Dutch Roll

FlySince9

FlySince9

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Jerry
I used to rent a Cessna 172XP that inevitability would enter, what could only be described as, a dutch roll during long, slow descents. At the time, I assumed I was in a Dutch roll, but since the maneuver was never covered during my Primary Training, I had no idea how to recover from it. Nothing I tried seemed to work. Since this was a high wing Cessna, and not a swept wing jet, I assumed that the issue was related to some kind of rigging issue. I loved flying that aircraft, but this was an annoying side show I had to endure. I haven't experienced this phenomenon in any other airplane, since.

I recently came across an article that gave the following, as a solution:

"What you do is move the wheel back and forth a few degrees as quick as you can. This small and rapid movement of the ailerons disrupts the rolling motion and uncouples it from yaw...

... You need to wiggle the wheel like a madman and then the Dutch roll stops like magic."

Any thoughts?
 
I was taught to correct with aileron. Look out your side of the plane and simply use aileron to counter that wing going down. That was in a swept wing and I've never heard of Dutch Roll in a straight wing. The mechanics are wrong.

In a swept wing as the plane yaws one wing is presented more to the airstream than the other. That causes lift and drag so that wing goes up and yaws the plane back. Then the other wing has the same issue and it goes up and back. Thus the oscillation begins and the correction is what I described.
 
When did the put swept wings on a Hawk XP? I'm confused as to the situation you're talking about.
 
That doesn't sound like a normal situation. I would have them check the rigging.
 
FlySince9 said:
Why don't you read the post before you answer?
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I did, Dutch Roll as you relate it being a situation of instability is an issue with swept wing aerodynamics. As Dutch Roll applies to a straight wing relates to a single event when you don't use the rudder to coordinate your turn leads. With a straight wing plane in correct rig this situation will stabilize itself at wings level.
 
Henning said:
I did, Dutch Roll as you relate it being a situation of instability is an issue with swept wing aerodynamics. As Dutch Roll applies to a straight wing relates to a single event when you don't use the rudder to coordinate your turn leads. With a straight wing plane in correct rig this situation will stabilize itself at wings level.
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I'm trying to indicate that I understand all that (jets, swept wing, etc). I'm wondering why I was experiencing the phenomenon. Bad Rigging, etc. And if anybody else had ever experienced it due to bad rigging, etc. It was not due to rudder input (manually) because it normally occurred during long, slow, descents like an extended final approach.
 
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Our Seneca will do it in rough air, keep your feet alive on the rudder and it stops.
 
FlySince9 said:
I'm trying to indicate that I understand all that (jets, swept wing, etc). I'm wondering why I was experiencing the phenomenon. Bad Rigging, etc. And if anybody else had ever experienced it due to bad rigging, etc. It was not due to rudder input (manually) because it normally occurred during long, slow, descents like an extended final approach.
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Well, it's interesting as you're basically saying you are getting an oscillation of the rudder or ailerons with a change in either pitch or speed.

If your rudder has a trim tab on it I suggest you check it for play, this could be caused there. I'd also look at the horn fairing on the rudder cap, defects/cracks there could cause an aerodynamic osscilating condition with changes in pitch and speed.

After that, without doing some experiments to find out what was actually occurring, I'd get out the rigging manual and start rigging and get the plane all squared away.
 
Agree with Henning. I was thinking of excess play in the cables.
 
Have the wings ever been off that Skyhawk?

Getting the wing angle of incidence wrong on one wing would do that.

If its as bad as you suggest, you need to find a qualified Cessna rigger and start measuring things. Something is way out of whack.
 
denverpilot said:
Have the wings ever been off that Skyhawk?

Getting the wing angle of incidence wrong on one wing would do that.

If its as bad as you suggest, you need to find a qualified Cessna rigger and start measuring things. Something is way out of whack.
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As this was a rental, I don't know about the wings. But I was pretty sure it would relate somehow to rigging. Something as small as cracks in the rudder cap fairing never occurred to me. Mostly I was just curious if anyone else ever experienced this in a straight wing airplane Cauz it was pretty weird...
 
FlySince9 said:
As this was a rental, I don't know about the wings. But I was pretty sure it would relate somehow to rigging. Something as small as cracks in the rudder cap fairing never occurred to me. Mostly I was just curious if anyone else ever experienced this in a straight wing airplane Cauz it was pretty weird...
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Anything that can catch air on one side and not the other.
 
I'd complain to the FBO. Something wrong with the plane. The only time I've experienced a Dutch roll was when I was a student pilot. I'd heard of them and asked my CFI to demonstrate one. He did and that's the closest I've come to getting airsick. Then I tried on and had no trouble. Something about having your hand on the yoke. That is the only time in a light plane.
 
In general, the thing that reduces damping of the Dutch Roll mode of oscillation with a swept wing is the same as a wing with a lot of dihedral. So, a straight wing with too much dihedral will exhibit Dutch Roll - I regularly fly one such type.

As for this 172, I don't know whether I have missed anything or not but I'd want a clearer description of its behavior than a simple statement that it can only be described as a Dutch Roll. To me, that does not describe its behaviour at all. An undamped or lightly damped lateral/directional oscillation? At what frequency? What roll to yaw ratio? Rudder oscillating too?
 
djpacro said:
In general, the thing that reduces damping of the Dutch Roll mode of oscillation with a swept wing is the same as a wing with a lot of dihedral. So, a straight wing with too much dihedral will exhibit Dutch Roll - I regularly fly one such type.

As for this 172, I don't know whether I have missed anything or not but I'd want a clearer description of its behavior than a simple statement that it can only be described as a Dutch Roll. To me, that does not describe its behaviour at all. An undamped or lightly damped lateral/directional oscillation? At what frequency? What roll to yaw ratio? Rudder oscillating too?
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From what I recall, some, but not a lot of yaw. About 5 degree roll one way then the other at about 10 seconds per full cycle. Also slight pitch oscillations while transitioning from roll-to wings-level to-roll. That's about the best I can describe it. As I slowed to final approach speed and dropped some flaps it seemed to settle down and eventually stop completely. The weird thing about this was that I only noticed it during long, straight-in descents. I didn't notice it in a standard traffic pattern.
 
I was thinking of excess play in the cables.
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If that's suspected, then the aircraft shouldn't be flown, period. Flutter may be the next problem, and flutter goes from nothing to fatal in very short order, because it removes the control surfaces and breaks up the aircraft.

Uneven tension in the control cables wouldn't normally do what's being described. What's being described isn't being described well enough to tackle here, other than to suggest the poster keeps his hands on the controls and feed on the rudders and works them more.

The only place i can see it being an issue would be play in the control column inteface, rather than cable rigging. Sometimes chain play or a loose control column might replicate what's being described, but the original poster hasn't described anything that sounds like a dutch roll.

Fix it with rudder. Forget pulsing the controls or giving it sudden inputs. That's all wrong, especially if you did have play in the control rigging.

First, make sure you're on the rudders. Rudders do tend to have very low cable tension. Press on them uniformly with both feet, and then control them. Stop any yawing motion. Use your aileron. Stop any rolling motion. Dutch rolling is a function of swept wings, although a straight wing airplane can dutch roll, but usually not without pilot in put, large aileron deflection (adverse yaw), or really sloppy handling. The descriptions thus far don't sound like dutch roll.

If you have questions about the integrity of the airplane, don't fly it.
 
DouglasBader said:
If you have questions about the integrity of the airplane, don't fly it.
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As I've said, this is a plane I used to rent. I don't rent at that FBO anymore. It is something I was curious about, because I've never had that situation in any other airplane, and I was wondering if anyone else had ever experienced such a thing. The responses thus far are pretty much what I expected, but I don't think it had anything to do with my control inputs as I have never had it happen (in my 30+ years of flying Cessnas) except in that, one, airplane...
 
Often airplanes talk to you in small ways before they talk to you in big ways. What you saw might have been a warning of things to come.

I worked at a place that had Senecas that were used for a lot of rough field work, including a lot of dirt airstrips. One of the airplanes had been reported to make a popping sound when the flaps were extended, but it only did it in flight, and the problem didn't duplicate itself on the ground. One day the Director of Maintenance, when the airplane happened by the maintenance base, asked a pilot to take him around the pattern and demonstrate this issue. He later told me that as they taxied to the runway, he felt the flooring shift beneath his feet. The DoM immediately told the pilot to take him back to the hangar.

The spar, as it turned out, was c racked completely through in three places. The popping sound was the spar grinding against itself as the wing parts shifted position when the flaps were deployed. How long it would have been until a really unpleasant failure would have occurred isn't clear, but it definitely would have happened.

Airplanes talk to you. What they're trying to say isn't always clear.

Not long ago an airplane I flew experienced a flickering oil pressure light for the number four engine, on each start. We have time limits when it should stabilize, and it did. The other engines didn't do this, however, and even though it raised eyebrows, it was within limits and the airplane continued to be operated. Recently it failed enroute, or rather, had to be shut down when it lost all pressure (and oil). A failed oil line was discovered, and replaced. The engine was operationally checked, and it checked normal. Two days later the same engine failed again, in the same way. The airplane was immediately taken off the line and ferried half-way around the world to a maintenance base for a much closer examination.

When you start to see trends, small things, abnormalities, these are often some of the only indications of issues before they become big things.

Perhaps you saw such a thing.

When you see such things, write them up. Owners and operators are frequently unhappy about these writeups, but my policy has always been to write them up and make photocopies. At a minimum, at least you've done your part in identifying and reporting the problem.
 
DouglasBader said:
When you see such things, write them up. Owners and operators are frequently unhappy about these writeups, but my policy has always been to write them up and make photocopies. At a minimum, at least you've done your part in identifying and reporting the problem.
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Sounds like GREAT advice. I didn't write it up, but I did mention it to the operator. Don't think they acted on it though... Oh well, C'est la vie!
 
I'm constantly amazed the condition of planes that people fly; cross controlled to keep straight, grinding noises as you move stiff controls, throttles split by inches... Then they complain about their maintenance and repair bill when the stuff finally gives up the ghost.

Our planes are extremely durable and will fly with a lot of crap broken, that is no excuse to keep doing so. A reasonably new pilot took me for a ride in a 172 once, it made a top of 2100 rpm and was obviously low on power. I asked him why we were flying a defective airplane that wouldn't make engine spec and how much they were charging to rent him broken crap?
 
Henning said:
I'm constantly amazed the condition of planes that people fly; cross controlled to keep straight, grinding noises as you move stiff controls, throttles split by inches... Then they complain about their maintenance and repair bill when the stuff finally gives up the ghost.

Our planes are extremely durable and will fly with a lot of crap broken, that is no excuse to keep doing so. A reasonably new pilot took me for a ride in a 172 once, it made a top of 2100 rpm and was obviously low on power. I asked him why we were flying a defective airplane that wouldn't make engine spec and how much they were charging to rent him broken crap?
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Exactly why I don't rent anymore...Too many hands in the pot. At least in a club, our members usually take ownership of such issues and follow up on them until they are resolved.
 
FlySince9 said:
Exactly why I don't rent anymore...Too many hands in the pot. At least in a club, our members usually take ownership of such issues and follow up on them until they are resolved.
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That's good but not a guarantee either. I see FBOs, clubs and private owners do the same thing. Clubs are excellent, but they are not all equal by any means.
 
FlySince9 said:
From what I recall, some, but not a lot of yaw. About 5 degree roll one way then the other at about 10 seconds per full cycle. Also slight pitch oscillations while transitioning from roll-to wings-level to-roll. That's about the best I can describe it....
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Thanks FlySince9. From memory the natural frequency of the 172's DR is much higher and I would've expected a majority of yaw in it so let's not call it a DR.

As others have suggested, the operator may not have been bothered but your info should've enabled the cause to be found. Things don't normally get better.

Perhaps some here may like to revise assumptions of the cause. I've seen the effect of loose rudder trim tabs, excessive free play in elevator tabs, sloppy aileron cable pulleys etc - as have others.
 
That's just it, without flying and looking at it, all we can do is guess as there are so many possibilities.
 
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