On 3/30/2020 9:35 AM, Dr. Feng Hsu via KRnet wrote:
The plane wants to go nose down from the first flight so we installed a
fixed trimtab on the elevator. Problem solved. I think we made the elevator
a bit to heavy, so the weight of the ellevator is forcing the nose
down.....I think.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Stef,
I don't think it is the weight of the elevator causing the nose down
pitching, it is the angle of incidence of the horizontal stabilizer,
which you can't change at this point.
Consider this. The early Cessna 170's (rag wing) did not have a
balanced elevator. Setting on the ground ,with engine shut down, if you
pulled back on the yoke to put the elevator at neutral and then let go
of the yoke the elevator would drop to the down limit with a noticeable
bang. Air loads in flight tend to push the elevator to a more neutral
position.
A more relevant example is my KR. It has a balanced elevator and on the
ground the elevator tends to stay near the neutral position. I can
place my adjustable trim tab in the neutral position, take off and climb
to altitude, push the nose over to cruise (150 mph), and if I let go of
the stick, I will get a violent nose down pitch. It is enough to throw
things up off the seat A bit of nose up trim removes the stick forces.
In flight photos show the elevator still in the near neutral position so
a very small change makes a big difference at higher speeds. Once
established in cruise I seldom touch the trim until I'm in the landing
pattern or sometimes on a long letdown from altitude. That same setting
is sufficient for takeoff and climb out and then adjust again for
cruise. One or one and one half degree more nose down angle of
incidence on my horizontal stabilizer would make it more efficient in
cruise.
Bottom line. If you're lucky you will set the horizontal stabilizer
incidence to the correct angle for your normal cruise speed with an
ideal C.G. location and the elevator at neutral. That would create the
least amount of drag and give you the best speed. Unfortunately life is
not that simple. Every other speed, power setting, and C.G. location
would require a different angle of incidence on the horizontal
stabilizer. A number of aircraft designers did exactly that by making
the horizontal stabilizer adjustable. All the early Pipers through the
TriPacer and the early C182's come to mind. So, you have two options.
One is a fixed tab to get zero stick forces at one given speed and C.G.
location or two, some type of an adjustable tab or other device to
relieve stick forces at different speeds. Your KR is no different than
any other airplane flying that requires pitch trim, adjustable or fixed,
to relieve stick forces.
One important thing to remember is if you use an adjustable trim tab
make sure there is no slop in the system. Any free play where the trail
edge can move up and down can induce flutter and you don't even want a
hint of that. That's the reason I removed my steel cable operated tab
setup and went with electric servo in the elevator. With a coupling of
only a few inches between the servo an the tab I was able to remove all
free play in the system. I recall the P51 that crashed at Reno
developed a problem in the trim tab that caused flutter and that didn't
end well.
Second bottom line. Don't worry about needing some pitch trim. All
aircraft have that need. If you can make C.G. changes to eliminate as
much stick forces in cruise as possible, great. That would be ideal.
If not, go with what you have as long as it is safe.
Larry Flesner
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