash. Don't forget to reverse the prop too.


Vriendelijke groeten Ron
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Me and math are not on talking terms most times of the day I thought it was about 50% more thrust.

Or just reverse the mounts so it pushes down.

I don't know what is going on for sure since the experimental method is flawed, but there's no doubt in my mind that two props running close together with intersecting airflow are going to have some effect on each other.

I'd expect the turbulence to reduce thrust, but there may be some form of wave form intersection in effect.
(helical sheet?)

If it means I could build an octocopter that works on well proven quad firmware, but with 50% more lift I'd like to explore the possibility.

PS. So do I need to use a 20amp ESC for two 10amp motors? I'd expect to, but I don't fancy trying that blind.
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Old May 01, 2012, 02:22 PM
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Any chance in the video that the weight of the vice has exceeded the capability of the load cell in the scale and when the Tare button was pushed, the resulting zero indication is meaningless?
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Old May 01, 2012, 04:34 PM
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Originally Posted by Martyn McKinney View Post
Any chance in the video that the weight of the vice has exceeded the capability of the load cell in the scale and when the Tare button was pushed, the resulting zero indication is meaningless?
Doubt it. If I overload my cheapo lidl 5Kg kitchen scales I just get an error display. They stay accurate +/-5g until they bug out.
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Old May 01, 2012, 09:53 PM
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There's no trick of physics or anything of that type here, and you'll get the most thrust from having your props angled straight down. Somewhere along the line there's a setup or measurement error.

The angles don't cause anything special because thrust is being produced at the prop blades and anything that happens past there has no significant effect. It's pretty much the same as if you tied strings to the motor shafts and pulled on them.

Sir Newton taught us all about this stuff...
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Old May 01, 2012, 09:56 PM
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Originally Posted by C₄H₁₀ View Post
The angles don't cause anything special because thrust is being produced at the prop blades and anything that happens past there has no significant effect.
Correct me if I'm wrong about this.

Are the motors rotating in different directions?

If so, this would put a twisting moment on the platen which could alter the reading dependent on the angle of the motor. If the motors rotated in the same direction there would be no net moment.

The first motor reading is likely incorrect. This is the way that thrust reversers work on jet aircraft, by putting an obtacle in the path of the airflow to reverse it.
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Old May 02, 2012, 09:01 AM
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Originally Posted by C₄H₁₀ View Post
There's no trick of physics or anything of that type here, and you'll get the most thrust from having your props angled straight down. Somewhere along the line there's a setup or measurement error.

The angles don't cause anything special because thrust is being produced at the prop blades and anything that happens past there has no significant effect. It's pretty much the same as if you tied strings to the motor shafts and pulled on them.

Sir Newton taught us all about this stuff...
I'd like to agree with Newton, but this isn't the same as pulling on strings. There are factors like disturbed air columns, pressure differentials and intersecting helical sheets at play here.
The angle in combination with the pitch used in the test may be causing the helical sheet (similar to a wavelength) coming from the props to mesh together sweetly where they cause interference (turbulence) with different angles and pitch.
I concede that these may well be restricted to ground effect, that this "tuned" air column is at just the right angle to bounce up and push back, but other videos from copterrichie on youtube (r.Scot here) don't appear to support this.

I'm not arguing that there is more thrust, I'm just saying there appears to be in that video and I'm looking for an explanation of why.
The set-up of the test doesn't account for the discrepancy.

Admittedly copterrichie is more concerned with demonstrating the best result for control of a V-tail, and his video "TriFour V-Tail Demonstration" is a pretty good argument in favour of 60º for that.
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Old May 02, 2012, 09:05 AM
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Originally Posted by Martyn McKinney View Post
Correct me if I'm wrong about this.

Are the motors rotating in different directions?

If so, this would put a twisting moment on the platen which could alter the reading dependent on the angle of the motor. If the motors rotated in the same direction there would be no net moment.

The first motor reading is likely incorrect. This is the way that thrust reversers work on jet aircraft, by putting an obtacle in the path of the airflow to reverse it.
It's for a V-tail quad with a fixed tail, so they must be (and appear to be) counter rotating.
I don't see how it could be twisting a wooden block clamped in a vies, although 60º would exert the greater twisting force there too much mass countering the action.
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Old May 02, 2012, 09:21 AM
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Originally Posted by docinfrance View Post
I don't see how it could be twisting a wooden block clamped in a vies,
The force (torque) from the motors doesn't twist the wooden block.

The torque from the motors puts a force on the block and the vice which pushes its front or back downward on the platen.

A motor with propeller mounted on a fuselage or wing doesn't twist the fuselage or wing, yet the aircraft still exhibits the effect of torque, the difference being that an aircraft can move in space while the vice is constrained.

The effect in this case would be the same as pushing your finger down on the front or back of the vice.
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