By Dave Esser
Reprinted from Woman Pilot magazine
Stability is the resistance something exhibits against a displacement force, and has been touted in everything from personal relationships to the cosmic forces of the universe. In the context of aircraft, stability plays a key role in the ability to maintain control. The opposite of stability is maneuverability. If an aircraft had infinite stability, it would have no maneuverability. In this case, there would be no way to change the flight path. A desirable middle ground must be designed, depending on the type of aircraft. Where an aircraft trainer or jet transport aircraft may require greater stability for safety and passenger comfort, a jet fighter needs maneuverability for combat situations.
Positive stability means that when the aircraft is displaced it tends to return to the original attitude. Neutral stability would result in the attitude remaining constant after displacement, neither returning nor continuing to displace. Negative stability would result in the attitude continuing to displace or diverge. (See Figure 1.)
Stability is also categorized as both static and dynamic. Imagine an aircraft in level flight having the pitch increased by the pilot. If, after increasing the pitch, the pilot releases the control yoke, the nose will return to level flight, demonstrating positive static stability. Those familiar with this demonstration know that the nose of the aircraft does not simply return to level, but overshoots and enters a descent, followed by a series of shallower climbs and descents until level flight is eventually reached. These oscillations of smaller and smaller amplitude are a function of the aircraft’s positive dynamic stability. If the aircraft had neutral dynamic stability, the oscillations would continue at the same amplitude indefinitely. If an aircraft had negative dynamic stability, the amplitude of the climbs and dives would get steeper and steeper. (See Figure 2.)
Pitch stability, also called longitudinal stability, is accomplished by keeping the center of gravity (CG) forward of the center of lift (CL). The center of lift is a point on the wing where all the lift force seems to act through. This “average” point is synonymous with the center of gravity, but normally is in the upward direction. Because the upward force of the CL is aft of the CG, the nose-down torque must be balanced by a tail-down force. The tail-down force is directly related to the airspeed flowing over the horizontal stabilizer airfoil, and thereby creates a stabilizing situation. (See Figure 3.)
Imagine the example given above, where an aircraft is pitched upward. The nose drops after the control yoke is released because the airspeed decreased when the aircraft was climbing, and there is then less tail-down force. When the nose drops, the aircraft enters an airspeed-increasing descent, resulting in more tail-down force and a climb. Due to frictional air resistance and inertial forces, the airspeed in the second climb is not as high as in the first, the climb angle is not as steep, and the subsequent descent is also not as steep. The oscillations eventually dampen out, returning the aircraft to level flight.
The CG of an aircraft must remain within certain forward and aft limits. The farther forward the CG, the more down force necessary to balance. As this tail-down force increases, the effective gross weight of the aircraft increases. The aircraft in Figure 3 weighs 2,300 pounds with 200 pounds of tail-down force. The amount of lift necessary in this case is 2,500 pounds. As the CG moves aft, the required tail-down force decreases and the amount of lift necessary also decreases. Thus, a forward CG aircraft will have good pitch stability, but will have the performance characteristics of a heavier aircraft, such as slower cruise speeds and higher fuel consumption for a given flight distance. Another safety consideration of a forward CG is the nose-down tendency in a stall recovery. In summary, the CG effect means that forward CG is safer, and aft CG provides better performance. Again, it should be emphasized that the CG must remain within published limits to be safe.
Lateral stability, or bank stability, is enhanced by dihedral. Dihedral is the upward angle of the wings. If the aircraft is inadvertently banked or displaced by turbulence, the ensuing slip toward the lower wing and the subsequent shielding of the upward wing by the fuselage results in a higher angle of attack and more lift on the lowered wing, restoring level flight. (See Figure 4.)
The amount of required dihedral is less in a high-wing aircraft than in a low-wing aircraft. In a high-wing aircraft, the weight is suspended below the wing, similar to a pendulum suspended from above. In a low-wing aircraft, the weight is centered above the suspending wings. In the case of the low-wing aircraft, think of balancing a baseball bat on your hand–it can be done, but is very unstable. When holding the end of the baseball bat suspended below your hand, the bat will seek equilibrium without any balancing effort from you.
When an aircraft rolls into a shallow bank of around 5 degrees, lateral stability results in the wings rolling level on their own. During steep banks, the tendency is for the aircraft to overbank. This is because the raised wing has to cover a greater distance, in the same way that the outer of two cars side by side on a racetrack has to travel a greater distance and therefore must travel faster to complete the circle at the same time. The faster airspeed of the outside raised wing results in greater lift and the overbanking tendency. During medium banked turns, these two tendencies cancel out, and the bank remains constant.
The last stability is directional stability, which is primarily a result of the vertical stabilizer serving the same function as the feathers on an arrow. (See Figure 5.) If a slip is encountered, the resulting force on the vertical stabilizer restores a zero sideslip condition. Also assisting in directional stability is wing sweep. The forward-swept wing has a more direct ram pressure of the relative wind. This creates more drag and restores the direction of the nose after displacement. Certain aircraft experience Dutch roll instability when the swept wing moving forward increases the lift as well as the drag. The result is the wing raising and retarding, which lowers and advances the other wing, which now has more lift and drag, and the process continues. This can be very annoying for the passengers. Dutch roll instability is usually overcome with design changes.
With all this talk about how great stability is, we should give some time to the jet fighters that crave maneuverability. In these aircraft, engineers design in instability. However, it is more euphemistic to say maneuverability rather than instability. When the pilot makes a control input, the aircraft responds with a dramatic change in attitude and flight path, which is rather comforting when people are shooting at you! Some aircraft are so unstable that it is not possible for a human to fly them. These aircraft are designed with stability augmentation systems that use computers and automatic pilots. In an effort to reduce weight and increase performance, many jet transport aircraft have a vertical stabilizer that is smaller than what would otherwise be required. In these aircraft, an autopilot is connected to the rudder to perform the aerodynamic directional stability functions of a larger, heavier vertical stabilizer. Future aircraft will most likely have more aerodynamic maneuverability and computerized autopilot systems to make them easy to control, thereby providing the best of both worlds: stability and performance.
Dave Esser is a professor and the associate chair in the Aeronautical Science Dept. at Embry-Riddle Aeronautical University. He may be contacted at firstname.lastname@example.org. Embry-Riddle, the world’s largest, fully accredited university specializing in aviation and aerospace, educates 24,000 students annually through the master’s level at residential campuses in Daytona Beach, Fla., and Prescott, Ariz., at more than 120 teaching centers in the United States and Europe, and through distance learning. David Esser Professor and Associate Chair Embry-Riddle Aeronautical University 600 S Clyde Morris Blvd Daytona Beach, FL 32114-3966
United States Dr. Esser was a member of the Embry-Riddle Aeronautical University Flight Department from 1981 to 1995, and has been a member of the Aeronautical Science Department since that time. His undergraduate degrees are from Embry-Riddle in Aviation Management, Computer Science, and Aeronautical Science. In 1989, he was awarded the M.S. in Aeronautical Science from Embry-Riddle, graduating at the top of his class. Professor Esser completed the Ph.D. Degree in Organization and Management Leadership from Capella University, again graduating at the top of his class. His dissertation pertained to airline Advanced Qualification Training and Line Check Safety Audits to validate Threat and Error Mitigation techniques. It also involved Flight Operations Quality Assurance and Aviation Safety Action Program data collection. He holds FAA Airline Transport Pilot certificate with a Type Rating in Airbus A319/320: Advised Graduate Research Projects and Thesis in Topics of CRM, AQP, Flight Data Monitoring, and FOQA.
Publisher’s Note: The original article will appear in a copy of the back issues on Woman Pilot magazine soon on womanpilot.com