[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-19]
Directional Stability (Yawing)
– about vertical axis
– turning to its original flight path.
Prime contributors of positive directional stability
- Area and location of the vertical fin
(Size and further aft of fin ∝ Directional Stability)
- sides of the fuselage aft of the CG
(Area aft of CG ∝ Directional Stability)
Generally speaking, the larger the size of fin, the greater the aircraft’s directional stability and the greater the area aft of CG, the more positive directional stability.
The above two primary factors make the aircraft act like the “weather vane”
Weather vane – means the nose tends to point into the relative wind
For example, if the aircraft has a slight rotation about its vertical axis to the right, the air is striking the left side of the fin at an angle. This causes pressure on the left side of the fin, which resists the turning motion and slows down the aircraft’s yaw. However, it is interestingly that the restoring tendency might be relatively slow in developing and ceases when the aircraft stops skidding – the nose of the aircraft will not return to the original heading on its own accord – pilot must reestablish the initial heading.
Other factor (minor improvement) for directional stability is the sweepback design. When aircraft yaw to one side, the airspeed of the forward wing increases and it acquires more drag than the back wing. The additional drag on the forward wing pulls the wing back and so the nose of the aircraft turning back to its original path.
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