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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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