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#1
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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) |
#2
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(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 |
#3
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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 (Remove DONTSPAMME from address to email reply.) ___ Make a commitment to learn something from every flight. Share what you learn. |
#4
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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. |
#5
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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) |
#6
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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 (Remove DONTSPAMME from address to email reply.) ___ Make a commitment to learn something from every flight. Share what you learn. |
#7
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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. |
#8
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"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 |
#9
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"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.) ___ Make a commitment to learn something from every flight. Share what you learn. |
#10
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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 email: (put Cubdriver in subject line) The Warbird's Forum www.warbirdforum.com The Piper Cub Forum www.pipercubforum.com Viva Bush! blog www.vivabush.org |
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