Control of an aircraft is achieved by the position of the wing and the
various control surfaces on the fuselage and careful distribution of the various
moveable components (mainly the pilots). The basis for stability is placing the
point where all of the mass of the glider and the occupants might be calculated
to act (centre of gravity) ahead of the centre where the
wing lifts (centre of pressure) and placing the pitch and
yaw control surfaces on the opposite side of the centre of gravity relative to
the centre of pressure. We then can make minor changes in the pitch
attitude and the aircraft will still be stable. By stable we mean that if the
nose of the aircraft drops that the horizontal stabilizer will move in the
opposite direction and generate a downward force. In this way the aircraft will
always seek to maintain a certain angle of attack. This can be altered slightly
with changes in the position of the elevator.
The aircraft will be stable in yaw if the vertical fin is sufficiently large. If the aircraft tries to track sideways then the wind will push on this fin and the aircraft will return to a straight path. Again minor changes can be made with the use of the rudder.
If the wings are attached at an angle such that the tips are higher relative to where the wings meet the fuselage, the aircraft is said to have a positive dihedral. If the aircraft starts to roll then the down going wing will meet the air with a greater angle than the upgoing wing and thus generate more lift. This will act to help level the wings of the aircraft and inhibit turning in that direction. This effect is further promoted by designing the aircraft such that the fuselage is underneath the wings. This creates an effect known as keel effect which causes the weight of the aircraft to swing towards the centre promoting stability in roll.