Effect of CG on Dutch Roll

[JammedStab]

Does the CG of an aircraft have an effect on tendency to Dutch roll and if so what CG is worse?

[411A]

Does the CG of an aircraft have an effect on tendency to Dutch roll and if so what CG is worse?

No, generally not.
However, if an aircraft has a tendency to dutch roll due to its design deficiency, aft CG can make it somewhat worse.
This can be confirmed by flying a type that did dutch roll (sometimes, divergent), the early models of the Boeing 707.
Take the long body early intercontinental model, the -320 straightpipe, as a good example.
A dutch rolling son of a gun, if you got it going.
However, with the center tank full (CG forward), more stable.
Center tank empty (but with wing tanks nearly full), aft CG prevailed, and the dutch roll became more severe.

Now, I fully expect that some will disagree with my statements, however, I flew these old airplanes, so I do know many of their undesirable tendencies....first hand, make no mistake.

[Jaxon]

I think it cannot be any other way, forward CG is more stable. More leverage for the forces from the tail to hold sway (pardon the pun).

[BOAC]

You may fair better on the Flight Testing forum, JS?, where you should get an expert opinion, but as a 'ham' I would go with 'Jaxon', since I recall DR occurs when lateral stabitily exceeds directional, so anything that enhances the latter (eg forward cofg) will reduce DR?

[Tester07]

No, generally not.

aft CG prevailed, and the dutch roll became more severe

So that would be a 'yes' to the original question, then, surely?

I think many aircraft will exhibit a tendency to Dutch Roll if you reduce directional stability sufficiently, which often effectively occurs on final approach at low airspeed, or alternatively if you moved the CG aft.

[411A]

So that would be a 'yes' to the original question, then, surely?

No, not in my opinion/experience.
The aft CG position only aggravates the situation, it is the original design deficiency of the specific type that is the problem.

Is that better, BOAC?

[BOAC]

Wrong forum, 411A?

[MilFlyBoy]

Directional Stability and Dutch Roll

Aft C of G reduces directional stability. Dutch Roll is worsened when this occurs as the tail restoring moment is reduced. Dutch roll is significant when directional stability is low compared to lateral stability.

At higher weights in the 707-320B Dutch roll is worsened as the Moment of inertia about the yaw axis is also increased. In the case of the centre tank on the 707 the Moment of inertia effect is more than the Forward C of G effect. Also the Centre of lift is moved forward at High AoA(due weight) and so the drag effects due to subsequent yaw are worsened.

[FAAFTP]

Dutch roll

The vertical tail provides the damping for dutch roll (that is why we put the yaw damper on the vertical tail to eliminate the DR). The longer the "arm" is to vertical tail from the cg, the more effective it will be. Hence, if the cg is forward, the moment arm to the vertical tail is the greatest providing the largest restoring moment.

In addition, the "fueslage tube" that is forward of the cg is destabilizing (e.g. doesn't help the aircraft turn back into the wind) whereas that part of the fuselage that is aft of the cg is stabilizing. Thus, moving the cg forward allows more of the fuselage to contribute to overall stability.

This being said, as the aircraft is yawing, the amount of wing sweep and dihedral in the wing will greatly affect the phi to beta ratio (e.g. the amount of rolling versus yawing motion encountered) and the overall behavior of the DR as well.