Pusher Prop Wash Compensation - RC Groups
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Mar 13, 2007, 04:20 PM
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JochenK's Avatar
Discussion

Pusher Prop Wash Compensation


I've mentioned several times that I like to compensate the effects of the prop wash to get my gyros flying straight. Today I tested a new setup, and while I was at it, I made a short video to demonstrate my point.

https://www.rcgroups.com/forums/show...93#post7087583

The gyro is suspended by a string tied to the top of the rotor shaft. Another string tied to the tail boom at the bottom of the rotor mast keeps the gyro in position. The first part of the video shows you what happens if you mount the motor normally, in line with the tail boom. In the second part of the video the motor is offset to the right of the gyro by about 8 mm (1/3") and is compensating the prop wash effects.

Jochen
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Mar 13, 2007, 06:48 PM
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leadfeather's Avatar
Looks very good! Waiting for results from flight testing.


Dan
Mar 14, 2007, 03:11 AM
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Jochen,
fantastic work and video. Pushing thumbs for the proof of concept- flights.
Stephan
Mar 14, 2007, 04:53 AM
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JochenK's Avatar
Dan, Stephan,

I think the concept of prop wash compensation is valid enough. I've been using it for quite some time.

It's just that up to now I've tried to overcome the prop wash effects by adjusting the motor's thrust vector to the right. This worked quite alright in it's way, but was a bit tricky. In this kind of set-up the prop wash pushes the rudder to the right, against the direction of the original spin, just as it is supposed to do. But at the same time, the thrust vector of the motor, which passes the c.g. on the left hand side, creates a torque in the direction of the original spin, thus annihilating some of the good work the prop wash is doing.

That's why I've now decided to use a 'clean' solution of the problem by just offseting the motor to the right of the gyro without changing the basic direction of it's thrust vector.

Jochen
Mar 14, 2007, 05:15 AM
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leadfeather's Avatar
Good work Jochen!

Now, if you can also cancel the torque of the propeller????

I have been contemplating turning vanes behind the propeller to cancel torque but I haven't tried it.

Dan
Mar 14, 2007, 06:36 AM
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JochenK's Avatar
Dan,

I'm trying to compensate motor torque by mixing a bit of throttle onto roll. This works sufficiently well and has only one drawback: when you've switched the power off and want to start the motor again, you have to do this slowly. Otherwise motor torque compensation kicks in before motor torque with unwanted results. Once the motor is running, it's no problem.

Jochen
Mar 14, 2007, 09:04 AM
Registered User
http://www.mindspring.com/~cramskill/propefct.htm

Here is a good explaination of prop effects.
But there is no solution only compensation.
Wyldman
Mar 14, 2007, 09:28 AM
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StephanB's Avatar
Quote:
Originally Posted by JochenK
Dan,

I'm trying to compensate motor torque by mixing a bit of throttle onto roll. This works sufficiently well and has only one drawback: when you've switched the power off and want to start the motor again, you have to do this slowly. Otherwise motor torque compensation kicks in before motor torque with unwanted results. Once the motor is running, it's no problem.

Jochen
Jochen, what TX are you running? You may check the manual to see, whether you can set an offset on that mix-function.
Stephan
proud owner of a brand new Futaba FF9synth.
Mar 14, 2007, 10:50 AM
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leadfeather's Avatar
Jochen,

It could be an illusion but in the video clip it looks like the prop is on backwards???

In the link about propeller effect, the author speaks about correcting with right rudder, yet you have offset the thrust to give you left yaw????

Dan
Mar 14, 2007, 11:37 AM
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JochenK's Avatar
Wyldman,

thanks for the info.


Stephan,

MPX 3030 and MPX 4000. This is no great problem, when the motor is running everything is sufficiently synchronized. It's just the delay of the restarting motor you have to look out for.


Dan,

it's really an illusion. When you put the prop on backwards, you get a totally different sound, more swishing instead of humming. But you're right, in the video it looks like the prop is the wrong way round.

The prop wash works a bit different for the Micromum, because the whole rudder is below the motor's thrustline. Seen from behind, you've got a cw rotating wash leaving the prop. Now think of a circle (the prop wash) with a line drawn down from its center, a bit longer than the radius (the rudder). Then use your (mental) finger to trace the perimeter of the circle in a clockwise direction and see which side of the rudder it hits - the right side, pushing the tail of the Micromum to the left. To counteract this I need left rudder (which I haven't got) or an offset of the motor to the right.

Was able to get a few flights in today, and even if it was too windy to say anything definite. the new system seems to work as it should. During the less windy moments I was able to take the hands off the controls for several seconds a time.

Jochen
Last edited by JochenK; Mar 14, 2007 at 11:44 AM.


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