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Axes of Stabillity

Asked by: 2314 views Aerodynamics

How do elevators provide longitudinal stability, while ailerons provide lateral stability? Rudders provide vertical stability, which makes sense, since they control movement around the vertical axis. The other two seem to be flipped. If stability is determined by which control surfaces prevent rotation that isn't the axis of rotation, then why can't the rudder replace the elevators for longitudinal stability, for example? What is an easy way to think of stability?

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1 Answers



  1. Skyfox on Feb 03, 2017

    Stability is how an object (in this case, an aircraft) reacts to a change in its position. That’s divided up into static and dynamic stability. Positive static stability means it will try to return to its original position; neutral static stability means it will hold that position after it’s disturbed; negative static stability means it will continue moving in the direction of disturbance far beyond what the initial input of that disturbance was. Dynamic stability refers to what that disturbance does over time and only comes into play when there is positive static stability. Positive dynamic stability means the disturbance will dampen down to nothing over time. Neutral dynamic stability means the disturbance will remain the same and the aircraft will bob back and forth at the same amount. Negative dynamic stability means the disturbance will grow worse over time which can make an aircraft rip itself apart.

    The longitudinal axis runs from the tip of the nose to the tip of the tail, and longitudinal stability refers to that line pitching up and down. The lateral axis runs from wingtip to wingtip, and lateral stability refers to that line rolling one way or the other. While the rudder does affect both of those axes in a horizontal way, the vertical axis is likewise affected by both the ailerons and elevator in terms of tilt while the rudder affects rotation around that axis.

    Anyway, there’s a difference between what the name of something is and what the thing actually does. A rudder can’t control the pitch or roll and so can’t maintain stability around those axes. However, some aircraft have a V-tail which has only two control surfaces that serve the functions of both the elevator and rudder. From what I understand about it, that configuration is mechanically complicated but has the benefit of cutting down on some of the drag that a third control surface would generate.

    I agree, it does seem to be flipped since vertical stability refers to rotation around that axis while longitudinal and lateral stability refer to rotation around the opposite axis and not the axis each is named after. That’s just how it is.

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