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DESIGN CHARACTERISTICS




Axes of an Aircraft

There are three axes of an aircraft, all three of axes pass through the center of gravity (CG) section of the aircraft and are 90 degrees to each other. These axes are:

  • Lateral axis (pitching)

  • Longitudinal axis (rolling)

  • Vertical axis (yawing) - is the left and right movement of the aircrafts nose.




Stability

Stability is the ability of an aircraft to correct itself from conditions that disturb its original path, such as turbulence, flight control inputs. Training aircraft, such as Cessna or Piper, etc. are built to be stab, whereas fight jet aircraft F-16, F-18, F-35, etc. are built to be unstable so they can be more maneuverable. There are two types of stability:

  • Static

  • Dynamic


Static Stability - refers to the aircraft’s initial response when disturbed from a given pitch, yaw, or bank.

  • Positive - the initial tendency of the aircraft to return to the original state after being disturbed.

  • Neutral - the initial tendency of the aircraft to remain in a new condition after its has been disturbed.

  • Negative - the initial tendency of the aircraft to continue away from the original state after being disturbed.


Dynamic stability - is how the aircraft response over time. just like Static stability, Dynamic also has three types of stability.

  • Positive - over time, the motion of the displaced object decreases in amplitude and, because it is positive, the object displaced returns toward the equilibrium state.

  • Neutral - once displaced, the displaced object neither decreases nor increases in amplitude. A worn automobile shock absorber exhibits this tendency.

  • Negative - over time, the motion of the displaced object increases and becomes more divergent.



Longitudinal and Lateral Stability


Longitudinal Stability - or pitching movement of the aircraft nose up and down. If the aircrafts longitudinal stable is unstable the aircraft could pitch up into a steep climb and stall or dive to steep. These can make an aircraft very dangerous to fly.


Lateral Stability - or rolling effect extends from the aircrafts nose to its tail. there are four types of design factors that make up lateral stability. We will only go over to important ones.

  • Dihedral

  • Sweepback


The dihedral is the upward angle of the wings, or where the outer tip of the wing is higher than that of the wing roots. When flying an aircraft with this wing design the pilot must be aware of the wing design and how the wind affects it.


When a gust of wind causes the airplane to roll, the dihedral design causes the airplane to sideslip. If the relative wind is from the side of the aircraft, the wing slipping into the relative will have an increase in the angle of attack (AOA), creating more lift and causing the aircraft to roll.



Sweepback wings are an advantage to aircraft. If an aircraft encounters a disturbance in flight, such as turbulence and one of the sweepback wings drops or slips, that wings leading edge becomes more perpendicular to the relative wind. As a result, the low wing acquires more lift, and the aircraft returns to its original flight attitude.



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