Aircraft Stability

Balanced Flight:

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• In steady flight, the sum of these opposing forces is always zero (remember Newton's third law)
• Balanced flight does not mean the four forces are equal to one another but rather that the opposing forces are equal to, and thereby cancel, the effects of each other [Figure 1]
• Simply stated: thrust equals drag and lift equals weight:
• 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 each be broken down into two components
  • Force vectors during a stabilized climb shows thrust has an upward component [Figure 2]
  • In glides, a portion of the weight vector is directed along the forward flight path and, therefore, acts as thrust

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Static Stability:

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

  • Positive Static Stability:

    • Tendency to return to original position
    • If an airplane yaws or skids, the sudden rush of air against the fuselage and control surfaces quickly forces the airplane back to its original direction

  • Neutral Static Stability:

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

  • Negative Static Stability:

    • Tendency to continue away from 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

Dynamic Stability:

• 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 new position

  • Negative Dynamic Stability:

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

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, and it is 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)
• Aerodynamic center aft of C.G. is a stabilizing moment
• 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:

  • Dihedral is evident when an aircraft rolls, creating a side-slip (assume no rudder)
  • One of the wings is lower than the other and this creates a difference in the angle of attack experienced by each wing
  • The lower wing has an increase in angle of attack which causes it to create more lift and therefore rise while the opposite is true for the higher wing [Figure 7]
    • 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

• Swept Wing Effect:

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

Vertical Stability:

• The vertical axis is an imaginary line running from the top of the plane to the bottom of the plane, rotation about this axis is called "yaw" and is controlled by the rudder
• Tendency to resist yawing
• Yawing moment
• Accomplished through rudders

Directional Stability:

• Stability around the vertical axis
• Vertical tail accomplishes this
• You must have more surface area behind the CG than in front of it

• Dutch Roll:

  • Coupling of the lateral and directional axes causes Dutch roll
  • Dutch roll is a combined yawing-rolling motion of the aircraft and may be considered only a nuisance unless allowed to progress to large bank angles
  • Large rolling and yawing motions can become dangerous unless properly damped
  • Side-slip disturbance will cause the aircraft to roll
  • The bank angle, in turn, causes 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, it is damped by normal aircraft stability
  • Poor Dutch roll characteristics may make the aircraft susceptible to pilot induced oscillations (PIO)
  • Lateral-directional PIO is most common when the pilot chases line-up in the landing configuration

Four Left Turning Tendencies:

1. 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

2. 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
   • In a tail-wheel 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 the nose is raised for climb it will produce a force to the right
   • When the nose is lowered for a descent, it will produce a force to the left
   • In the helicopter community, gyroscopic precession is also called Phase Lag

3. 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

4. Slipstream:

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

Maneuver vs. Controllability

• Controllability and Maneuverability are conflicting ideas and the two must be balanced by the designers for the purpose of the aircraft
• Nothing in aviation is free and the price for higher lift is always higher drag

• Maneuverability:

  • Permits you to maneuver the plane 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

• Adverse yaw is caused by imbalanced drag between the wings which causes a yaw moment on the aircraft, opposite the direction of turn
  • Any time the ailerons are used, adverse yaw is produced
• When the outboard aileron is deflected down, 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 aileron is deflected 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 ball indicates this yaw by sliding to the inside of the turn which we refer to as a slip
• The rudder is used to offset the unequal drag of the wings that is created only when the ailerons are deflected
• Unbalanced drag only exists while the ailerons are deflected and the airplane is in the act of rolling
• What that also says is that when the airplane is in a steady bank, as when established in a turn, the ailerons are neutral so the lift on the two wings is balanced
  • The drag is also 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

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
In point of fact, 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

• Turning tendencies:

  • Left turning tendencies are a phenomena primarily effecting single engine propeller aircraft
  • Although jet aircraft have various forces acting upon it during flight, you can consider these forces negligible
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References:

• Pilot Handbook of Aeronautical Knowledge
• 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