why is longitudinal stabilty about the lateral axis??

[fudgy2000]

why is longitudinal stabilty about the lateral axis and involves pitching. Why is lateral stability concerned with the longitudinal axis i.e rolling???

seems backwards to me??????:( Why is this???

Wouldnt it be easier to say longitudinal stability is concerned with the longitudinal axis, therefore rolling plane????

[Notso Fantastic]

Because you roll laterally around the longitudinal axis, and you pitch longitudinally about the lateral axis. The axis about which you move is at right angles to the direction of movement. If you move your arm out sideways, it moves about an axis fore/aft through your shoulder, whilst your arm moves sideways. The longitudinal axis of a plane will always go from nose to tail- the only way you can move around this axis is laterally, ie roll. How can you change this?

[*Lancer*]

Longitudinal (in)stability = the longitudinal axis moves. The pivot point is the lateral axis (which is fixed)

[Milt]

Lateral Axis.

Does anyone know how to determine where the lateral axis is situated? And I guess it moves around a bit with alpha.

Likewise the lateral centre of pressure.

I can only think of determining the lateral axis in a wind tunnel or using the extremely complex computer software used for aircraft design these days.

[John Farley]

Fudgy

The simple answer is as Notso and Lancer say.

Additionally, the longit one is EITHER aligned with the body of the aeroplane in which case the set of axes are termed the Body Axes OR aligned with the airflow approaching the aeroplane when the set are called the Wind Axes. In the case of the wind axes you have to know angle of attack and angle of sideslip to position them.

All axes go through the CG – which answers Milt’s concern

[lineboy_nz]

yeah... i always thought the 'plane only moves throught its center of gravity. also... using a 90 degree offset gives the most accurate reading of its pitch, yaw and roll. IMVHO.

[Milt]

John Farley

Aircraft Axes. You prompt me to become more specific.

To say that all axes go through the cg is somewhat simplistic for convenience. The definitions become easier. OK for simple theory and non manoeuvering flight.

Throw in manoeuvre and things get very complex. The actual lateral axis during vertical manoeuvres normal to the wings will move around under the influences of wing/body lift and tail power.Trying to determine what the vertical and longitudinal movements of the lateral axis might be must be an aerodynamicist's nightmare.Too many variables.

Consider the vertical/yawing axis seperately. In steady flight this axis has no relationship to cg whatsoever. The symmetry or asymmetry of wings level flight is determined by the balance between three collective forces; Thrust, Drag along the longitudinal axis and the sideslip forces controllable by rudder acting through the Lateral centre of pressure. Weight/Mass doesn't get a look in as you cannot resolve weight through 90 degrees. Mass only comes into play during manoeuvre when you have lateral or other accelerations. Did anyone ever determine your axis of yaw when you rotated a Harrier around a fixed point on the ground in nil wind. Can't think of a reason for wanting to know that axis except that to rotate around it would require minimum control variations to hold position.

Then consider the Longitudinal Axis seperately. It would be a big coincidence if the actual longitudinal axis passed through the cg particularly its vertical position. If it did there would be less to worry about from cross coupling during rapid rolling.

Consider the three axes collectively and you have a a whole host of interactions to be sorted out by natural stabilities and/or the flight computers.

Conclusion. All steady state actual axes are influenced by the shape of the aircraft, the airflow around the shape and the cumulative direction of thrust and a few other incidental forces. Manoeuvres introduce accelerations on the mass of an aircraft usually represented at the cg. This cg has a three dimensional position with two of these dimensions (vertical and lateral) being conveniently brushed aside by most aviators.

I will be delighted if you can pick holes in the above.

Should I give up on trying to hang on to g for acceleration and hence cg for centre of gravity? It seems to be a lost cause!

[ft]

Milt,
I for one would think twice before trying to argue aerodynamics or dynamics of flight with mr. Farley here. He's one of those people who have an annoying habit of spoling any and all discussions by actually knowing the facts. :)

An axis is not a physical thing. You can define an axis arbitrarily and have it far, far away from the CoG. This, however, is making things hard on yourself if you're interested in flight dynamics. Having all axes used be the rotational axes, through the CoG, makes things a lot easier*!

The definition of e g the yaw axis then will simply be the axis an aircraft will rotate around when you apply a yaw moment to the airframe.

That way, you can divide the forces acting upon an aircraft in flight into torque couples (moments) and forces. The former will rotate the aircraft about the CoG (i e the axes) and the latter will accelerate the CoG.

If the axes don't go through the CoG, you will have to consider acceleration of the CoG every time the attitude is changing... and you'll need to deal with torque about your origo to keep the attitude constant when the CoG is accelerating.

And no, with a CoG-fixed origo the axes don't move with alpha, beta or anything else, nor do they need to move relative to the airframe to achieve this stunning simplicity - provided the CoG doesn't move.

Mass does come into play even when you are only changing the attitude of the aircraft. Mass is what causes rotational inertia.

An interesting note on the actual axis to roll around is that some jet fighter FCS's change the roll axis when you go to guns mode. Rather than rolling around the CoG, the FCS makes the aircraft roll around the barrel of the gun. Neat trick, IMO.

Cheers,
Fred

*) When working with the dynamics, that is. The origin of the coordinate system used for component placement and so on in construction will probably very rarely be anywhere near the CoG - but that coordinate system is not the one we're talking about here, is it?