Feb. 22, 2019, ©. Leeham News: After discussing an airliner’s pitch stability we now turn to the yaw stability. A stable aircraft in yaw means we don’t want the aircraft to wag its tail sideways while flying.
The airliner shall fly straight ahead during take-off, climb, cruise, descent and landing, even when we have problems with an engine and must throttle it back or shut it down on one side.
An aircraft should fly straight ahead regardless of disturbances like side wind, uneven thrust from the engines or when one engine has problems and goes inoperative ( OEI, One Engine Inoperative).
To achieve this stability we must have larger and more effective vertical surfaces behind the center of gravity than ahead of it, Figure 1.
A side wind or a sideslip from the aircraft has the air flowing around the nose, engines and the fuselage (over the top and under the belly). Any smoothly curved surface, be it the nose, fuselage barrel or the engine nacelles, will create a side force on the leeward side from such an angled flow curving around it.
The airflow is then hitting the aircraft with an Angle of Yaw and just like the Angle of Attack creates a suction peak just behind the cockpit of the aircraft in the pitch axis (Figure 2), it will create a suction peak on the leeward side of the forward fuselage. The same happens with the engines.
An aircraft designer, therefore, uses a large and efficient vertical surface to make sure these forces are counterbalanced aft of the center of gravity (the point around which the aircraft moves in flight).
A large vertical tail creates drag and it adds weight. The vertical tail size is, therefore, the minimum size which gives enough “weathercock” stability to the aircraft in all the side force scenarios the aircraft can encounter.
The side force needed is a moment about the center of gravity. Therefore both the size of the vertical tail and its distance from the center of gravity is important. This is also why the shortest member of an aircraft family is the most critical when it comes to yaw stability.
The Airbus A330 is a good example where the shorter A330-200 has a 1m higher vertical tail than the longer A330-300. The short Boeing 747 SP is another example where the vertical tail grew substantially as the aircraft was shortened.
In the next Corner, we will dig deeper into airliner Yaw stability.