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tc1917(at)bellsouth.net
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 Posted: Mon Sep 27, 2010 2:17 am Post subject: tail wag |
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Havent said much on the list of late. Been lurking. Firstly; my sling shot
w/912ul does not have any tail wag at all. I can and do just sit back with
my feet off or slightly on the peddles. When the wind hits me wrong, I can
feel the rudder react but not wag. I dont use my rudder much in the air.
Secondly; I installed the double X-hinge system to the rudder when I built
it. Tricky but glad I did. In case anyone remembers, I have a roll dynamic
noise in my engine. I have been able to get about half of it out or quiet
it down by half, whatever. It cost me another five hundred for a muffler
and I designed an after-muffler which helps. It is now acceptable to me. I
can fly my little bird hands off for quite a while before I get the P factor
from the blade and it will turn slightly to the right. Easily controlled
with a slight touch of the rudder. I do thank everyone for their ideas on
my engine roll but nothing worked short of spending a lot of money. It is
internal in the engine, probably timing or the crank being off a hair or
whatever but the harmonics are there and it is not the only one. No one has
a cure or more correctly, anyone with the knowledge and ability to do
something about it really cares. Only company who offered to help was
Lockwood Aviation way down in Florida and I would either have to take the
engine off and send it to them or take the plane there and have them test
and take the engine off. Either way, lots of money and no guarantee. Some
times we get caught in a big black hole and we just have to keep taking care
of ourselves. Sorry for the length. Take care and keep flying. Ted Cowan,
SlingShot, 912ul, zoom zoom.
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stolspeed(at)gmail.com
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| Posted: Mon Sep 27, 2010 2:44 pm Post subject: Tail Wag |
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I haven't flown a Kolb yet, but aspire to own one, thus am starting to follow this list.
This mention of tail wag is most interesting. We've had some experience with tail wag in a Australian design known as the Boorabee. It's also a pusher configuration like the Kolb, with a complete fibreglass enclosure. The enclosure is a smooth aerodynamic shape. Flight characteristics are excellent, except for a tendency to want to fly with a yaw bias, in either direction. To explain - It will be stable with a yaw bias of several degrees to one side, then if corrected with rudder will be stable with an equal yaw in the other direction. It doesn't wag between to two sides, but is stable at either side. This was completely corrected by installing vortex generators to sides of the fuselage.
It seems that the cause of the problem was the sides of the fuselage where they taper in at the back. For example, once the aircraft yawed a bit to the right, the airflow on the rear right side tended to stall, while the left side was acting like an airfoil and causing a lateral 'lift' force in that direction, and thus sustaining the yaw. As soon as the rudder was used to over-center the aircraft, the effects reversed and it then stable in the other direction. The VGs stabilized the airflow, and the aircraft now flies straight with no rudder input.
I wonder if it's a version of this effect that's effecting the Kolbs that experience tail wag?? I note that the Kolbs have a lot longer tail moment arm than the Boorabee. Could it be that the longer tail over-rides the yaw effect caused by the enclosure, causing it to over-center, and then wag back and forth?? It sounds like a marginal situation, which may account for the reason that some Kolbs feel it and some don't. There's lots of mysteries and surprises in aerodynamics, and only real life flight experience tells the real story.
The VGs that they used on the Boorabee are home-made of bent aluminum, about 25mm (1") high. As I remember, they had about five double ones down each side of the fuselage, just behind the widest point. I'm not sure that the micro VGs about 12mm high would be high enough to reach outside the boundary layer at that point, but maybe....
I manufacture micro VGs, www.stolspeed.com , and am offering them free to anyone with a Kolb wanting to test if they can help reduce the tail wag effect. It's certainly worth a try......
JG
www.stolspeed.com
[quote][b]
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willuribe(at)aol.com
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| Posted: Thu Sep 30, 2010 6:43 pm Post subject: Tail Wag |
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Greetings,
I never experienced tail wag and I didn't always fly with my feet on the rudder pedals. I have the standard springs that came with the kit.
Regards,
Will Uribe
FireStar II
[img]cid:tie_8CD2F292A1DCD08_1498_B14D[/img]
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JetPilot
Joined: 10 Jan 2006
Posts: 1246
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John Hauck
Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)
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| Posted: Sun Oct 03, 2010 10:56 am Post subject: tail wag |
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I think tail way is an inaccurate description of rudder flutter.
On a 912 powered Kolb the prop blast hits the vertical stabilizer and rudder
on the left side. When the rudder is pushed to the right by the prop blast
it hits the airstream on the right side of the vertical stabilizer which
kicks it back to the left. Once this starts, usually because there is no
pressure on the rudder pedals, it accelerates with larger and larger
excursions. I have never let it progress to a dangerous level, and I really
do not know what that level would be. However, a little foot pressure on
the rudder pedals stops it immediately.
On a Rotax two stroke engine the prop blast hits the right side of the
vertical stab and rudder. Rudder is pushed left by the prob blast and the
air stream kicks it back to the right, and so it starts and grows.
Before I installed two springs on each rudder pedal, I would usually become
aware of the rudder flutter when I noticed the wing tips shuffling slightly
fore and aft. Doesn't take much rudder.
Again, my own personal opinion and observations of rudder flutter on all my
Kolbs; US, FS, MKIII, and most other Kolbs I have flown over the years. I
think it is more annoying than dangerous unless allowed to progress
unchecked.
john h
mkIII
Rock House, OR
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_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama |
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