So what is Dutch roll?
Yes, you may guess but it’s not the food here, but rather something to do with the motion of an aircraft. In flight world, the oscillatory motion following heavily damped roll subsidence mode is the lateral-directional oscillatory motion known as Dutch roll. In simple words, it is the combination of continuous yawing and rolling oscillation. This would indicate to be undesirable, after all, who wants wiggle in their aircraft, and not one direction but two at once. The origin of its name is believed to come from the motion of a classic Dutch skating technique. This mode of motion was described by G.H. Bryan in 1911, and later by 1916 was being called the “Dutch roll.” In many aircraft, it happens naturally (due to its directional stability), while in some it may happen accidently. Allow me to make one thing clear at this point, Dutch roll is not an aircraft deficiency but is the inevitable result of having directional stability, which we all concede to be a very desirable characteristic.
The lateral-directional characteristics equation of flight dynamics gives us five roots or the five modes of its motion. One root is zero, the other is large and negative corresponding to heavily damped roll subsidence mode. One root is small and may be positive or negative depicting spiral mode. The remaining two are a complex pair- motion associated with them is the Dutch roll.
Pilots exercise Dutch roll for the acquisition of maneuver knowing how much cross-control input they need to maintain the center line on landing and takeoff in a crosswind landing. This technique is better referred to as “rolling on a heading”. The aircraft is alternately rolled up to 60 degrees left and right while rudder is applied to keep the nose of the aircraft pointed at a fixed point. More precisely, this is a rudder coordination exercise, to teach a student pilot how to ameliorate for the effect known as adverse aileron yaw during roll inputs.
Why does it happen?
- Sweep back and Dihedral effect
Most aircraft are designed with swept wings; is the primary mechanism that gives the roll effect to an airplane which may only receive a yaw input. If you look at this picture:
You can see that both wings have a backward sweep to them. Now, if you introduce a yaw to the aircraft, one wing will extend out more directly into the wind stream, while the other wing will be even more swept. This effectively makes one wing longer and the other wing to be shorter. Obviously, longer wing will generate more lift, and the shorter one will generate less lift, this effect is known as a dihedral effect. The higher the dihedral effect, the more it will roll as it oscillates. And since there is asymmetric lift around the roll axis, the airplane will roll, and continue to roll. And as we know, with more lift comes more drag, so that will counter the lift and pull the wing back Causing an effect known as “Dutch Roll”.
- Weaker Yaw Stability
Aircraft with extremely low yaw damping are a relatively new phenomenon, however, resulting from high speed and high altitude low damping leads to a nasty Dutch roll that is very difficult to control. Also when the wind blows over the vertical stabilizer, it generates lift in opposite direction to the yaw produced by the wing’s drag. This yaws the nose back to its initial position.
- Strong Roll Stability
Strong roll stability causes the aircraft lift vector to tilt in a direction causing sideslip. In such a condition instead of air blowing straight down to the nose, comes from the sideslip direction. This enhances the differential lift in two wings causing the aircraft to roll while yawing.
What to do during Dutch roll?
At high altitude, Dutch roll is almost certain to happen; in a jet aircraft if the Yaw dampener is turned off. Hence, the first thing to inspect when an aircraft begins to exhibit Dutch roll is whether the yaw-dampener is off. The pilot should then try to minimize the yawing oscillations by holding the rudder pedals in the neutral position. Then, apply aileron (spoiler) control opposite to the roll. A perfect technique to use is short stabs of ailerons applied opposite to the roll. Try to give one quick jolt on each cycle (i.e. turn the wheel toward the rising wing, and then return it to neutral.) Finally accelerate to a higher speed, where directional stability will be better, or descend into the more dense air, for the same reason.
For normal flying, the rolling tendency of the Dutch roll is purely a grouchy and unnecessary for control. Pilots use ailerons and/or spoilers for assuring the roll axis is in the correct position. However, if you go through a failure in the roll axis, such as a hard-over roll control actuator, or hydraulic failure? In those situations, an adroit pilot isn’t finished. By proper control of yaw, the pilot can retrieve control in roll through the “grouchy” roll component of the Dutch roll mode. Proper use of the rudder pedals as an alternate way of rolling an aircraft can entail the difference between recovery and loss of control. The use of rudder pedals for roll control is not without risks, as the fatal crash of May, 2013, of a US Air Force KC-135 has been ascribed to the flight crew’s inability to recover from a “Dutch roll.
So that’s Dutch roll; an excellent maneuver to give you the fell of how much control input you need to keep plane aligned with the center line on landing to takeoff in the florid landscape.
Article By: ANIL KUMAR KAMAT
IOE, Thapathali Campus; Department of Mechanical Engineering Undergradute Student