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

Chapter 16

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

–Adverse yaw

–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:



–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
  • Aircraft manufacturers therefor need to design in certain characteristics
  • 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
  • Adverse Yaw
  • 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.
  • Adverse Yaw
  • 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.
  • Coupled Effects (Bad Things)
  • There are 3 types of motion associated with the coupling of yaw and roll:

–Spiral divergence

–Directional divergence

–Dutch roll

  • Bad things (Coupled Effects)
  • 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
  • Bad things
  • 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.
  • Bad things
  • 3. Dutch roll is directional and lateral oscillation caused by strong dihedral effect and weak directional stability
  • If the plane is slipped to the right it will yaw to the right
  • At the same time the right wing will develop more lift and the plane will roll left
  • The up going right wing will cause a slip to the left
  • This causes the whole process to repeat itself
  • When static directional stability is strong, Dutch roll is less of a problem
  • Spiral divergence is more of a problem in this case, but is desirable to Dutch roll
  • Good Things
  • Spiroid Winglets, which look like a large loop of rigid ribbon material attached to each wingtip, cut fuel consumption by 6% – 10% in cruise flight. Initial flight tests of the Spiroid concept on a GII reportedly reduced cruise fuel consumption by more than 10%. The Spiroid eliminates concentrated wingtip vortices, which represent nearly half the induced drag generated during cruise.
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