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Old Dec 14, 2012, 01:29 AM
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Question
T-28 trojan wobbles tail in flight like a fishing lure

So basically my T-28 trojan wobbles tail in flight like a fishing lure. faster it goes the worse the wobble. it appears the wobble is most pronounced at the rear of the ship. Any thoughts on what might cause that?

First post owns page one.
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Old Dec 14, 2012, 04:19 AM
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That's what's known as 'Dutch Roll'. You will probably notice it as a wagging of the tail and maybe a slight rocking of the wings it's probably worse as you exit turns. You will be able to set it off wagging by giving a quick pulse of rudder in straight and level flight.

It's a form of dynamic instability that you get on planes that have strong lateral stability (e.g. lots of dihedral) and relatively weak directional stability, a side effect of Parkzone deliberately making the T-28 easy to fly by using lots of dihedral. It's nothing to worry about really, just a sign that the plane is has lots of lateral stability and so has a strong 'self righting' tendency.

The only way to 'fix' it would be by reducing dihedral and/or adding more area to the vertical stabiliser. These days a gyro on the rudder could be used I guess but that might interfere with turning the plane.

http://en.wikipedia.org/wiki/Dutch_roll
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Old Dec 14, 2012, 07:18 AM
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So all PZ T-28 Trojans have this behaviour?
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Old Dec 14, 2012, 08:03 AM
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I'd assume so, I know the PZ Corsair does
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Old Dec 14, 2012, 08:55 AM
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I get that effect sometimes when I fly my FMS 1400MM T-28. I was thinking maybe it was the wind or something else. But sometimes my T-28 will track straight as an arrow and then other times the tail will wag a little
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Old Dec 14, 2012, 10:45 AM
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I've never noticed it on my PZ T-28 Trojan.
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Old Dec 14, 2012, 11:27 AM
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Mine has never done it either. If there is a misalignment of the tail feathers, would that be a contributing factor?
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Old Dec 14, 2012, 01:40 PM
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On my little T28, the feedback potentiometer in the servo became contaminated and it exhibited this behavior, I could see the rudder wagging back and forth.
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Old Dec 14, 2012, 06:29 PM
Grumpy old git.. Who me?
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Obviously if the rudder servo faulty and jittering back and forth that could also cause this problem but I assume the OP would have spotted that

Still sounds like a classic case of Dutch Roll to me. Not having flown a T-28 first hand I cant say for sure they all suffer from Dutch roll but I know for 100% sure that the PZ corsair Dutch rolls like crazy.
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Old Dec 14, 2012, 06:49 PM
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Heh, on mine it wouldn't happen without the slight vibration from the motor and the servo having to hold position against the slipstream.
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Old Dec 14, 2012, 09:13 PM
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Heck, hang a treble hook from the belly and see if you catch anything!
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Old Dec 14, 2012, 11:56 PM
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I've been flying mine for a long time and never noticed it. If it is very pronounced, I'd say something is wrong and that is not normal.

I do notice it on my Parkzone Icon, but that is a well known issue which has been discussed in the thread on that plane. I don't recall seeing this discussed in a T28 thread, but you should do a search and see.
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Old Dec 15, 2012, 12:26 AM
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JMHO, I think that the Vertical Fin on that T28 is too high and too big. Note that all of all of this height is above the thrust line. Could be the spiralling propwash hitting that huge vertical.

Since this fishtail phenomenon doesn't exhibit itself in all T28s, its vertical fin could be marginally too big. So depending on the throttle level, the fishtailing could come and go.

When I was designing pattern ships way back, I'd play around with dorsal and ventral fins, as well as varying vertical fin heights and areas.
I think the T28 can get by with a less tall vertical fin, or transfer some of that vertical fin surface below the stabilizer (ventral fin)
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Old Dec 15, 2012, 01:06 AM
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Replace the servo.
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Old Dec 15, 2012, 04:23 AM
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Quote:
Originally Posted by chucksolo69 View Post
I've never noticed it on my PZ T-28 Trojan.
Mine neither, my PZ T-28 flies smooth.
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