Dutch Roll

So what is your understanding of a Dutch Roll? It is interesting to me
the various understandings of this manuever, how it is performed and
its origins.
I've got my own that I was taught by a CFI over 45 years ago and have
taught to virtually every student I've had over the years, including
helicopters.
Ol Shy & Bashful CFI - Airplanes and Rotorcraft Helicopters,
Instrument Rotorcraft and Airplanes, Multiengine, Gold Seal (1967)

(SelwayKid) wrote

So what is your understanding of a Dutch Roll? It is interesting to me
the various understandings of this manuever, how it is performed and
its origins.
I've got my own that I was taught by a CFI over 45 years ago and have
taught to virtually every student I've had over the years, including
helicopters.

Well....a Dutch Roll is probably not what you understand it to be,
particularly if you have not flown swept-wing transport aircraft.
The aileron/rudder drill sometimes taught to student pilots is not
a Dutch Roll.

Copied from the following web site:

http://142.26.194.131/aerodynamics1/...ity/Page5.html

Dutch Roll
Many swept wing aircraft suffer a dynamic instability problem known as
Dutch Roll.

Dutch roll happens when the aircraft has relatively strong static
lateral stability (usually due to the swept wings) and somewhat weak
directional stability (relatively.) In a Dutch roll the aircraft begins
to yaw due to a gust or other input. The yaw is slow damping out so the
aircraft begins to roll before the yaw is stopped (due to the increased
speed of the advancing wing and the increased lift due to the swept wing
effect.)

By the time the yaw stops and begins to swing back toward zero slip the
aircraft has developed a considerable roll rate and due to momentum plus
the slip angle the aircraft continues to roll even once the nose has
begun returning to the original slip angle.

Eventually the yaw overshoots the zero slip angle causing the wings to
begin rolling back in the opposite direction.

The whole procedure repeats, sometimes with large motions, sometimes
with just a small churning motion. Like all dynamic stability problems,
Dutch roll is much worse at high altitudes where the air is less dense.

Dutch roll is almost certain to happen in a jet aircraft if the Yaw
dampener is turned off at high altitude. Therefore, the first thing to
check if an aircraft begins to exhibit Dutch roll is that the Yaw
Dampener is on. The pilot should then try to minimize the yawing
oscillations by blocking the rudder pedals (i.e. hold the rudder pedals
in the neutral position.) Next apply aileron (spoiler) control opposite
to the roll. The best technique to use is short jabs of ailerons applied
opposite to the roll. Try to give one quick jab on each cycle (i.e. turn
the wheel toward the rising wing, then return it to neutral.) Finally
accelerate to a higher speed, where directional stability will be
better, or descend into more dense air, for the same reason.

Bob Moore
ATP B-707 B-727
FI Airplanes/Instruments

Bob Moore wrote:

Well....a Dutch Roll is probably not what you understand it to be,
particularly if you have not flown swept-wing transport aircraft.

The term is correctly used for both the intentional flight
maneuver that you think he's thinking of (alternating motion
on the roll axis without corresponding yaw) and the dynamic
instability problem you describe below. It's also properly
used for the motion of a ship with a following sea.


Todd Pattist
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The term is correctly used

Can you specify on what basis you determine that the term is used
"correctly"?

It's true that the piloting community uses "Dutch Roll" in the way the
OP did, but the piloting community uses lots of words with implied
meanings at odds with the aerodynamic literature. This usage
interferes with their ability to gain a more sophisticated
understanding later on, due, I suppose, to the law of primacy.

I agree with Bob Moore (and Bill Kershner, et al) that Dutch Roll
shouldn't be used in this context.

Dutch roll is roll due to yaw in a swept wing aircraft. It is more
pronounced than that experienced in a relatively straight wing aircraft.
Also, since swept wing aircraft are more frequently flown at high altitudes
where the air is thinner and mach transitions can occur it is generally
considered only a swept wing phenomenon. see
http://142.26.194.131/aerodynamics1/...ity/Page5.html

--

B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"SelwayKid" wrote in message
m...
So what is your understanding of a Dutch Roll? It is interesting to me
the various understandings of this manuever, how it is performed and
its origins.
I've got my own that I was taught by a CFI over 45 years ago and have
taught to virtually every student I've had over the years, including
helicopters.
Ol Shy & Bashful CFI - Airplanes and Rotorcraft Helicopters,
Instrument Rotorcraft and Airplanes, Multiengine, Gold Seal (1967)

Greg Esres wrote:

Can you specify on what basis you determine that the term is used
"correctly"?

Simply that it seems to be successfully used for both
meanings without any real confusion I can detect. CFI's
suggesting coordination exercises are using the OP's meaning
and aerodynamicists are talking about the dynamic
instability problem.

I suspect the OP had never heard the aerodynamic definition,
and the first reply he got assumed the same thing, so in
this group, it's likely to have the coordination exercise
definition. If you think it's "incorrect" while I think
it's "correct" that's interesting, but I suspect it's too
late to stop it from being used the way it is currently
being used, even if we wanted to.

Language has a tendency to go its own way and get labeled as
correct or incorrect after the fact :-)

BTW, do you think the coordination exercise label was
initially adopted by someone who'd heard of the aerodynamic
usage, but didn't understand it, or do you think they
developed independently?

It's true that the piloting community uses "Dutch Roll" in the way the
OP did, but the piloting community uses lots of words with implied
meanings at odds with the aerodynamic literature. This usage
interferes with their ability to gain a more sophisticated
understanding later on, due, I suppose, to the law of primacy.

I didn't have any problem when I first learned of another
usage. I'd already learned the nautical usage, so the two
aviation usages were just added into the mix. I believe int
h law of primacy, but we're talking intellectual stuff here,
not how to react when your wing falls off :-)


I agree with Bob Moore (and Bill Kershner, et al) that Dutch Roll
shouldn't be used in this context.

Todd Pattist
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If you think it's "incorrect" while I think
it's "correct" that's interesting,

No, no, just "non-standard". "Correct" is a fuzzy term. ;-)

BTW, do you think the coordination exercise label was
initially adopted by someone who'd heard of the aerodynamic
usage, but didn't understand it, or do you think they
developed independently?

Hmmm....interesting question. Seems low probability they'd develop
independently. The originator of the term might well have understood
the stability use of the term, but since the motions bear a
resemblence to each other, might have used it anyway.

Actually, I had never heard of the term in a nautical sense (I'm not a
boater.) Given that boating is older than aviating, seems more likely
that the term was originally use to describe a motion without regard
to its origin. (Or is the boating use a stability issue too?)

But now that it exists as a stability issue in aviation, it seems
prudent to use it only in that sense. I can't tell you how many hours
of confusion it has caused me in the past when the author of an
aerodynamics text used a word carelessly and sent me along dead-end
trails.

Possibly an analogy is the description of the 4 left-turning
tendencies of an airplane. Collectively they're often called
"torque", but only one is caused by the torque of the engine, and is
also called torque. So is calling p-factor "torque" incorrect? Not,
I suppose in the sense that it creates torque around the vertical
axis, and the engine torque creates torque around the longitudinal
axis. But this generated ambiguity in the word "torque" makes it a
bit confusing to talk about the subject.

"Todd Pattist" wrote in message
...

Language has a tendency to go its own way and get labeled as
correct or incorrect after the fact :-)

I spent a while writing the physics bit of the New Penguin Dictionary of
Science. The hardest part was knowing whether to be prescriptive (tell them
what the usage *should* be) or descriptive (describe what the common usage
*is*). It's a judgement call in almost every case -- for example, I had no
qualms about defining "weight" quite carefully to distinguish it from
"mass", even though many people say "weight" when they mean "mass". But
should I really make a fuss about the difference between "spectrograph" and
"spectrometer" when everyone uses the terms interchangeably?

I guess "Dutch Roll" is pretty close to the line.

Julian

"Julian Scarfe" wrote:

I guess "Dutch Roll" is pretty close to the line.

I encountered the term first in sailing. Then I ran into
the term used for the aviation coordination exercise and I
have to admit that it seemed odd as the motions weren't
really all that similar. Then I ran into the technical
aerodynamic usage, which describes a motion more similar to
the nautical motion. I suppose if I'd encountered the
technical aviation usage first, I might have felt the
coordination usage was "wrong," but I've never looked at it
like that. I've always seen it as the same adopted name for
two distinct things.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
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Make a commitment to learn something from every flight.
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It's also properly
used for the motion of a ship with a following sea.

Actually, a ship with a following sea pitches up and down, and in the
worst case is pooped.

A ship with a sea off the stern wallows, but is still pitching. It's a
corkscrew motion, say rolling to port while diving down, then rolling
to starboard while climbing up. Very sick-making.

I never though of either motion as a Dutch roll, and it is not really
similar to Dutch roll in an aircraft.

all the best -- Dan Ford
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