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mclayton(at)rochester.rr.
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 Posted: Sun Feb 11, 2007 6:13 am Post subject: KF2 Tail |
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I am in the process fo rebuilding a KF2 damaged in an accident (S/N
89), and since I am basically starting over to build the aircraft, I
want to both increase the height of the vertical stabilizer and
rudder, and add ribs to them and the horizontal tail surface. I am
also considering adding electric elevator trim, since the servos
weigh about 3-4 oz, and should not add an excessive amount of weight.
Has anyone made this kind of mod? Any suggestions as to what airfoil
to use? How about the leading and trailing edge shapes? What shape
should the modified vertical surfaces have (eg, squared off on top,
lengthened fore-aft, etc). Also, what about the trailing edges?
Any ideas and pics would be very welcome at this point.
Mike
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lcfitt(at)sbcglobal.net
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| Posted: Sun Feb 11, 2007 6:10 pm Post subject: KF2 Tail |
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Mike, I'll take a stab at an answer at least as far as the electric trim.
When I built, I was a pretty ignorant soul. All I knew was the C-170B I
trained in had elevator trim and didn't understand that trim on the typical
I through IV Kitfox was with flaperon. I had assuned that elevator trim was
muscle induced and I didn't need sore muscles.
I ordered the Speedster elevator with the trim tab cut-out, but it wasn't in
my budget to order the tab or the servo.
When it came time to work on that part of the airplane, I was dismayed to
find that there was a 3/4" U channel that forned the forward edge of the tab
cutout on the elevator. This was the hinge attachment point and helped
create the space needed for the MAC servo. At this time I was unaware that
the typical rib profile in horizontal stab and elevetor was essentially from
leading edge to trailing edge in both, forming essentially a thin figure
eight in profile with the intersection at the hinge line - two separate
airfoils joined at the hinge line.
What I proposed to do was create a single airfoil with a gap closure
encorporated at the hinge line. This I did with 1/4" medium density foam
ribs with a very lightweight glass cloth on the faces. I determinde the
airfoil shape with a most unscientific method. I fixed the elevator and
horizontal stabilizer in the neutral positon with clamps. The leading edge
of the stabilizer is 1/2" tubing , the trailing edge of the elevator is
3/8". with the 3/4" channel in there somewhere. I took a piece of aluminum
strap, probably 1"X1/16" by several feet long and clamped it on the trailing
edge edge - long end forward. I thin proceeded to add weight to the forward
free end until it contacted the leading edge. This produced a curve with
the radius progressively shorter toward the leading edge as is found in most
airfoils. This curve was then traced onto cardboard rib templates and
mirrored for the bottom surface. Several of these made up the various ribs
needed. The gap seal ws a bit more complicated. I am attaching a drawing
of the gap seal method I used.
Hope this helps.
Lowell
do not archive
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Bob
Joined: 24 Oct 2006
Posts: 89
Location: Damascus, Maryland, USA
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| Posted: Mon Feb 12, 2007 6:03 am Post subject: Re: KF2 Tail |
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Directly to your question, can you run an exterior link? Running a mechanical link out in the airstream on these slow little planes is often the easiest and lightest way to go.
Be extremely careful of potential problems with gap seals and anything else that may hinder elevator movement, especially remembering that the horizontal and elevator can flex under load. Cirrus lost an airplane and test pilot a few years ago when the tight fit of the aileron rubbed the trailing edge of the wing. Ice on a tight fitting hinge or gap seal can do the same, and the further you are from the center of the hinge, the more mechanical advantage you give to a blockage.
Bob
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