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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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