Stability is the ability of aircraft to maintain or return to a particular flight condition, after it being disturbed, without any effort by the pilot. Stability is sometimes also called Inherent Stability. All passenger aircraft and pilot training aircraft are designed to have high inherent stability. While various military and aerobatic aircraft are designed with very little inherent stability.
There are two types of stability
- Static Stability
- Dynamic Stability
Static stability is the initial tendency of aircraft to return to its original position after being disturbed. It can either be Positive, Negative, or Neutral.
- Positive Static Stability:
When aircraft returns to its original attitude after being disturbed, then it has positive static stability or is statically stable.
- Negative Static Stability:
Aircraft which have negative static stability tend to move away from their original position after being disturbed. It is also said to be statically unstable.
- Neutral Static Stability:
Aircraft which attains a new position after being disturbed, and neither move away nor move toward its original position is said to be statically neutral and have neutral static stability.
Dynamic stability is the movement of the aircraft over time in response to disturbance. We can surely assume that sometimes aircraft may not directly move back to its equilibrium position after disturbance. But it will oscillate around that position and then finally obtain its equilibrium position.
Suppose an aircraft is flying in level flight and a gust of wind pitches its nose up. Because the aircraft is statically stable it will try to move back to its equilibrium position. But by doing so, it will miss its position and need to pitch up its nose. This process continues until eventually aircraft stops oscillating and fly in its equilibrium position.
Dynamic stability also has 3 types
- Positive Dynamic Stability:
When an aircraft’s oscillation dampens over time after being disturbed then the aircraft is said to have Positive Dynamic Stability.
- Negative Dynamic Stability:
When aircraft oscillation increases overtime after being disturbed then the aircraft is said to have Negative Dynamic Stability.
- Neutral Dynamic Stability:
When aircraft oscillation neither increases nor dampens over time then the aircraft is said to have Neutral Dynamic Stability.
Note: Two terminologies determine the degree of stability, they are
Stick Fixed: – In this condition, the movement of control surfaces is zero, which means they are not allowed to move freely, stability obtain naturally by the aircraft.
Stick Free: – In this condition, control surfaces are free to move to stabilize the aircraft.
Aircraft have three axis along which movements of aircraft takes and stability along all these three axis is different from each other.
- Longitudinal Stability
- Lateral stability
- Directional Stability
Longitudinal stability concerns the movement of aircraft about its lateral axis and along its longitudinal axis, i.e., pitching movement of aircraft. A horizontal stabilizer is used to stabilize an aircraft disturbance that causes the nose of the aircraft to pitch up or down.
When aircraft flies the center of lift can be either be forward of the center of gravity or behind of center of gravity. This causes the nose of the aircraft to either pitch up or down during level flight. To prevent this from occurring the horizontal stabilizer provides upward or downward force at the tail plan. The direction of the force will depend upon the position of c.g. The force produced by the horizontal plain does not need to be very large because of its distance from c.g. which provides leverage to the force of the horizontal stabilizer.
More the distance between horizontal and center of gravity greater is the stability and vice versa.
Lateral stability concerns the movement of the aircraft about the longitudinal axis and along the lateral axis. It is associated with the rolling of an aircraft, to put it simply, it prevents aircraft from rolling. Various factor affects the rolling of an aircraft and therefor lateral stability becomes very complicated. We can break down lateral stability into three parts
- Rolling Stability Spiral Stability
- Spiral Stability
- Dutch Roll Stability
Rolling Stability: It is the ability of an aircraft to return to its normal flight position after being disturbed which causes it to roll. Simply it is a resistant offer by the aircraft to roll.
Suppose the aircraft is rolling towards the starboard side, then the starboard (Right) side wing will go down and the port (left) side wing will go up. Now the air which is striking on the surface of the wing will have two components. For the port side wing, one component will act vertically downward and one that is directly striking the wings leading edge. The resultant of these components will decrease the angle of attack of the port side wing.
While on the down-going wing the one component will act vertically upward direction. This will cause the angle of attack to increase on the starboard side wing and decrease in the port side wing. Hence, lift increases on the starboard side wing and decrease on the port side wing. Thus, the degree of roll decreases as the bank of aircraft increases. This dampening effect is proportional to the degree of roll and decreases as the aircraft approaches level flight. Therefore, the disturbed aircraft takes the bank position and due to inertia continues to fly at that position.
Since lift is now acting slightly at an angle from its vertical position, it divides into two components. One acts vertically upward, balancing the weight of an aircraft, and the other acting sideways. Initially, there is no force to balance the sideward component of lift and the upward component is less than weight, the aircraft moves sideways and downward direction. This is called sideslipping of an aircraft. This is prevented by giving the wings a dihedral angle and making them swept back. For simplicity we will not discuss about these in this article.
Spiral Stability: It is the resistance offered by the aircraft towards the downward spiralling motion when it is disturbed. When the aircraft is sideslipping the relative velocity of the sideslip, strikes the vertical fin and produces lift such that the force on the vertical fin will try to yaw aircraft in the direction of the down going wing.
Due to yawing of aircraft the outer wing will accelerate and produce more lift and bank the aircraft even more. If the rolling stability is low then this condition can become out of control and can be led to an accident. To prevent this, aircraft directional stability must be less than roll stability.
Dutch Roll Stability: This type of instability occurs when an aircraft’s directional stability is less than its lateral stability in a swept-back wing. This type of instability is more serious than spiral instability.
Suppose a swept-wing aircraft is disturbed and yaw to the right, the left-wing will accelerate and produce more lift while the right-wing will slow down and produce less lift. This imbalance in lift further intensifies when the left-wing becomes less swept and introduces more of its area (effective area) against the relative airflow thus increasing lift, while on the other hand, the right-wing becomes more swept its effective area decreases thus producing less lift. This result rolling of aircraft in the direction of yaw.
As the aircraft rolls, a point comes where the advancing wing (in this case left) produces more drag than lift. This makes the aircraft yaw toward the left side. Now the right-wing will advance and produce more lift and aircraft roll toward the left. This unstable motion of aircraft is because the lateral stability of aircraft is more than the directional stability thus it does not give time for the aircraft to become stable after yaw.
It is the tendency of aircraft to return to its original state after being disturbance cause it to yaw. Directional stability is mainly obtained by the vertical fin of an aircraft. When an aircraft is disturbed and starts to yaw in any direction, it places the vertical fin at an angle to the relative airflow. The vertical fin then produces force in the form of lift which tends to restore the aircraft to its original position.
The information provided in this blog does not contain the whole stability topic. This blog is made to give you just a little idea about stability without any mathematical equations. Stability as a topic is very vast and it not possible to cover that in just a single blog. Thank you! For reading.