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Old 02-23-2015, 09:39 AM
douglassmt douglassmt is offline
Join Date: Nov 2008
Location: Missoula, MT
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Default Stroboscopic/Wagon Wheel effect

The prop is turning clockwise as viewed from the cockpit. You are seeing the stroboscopic or "wagon wheel" effect.

Originally Posted by AlexPeterson View Post
Now to figure out why your engine is running backwards... Does anyone have an explanation as to why the yaw of the nose wheel is against the swirl of the prop, at least in these two cases (OP + video)?
Bryan Douglass
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Old 02-23-2015, 10:36 AM
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PerfTech PerfTech is offline
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Originally Posted by Snowflake View Post
And if you really can't find the culprit at the end of the day, you could always add a simple aluminum trim tab to the back of the wheel pant, just like on the rudder...
...This what I was thinking as well! It would be very interesting just for a test to see the effect. But wait! Here is a potentially new product. "The all electric wheel pant trim kit"
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Old 02-23-2015, 12:13 PM
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vfrazier vfrazier is offline
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Did you check to make sure that you haven't lost the little plug that covers the valve stem access for your tire?

Also, before you go cutting and/or adding things, try taping over the seams in the nose wheel pant to see if you've got a strange airflow going on.
Vince Frazier
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Old 02-23-2015, 02:07 PM
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JoeBlank JoeBlank is offline
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Location: Molalla, Oregon (KOL05)
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The NW video was very revealing and confirms some suspicions that I've had.

From my own experience, during an RV-7A landing, I experience a significant shimmy event, which I believe was initially caused by low nosefork breakout force. There was a pretty good gusting crosswind from the right that day and immediately after de-rotation/NW touchdown, there was a significant vibration from the nose gear. Fortunately with those winds, the ground speed was quite low at de-rotation and the event didn't last long. It was later found that the breakout force had creeped to a lower than standard value probably due to wearing of the Belville washers.

On another occasion, while formation landing in trail, I watched the same type of event occur on an RV-6A directly in front of me. Since we are usually quite close (<300-500'), it was quite easy to see the nosewheel fairing swinging back and forth in the breeze on this aircraft while the pilot dealt with a left crosswind, power changes, and associated turbulence from aircraft directly in front of them. Normal stuff if you are used to doing formation arrivals... In the flare as the pilot pulled the power to idle, the nosewheel cocked to more align with the crosswind. Then at de-rotation/touchdown it was quite evident that the nosewheel acted like an unbalanced shopping cart wheel and vigorously vibrated for 3-5 seconds as it dissipated energy trying to align itself with the opposing forces of crosswind, runway, and prop blast. I later asked the pilot if they had a nosewheel shimmy event, to which they replied to surprised, "Why Yes, how did you know..?" Clearly the breakout force was not set high enough...

And yes, nose, main, and tailwheels do spin in flight, although not as fast as you might guess. If you even are in formation, especially Close Trail looking up at a tailwheel, you will notice that some (with good bearings) spin slowly in flight. Nosewheels also do spin sometimes depending on the model and torque setting used by the builder. I'm guessing that this is not a factor as the rotation is generally quite slow.
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Old 02-23-2015, 03:38 PM
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rv7boy rv7boy is offline
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Originally Posted by swisseagle View Post

Today I was setting the preload force as Vans mentioned to the 22 lbs. I did not flown the plane, still the last steps to do. I remembered there was a video that showed different preload-szenario's.

I hope it is fine if I link this video form unknown source again:

I hope it helps.
The "unknown source" of the YouTube video is the Unrein family (Brian and Brandi) who built their RV-10 and live in the Atlanta area.
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Old 02-23-2015, 06:10 PM
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AlexPeterson AlexPeterson is offline
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Originally Posted by douglassmt View Post
The prop is turning clockwise as viewed from the cockpit. You are seeing the stroboscopic or "wagon wheel" effect.
Uh, yeah, I've been watching my prop go clockwise for quite a few hours now, so I kind of have that part figured out.

Prop swirl would make the nose wheel yaw the other way from what is being experienced; hence the whole discussion about the cause.
Alex Peterson
RV6A N66AP 1600+ hours
Maple Grove, MN
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