dutch roll

[TAMATERAI]

A simple question;

Why does a wing which experiences dutch roll does not continue to roll (same side) but the effect reverse?

Thanks

;)

[Genghis the Engineer]

Simple answer, if it did, the aircraft wouldn't get certified. It is possible, I've once seen a prototype that bad, but the characteristic is considered unacceptable if it doesn't return through straight and damp out within 7 cycles maximum.

Do you want the complicated answer, or is that sufficient?

G

[Miserlou]

After a disturbance about the vertical axis (yaw) one wing is going slightly faster than the other and therefore creating slightly more lift. Slightly more lift results in slightly more drag which causes the wing to slow down and voila the effect is reversed.

[False Capture]

A simple answer;

If it didn't reverse it's direction then it wouldn't be called a 'Dutch Roll'. :ok:

[Alex Whittingham]

Just for fun, a little video clip of dutch roll we made for a website...

click to run (http://www.bristol.gs/download_files/Dutch roll.avi)

[Notso Fantastic]

That doesn't look much like Dutch Roll to me! There should be significant yawing motion- I'm not sure of the prominence sideflow has on the tailfin, but on the VC10 during training, the yaw damper used to get switched off at high altitude, and a control waggle would kick the Dutch Roll off, which would grow into a divergent oscillation and be quite violent. One form of horror show was to view the tailplane through the periscope that used to go out through either side of the fuselage roof at the back to view the engines. It was a ghastly sight to see the whole fin and tailplane waggling from side to side during a Dutch Roll.
The technique for ending the Dutch roll (without being boring and putting the yaw damper back on) was to give a violent aileron input as the wings rolled level).
*A bonus point to anyone who remembers this self same periscope could go out through the bottom of the fuselage electronics bay to inspect the undercarriage!

[Genghis the Engineer]

Looks exactly like DR to me. It's a combined rolling:yawing motion, but depending upon the characteristics of an individual aircraft type can be predominantly in roll, predominantly in yaw, or a balanced mix of the two.

I remember the VC10 periscope, spent much time looking through it once whilst trying to deal with the problem of a fluttering beavertail between two engines on one side.

G

[Alex Whittingham]

Because it was made for a website it's only a little animation, and the looping makes it a bit jerky. The VC10 had quite an exaggerated dutch roll, as did the Victor before it. Any 737 pilots care to comment?

[Old Smokey]

Dutch roll is characteristic of aircraft with stronger lateral stability than directional stability.

If the aircraft encounters a disturbance causing yaw (the pilot can do this by pushing a rudder pedal), the outer, faster travelling wing will rise due to lift increase, much more markedly so if a swept wing. There is no significant change in wing angle of attack, but as the yaw continues, the 'weak' fin encounters an increasing angle of attack, and returns the aircraft towards normal, but causing greater lift on the opposite wing, causing a reversal in roll direction as the aircraft commences a yaw in the opposite direction....until the fin encounters an increased angle of attack from the opposite direction, and returns to normal, overshoots into another yaw in the original direction, and so it goes on.

Some aircraft will self correct after a few oscillations, whilst others become more and more divergant to the point of structural failure.

As a historical note, I believe that the term "Dutch Roll" originated from a Boeing engineer's observation of the likeness between the Dutch girl's swivelling bottoms whilst skating, and the new unstable condition discovered on the B47.

[TAMATERAI]

Thanks for all these answers.... Great;)

[Genghis the Engineer]

Dutch roll is characteristic of aircraft with stronger lateral stability than directional stability
Negative. DR can (does!) occur in any FW aircraft; it is driven by both lateral and directional stabilities, but is most noticeable where there is poor damping in one or both axes.

The ratio of roll to yaw (best observed by sighting along the wingtip, which will describe an oval) is approximately the same as the ratio of lateral to directional stability.

I've never seen an aircraft that didn't DR, but not many when it was a problem. A little bit of mild DR in a light aircraft is nothing more than an irritant (I can think of one which does so constantly, albeit with small amplitude), any undamped DR in a fighter is unacceptable because it prevents you flying an accurate guns tracking task. In an airliner you'd want it reasonable well damped, otherwise it'll make the steerage passengers airsick, but is unlikely to cause a safety problem unless it's very bad.

G

[Romeo Tango Alpha]

I think what Old Smokey was saying is that it is more APPARANT in laterally "weak" aircraft. You sort of agreed at the same time by saying that it is a function of aircraft with poorer dampening in axes - ergo lateral "unstable" aircraft ARE more susceptible.

Yes, it can happen in ANY aircraft - nobody disputed that.

Dutch Roll in a Lear 45 is INTERESTING, yet fairly quickly recoverable, by riding the controls WITH the roll and yaw.

I believe the B-52 suffered VIOLENT Dutch Roll, mainly as a result of it's MASSIVE fin, and moderately high aspect ratio wing (which apparently also suffered tip flutter at higher Mach numbers). Boeing SHORTENED the vertical stab on the G model onwards, and the "problem" is apparently less severe. The earlier models with the tall fin had been known to suffer twist fatigue of the rear fuselage, and fin fatigue.