Dutch roll and swept wings

[saxman66]

I'm having trouble understanding dutch roll and what causes it and how you would get out of it. Anyone have any simple explanations?

Also wondering about swept wing aerodynamics. Why are they better, and why aren't they?

[tgrayson]

Originally Posted by saxman66

I'm having trouble understanding dutch roll and what causes it and how you would get out of it. Anyone have any simple explanations?

Also wondering about swept wing aerodynamics. Why are they better, and why aren't they?

Dutch roll is a battle between the lateral and directional stability of the aircraft. Lateral stability is the tendency of an aircraft to roll wings level if it gets into a bank. Directional stability is the tendency of an aircraft to weathervane into the relative wind.

Both of these types of stability depend on a side slip to function. Lateral stability, for instance, is classically provided by wing dihedral. When the aircraft side slips in one direction, the dihedral tends to raise the low wing. Directional stability, which depends on the vertical stabilizer, wants to weathervane into the wind. Both effects want to eliminate side slip, but do it in different ways.

Normally, directional stability is stronger, and when a side slip develops, the airplane weathervanes and it’s over. When lateral stability is unusually high, though, it asserts itself a bit too strongly and will do so out of phase with the directional stability. In other words, they correct in different directions.

For instance, let’s say I push the left rudder to the floor. Just prior to releasing it, the aircraft is in a side slip to the right. The aircraft wants to yaw right, but it wants to roll left. If I release the rudder, it will do just that. In most straight wing airplanes, it will sort itself out with only small oscillations, but in airplanes with high lateral stability, the oscillations can go on for quite a while. They might possibly get worse.

How do you get out of it? Turn on the yaw damper. ;-)

The reason that swept wing airplanes are prone to this and the reason that swept wings exist at all have similar origins.

The most common way to think about this is that the true airfoil of a wing is formed via a line perpendicular to the leading edge to the trailing edge. Normally, the airflow does not flow along this line this because the airflow comes from straight ahead of the airplane. The airflow is effectively coming at the airfoil at an angle, so the airfoil only sees a portion of the actual velocity of the relative wind. This has the effect of reducing the maximum velocity over the top of the wing, delaying critical mach. For instance, if critical mach of a straight wing were .7, then a 30 degree wing sweep would delay it until .7/cos(30) = .81. Pretty significant!

A side effect of this is that when the aircraft is in a side slip, the airfoil is suddenly seeing the entire velocity of the relative wind and lift is increased. This tends to roll the airplane out of the side slip.

[bLizZuE]

yea that