Netters
It has been my experience with the aircraft I have flown that have a
traditional empennage, or non-T tail aircraft that they tend to be more
sensitive to power changes due to the combined effect of prop wash & relative
wind over the elevator. The arm should not be that significant because the
nose drop is caused by a decreasing elevator authority which will been seen in
all. The tail boom for typical C152 is quite a bit longer than the KR yet it
experiences the same nose drop, if no changes are made. Note for the 152 it
has 2 trim tabs to get trim authority, where the 172 only has one. This may
explain why experience in these aircraft lead one to believe that all aircraft
can be trimmed for all speeds. Once the aircraft begins to change attitude,
the arm of that elevator becomes VERY significant , slower speeds requiring
much larger inputs by comparison to faster speeds given the same plane each
time. Since our little planes are very sensitive to occupants, it is not
surprising that we have a very forward CG during solo flights, but this will
change dramatically when 2 are aboard. Larry, I would recommend what I plan to
do, which is add weight next to you, in 50 pound increments and note the
performance changes. You should find several things happen, based on rules of
aerodynamics:
On a fwd CG, the plane should:
1) Feel stable, trim easier
2) Have slightly lower cruise speed
3) Have slightly higher stall
speeds, and takeoff/landing speeds
4) Can have difficulty lifting the
nose if it gets too slow
On a rear CG it should:
1) Feel very active, not want to
trim stay trimmed, unstable
2) Have higher cruise speed at the
same power settings
3) Have lower takeoff/landing, and
stall speeds
4) Will tend for the nose to rise
on its own, and may make it difficult to correct a stall
2 things are typically working at all times to combine for what actually is
happening at any one time, and yes this is an over simplification AGAIN. One
is the CG and its placement, and the other is the Center of Pressure, or Lift.
These 2 forces have the most dramatic effect on what the plane does in any
given configuration or setting. Once loaded the CG does not "move". However
the CP DOES move depending on wing planform and angle of attack. So as speed
changes in level flight and the angle of attack changes to maintain that
attitude, the CP moves in response. If the spread between the CG & CP gets
greater the aircraft becomes more stable, all other things equal, if the CP
moves to the CG, or gets closer, as is typical in slow flight, it gets less
stable. The combination of the slow speed effecting the elevator authority,
and the instability makes for the rapid reaction of any short coupled aircraft.
The Rockwell Commander single engine reacts the same way.
I bet Larry will find as passenger weight is slowly added that the
dramatic nose down will start to occur less at the same speeds, and the plane
will fly faster, but with alittle more attention.
Colin & Bev Rainey
KR2(td) N96TA
Sanford, FL
crain...@cfl.rr.com
http://kr-builder.org/Colin/index.html
