DESCRIBE the characteristics of damped, undamped, and divergent oscillations, and the combination of static and dynamic stabilities that result in each
EXPLAIN the relationship between stability and maneuverability
- More stable = less maneuverable
- A stable airplane tends to stay in equilibrium and is difficult for the pilot to move out of equilibrium.
- The more maneuverable an airplane is, the easier it departs from equilibrium, and the less likely it is to return to equilibrium.
STATE the methods for increasing an airplane's maneuverability
- Give it weak stability
- Give it larger control surfaces.
STATE the effects of airplane components on an airplane's longitudinal static stability
- Longitudinal stability is the stability of the longitudinal axis around the lateral axis (pitch stability)
- If the Aerodynamic Center (AC) is behind the COG, it will have a positive impact on longitudinal static stability because of its initial tendency to return to equilibrium.
- If the AC is ahead of the COG, it will have a negative impact on longitudinal static stability.
- AC is usually located ahead of the CG.
- It is a negative contributor to longitudinal static stability.
- The Horizontal Stabilizer
- Designed for lateral axis, its contribution to longitudinal static stability is determined by the moment it produces around the CG.
- Since it's AC is well behind the plane's CG, the horizontal stabilizer has the greatest positive effect on longitudinal static stability.
- The pitching moment can be increased by increasing the distance between the airplane's CG and the stabilizer's AC, or by enlarging the horizontal stabilizer.
- Shorter planes need a larger stabilizer and vice versa.
- The Neutral Point
- the location of the center of gravity along the longitudinal axis that would provide neutral longitudinal static stability.
- It can be thought of as the aerodynamic center for the entire plane.
- The NP is fixed on conventional planes, but CG can change.
- As the CG is moved aft, the airplane's static stability decreases.
- The NP defines the farthest aft CG position without negative stability.
- Once the NP is aft of the NP the airplane becomes unstable.
EXPLAIN the criticality of weight and balance
If the CG is aft of the NP, the plane becomes unstable and difficult to control in flight.
STATE the effects of airplane components on an airplane's directional static stability
- Directional Static Stability
- the stability of the longitudinal axis around the vertical axis. (yawing)
- When an airplane yaws, its momentum keeps it moving along its original flight path for a short time. (sideslip)
- -Straight Wings
- the advancing wing on a straight winged plane has a momentary increase in airflow velocity as it moves forward.
- Parasite drag increases and pulls it back to its equilibrium position.
- The retreating wing has less velocity and less parasite drag, which helps bring the nose to the relative wind.
- Straight wings have a small positive effect on directional static stability.
- -Swept Wings
- Swept wings will further increase directional stability.
- The advancing wing not only experiences an increase in parasite drag, but also an increase in induced drag due to the increase chordwise flow.
- The retreating wing experiences more spanwise flow.
- The result is an airplane that comes back into the relative wind.
- AE is forward of the airplane's CG.
- When the airplane enters a sideslip, an angle of attack is created on the fuselage.
- The lift created at eh fuselage AC pulls the nose away from the relative wind, increasing sideslip angle.
- Fuselage is a negative contributor to directional stability.
- Vertical Stabilizer
- Greatest Positive contributor to directional stability.
- Sideslip creates an increase in AOA, creating horizontal lifting force on the stabilizer that is multiplied by the moment arm distance to the CG.
- The moment will swing the nose of the plane back into the relative wind (equilibrium).
- Inverse relationship between tail size and moment arm length. The smaller the distance, the larger the stabilizer must be.
- 2 small can accomplish the same effect as one large.
STATE the effects of airplane components on an airplane's lateral static stability
- Lateral Static Stability
- The stability of the lateral axis around the longitudinal axis. (roll)
- -Dihedral effect
- dihedral wings cause an increase in AOA and lift on the down-going wing.
- The up-going wing has a reduced AOA and a decrease in lift.
- The difference creates a rolling moment that rights the plane and stops the sideslip.
- Diehedral wings are the greatest positive contributors.
- Straight are neutral, anhedral are the greatest negative contributors.
- -Wing Placement
- A high mounted wing is a positive contributor
- A low mounted wing is a negative contributor to lateral static stability.
- -Wing Sweep
- Swept wings are laterally stabilizing.
- Vertical stabilizer
- Tends to right the plane since the tail is above the plane's CG when it senses an AOA and produces lift
STATE the static stability requirements for, and the effects of, directional divergence
- Directional divergence is a condition of flight in which the reaction to a small initial sideslip results in an increase in sideslip angle.
- Caused by negative directional static stability.
- If the vertical stabilizer becomes ineffective for some reason, directional divergence could cause out of control flight.
- Most planes have strong directional stability to prevent this from occurring.
STATE the static stability requirements for, and the effects of, spiral divergence
- Occurs when a plane has strong directional stability and weak lateral stability.
- For example, a plane is disturbed so that its wing dips and starts to roll to the left.
- Since it has weak lateral stability it cannot correct itself and the flight path arcs to the left.
- The plane senses a new relative wind from the left and aligns itself with the new wind by yawing into it (strong directional stability).
- The right wing is now advancing and the increased airflow causes the plane to roll even more to the left.
- The plane will continue to chase the relative wind and will develop a tight descending spiral.
- It is corrected by control input from the pilot
STATE the static stability requirements for, and the effects of, dutch roll
- Dutch roll is the result of strong lateral stability and weak directional stability.
- The plane responds to a disturbance with both roll and yaw motions that affect each other.
- For example, a gust causes the plane to roll left, producing a left sideslip.
- The strong lateral stability increases lift on the left wing and corrects it back to wings level.
- At the same time, the nose of the airplane yaws left into the sideslip relative wind. This leaves the airplane wings level, with the nose cocked to the left.
- Weak directional stability now swings the nose to the right to correct it back to the relative wind.
- The left wing advances faster, causing the plane to roll right, and the scenario repeats itself.
EXPLAIN how an airplane develops pilot induced oscillations
- It occurs when a pilot is trying to control airplane oscillations that happen over approximately the same time span as it takes to react.
- A pilot tries to push the nose-down to correct the plane. The input may coincide with the stability correction, causing the nose to over correct and end up low.
DEFINE asymmetric thrust
- Thurst unequal on different part of the plane.
- For example, an engine fails and the thrust is unequal.
- it will create a yawing moment toward the dead engine.
EXPLAIN how an airplane develops phugoid oscillations
- Phugoid oscillations are long period oscillations (20 to 100 seconds) of altitude and airspeed while maintaining a nearly constant AOA.
- Upon being struck by an upward gust, the airplane would gain altitude and lose airspeed.
- When enough airspeed is lost, the airplane will nose-over slight, commencing a gradual descent, gaining airspeed and losing altitude.
- When enough airspeed is regained, the plane will nose-up slightly and restart the process.
DEFINE proverse roll
- the tendency of an airplane to roll in the same direction as it is yawing.
- yawing left will cause the right wing to accelerate faster, causing it to increase lift and roll left.
DEFINE adverse yaw
- The tendency of an airplane to yaw away from the direction of aileron roll input.
- When the airplane rolls, it has more lift on the up-going wing than the down-going wing.
- This causes an increase in induced drag on the up-going wing that will retard that wing's forward motion and cause the nose to yaw in the opposite direction of the roll.