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There have been a lot of terms thrown around here like CG,
Neutral Point, Stability, Aerodynamic Center and MAC but how they interact can
be better understood. I will attempt to clarify and simplify for those that have
not been around this block.
In particular, stability, Neutral Point and why they work
the way they do. In fact, NP is defined as that CG condition where the airframe
will not correct itself in pitch. This is good for aerobatic and combat aircraft
but not for day to day flying.
It is common knowledge that if the CG is at or behind
the NP, the airframe has zero of negative stability (if the nose goes up, it
will keep going up as the airspeed decreases unless elevator input brings it
back down and vice-versa. What is not common knowledge is why.
The basis of all this is in the fact that as a
typical wing increases it's Angle of Attack, it's center of pressure (center of
lift) moves aft. This produces a moment that becomes more negative pushing the
nose back down and vice-versa. At some speed the nose will be happy at some
attitude and in steady state flight.
There are some airfoils that do not exhibit this behavior
and even show the opposite behavior and are not suitable for use as main wings.
Add an elevator to a wing and positive stability behavior can be enhanced
increasing the usable selection of available airfoils if the CG is kept
forward of the center of lift. This, of course requires the elevator to produce
down force to handle the CG in front of the wing's center of lift. Now if the
nose is disturbed upward, the center of lift moves to the rear helping the
nose come back down and the elevator experiences a less negative AoA producing
less down force adding to the restoring force bringing the nose back
down.
This self stabilizing type of flight is what allows one to
trim the airframe for "hands off" flight. The greater the stability, the
more "hands off" you will be.
Adding reflex to a wing, any wing, will reduce the center
of lift travel with pitch changes and reduce stability.
Also because reflex moved the center of lift forward, closer to the
CG, you require less down force from the elevator leaving less margin
for elevator provided stability.
Sooo . . . If you already have an aft CG and you moved
your center of lift forward closer to the CG by using reflex, you can expect
less pitch stability . . . . Want some stability back ? . . . . loose some
of that reflex until you burn off some fuel and move the CG
forward.
The Mean Aerodynamic Center of a
wing is a point on the wing chord which results in a constant moment when the
wing angle of attack is changed. In other words
that is the point where there is no pitch restoring force from the main wing and
keeping the nose level becomes hard work. Now, since most airfoils have a slight
negative moment about the Mean Aerodynamic Center, some elevator down force is
still required to keep the nose level. A nose up disturbance will not change the
moment of the wing (no restoring force) but there will be a reduction of down
force from the elevator and some stability will be evident. Move the
CG even further back and you get to the Neutral point, a point where the entire
airframe has NO pitch restoring force at all (making your plane a hand full to
fly).
A more detailed treatment of stability can be found
here;
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