# LESSON 17 Chapter 16 Directional and Lateral Stability and Control ANA Chapter 4

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## Directional and Lateral Stability and Control

• Rudder/Vertical Tail
• Directional stability about the vertical axis is obtained in our airplanes by use of the rudder/vertical stabilizer.
• Sideslip angle or beta is the angle of the relative wind and the longitudinal axis.
• Rudder Stability
• This plane is yawing to the left
• Note the relative wind sets up an angle of attack on the tail generating lift to the left
• This straightens the nose to the right
• This is known as weathercock stability or weathervaning
• Directional Stability
• The degree of directional stability is proportional to the size of the vertical stabilizer and the distance from the CG
• Increase either or both and an increase in directional stability will result
• The Wings
• The wings may be a small stabilizing influence if they are sweptback.
• When the relative wind is shifted, the upwind wing produces more drag than the other because of increased frontal area being presented to the relative wind
• This causes the yaw towards the relative wind.
• The Fuselage
• The fuselage may be a destabilizing influence on yaw.
• If the surface area ahead of the cg is larger than that behind, it will tend towards negative stability.
• In a sideslip, the wing swings forward of the cg and is a destabilizing influence
• Engine nacelles
• Engine nacelles can be a destabilizing force if ahead of the cg in a sideslip
• If the plane is in a slip the relative wind must turn to be aligned with the axis of the engine
• A sideways force is developed when the direction of the airflow is changed
• If the engines are aft of the cg (think business jet) then the force is stabilizing for both pitch and yaw
• Can you say “Destabilization”
• These two, the fuselage and the engine nacelles have the greatest potential for destabilization.
• The Vertical Tail
• The vertical stabilizer is the primary stabilizing influence.
• A dorsal fin may be added to increase the stabilizing force with a lower drag penalty than increasing rudder size
• The dorsal fin also delays the stall of the vertical tail at high sideslip angles
• Strakes may be added under the fuselage to improve overall yaw characteristics.
• Also in cases where yaw is a problem especially at higher speeds, a yaw damper may be required to be operational on the aircraft.
• The Vertical Tail
• When at high angles of attack, there is a reduction in the vertical tails’ contribution and stability is decreased.
• There is a decay in directional stability in planes with low aspect ratios.
• The sweepback needs such high angles of attack that a lack of directional stability results
• The Vertical Tail
• There are 5 conditions critical to the directional control forces exerted by the rudder:

–Spin recovery

–Slipstream rotation

–Crosswind takeoff and landing

–Asymmetrical thrust (multiengine)

• Vertical Tail or Keel Effect
• Side forces on the tail cause a weather vane effect which tends to bring the plane back into alignment with the relative wind.
• If there is a large enough portion of the rudder above the longitudinal axis, then there is a rolling moment induced by the side ways force of the relative wind on the tail; this is a stabilizing force.
• Vertical Stability
• Directional stability is influenced by putting more vertical surface behind the CG
• This allows the airplane to weather vane for positive directional stability
• If there were the same area in front of as behind the cg it would have neutral directional stability
• If more area is ahead of the cg, negative directional stability would result
• Vertical Stability
• In order to assure directional stability, some aircraft have strakes, ventral fins or dorsal fins
• Float planes will usually have vertical surfaces attached to the elevator to offset the added surface area of the floats
• In order to keep the vertical surface area in balance on the 747 that carries the shuttle, large vertical fins were added to the tail
• Vertical Stability
• When a gust causes the aircraft to yaw to the right, the left side of the vertical surface will cause a force to be applied back to the left
• The change in direction is behind the change in heading
• An oscillation to develops and the plane yaws back
• This means the longitudinal axis is pointed slightly right to direction of travel
• The plane is now skidding sideways
• A force is again applied because of the skid and a new heading is the result
• The pilot will have to initiate a heading change after this yaw upset
• Cg
• Has a minimal effect on directional stability
• CG mostly effects longitudinal stability
• Lateral Stability and Control
• Lateral stability is the stability displayed about the longitudinal axis of the airplane or specifically the stability in the roll.
• There are 4 main design factors that make a plane laterally stable:

–Dihedral

–Sweepback

–Keel effect

–Weight distribution

• Lateral Stability
• The different thing about Lateral stability is that there is really no aerodynamic force in a roll that will cause the airplane to right itself
• In addition there is no force that will continue the roll once it has begun
• Most airplanes are neutrally stable in the roll
• The equation to figure rolling moment is:

–Where L’cg is rolling moment about the cg (ft-lbs

–CL’(cg) is coefficient of rolling moment about the CG

–q is dynamic pressure

–S is wing area

–b is wing span

• Dihedral or Anhedral
• Dihedral is a stabilizing design, whereas Anhedral is a destabilizing design.
• The stabilizing effect of dihedral occurs when a sideslip is set up as the result of turbulence or gust displacing the plane.
• Dihedral
• The side slip results in the downward wing having a greater angle of attack than the upward wing. The extra lift then rights the airplane.
• The most common way to produce lateral stability is to use dihedral
• Manufactures build in a 1 to 3 degree angle
• Dihedral
• Dihedral involves a balance of lift created by each wing
• If a gust causes roll, the aircraft will sideslip in the direction of the bank
• Since the wings have dihedral the air strikes the lower wing at a much greater α
• This causes more lift to be generated on the lowered wing making it rise
• Once level the lift is equal again
• Dihedral How Does It Work?
• As you can see in this exaggerated diagram, the sideslip that sets up causes an increase in α
• There is a change in the relative wind due to the slip
• The lowered wing has a higher α due to the relative wind changing from directly 90 degrees to an angle off the wing tip
• In addition the lowered wing has a greater vertical lift component
• The raised wing has a greater horizontal lift component
• This causes the imbalance in lift between the two wings
• Dihedral
• If we look at the force vectors for a wing with dihedral we see that some of the lift the wing generates is tilted into the horizontal
• This horizontal vector requires more lift from the wing than if it had no dihedral
• This concept however is slight, the main reason dihedral works is due to the sideslip and increase in α
• There are some penalties that go along with too much dihedral:

–Less vertical component of lift

–More drag (higher α to make up for loss of lift)

–More aileron force to roll

• Wing position
• A high wing sets up a pendulum type of situation this can result in the equivalent of a 1 to 3 degree dihedral.
• So not as much dihedral is needed.
• In some planes, negative dihedral is needed.
• The low wing however the reverse is true.
• Still other airplanes have both dihedral and anhedral
• Wing Sweepback
• When a side slip is set up in a sweepback wing, the upwind side wing will have a greater angle of attack because of the more favorable relative wind.
• Lateral control
• Ailerons, elevons and ailevator (delta wing aircraft) are all ways the pilot can use to control the plane.
• The elevons or ailevator work together when elevator action is required and work opposite when roll action is required.
• A negative aspect of lateral control is the overbanking tendency which shows up at bank angles of greater than 30 degrees.
• Lateral control
• The outer wing is moving faster than the inner wing and thus developing more lift.
• This causes the plane to want to continue the roll and it takes opposite aileron to counteract it.
• This may lead to the curious condition of cross controls but having a coordinated airplane.
• Directional-Lateral Coupling
• Dutch Roll aka Free Directional Oscillations
• Roll and Yaw are not separate forces, they act together
• If the plane yaws, it will invariably roll as well
• When the plane yaws, one wing speeds up while the other slows down
• The yaw will cause greater lift on one wing more than the other
• This causes a rolling moment which in turn causes a sideslip to be set up whereby the process starts all over again.
• Directional-Lateral Coupling
• One of the best examples of this is Adverse Yaw
• Because the yaw is produced in the opposite direction of the turn it is referred to as adverse
• When rolling into a turn, the upward wing’s lift vector is tilted aft because of the change in the relative wind components being up and parallel to the flight path
• The downward wing’s lift vector is tilted forward because of the change in relative wind components being down and parallel to the flight path.
• These two forces oppose the turn entry and cause adverse yaw.
• Aileron drag is another common cause of adverse yaw.
• Frise ailerons and differential aileron travel are common ways of offsetting the effects of aileron drag.
• Using spoilers to turn solves this problem.
• There are 3 types of motion associated with the coupling of yaw and roll:

–Spiral divergence

–Directional divergence

–Dutch roll

• 1. Spiral Divergence happens when directional stability is greater than lateral stability.

• What happens is there is little sideslip when the aircraft is displaced and next to no dihedral effect.
• The plane enters an ever tightening spiral.
• Normally the pilot would correct this, however, in some situations, (IFR) it could prove to be trouble
• 2. Directional divergence results from negative directional stability
• The plane develops a sideslip after being disturbed which causes a further yaw or roll
• The plane will eventually end up yawing sideways to the relative wind.