What's wrong with a dutch roll in a passenger jet?

bflynn

bflynn

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Brian Flynn

First, I understand the aerodynamics of a delta wing in a yaw. The leading wing is more directly pointed into the airstream so it gets more lift. The trailing wing is less perpendicular to the airstream, so it gets less lift. But isn't this also a way (the way?) to initiate a turn, by using a little yaw to induce a little roll?

But why would dutch rolls require an airframe to be grounded for inspection? I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
 
bflynn said:

First, I understand the aerodynamics of a delta wing in a yaw. The leading wing is more directly pointed into the airstream so it gets more lift. The trailing wing is less perpendicular to the airstream, so it gets less lift. But isn't this also a way (the way?) to initiate a turn, by using a little yaw to induce a little roll?

But why would dutch rolls require an airframe to be grounded for inspection? I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
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No, you use the ailerons to bank the plane the way you want to turn. The increased lift on the outside of the turn wing causes more induced drag which pulls the nose away from the turn, so you use rudder to bring the nose back to the turn. I find it works best for me if I lead with the rudder, but it isn't to induce a roll.
 
Prolly more of a flight control computer thing, cause it ain’t supposed to happen.

Jets actually go divergent pretty quick at altitude with out a yaw damper…. And that’s B A D.
 
bflynn said:
But why would dutch rolls require an airframe to be grounded for inspection? I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
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The coordination training maneuver commonly referred to as a dutch roll (where you roll back and forth to figure out the magnitude/phasing of the rudder so that the nose doesn't appear to move) isn't a "real" dutch roll.
 
bflynn said:
I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
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You weren't taught dutch rolls. Dutch rolls are a form of negative dynamic stability, and occur when yaw and roll stability negatively interact with each other. It is why swept wing jets have a yaw damper and the yaw damper can't be MEL'd on most of them.
 
I’ve never understood the rolling on a heading thing. How is it not just a slip? And how can that possibly help you learn to turn coordinated, when it’s the very definition of uncoordinated. I mean, yeah for a second you need to be coordinated, but if you hold that bank, you have to go uncoordinated to stay on an heading. Obviously I’m missing something.
 
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Salty said:
I’ve never understood the rolling on a heading thing. How is it not just a slip? And how can that possibly help you learn to turn coordinated, when it’s the very definition of uncoordinated. I mean, yeah for a second you need to be coordinated, but if you hold that bank, you have to go uncoordinated to stay on an heading. Obviously I’m missing something.
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Because if you aren't coordinated the nose will yaw away from the direction of roll. It's called adverse yaw, and will result in a slip. Practice it in slow flight and it will be more obvious.
 
dmspilot said:
Because if you aren't coordinated the nose will yaw away from the direction of roll. It's called adverse yaw, and will result in a slip. Practice it in slow flight and it will be more obvious.
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I know that.
 
Salty said:
I’ve never understood the rolling on a heading thing. How is it not just a slip? And how can that possibly help you learn to turn coordinated, when it’s the very definition of uncoordinated. I mean, yeah for a second you need to be coordinated, but if you hold that bank, you have to go uncoordinated to stay on an heading. Obviously I’m missing something.
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Take me up in the Yak and I’ll show you. :)
 
Salty said:
I’ve never understood the rolling on a heading thing. How is it not just a slip? And how can that possibly help you learn to turn coordinated, when it’s the very definition of uncoordinated. I mean, yeah for a second you need to be coordinated, but if you hold that bank, you have to go uncoordinated to stay on an heading. Obviously I’m missing something.
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You're doing it wrong. You don't hold bank. When you reach desired bank angle, you immediately roll in the opposite direction.
 
I remember doing this incorrectly a student. Instructor said to roll but don't let the nose move. That's easy! Left aileron and right rudder! :idea:

Later, after I got my ticket, I was flying with a different instructor who pointed out, "oh, you're just slipping... my flight controls" and demonstrated it correctly.

Rolling in/out of steep turns is another opportunity to practice this.
 
GeorgeC said:
I remember doing this incorrectly a student. Instructor said to roll but don't let the nose move. That's easy! Left aileron and right rudder! :idea:

Later, after I got my ticket, I was flying with a different instructor who pointed out, "oh, you're just slipping... my flight controls" and demonstrated it correctly.

Rolling in/out of steep turns is another opportunity to practice this.
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I do dutch rolls every time I fly acro, which is 2-3x per week. En route to the practice area, I do 1 minute of rolls as a warmup. I have a dot of tape on my windshield bracket, directly in line with my eyes. I put that on a cloud or object on the horizon and try to keep it stationary as I roll back and forth 45 to 45.
 
Because the yaw damp system is malfunctioning or not working and there is no deferral in the MEL.
 
GeorgeC said:
The coordination training maneuver commonly referred to as a dutch roll (where you roll back and forth to figure out the magnitude/phasing of the rudder so that the nose doesn't appear to move) isn't a "real" dutch roll.
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I was gonna chime in with that without reading on first but read on. That thing where ya pick a bug on the windshield and keep it right there using rudder when ya roll back and forth was an eye opener to me, even though the CFI who did it used the wrong terminology.
 
Many years ago, our flight delayed boarding, and the 2 pilots departed the holding area. I followed, and asked what the problem was. "Failed yaw damper, we have 2, and both have to be checked good, or no departure. We lost one enroute, but it is legal to continue short distances with one. Lose both, land at the nearest field large enough for a safe landing, as the plane is very difficult to control without the damper".

I was happy to kill an hour or so as the technicians made the required repair.

Presumably, the airliner we are discussing lost all its yaw dampers, and started Dutch rolls, but was caught by the pilots, and they made a safe landing. If the airport did not have parts and technicians, the plane was grounded.
 
A swept wing aircraft can react abruptly (and not in a good way) to a skidding turn, whereas in a dutch roll, the aircraft tries to correct its flight path, which can also lead to a loss of control. It's been 15 or 20 years ago, but I was doing my evening rounds at MIA and witnessed a B-707 experiencing a yaw damper failure immediately after takeoff. I could tell that something was amiss, because the aircraft looked like it was undergoing some sort of spasming, jerking left and right on its roll axis. It did not climb as one would have expected and disappeared low-level towards downtown Miami. Some eye witnesses thought that it was low enough to have clipped the rotating guitar mounted above the Hard Rock Cafe. Anyway, the appropriate circuit breaker was pulled and it continued on its way to its ultimate destination, somewhere in Africa. This particular aircraft was a real "Ramp Queen", having sat on the ramp on the north side of MIA for almost a year.
 
In 1959, a new Braniff 707 on its acceptance flight crashed as after one of the pilots initiated an unrecoverable Dutch roll. Three of the aircraft's four engines were torn off the wings, and shortly afterwards it crashed on a riverbank. Four crewmembers were killed.

The instructor-pilot initiated a Dutch Roll in which the roll-park angle of the aircraft reached 40 to 60 degrees. This bank angle is in excess of limitation set by the company for demonstration of his maneuver. The pilot-trainee, who was to make the recovery, rolled full right aileron control while the right rank was still increasing. The instructor-pilot immediately rolled in full opposite aileron.

The airplane stopped its right roll at a point well past a vertical bank and then rolled to the left even more violently. Several gyrations followed and after control of the aircraft was regained, it was determined that three of the four engines had separated from the aircraft and it was on fire.

The fire rapidly reduced controllability of the aircraft and an emergency landing was attempted, however, the aircraft struck trees and crashed short of the intended landing area because power on the engine remaining had to be shut down to keep the aircraft wings level. The aircraft was destroyed and four crew members were killed while four others were injured.

 
GeorgeC said:
The coordination training maneuver commonly referred to as a dutch roll (where you roll back and forth to figure out the magnitude/phasing of the rudder so that the nose doesn't appear to move) isn't a "real" dutch roll.
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Thinking about it, it's an "anti-dutch roll".
 
Ed Haywood said:
You're doing it wrong. You don't hold bank. When you reach desired bank angle, you immediately roll in the opposite direction.
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Exactly.

And you should do that from near stall to near Vne with varying degrees of bank and varying roll rates. Try near stall with a shallow bank but maximum roll rate. Your feet will be flying.

Coordinated it making the nose do what YOU want it to do in yaw, not what the plane wants to do.
 
Pinecone said:
Exactly.

And you should do that from near stall to near Vne with varying degrees of bank and varying roll rates. Try near stall with a shallow bank but maximum roll rate. Your feet will be flying.

Coordinated it making the nose do what YOU want it to do in yaw, not what the plane wants to do.
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About a year ago, my instructor had me doing stalling Dutch rolls. The plane never went inverted but it came close. It's good practice for waking up your feet on the rudder pedals.
 
Crashnburn said:
About a year ago, my instructor had me doing stalling Dutch rolls. The plane never went inverted but it came close. It's good practice for waking up your feet on the rudder pedals.
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I think what you are referring to is called a falling leaf stall where you hold it in a stall and pick up each wing with rudder as they fall. Still not a Dutch Roll.
 
Boeing had to replace the vertical stabilizer and rudder on the original 707's with larger units because of excessive dutch roll. The crash documented above was one of the events that led to that action.
 
kyleb said:
Boeing had to replace the vertical stabilizer and rudder on the original 707's with larger units because of excessive dutch roll. The crash documented above was one of the events that led to that action.
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My old boss’ first job out of Georgia Tech was riding in the back as a test engineer right after they reinforced the tail. He told me a funny story that his first day on the job he showed up when he was told to and no one was around. Turns out the pilots were upstairs negotiating for more money. :)

It was the same airplane that Tex Johnston famously roll and is on display at the Smithsonian.
 
Dave Theisen said:
I think what you are referring to is called a falling leaf stall where you hold it in a stall and pick up each wing with rudder as they fall. Still not a Dutch Roll.
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No, it was a stalled Dutch roll. Use the rudder to hold the nose steady while you bank left, then immediately bank right.
 
bflynn said:

First, I understand the aerodynamics of a delta wing in a yaw. The leading wing is more directly pointed into the airstream so it gets more lift. The trailing wing is less perpendicular to the airstream, so it gets less lift. But isn't this also a way (the way?) to initiate a turn, by using a little yaw to induce a little roll?

But why would dutch rolls require an airframe to be grounded for inspection? I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
Click to expand...
I might have missed it but i believe it was "un commanded". Big Planes operate there flight controls hydraulically / flyby wire. A lot more complexity.
 
bflynn said:

First, I understand the aerodynamics of a delta wing in a yaw. The leading wing is more directly pointed into the airstream so it gets more lift. The trailing wing is less perpendicular to the airstream, so it gets less lift. But isn't this also a way (the way?) to initiate a turn, by using a little yaw to induce a little roll?

But why would dutch rolls require an airframe to be grounded for inspection? I was taught forward slip coordination with dutch rolls and I think they're fun to do. We do them for crosswind landings, without the rolling.
Click to expand...
What airline has an airplane with a true delta wing??
 
Salty said:
I’ve never understood the rolling on a heading thing. How is it not just a slip? And how can that possibly help you learn to turn coordinated, when it’s the very definition of uncoordinated. I mean, yeah for a second you need to be coordinated, but if you hold that bank, you have to go uncoordinated to stay on an heading. Obviously I’m missing something.
Click to expand...
Even if you don't hold the bank, it's physically impossible to roll back and forth with the nose truly locked on a point even at modest bank angles without having to cross control (slip) a bit as you reverse the roll. This moves the ball all over the cage. Yeah it's kinda silly and doesn't teach how you should actually be using the rudder. If you do the same exercise using the rudder properly with the ball in the center the whole time, the airplane will actually do slight S-turns rather than being locked on a point. The true nose on point exercise is good for aerobatic training, not so much student pilot training IMO.
 
B707/KC-135 Flight Manual excerpt about Dutch Roll

Dutch Roll.jpg

Now, this is pre-SHELL77, so I'm not sure what the Dash One says now, but I'm sure it's been re-written due to that accident.
 
RoscoeT said:
Even if you don't hold the bank, it's physically impossible to roll back and forth with the nose truly locked on a point even at modest bank angles without having to cross control (slip) a bit as you reverse the roll. This moves the ball all over the cage. Yeah it's kinda silly and doesn't teach how you should actually be using the rudder. If you do the same exercise using the rudder properly with the ball in the center the whole time, the airplane will actually do slight S-turns rather than being locked on a point. The true nose on point exercise is good for aerobatic training, not so much student pilot training IMO.
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This was to wake up my feet on the rudder pedals. It worked. I spent about 30 minutes doing that, then did my first two spins and my first to rolls. So, I guess I had my first aerobatics lesson.
 
So, a dutch roll in an airliner is a feedback loop with a delay, so if you try to get out of it by feel you're going to make it worse. The write up above is an instruction for the 707 on how to apply specific control inputs that seem counterintuitive, but that actually will dampen it. Is that about right?

Years ago I read about Tex Johnson being a passenger on a commercial flight when this was happening, and he went up front to "tell them how to fix it" and then everything was fine. That seemed like urban folklore until reading the recovery description.
 
Albany Tom said:
So, a dutch roll in an airliner is a feedback loop with a delay, so if you try to get out of it by feel you're going to make it worse. The write up above is an instruction for the 707 on how to apply specific control inputs that seem counterintuitive, but that actually will dampen it. Is that about right?

Years ago I read about Tex Johnson being a passenger on a commercial flight when this was happening, and he went up front to "tell them how to fix it" and then everything was fine. That seemed like urban folklore until reading the recovery description.
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I could be wrong here, but…. I think what the GA does for coordination practice, is not a *true* Dutch roll.
I’m not even certain that a straight wing airplane can do a true Dutch roll.

As always, I will likely be proven wrong.
 
Sluggo63 said:
B707/KC-135 Flight Manual excerpt about Dutch Roll

View attachment 130206

Now, this is pre-SHELL77, so I'm not sure what the Dash One says now, but I'm sure it's been re-written due to that accident.
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“Usually engine separation.” I’m not sure I want to know how many experiments they performed to come up with the word “usually” here.
 
One thing worth mentioning, in the case of yaw damper failure, the rear section is in effect "the tail end of the whip" severe enough to cause injuries to those in the cabin.
 
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