Aircraft Stability

Balanced Flight:

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• In steady flight, the principles of flight demonstrate the relationship between forces acting upon an aircraft[ Figure 1]
• Simply stated: thrust equals drag and lift equals weight, but more appropriately stated:
  • The sum of all upward components of forces (not just lift) equals the sum of all downward components of forces (not just weight)
  • The sum of all forward components of forces (not just thrust) equals the sum of all backward components of forces (not just drag)
• This refinement addresses how any time the flight path of the aircraft is not horizontal, lift, weight, thrust, and drag vectors must break down into two components
  • Force vectors during a stabilized climb show thrust have an upward component [Figure 2]
  • In glides, a portion of the weight vector is along the forward flight path and, therefore, acts as thrust

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Static Stability:

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• Static stability is the initial tendency of the aircraft once disturbed
• Stability can be described as either positive, negative, or neutral [Figure 3]

  • Positive Static Stability:

    • Positive static stability is an aircraft's initial tendency to return to its original position once disturbed
    • If an airplane is in a turn and the controls are released, the aircraft neither rolls out nor gets steeper

  • Neutral Static Stability:

    • Tendency to remain at the new position
    • If an airplane is put into a turn and the controls are released, the aircraft remains in that turn but neither rolls out or gets steeper

  • Negative Static Stability:

    • Tendency to continue away from the original position
    • If an aircraft is rolled to a high bank angle, letting go of the controls results in the aircraft continuing to roll further

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Dynamic Stability:

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• Dynamic stability is the tendency of the aircraft over time
• An aircraft must have positive static to have dynamic stability [Figure 4]

  • Positive Dynamic Stability:

    • Positive dynamic stability is the tendency of an aircraft to dampen toward original position once disturbed

  • Neutral Dynamic Stability:

    • Neutral dynamic stability is the tendency of an aircraft to dampen back to its original position once disturbed to a new position

  • Negative Dynamic Stability:

    • Negative dynamic stability is the tendency of an aircraft to trend away from its original position once disturbed

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Longitudinal Stability:

• The longitudinal axis is an imaginary line running from the nose to the tail of the aircraft, motion about this axis is called "roll," controlled by the ailerons
• Longitudinal stability is the tendency of an aircraft to return to the trimmed angle of attack
• Accomplished through elevators and rudders
• Contributors:
  • Straight wings (negative)
  • Wing Sweep (positive)
  • Fuselage (negative)
  • Horizontal stabilizer (largest positive)
• An aerodynamic center aft of Center of Gravity (C.G.) is a stabilizing moment
• An aerodynamic center forward of C.G. is a de-stabilizing moment

Lateral Stability:

• The lateral axis is an imaginary line running from wing tip to wing tip; movement about this axis causes the nose of the aircraft to raise or lower and is caused by moving the elevators
• Lateral stability is the tendency of an aircraft to resist roll

• Dihedral Effect:

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  • Dihedral is evident when an aircraft rolls, creating a side-slip (assume no rudder)
  • One of the wings is lower than the other, creating an angle of attack difference for each wing
  • The lower wing has an increase in the angle of attack, which causes it to create more lift and therefore rise, while the opposite is true for the higher wing [Figure 5]
    • The net result is the aircraft rolling away from the side-slip, thus resisting roll and attempting to bring the wings back to level
  • Use of the rudder will smoothen the turn and overcome these forces as well as others, such as adverse yaw

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• Swept Wing Effect:

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  • Side-slips create more direct relative wind to the upwind swept wing, which creates a roll toward wings level [Figure 6]

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Vertical Stability:

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• The vertical axis is an imaginary line running from the top of the plane to the bottom of the plane
  • The rudder controls rotation about this axis and is called "yaw" [Figure 7]
• Tendency to resist yawing
• The more surface area behind the CG, the more directional stability

• Dutch Roll:

  • Coupling of the lateral and directional axes causes Dutch roll
  • Dutch roll is a combined yawing-rolling motion of the aircraft but may only be a nuisance unless allowed to progress to large bank angles
  • Large rolling and yawing motions can become dangerous unless properly damped
  • The side-slip disturbance will cause the aircraft to roll
  • The bank angle, in turn, causes a side-slip in the opposite direction
  • While not unstable, this continual trade-off of side-slip and angle of bank is uncomfortable
  • Dutch roll may be excited by rough air or by lateral-directional over-controlling
  • Once induced, normal aircraft stability dampens the effect
  • Poor Dutch roll characteristics may make the aircraft susceptible to pilot induced oscillations (PIO)
  • Lateral-directional PIO is most common when the pilot attempts to line up in the landing configuration

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Four Left Turning Tendencies:

1. Most general aviation engines rotate clockwise as the pilot would see it from the cockpit looking out the windscreen
2. The principles of p-factor, gyroscopic precession, torque and slipscreem result in a left-turning tendency in a clockwise rotating propeller
   • In those engines configured to rotate the propeller counter-clockwise, these principles become right-turning tendencies

3. P-factor:

   • Also referred to as asymmetric loading
   • P-factor is a complex interaction between aircraft, relative wind, and rotational relative wind
   • The descending blade has a higher AoA and therefore increased thrust

4. Gyroscopic Precession:

   • Gyroscopic precession is the force applied (which moves a propeller out of its plane of rotation) is felt 90° from that location, in the direction of rotation
   • Gyroscopic precession is more prevalent in tailwheel airplanes at lower airspeeds with high power settings (takeoff)
     • In fact, this force is considered a right-turning tendency in tricycle gear aircraft
   • In a tailwheel plane, on the take-off run, when the tail comes up, it will produce a left-turning tendency, as the top of the propeller is "pushed" forward and the bottom is "pulled" aft
   • When raising the nose for climb, precession will produce a force to the right
   • When lowering the nose for descent, precession will produce a force to the left
   • In the helicopter community, gyroscopic precession is also called Phase Lag

5. Torque:

   • Torque is the force generated when the clockwise rotation of the blade forces the aircraft to rotate counter-clockwise
   • It is greatest at low airspeeds with high power settings and a high angle of attack

6. Slipstream:

   • The corkscrew wind strikes the tail (rudder) on the left side

Maneuver vs. Controllability

• Maneuverability and controllability are conflicting ideas, and the designers must balance the two for the aircraft
• Nothing in aviation is free, and the price for higher lift is always higher drag

• Maneuverability:

  • Maneuverability permits you to maneuver the aircraft easily and allows aircraft to withstand stress
  • Dependent on:
    • Weight
    • Flight control system
    • Structural strength
    • Thrust

• Controllability:

  • Aircraft ability to respond to control inputs w/ regard to attitude and flight path

Adverse Yaw:

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• An imbalanced drag between the wings which causes a yaw moment on the aircraft, opposite the direction of turn is called adverse yaw [Figure 8]
  • Any time the ailerons move, adverse yaw occurs
• When the outboard aileron deflects downward, lift on the outboard wing increases, and lift on the inboard wing decreases, which causes the airplane to roll
  • In a turn to the right: the right aileron is up, and the left aileron is down
  • In a turn to the left: the left aileron is up, and the right aileron is down
• However, as a downward-deflected aileron is increasing the airfoil's lift, it is also increasing the drag
• When the inboard aileron deflects down, lift and drag are increasing (more so on the outboard wing)
  • This slows the outboard wing, and the rudder must be used in the direction of the turn to overcome the outboard wing's increased drag to keep that drag from holding the wing back
• With no rudder input, the nose will yaw outboard (to the outside of the turn) while rolling into the turn
• The turn coordinator ball indicates this yaw by sliding to the inside of the turn
  • We refer to this as a slip
• The rudder offsets the unequal drag of the wings created only when the ailerons deflect
• Unbalanced drag only exists while the ailerons deflect, and the airplane is in the act of rolling
  • This means that when the airplane is in a steady bank, the ailerons are neutral, so the lift and drag on the two wings are balanced
• That being the case, the rudder generally isn't needed while actually in the turn
• Also, since the airplane is in a steady-state condition (banked), generally, no aileron deflection is needed to maintain that condition
• The farther out the wings are (ailerons), the more of a moment this drag will have

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Aircraft Stability Knowledge Quiz:

Conclusion:

• By using the aerodynamic forces of thrust, drag, lift, and weight, pilots can fly a controlled, safe flight
• Why Adverse Yaw Matters:
  • When you turn, stall speed increases
  • If you're experiencing adverse yaw without having the correct amount of rudder in to counter, then you are uncoordinated
  • If you get slow, uncoordinated with a higher stall speed, then you can find yourself in a spin
• Considering only level flight, and normal climbs and glides in a steady-state, it is still true that lift provided by the wing or rotor is the primary upward force, and weight is the primary downward force
• Left turning tendencies are phenomena primarily affecting single-engine propeller aircraft
  • Although jet aircraft have various forces acting upon them during flight, you can consider these forces negligible
• Aircraft are more stable in right turns due to left-turning tendencies
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References:

• Federal Aviation Administration - Pilot/Controller Glossary
• Flight Training.aopa.org - Technique, The Basic Turn
• Instrument Flying Handbook (2-11) Coordination of Rudder and Aileron Controls
• Pilot Handbook of Aeronautical Knowledge