|
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;
|