# Aircraft Stability

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I'm self teaching myself all 7 books of the PPL before going onto the ATPL stuff at oxford. Having a little difficuilty understanding how each axes - Longitudinal, lateral and vertical correspondes to a type of stability!?

For example, take the lateral axis how can that be longitudinal stability? Is there an easy way to picture all the axis and there stability type?

Thanks.

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Good source here:

http://www.flightlab.net/Flightlab.net/Home.html

under

and this one, which is a Classic already :-):  http://www.av8n.com/how/

But, regarding the axis, the lateral one is the imaginary line that crosses the aircraft from wing tip to wing tip going through the center of rotation, in pitch - the aircraft pitches up or down rotating around that axis, with positive values up, negative down ( by convention )

Then, the longitudinal axis is the symmetry axis along the aircraft fuselage, and the aircraft banks around it, left or right, right being positive ( by convention )

Finally, the vertical axis is orthogonal ( 90º) to the other two and crosses the fuselage through the Center of rotation upside-down, and the aircraft yaws around it, left or right, right being positive ( again by convention )....

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Just got that light bulb lol so you could say to help you imagine it, is, putting a stick through the longitudinal axis and twisting it at both ends and obseving the plane rotating at its axis?

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Nope, the stick would be the axis of rotation ( in bank in that case... )...

The texts and images in the links will certainly make it even more evident :-)

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Thats what i mean't, its the planes axis and in this case its lateral stibility (roll)

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Vernon, the axis give you only the rotation part of the "math"... Stability ( static and dynamic ) that's the next step you'll have to learn.

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Positive static stability

Neutral * *

Negative * *

Currently on that chapter now. Can you only get positive static with dynamic stability

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In general you have three types of static and dynamic stability:

Static: Stable, Indifferent or Unstable

Dynamic: heavily damped, damped, indifferent...

You usually have positive static stability associated with (positive) damped dynamic stability, but other combinations are possible.

But to be more precise, there are actually 6 types of dynamics response in stability:

- aperiodic stable

- aperiodic neutral

- aperiodic unstable

- periodic stable

- periodc neutral

- periodic unstable

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Hi.

Put simply...

Static stability is concerned with an aircraft's tendency to return to its former state after a brief disturbance like a strong gust.

Dynamic stability is concerned with the oscillations the plane will make after such a disturbance.

Positive, neutral and negative just mean 'returns to its previous state', 'stays in its new state' or 'gets worse'.

*** ***

The first of these sites shows the six combinations clearly without graphs or maths. The second is one part of an easily readable work on handling aircraft:

*** ***

Best regards,
Dave

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