theory of flight airplane axes

Sep 2012Lesson 2.3

Theory of Flight

Airplane Axes

Reference

From the Ground UpChapters 2.1.3, 2.1.4:The Axes of an Airplane, StabilityPages 23 – 26

Introduction• There are three axes around which an

airplane moves. Each run through the C of G, are controlled by the pilot, and are stabilized by features in the aircraft.

Outline• Airplane Axes• Balanced Controls• Stability

Airplane Axes

Longitudinal Axis(nose to tail)

Vertical (or Normal) Axis(vertical through C of G)

Lateral Axis(wing tip to wing tip)

Longitudinal Axis

Movement around: RollControlled by: AileronsAttitude: Left/Right Bank

Lateral Axis

Movement around: PitchControlled by: ElevatorAttitude: Nose Up/Down

Vertical Axis

Movement around: YawControlled by: RudderAttitude: Nose Left/Right

Balanced Controls• Control surfaces sometimes balanced to help

pilot move them and reduce flutter

• Inset Hinge/Horn Balance– Part of control surface in front of hinge– Airflow helps pilot to move it

• Mass Balance– Streamlined mass in front of control surface hinge– Reduces flutter

Balanced Controls

Horn Balance Mass Balance

Stability• Stability

– Tendency of an airplane in flight to remain in straight, level, upright flight and to return to this attitude, if displaced, without corrective action by pilot

• Inherent Stability– Stability from design features of an aircraft– Affected by weight and C of G

Stability

Straight flight disturbedby upward gust of wind

Static StabilityInitial tendency to return to original position

Dynamic StabilityOverall tendency to return tooriginal position, after seriesof oscillations

Stability

Straight flight disturbedby upward gust of wind

Positive StabilityAircraft returns to original position

Neutral StabilityAircraft continues in disturbed position

Negative Stability (AKA Instability)Aircraft moves further away fromdisturbed position

Longitudinal Stability= Pitch Stability = Stability around Lateral Axis = Corrects unwanted pitch

Horizontal StabilizerAirflow hitting stabilizer pushes aircraft back to original position if disturbed

Longitudinal StabilityCenter of GravityC of G affects nose up/down tendenciesif too far rear/forward of C of P

* Airplane must never be tail-heavy, orunable to recover from stalls

Lateral Stability• Lateral Stability = Roll Stability = Stability around Longitudinal Axis

• Dihedral– Angle that each wing makes with horizontal– If wing is displaced downwards, airplane slips into that direction

causing more airflow to down going wing and lifting it

• Keel Effect– High-wing aircraft have weight below wings acting as pendulum to

return aircraft to original position if wing displaced

• Sweepback– If aircraft is disturbed and a wing dips, lower frontward wing is

exposed to more airflow– This creates more lift in frontward wing, thus picking it up

Lateral Stability

Keel Effect:

Directional Stability= Stability around vertical/normal axis = Corrects unwanted yaw

Vertical StabilizerAirflow hitting vertical stabilizer pushes aircraft back to straight flight when disturbed

Directional StabilitySweepbackAirflow hitting one wing is more perpendicular to relative airflow, creating more drag and pushing it back

Next Lesson

2.4 - Theory of FlightFlight Performance

From the Ground UpChapters 2.1.5, 2.1.6, 2.1.7:Flight Performance Factors, AirspeedLimitations, Mach NumberPages 26 - 33