## Stability Stability is how an aircraft corrects (or doesn't correct) from disturbances. A disturbance may be a control input, such as pushing the yoke forward. There are two key types of stability: static and dynamic. - [[Static Stability]]: `= [[Static Stability]].definition` - [[Dynamic Stability]]: `= [[Dynamic Stability]].definition` Both stabilities can be further broken down. --- ### Types of Stability #### Static Stability In a single axis, an aircraft can have: - Positive Static Stability: Initial tendency is to return to the original state. - Neutral Static Stability: Initial tendency is to remain in new condition. - Negative Static Stability: Initial tendency is to continue away from original state. ![[Static Stability Types.jpeg]] #### Dynamic Stability Similarly, dynamic stability can be positive, neutral, or negative depending on if it converges to equilibrium, oscillates consistently, or diverges. ![[Chart of Dynamic Stability Types.jpeg]] ---- ### Stability Around Each Axis #### Lateral Stability ***Definition***: `= [[Lateral Stability]].definition` A major goal of [[Lateral Stability]] is to have the aircraft's wings tend to return to wings level. There are four design factors that make an aircraft laterally stable: 1. [[Dihedral]]: `= [[Dihedral]].definition` (See `= [[Dihedral]].media` for more information.) 2. [[Sweepback]]: `= [[Sweepback]].definition` When a disturbance causes a wing to drop, the low wing presents a more perpendicular relative airflow, causing more lift and a return to wings level. 3. [[Keel Effect]]: `= [[Keel Effect]].definition` 4. ***Weight Distribution***: Having more keel area above the [[CG]] encourages the [[Keel Effect]]. #### Longitudinal Stability ***Definition***: `= [[Longitudinal Stability]].definition` A major goal of [[Longitudinal Stability]] is to have the aircraft's nose return to safe flying attitude if a force pushes the nose up or down. This is accomplished by having a [[CG]] forward of the [[CL]], and using a downward force from [[Downwash]] on the [[horizontal stabilizer]] while the aircraft is in steady flight.^[This is also why a CG which is too far back can be dangerous: without the tendency for the nose to return to safe flight, the aircraft may try to put itself into an ever-increasing stall.] [[Thrust]] line placement, either below or above the [[CG]], can also effect longitudinal stability when power is changed. ![[Longitudinal Stability.png]] **How Does Downwash Create Stability?** If power is reduced on the aircraft, less downwash is created, and there is less downward force on the horizontal stabilizer. This causes the nose to drop. But when the nose drops, the aircraft starts to gain speed until the downwash is again leveling the plane. These oscillations continue with positive dynamic stability, until the aircraft settles to a new attitude. #### Directional or Yawing Stability ***Definition***: `= [[Directional Stability]].definition` A major goal of [[Directional Stability]] is to minimize yawing at keep the plane pointed into the relative wind. Like an arrow or weather vane, if the plane yaws during flight, the [[fuselage]] and fin will help to keep the airplane return.^[Remember that a body free to rotate always turns about its CG.] ![[Fuselage and Fin for Directional Stability.jpeg]] ***Source***: [[PHAK Ch5]]