On 7/19/2022 2:12 PM, Mark Wegmet wrote:
As the aircraft gains speed during take off, and the primary airfoil
(wing) begins to generate lift, it applies sufficient force to
"rotate" the fuselage towards the horizontal
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
One out of seven correct (one other was correct but lacked explanation)
is not something to write home to Mom about for a bunch of aviators.
The center of gravity of a conventional gear airplane is aft of the main
wheels or the airplane would fall on its nose at rest. The center of
lift is also aft of the main wheels. As Mark stated, as the airplane
gains speed the wing continues to create more lift "aft" of the main
gear and causes the tail to rise. When we use forward stick early in the
takeoff roll we are simply aiding the wing with additional lift on the
tail but we must reduce that additional lift as we gain speed or the
airplane will nose over. By liftoff speed the wing is creating enough
lift that we have to apply a "down" force on the tail to change the AofA
to generate the lift required to climb. The tail surfaces are only ever
to compensate with force to bring the center of gravity in alignment
with the center of lift or to shift it fore or aft for a different angle
of attack to climb or descend. We are simply aerodynamically adding or
reducing weight on the tail to shift the CG either fore or aft of the
center of lift to change the angle of attack so the wing will create
more or less lift. When the "physical" balance of the airplane is
outside the CG range the tail surfaces (or canard) are simply not able
to create enough aerodynamic force to put the airplane back to a
balanced state and you lose control with either a climb or dive. In
cruise when we trim we are simply adding or reducing tail weight
aerodynamically to put the CG in alignment with the CofL.
If I take off with my pitch trim set to neutral, at cruise I have to
hold considerable back stick or adjust trim or my KR will pitch nose
over, almost violently. That could be corrected if my wing were moved
to 2 or 2.5 degrees angle of incidence instead of the plans 3.5 degrees
at the root and .5 degrees at the tip. I could also change the Angle of
incidence of the horizontal stabilizer. That was the method use on many
early Pipers using a jack screw to change the angle in flight. I'm
simply adding more tail weight (aerodynamically) to align the CG with
the center of lift. More weight means I need to create more lift, more
lift means more drag, which means a slower airspeed. Just the opposite
when tail heavy. Reduced weight on the tail means less required lift
generated, less lift, less drag, more speed.
Knowing what is happening aerodynamically to the airplane in flight can
help builders to know what items on an airplane or critical and which
aren't. Placing my ASI on the panel or the location of the trim switch
are not as critical as where to place the fuel tanks and their size.
As always, YRMV..............
Larry Flesner
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