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<< Lancair Builders' Mail List >>
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Guys,
This is a follow up to my posting a couple of weeks ago about an irregular
yaw oscillation problem in my Super ES. I got a lot of input and very good
suggestions. I really appreciate it. I got responses from individuals, test
pilots, Lancair and other pilots like me. I investigated most of them and
tried to test for some of them. Ended up trying mostly to figure out what
had changed on the plane because it had not always exhibited the yaw problem.
Finally tied it timewise to the onset of cold weather here in Indiana. This
led me to look at any changes I might have made for winter flying. About
that time Charlie Kohler, in one of his many e-mails to me about the problem,
asked if there might be any air flow inside the rudder cavity or near the
vertical tail. This made me look at fresh air vent modifications I had made
for cold weather so that my overhead vent console would be completely sealed
off.
I have an aluminum plenum box between my air intake scoop on the vertical
tail and my electric vent fan which supplies air to the overhead console.
This group of parts is located in the aft section of the fuselage just ahead
of the tail. The box has a powered door that can be used to close off the
intake air from outside the plane and allow air inside the fuselage to be
forced into the overhead vent system by using the fan.
I found that, even though I closed the door to stop the flow of air, I still
got some small amount of air into the system and, since I have switches,
lights and other non-sealed items in the overhead console, I still got some
cold air into the passenger compartment. Being quick to solve the problem, I
inverted the plenum box so that it sealed the vent fan from taking in any
air. This dumped the high pressure Naca scoop air into the cavity. No
problem, right? Wrong! It appears that the high pressure air being let
loose in the cavity could only escape toward the back and out the sides of
the vertical stab through the ports for the elevator torque tube. Seems this
turbulence was disturbing the airflow on each side of the stab and changing
the lift values and creating the yaw oscillation. After discussing this
theory with Charlie I taped over the Naca scoop so that air could not enter
the cavity. Have flown it about six hours since and at a variety of cruise,
climb and descent speeds and show no signs of the yaw. I believe the problem
has been identified. I can now fix it.
Thanks for all your help. I'm a bit embarrassed to say what caused the
problem, but it sure appears that I did it. Hopefully this can help others
when troubleshooting problems. Looking at recent modifications, changes,
fixes or temporary "winterization" is a good idea.
One other note: Lance Niebaur was the representative from Lancair who
participated in trying to solve the problem. I think that is an indication
that Lancair is concerned about the fleet and its safety.
Anyone who wants to contact me for further details can do so at 812-853-9800.
Thanks again, especially to Charlie Kohler. He has done a lot for me. I
appreciate it.
Jim Scales 98AF
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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