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Tail wagging

Just curious about the way my vampire sometimes wgas it's tail

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DH Vampire

DH Vampire

The plan build article by Tony Nijhuis - 15/7/09

Ian Jones16/07/2009 01:14:04
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It''s not caused me any problems but occasionally I''ve found that my vampire gets very slightly erratic in it''s direction, it''s not much best described as wagging it''s tail I think.  It might worse when the throttle is opened quickly but I''m not sure as it hasn''t been a great concern. I''m curious to know why it might do this though.
Phil Wood.16/07/2009 01:21:47
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If you open the throttle quickly you'll get a sudden torque which will try to turn the model......it may take a couple of seconds for it to pick up speed and get enough airflow over the fins to stabilise itself..
 
It may "wag" during these few seconds.
 
Polyphilla.
Tim Mackey16/07/2009 08:50:12
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I think it could be "fishtailing". This odd movement is very noticeable on certain models - my Sabre for instance does a super impression of a waggling fish if there is any wind.
I believe the full sized one had similat traits...but not sure about that...just what I had been told. Have no ide what causes it, but must surely be vertical stabilser / rudder related?
ken anderson.16/07/2009 10:02:48
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hello ian-same here i have a 'pushy cat which has twin boom's/tail it like's to have a waggle occasionally-i though it was dodgy radio but now relize it's a particular thing with the model....................
   ken anderson............ 
Bert16/07/2009 10:26:59
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Ian
Sounds like 'Dutch Roll'
 
I have an RBC Paddle that used to do that - the cure is to increase the size of the fin - or put up with it
 
Bert
Eric Bray16/07/2009 19:04:57
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Yes - lack of fin area, and due to the spindly booms, lack of fus side area!
i12fly16/07/2009 22:08:38
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Dutch roll is side to side rocking movement on the wings, the axis being along the fuselage. Seen it on free flight with too much dihedral.
Once flew as passenger in a Fokker (70?)  twin turboprop and that was doing a dutch roll,  only mildly but it was noticeable.
Not sure about the cause of fish-tailing, but I've seen it on a number of scale type models including a P51
Tony Nijhuis16/07/2009 23:51:09
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My Orginal prototype fish tailed so I added  larger and larger fins....finally got it to stop but boy it did look silly......bit like Dumbo.... Final prototype was a compromise with the fins being 15% larger than scale......
Tony 
Tim Hooper17/07/2009 14:22:08
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Posted by i12fly on 16/07/2009 22:08:38:.......... fish-tailing, but I've seen it on a number of scale type models including a P51


Also saw it on the prototype of the FPC Sea Fury (reviewed for QEFI).  At the designers suggestion I temporarily added some extra fin area in the form of a balsa sheet fillet, which cured the problem immediately.  Subsequent production models featured a slightly enlarged fin.

tim


 

Edited By Tim Hooper on 17/07/2009 14:22:37

Ian Jones17/07/2009 22:17:07
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Well thank you everybody for your helpful responses, especially from the man himself.
 
That clears it up then - it's a feature of the model that I'l happily continue to put up with.
 
Interesting that the fins on the real vampire seemed to be under constant modification - did it have the same "feature"?
Martin Harris17/07/2009 23:05:24
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Interesting - I'm sure I've heard that too large a fin area can cause tail wagging but this seems at odds with the experiences above. Something along the lines of excessive inertia behind the centre of pressure causing a divergent oscillation (sounds impressive anyway - what's happened to Gemma? I'm sure she would know the answer to this one.)
 
I wonder how much of it is caused by structural flexing - the fin bends one way under load - causing the rudder to deflect slightly the other way - causing an opposite yaw, deflecting the fin causing a rudder movement in the opposite direction and so on and so on until the limit of flexing (or structural failure) occurs.   If the theory I have heard is basically correct, perhaps the increase in fin area has actually increased the stiffness of the fin structures?
Eric Bray17/07/2009 23:31:29
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It is more likely to be yaw/roll couple. As the a/c yaws, the wing imparts a roll, but the a/c has already begun to yaw the other way, and thus a rolling oscillation develops, along with the waggly tail.
(How much is that doggie...)
Tim Mackey18/07/2009 00:05:40
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My Sabre does not have a rudder, so I doubt its connected with that Martin.
Ian Jones18/07/2009 00:29:21
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No rudders on the Vampire either. It's stable in the roll - more like the fish-tailing mentioned earlier.
 
It never looks scary.... so far anyway!
 
Ian Jones18/07/2009 00:56:22
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Oh nice looking Fury Tim.
 
The Spitfire fin might throw some light on it though - as it developed and was fitted more powerful engines the fin area was increased - a huge difference between the Mk1 & Griffon powered MK24. The later turbo prop powered Westland Wyvern had a large fin too. Perhaps that brings us back onto the engine torque theory?
 
 
Tim Mackey18/07/2009 08:24:37
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But the Sabre is a Jet - no prop wash over the fin at all, and no torque roll
Peter Miller18/07/2009 08:41:26
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The original Boeing 707 had a bad Dutch roll.That was why it kept losing engines. As a joke of the time said, The comet has its engines buried in the wing, the Boeing 707 has them buried in French fields.
 
BOAC found the cure and fitted a big underfin. They then patented the cure and sold the patent back to Boeing.
 
When I was at St Mawgan on Master Diversion Flight in about 59-60 We had BOAC doing their 707 crew training there. They had the modified 707. WE all got flights on it.
 
In passing, all the locals bellyache about the aircraft noise as the modern, quiet aircraft go over at about 4000 feet on their way into Stanstead. The old ladies complain they can't sleep for the noise. You can hardly hear them out side, let along indoors. We used to sleep with a 707 doing touch and goes all night long 200 yards from the night shift windows. (It was a sleeping duty)
Ian Jones18/07/2009 11:47:23
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Good point Timbo. Some jets do have large fins though.
 
Whatever I think Tony was right to find an acceptable compromise rather than have way off scale fins.
 
Having read all the very interesting comments made would the conclusion be that the fish-tailing I originally mentioned and the dutch rolling that has also been raised can have a number of causes, however the cure is to introduce vertical stability by added fin size, ventral fin(s) or winglets?
 
 
Bert18/07/2009 17:45:00
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Ian
 
Dutch roll and fishtailing are the same thing with the same causes. The cure is to reduce the dihedral if there is any, or to increase the size of the fin.
 
Adding winglets would probably make it worse.
 
Bert
 
Ian Jones18/07/2009 18:21:50
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Thanks, I'll make some more detailed observations next time out.

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