EASA Module 8 (1) (1) PDF - Flight Stability and Dynamics

8.4 - FLIGHT STABILITY AND DYNAMICS THE AXES OF AN AIRCRAFT The axes of an aircraft can be considered as imaginary Whenever an aircraft changes its attitude in flight, it axles around which the aircraft turns...

8.4 - FLIGHT STABILITY AND DYNAMICS THE AXES OF AN AIRCRAFT The axes of an aircraft can be considered as imaginary Whenever an aircraft changes its attitude in flight, it axles around which the aircraft turns like a wheel. must turn about one or more of three axes. Figure 4-1 At the center, where all three axes intersect, each is shows the three axes, which are imaginary lines passing perpendicular to the other two. The axis that extends through the center of the aircraft. lengthwise through the fuselage from the nose to the Elevator Aileron Lateral Axis Rudder Aileron Longitudinal Axis Vertical Axis A Banking (roll) control affected by aileron movement Normal Altitude Longitudinal Axis B Climb and dive (pitch) control affected by elevator C Directional (yaw) control affected by rudder movement movement Normal Altitude Lateral Axis Vertical Axis Normal Altitude Figure 4-1. Motion of an aircraft about its axes. 4.2 Module 08 - Basic Aerodynamics tail is called the longitudinal axis. The axis that extends to cause it to f ly (hands off ) in a straight-and-level crosswise from wing tip to wing tip is the lateral, or flightpath. Maneuverability is the characteristic of an pitch, axis. The axis that passes through the center, from aircraft to be directed along a desired f lightpath and top to bottom, is called the vertical, or yaw, axis. Roll, to withstand the stresses imposed. Controllability is pitch, and yaw are controlled by three control surfaces. the quality of the response of an aircraft to the pilot's Roll is produced by the ailerons, which are located at commands while maneuvering the aircraft. There are the trailing edges of the wings. Pitch is affected by the two kinds of stability, static and dynamic. elevators, the rear portion of the horizontal tail assembly. Yaw is controlled by the rudder, the rear portion of the STATIC STABILITY vertical tail assembly. Static stability refers to the initial tendency, or direction of movement, back to equilibrium. In aviation, it refers STABILITY AND CONTROL to the aircraft's initial response when disturbed from a An aircraft must have sufficient stability to maintain given AOA, slip, or bank. a uniform f lightpath and recover from the various Positive static stability—the initial tendency of upsetting forces. Also, to achieve the best performance, the aircraft to return to the original state of FLIGHT STABILITY AND the aircraft must have the proper response to the equilibrium after being disturbed (Figure 4-2) movement of the controls. Control is the pilot action of Neutral static stability—the initial tendency of DYNAMICS moving the flight controls, providing the aerodynamic the aircraft to remain in a new condition after its force that induces the aircraft to follow a desired equilibrium has been disturbed (Figure 4-2) flightpath. When an aircraft is said to be controllable, it Negative static stability—the initial tendency of the means that the aircraft responds easily and promptly to aircraft to continue away from the original state of movement of the controls. Different control surfaces are equilibrium after being disturbed (Figure 4-2) used to control the aircraft about each of the three axes. Moving the control surfaces on an aircraft changes the DYNAMIC STABILITY airflow over the aircraft's surface. This, in turn, creates Static stability has been defined as the initial tendency changes in the balance of forces acting to keep the to return to equilibrium that the aircraft displays aircraft flying straight and level. after being disturbed from its trimmed condition. Occasionally, the initial tendency is different or Three terms that appear in any discussion of stability and opposite from the overall tendency, so a distinction control are: stability, maneuverability, and controllability. must be made between the two. Dynamic stability refers Stability is the characteristic of an aircraft that tends to the aircraft response over time when disturbed from Positive Static Stability Neutral Static Stability Negative Static Stability Applied Applied Applied Force Force Force CG CG CG CG Figure 4-2. Three types of static stability. Module 08 - Basic Aerodynamics 4.3 a given AOA, slip, or bank. This type of stability also axis is considered to be the most affected by certain has three subtypes: (Figure 4-3) variables in various flight conditions. Positive dynamic stability—over time, the motion of the displaced object decreases in amplitude and, Longitudinal stability is the quality that makes an aircraft because it is positive, the object displaced returns stable about its lateral axis. It involves the pitching toward the equilibrium state. motion as the aircraft's nose moves up and down in Neutral dynamic stability—once displaced, the flight. A longitudinally unstable aircraft has a tendency displaced object neither decreases nor increases to dive or climb progressively into a very steep dive or in amplitude. A worn automobile shock absorber climb, or even a stall. Thus, an aircraft with longitudinal exhibits this tendency. instability becomes difficult and sometimes dangerous Negative dynamic stability—over time, the motion to fly. Static longitudinal stability or instability in an of the displaced object increases and becomes aircraft, is dependent upon three factors: more divergent. 1. Location of the wing with respect to the CG. 2. Location of the horizontal tail surfaces with respect Stability in an aircraft affects two areas significantly: to the CG. Maneuverability—the quality of an aircraft that 3. Area or size of the tail surfaces. permits it to be maneuvered easily and to withstand 4. In analyzing stability, it should be recalled that a the stresses imposed by maneuvers. It is governed body free to rotate always turns about its CG. by the aircraft's weight, inertia, size and location of 5. To obtain static longitudinal stability, the relation flight controls, structural strength, and powerplant. of the wing and tail moments must be such that, if It too is an aircraft design characteristic. the moments are initially balanced and the aircraft Controllability—the capability of an aircraft to is suddenly nose up, the wing moments and tail respond to the pilot's control, especially with regard moments change so that the sum of their forces to flightpath and attitude. It is the quality of the provides an unbalanced but restoring moment aircraft's response to the pilot's control application which, in turn, brings the nose down again. when maneuvering the aircraft, regardless of its Similarly, if the aircraft is nose down, the resulting stability characteristics. change in moments brings the nose back up. LONGITUDINAL STABILITY The Center of Lift (CL) in most asymmetrical airfoils (PITCHING) has a tendency to change its fore and aft positions with In designing an aircraft, a great deal of effort is spent a change in the AOA. The center of lift tends to move in developing the desired degree of stability around all forward with an increase in AOA and to move aft with three axes. But longitudinal stability about the lateral a decrease in AOA. This means that when the AOA Damped Oscillation Undamped Oscillation Divergent Oscillation Positive Static (positive dynamic) Displacement Time Positive Static Positive Static (neutral dynamic) (negative dynamic) Figure 4-3. Damped versus undamped stability. 4.4 Module 08 - Basic Aerodynamics of an airfoil is increased, the center of lift, by moving In aircraft with fixed-position horizontal stabilizers, the forward, tends to lift the leading edge of the wing still aircraft manufacturer sets the stabilizer at an angle that more. This tendency gives the wing an inherent quality provides the best stability (or balance) during flight at of instability. (Note: center of lift is also known as the the design cruising speed and power setting. Center of Pressure (CP). If the aircraft's speed decreases, the speed of the Figure 4-4 shows an aircraft in straight-and-level airflow over the wing is decreased. As a result of this f light. The line CG-CL-T represents the aircraft's decreased f low of air over the wing, the downwash longitudinal axis from the CG to a point T on the is reduced, causing a lesser downward force on the horizontal stabilizer. horizontal stabilizer. In turn, the characteristic nose heaviness is accentuated, causing the aircraft's nose to Most aircraft are designed so that the wing's CL is to pitch down more. (Figure 4-6) the rear of the CG. This makes the aircraft "nose heavy" and requires that there be a slight downward force on This places the aircraft in a nose-low attitude, lessening the horizontal stabilizer in order to balance the aircraft the wing's AOA and drag and allowing the airspeed FLIGHT STABILITY AND and keep the nose from continually pitching downward. to increase. As the aircraft continues in the nose-low Compensation for this nose heaviness is provided by attitude and its speed increases, the downward force DYNAMICS setting the horizontal stabilizer at a slight negative on the horizontal stabilizer is once again increased. AOA. The downward force thus produced holds the Consequently, the tail is again pushed downward and tail down, counterbalancing the "heavy" nose. It is as if the nose rises into a climbing attitude. the line CG-CL-T were a lever with an upward force at CL and two downward forces balancing each other, one As this climb continues, the airspeed again decreases, a strong force at the CG point and the other, a much causing the downward force on the tail to decrease lesser force, at point T (downward air pressure on the until the nose lowers once more. Because the aircraft is stabilizer). To better visualize this physics principle: If dynamically stable, the nose does not lower as far this an iron bar were suspended at point CL, with a heavy time as it did before. The aircraft acquires enough speed weight hanging on it at the CG, it would take downward in this more gradual dive to start it into another climb, pressure at point T to keep the "lever" in balance. but the climb is not as steep as the preceding one. Even though the horizontal stabilizer may be level when the aircraft is in level flight, there is a downwash of air from the wings. This downwash strikes the top of the stabilizer and produces a downward pressure, which at CG a certain speed is just enough to balance the "lever." The Balanced tail load faster the aircraft is flying, the greater this downwash and the greater the downward force on the horizontal Cruise Speed stabilizer (except T-tails). (Figure 4-5) CG Lesser downward tail load Low Speed CG Greater downward tail load High Speed Figure 4-4. Longitudinal stability. Figure 4-5. Effect of speed on downwash. Module 08 - Basic Aerodynamics 4.5  Lift Thrust CG Thrust CG Weight Below center of gravity Normal Downwash Lift Thrust CG CG Thrust Weight Through center of gravity Reduced Downwash Figure 4-6. Reduced power allows pitch down. Thrust After several of these diminishing oscillations, in CG which the nose alternately rises and lowers, the aircraft finally settles down to a speed at which the downward Above center of gravity force on the tail exactly counteracts the tendency of the Figure 4-7. Thrust line affects longitudinal stability. aircraft to dive. When this condition is attained, the aircraft is once again in balanced flight and continues in stabilized flight as long as this attitude and airspeed Lift are not changed. Thrust CG A similar effect is noted upon closing the throttle. The downwash of the wings is reduced and the force at T in Figure 4-4 is not enough to hold the horizontal Cruise Power stabilizer down. It seems as if the force at T on the lever were allowing the force of gravity to pull the Lift nose down. This is a desirable characteristic because Thrust CG the aircraft is inherently trying to regain airspeed and reestablish the proper balance. Power or thrust can also have a destabilizing effect in Idle Power that an increase of power may tend to make the nose Lift rise. The aircraft designer can offset this by establishing a "high thrust line" wherein the line of thrust passes Thrust CG above the CG. (Figures 4-7 and 4-8) In this case, as power or thrust is increased a moment Full Power is produced to counteract the down load on the tail. On the other hand, a very "low thrust line" would tend to Figure 4-8. Power changes affect longitudinal stability. add to the nose-up effect of the horizontal tail surface. LATERAL STABILITY (ROLLING) Conclusion: with center of gravity forward of the center Stability about the aircraft's longitudinal axis, which of lift and with an aerodynamic tail-down force, the extends from the nose of the aircraft to its tail, is aircraft usually tries to return to a safe flying attitude. called lateral stability. This helps to stabilize the lateral or "rolling effect" when one wing gets lower 4.6 Module 08 - Basic Aerodynamics than the wing on the opposite side of the aircraft. edge slopes backward. When a disturbance causes an There are four main design factors that make an aircraft with sweepback to slip or drop a wing, the aircraft laterally stable: dihedral, sweepback, keel low wing presents its leading edge at an angle that is effect, and weight distribution. perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises, and the aircraft is restored DIHEDRAL to its original flight attitude. The most common procedure for producing lateral stability is to build the wings with an angle of one to Sweepback also contributes to directional stability. three degrees above perpendicular to the longitudinal When turbulence or rudder application causes the axis. The wings on either side of the aircraft join the aircraft to yaw to one side, the right wing presents fuselage to form a slight V or angle called "dihedral." The a longer leading edge perpendicular to the relative amount of dihedral is measured by the angle made by airflow. The airspeed of the right wing increases and it each wing above a line parallel to the lateral axis. acquires more drag than the left wing. The additional drag on the right wing pulls it back, turning the aircraft Dihedral involves a balance of lift created by the wings' back to its original path. FLIGHT STABILITY AND AOA on each side of the aircraft's longitudinal axis. If a momentary gust of wind forces one wing to rise and DYNAMICS KEEL EFFECT/WEIGHT DISTRIBUTION the other to lower, the aircraft banks. When the aircraft An aircraft always has the tendency to turn the is banked without turning, the tendency to sideslip longitudinal axis of the aircraft into the relative wind. or slide downward toward the lowered wing occurs. This "weather vane" tendency is similar to the keel of (Figure 4-9) a ship and exerts a steadying influence on the aircraft laterally about the longitudinal axis. When the aircraft Since the wings have dihedral, the air strikes the lower is disturbed and one wing dips, the fuselage weight acts wing at a much greater AOA than the higher wing. The like a pendulum returning the airplane to its original increased AOA on the lower wing creates more lift than attitude. Laterally stable aircraft are constructed so the higher wing. Increased lift causes the lower wing to that the greater portion of the keel area is above and begin to rise upward. As the wings approach the level behind the CG. (Figure 4-10) position, the AOA on both wings once again are equal, causing the rolling tendency to subside. The effect of Thus, when the aircraft slips to one side, the combination dihedral is to produce a rolling tendency to return the of the aircraft's weight and the pressure of the airflow aircraft to a laterally balanced flight condition when a against the upper portion of the keel area (both acting sideslip occurs. about the CG) tends to roll the aircraft back to wings- level flight. The restoring force may move the low wing up too far, so that the opposite wing now goes down. If so, the process Normal Angle of Attack is repeated, decreasing with each lateral oscillation until a balance for wings-level flight is finally reached. Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. The aircraft may be so stable laterally that it resists an intentional rolling motion. For this reason, aircraft that require fast roll or Dihedral Lateral Stability banking characteristics usually have less dihedral than those designed for less maneuverability. Greater Angle SWEEPBACK of Attack Lesser Angle Sweepback is an addition to the dihedral that increases of Attack the lift created when a wing drops from the level position. A sweptback wing is one in which the leading Figure 4-9. Dihedral for lateral stability. Module 08 - Basic Aerodynamics 4.7  CG CG CG Centerline CG Area Forward Area After Center of Gravity (CG) of CG Figure 4-10. Keel area for lateral stability. DIRECTIONAL STABILITY (YAWING) CG yaw Stability about the aircraft's vertical axis (the sideways moment) is called yawing or directional stability. Yawing yaw or directional stability is the most easily achieved Relative W ind stability in aircraft design. The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors which make the aircraft act like the well known weather vane or arrow, pointing its nose into the relative wind. Figure 4-11. Fuselage and fin for directional stability. In examining a weather vane, it can be seen that if heading. Therefore, after a slight yawing of the aircraft exactly the same amount of surface were exposed to to the right, there is a brief moment when the aircraft is the wind in front of the pivot point as behind it, the still moving along its original path, but its longitudinal forces fore and aft would be in balance and little or no axis is pointed slightly to the right. directional movement would result. Consequently, it is necessary to have a greater surface aft of the pivot point The aircraft is then momentarily skidding sideways, and than forward of it. during that moment (since it is assumed that although the yawing motion has stopped, the excess pressure on Similarly, the aircraft designer must ensure positive the left side of the fin still persists) there is necessarily a directional stability by making the side surface greater tendency for the aircraft to be turned partially back to aft than ahead of the CG. (Figure 4-11) To provide the left. There is a momentary restoring tendency caused additional positive stability to that provided by the by the fin. fuselage, a vertical fin is added. The fin acts similar to the feather on an arrow in maintaining straight flight. This restoring tendency is relatively slow in developing Like the weather vane and the arrow, the farther aft and ceases when the aircraft stops skidding. When this fin is placed and the larger its size, the greater the it ceases, the aircraft is f lying in a direction slightly aircraft's directional stability. different from the original direction. It will not return of its own accord to the original heading; the pilot must If an aircraft is flying in a straight line, and a sideward reestablish the initial heading. gust of air gives the aircraft a slight rotation about its vertical axis (e.g., the right), the motion is retarded and A minor improvement of directional stability may be stopped by the fin because while the aircraft is rotating obtained through sweepback. Sweepback is incorporated to the right, the air is striking the left side of the fin at in the design of the wing primarily to delay the onset of an angle. This causes pressure on the left side of the fin, compressibility during high-speed flight. In lighter and which resists the turning motion and slows down the slower aircraft, sweepback aids in locating the center aircraft's yaw. In doing so, it acts somewhat like the of pressure in the correct relationship with the CG. A weather vane by turning the aircraft into the relative longitudinally stable aircraft is built with the center of wind. The initial change in direction of the aircraft's pressure aft of the CG. f lightpath is generally slightly behind its change of 4.8 Module 08 - Basic Aerodynamics Because of structural reasons, aircraft designers at times PASSIVE AND ACTIVE STABILITY can't attach the wings to the fuselage at the exact desired Additional terms which are often used to describe point. If they had to mount the wings too far forward, the stability characteristics of an aircraft are Passive and at right angles to the fuselage, the center of pressure and Active. would not be far enough to the rear to result in the desired amount of longitudinal stability. By building The term "Passive Stability" refers to a situation in sweepback into the wings, however, the designers can which the vehicle is naturally (inherently) stable and move the center of pressure toward the rear. The amount does not require any artificial stabilization systems. of sweepback and the position of the wings then place the This would require positive static stability and positive center of pressure in the correct location. dynamic stability. The contribution of the wing to static directional The term "Active Stability" refers to the use of artificial stability is usually small. The swept wing provides stabilizing systems to improve the handling of vehicles a stable contribution depending on the amount of which do not exhibit sufficient passive stability. An sweepback, but the contribution is relatively small when example of such a system would be an aircraft automatic FLIGHT STABILITY AND compared with other components. stabilization system (Basic Autopilot). DYNAMICS FREE DIRECTIONAL OSCILLATIONS (DUTCH ROLL) Dutch roll is a coupled lateral/directional oscillation that is usually dynamically stable but is unsafe in an aircraft because of the oscillatory nature. The damping of the oscillatory mode may be weak or strong depending on the properties of the particular aircraft. If the aircraft has a right wing pushed down, the positive sideslip angle corrects the wing laterally before the nose is realigned with the relative wind. As the wing corrects the position, a lateral directional oscillation can occur resulting in the nose of the aircraft making a figure eight on the horizon as a result of two oscillations (roll and yaw), which, although of about the same magnitude, are out of phase with each other. In most modern aircraft, except high-speed swept wing designs, these free directional oscillations usually die out automatically in very few cycles unless the air continues to be gusty or turbulent. Those aircraft with continuing Dutch roll tendencies are usually equipped with gyro- stabilized yaw dampers. Manufacturers try to reach a midpoint between too much and too little directional stability. Because it is more desirable for the aircraft to have "spiral instability" than Dutch roll tendencies, most aircraft are designed with that characteristic. Module 08 - Basic Aerodynamics 4.9 4.10 Module 08 - Basic Aerodynamics  QUESTIONS Question: 4-1 Question: 4-3 Name the three axes of an aircraft. An increase in wing __________________ increases the lateral stability of the aircraft in flight. Question: 4-2 Question: 4-4 Static longitudinal stability of an aircraft depends on Stability about the vertical axis of an aircraft is known what three things? as __________________ or __________________ stability. Module 08 - Basic Aerodynamics 4.11  ANSWERS Answer: 4-1 Answer: 4-3 longitudinal. dihedral. lateral. vertical. Answer: 4-2 Answer: 4-4 Location of the wing with respect to the CG yawing. (center of gravity). directional. Location of the horizontal tail surfaces in relation to the CG. Area of the tail surfaces. 4.12 Module 08 - Basic Aerodynamics