Thursday, August 19, 2010

How the Dutch Rolls

There we were in this early summer moment, relaxing in the smooth air at 31,000 feet, headed east in the sunshine toward Philadelphia. We had climbed out of Kansas City, Missouri about an hour before and had about one hour left to reach Philly. Our exact position was 230 miles west of Johnstown, ‘P.A.’ (the one that had THE flood). The 73 knot tailwind was helping to produce our ground speed (GS) of 522, or 8.7 miles a minute, or a mile about every seven seconds. The airspeed tape indicated about 210 knots, but because of the thin air at altitude, our jet’s true airspeed was 439, computed by our ADC’s (air data computers). Our speed relative to the speed of sound was .74, or ‘Mach point .74’. Our total weight at this moment was 48,700 lbs, 1,700 lbs greater than our maximum landing weight of 47,000 lbs. Each of our two engines were burning almost 1,400 lbs per hour. What would our weight be upon landing in Philly in one hour? Do your quick Pilot mental math before proceeding further.

It would be ‘fourty-five nine’ as I like to say, or fourty-five thousand, nine-hundred pounds. Our little CRJ was packed to the gills with people and bags, and some or most of our ’pax’ were sleeping, no doubt. The droning on of our turbofans and the 8,000 feet above sea level equivalent altitude in the cabin had a part in the that. And the natural back and forth slight banking of our ’airship’ probably did too. I took a few moment to observe and appreciate this rhythmic motion of the left wing behind me, smoothly moving up and down against the horizon. Wads of symmetrically oriented cotton ball cumulus floated by far below, and seemed to frame the scene for contemplation on why.

Why do the wings of my CRJ always do this, rock back and forth? It’s always done this from the first day I started flying it, but it always seems to do it with the autopilot engaged. I’ve looked back when hand flying (just for a couple seconds, mind you!) and the wing is as steady as my hand can be. Well, with the autopilot on I took a video with my camera; let’s see how it looks on that. Observe the hazy horizon behind the wing tip, and you can see that it’s lazily rocking in a bank to the left and right, and maybe just yawing forward a back a hair.

As near as I can tell, this is the ‘natural frequency’ of a CRJ, like a ship or boat that lists or yaws back and forth naturally. A On a dependents cruise of my brother’s ship, a Destroyer in the US Navy, I observed his ship listing (banking) left and right a few degrees at a time. It’s period (time to complete one cycle) lasted about 20 seconds. I myself am not an aerodynamics expert, but I am a good BS’er. I know a little, of course; you can’t hardly call yourself a jet pilot without knowing a little about the difference between straight wing airplanes and swept wing jets.

I asked my excellent and very capable First Officer, who just happens to have a degree in Mechanical/Aerospace Engineering. He said that it had to do with an imbalance of lift of that between the left and right wings, due to slightly changing relative wind the left and right wings experience.  He explained that a jet has a spanwise airflow, meaning that airflow over and under the wings doesn’t really flow straight across, it flows at an angle across it. Straight wing airplanes don’t have spanwise airflow over their wings, and quite a bit more stable in flight than swept wing planes.

Let’s go dutch! After looking at the video, some other videos online, and reading about it, I think I know what it is. What my jet is experiencing and counteracting is Dutch roll, a classic aerodynamic term and dynamic situation swept wing jets inherently have. Asymmetrical, spanwise airflow imparts a yawing moment, or yawing tendency, to the aircraft. Yaw is a rotational force around an aircrafts’ vertical axis. Yaw makes a jets’ nose rotate to the left or right about the center of gravity (the jet‘s balance point, located next the wing, approximately). When an aircraft yaws in one direction the aforementioned difference in relative wind and imbalance of lift between the wings occurs.

See this video for a good look at authentic undamped dutch roll.

Explained similarly, dutch roll begins with the aircraft yawing one direction, then banking in the same direction. The yaw produces more lift on the wing which has moved ahead of the other. This imbalance of lift then produces the banking tendency. Eventually the dipped wing will produce more lift than the higher wing because of a higher ‘angle of attack’ (another explanation I won’t go into here) than what the higher wing has, and the wings will start to return towards level.

A ‘yaw damper’ is a fancy but necessary piece of flight control equipment on swept wing jets. It will apply the rudder in the opposite direction to counteract yawing, but because of the delayed reaction of banking occurring after yawing and as a result of it, the left and right banks occur. This is my best estimation of what is happening. The jet ends up banking slightly to the left and right as the yaw damper dampens out these oscillations. Rudder applied into the slipstream will produce yaw first, then bank, as the forward wing (in a right yaw the left wing will be forward of the right wing) produces more lift than the other one.

This neat website (HOW IT FLIES) shows what's happening to the wings in tandem aerodynamically during dutch roll. The wingtips actually trace loops against the horizon in opposite directions. If the plane yaws left the left wingtip will yaw aft and bank downward, then yaw forward and bank upward to wings level as the dutch roll is counteracted with rudder applied by the yaw damper. During this period the opposite right wingtip will yaw forward and bank upward, then yaw backward and back downward to wings level. It actually is a motion that is more oval or circular shaped, due to the changing aerodynamic forces involved. If the plane yaws right the opposite wingtip motions will occur (its confusing, so I won’t explain, see the ‘websight’ for more ‘insight’).

This is another British video which shows the circular loops the wingtips trace during dutch roll.

Dutch roll can be a dangerous phenomenon, and it was in the early days of swept wing jet fighters and airliners. In 1956 during a Braniff 707 customer acceptance flight control was lost after exceeding Boeing's maximum dutch roll demonstration bank angle (with the yaw dampers off).  Three of the four engines were shed from the airframe before they performed an emergency landing on a river bed.  Spending time flying with the yaw damper totally disabled (jets have two of them) can be a real eye opener, and it's something we demonstrate in the flight simulator.  Yaw dampers are so critical that my jet isn't allowed to fly passengers if both of our yaw dampers are broken.

As I’m trying to finish this post I’m riding on an ‘advanced’ Airbus A319, the ‘fly by wire’ one. I look at the wingtip against the horizon, and it’s the steadiest one I’ve seen yet. It does move, however; with the aircraft responding quickly, almost immediately, to keep the wings level. The swaying and correcting looks like a matter of inches; it’s impressive to watch on an airliner with a 111 foot foot wingspan. I presume the Airbus is better at damping out unwanted yaw and bank because it has more advanced avionics in the form of the ‘fly by (computerized-digital) wire’ technology.

Readers, I know it's been a while since I've posted, but I've been busy putting the nuts and bolts together for my business selling logbook binders and paper for computer pilot logbooks.  It's been going good, a few have sold on (click here to see it) and I'm about to start my own website.  Summer is passing by as well, and I've had a challenging time of it flying, and am ready for the season to change.  Lastly, God is still good and I still feel led to glorify him and make Him known to others through my job, my writing, and everything else I do.

I can't let you escape without a comparison of dutch roll to our own lives.  Observe your life events and you'll likely see that disturbances in your life balance can cause other parts of your life to 'roll away'.  Control the yaw, or your directional stability, better, and the rolls will cease or be minimized.  Christ is the one true light in the darkness (John 1:4-8) who can and will draw you in his direction.  Keep it pointed toward His light and he'll be your very own 'yaw damper'.  Thanks for reading, and God bless you!

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