Dutch roll

Last updated
An animated illustration of dutch roll DutchRoll AnimGIF 01.gif
An animated illustration of dutch roll
Dutch roll damping technique, scanned from U.S. Air Force flight manual. Dutch Roll Damping Technique.jpg
Dutch roll damping technique, scanned from U.S. Air Force flight manual.

Dutch roll is a type of aircraft motion consisting of an out-of-phase combination of "tail-wagging" (yaw) and rocking from side to side (roll). This yaw-roll coupling is one of the basic flight dynamic modes (others include phugoid, short period, and spiral divergence). This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases. Dutch roll stability can be artificially increased by the installation of a yaw damper. Wings placed well above the center of gravity, sweepback (swept wings) and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral, and transport-category swept-wing aircraft are equipped with yaw dampers. A similar phenomenon can happen in a trailer pulled by a car.

Contents

Stability

In aircraft design, Dutch roll results from relatively weaker positive directional stability as opposed to positive lateral stability. When an aircraft rolls around the longitudinal axis, a sideslip is introduced into the relative wind in the direction of the rolling motion (due to the lateral component of lift when the wings are not level). Strong lateral stability (due to the more-direct airflow past the down wing, which has been pivoted forward by the slip) begins to restore the aircraft to level flight. At the same time, somewhat weaker directional stability (due both to greater drag from the wing which is now generating greater lift, and by aerodynamic force on the vertical fin due to the yaw) attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since directional stability is weaker than lateral stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. The aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.

There is a trade-off between directional and lateral stability. Greater lateral stability leads to greater spiral stability and lower oscillatory stability. Greater directional stability leads to spiral instability but greater oscillatory stability. [1]

Mechanism

The most common mechanism of Dutch roll occurrence is a yawing motion which can be caused by any number of factors. As a swept-wing aircraft yaws (to the right, for instance), the left wing becomes less-swept than the right wing in reference to the relative wind. Because of this, the left wing develops more lift than the right wing causing the aircraft to roll to the right. This motion continues until the yaw angle of the aircraft reaches the point where the vertical stabilizer effectively becomes a wind vane and reverses the yawing motion. As the aircraft yaws back to the left, the right wing then becomes less swept than the left resulting in the right wing developing more lift than the left. The aircraft then rolls to the left as the yaw angle again reaches the point where the aircraft wind-vanes back the other direction and the whole process repeats itself. The average duration of a Dutch roll half-cycle is 2 to 3 seconds.

The Dutch roll mode can be excited by any use of aileron or rudder, but for flight test purposes it is usually excited with a rudder singlet (a short sharp motion of the rudder to a specified angle, and then back to the centered position) or doublet (a pair of such motions in opposite directions). Some larger aircraft are better excited with aileron inputs. Periods can range from a few seconds for light aircraft to a minute or more for airliners.[ citation needed ]

Tex Johnston describes the Dutch roll as, "...an inherent characteristic of swept-wing aircraft. It starts with a yaw. In a 35-degree swept-wing airplane, a yaw is accompanied by a simultaneous roll in the direction of yaw. The roll is caused by changing lift factors as the airflow path over the wing changes. For example, in a left yaw the left wing slews toward the rear so that airflow is displaced spanwise from its normal front-to-rear path over the airfoil section. That reduces lift. Simultaneously, the advancing right wing gets more chordwise flow, and so its lift is increased. In combination the two conditions create a left roll. Similarly, a yaw to the right results in a roll to the right. An oscillation is set up." [2]

Rolling on a heading

Dutch roll is also the name (considered by professionals to be a misnomer) given to a coordination maneuver generally taught to student pilots to improve their "stick-and-rudder" technique. The aircraft is alternately rolled as much as 60 degrees left and right while rudder is applied to keep the nose of the aircraft pointed at a fixed point. More correctly, this is a rudder coordination practice exercise, to teach a student pilot how to correct for the effect known as adverse aileron yaw during roll inputs.

This coordination technique is better referred to as "rolling on a heading", wherein the aircraft is rolled in such a way as to maintain an accurate heading without the nose moving from side-to-side (or yawing). The yaw motion is induced through the use of ailerons alone due to aileron drag, wherein the lifting wing (aileron down) is doing more work than the descending wing (aileron up) and therefore creates more drag, forcing the lifting wing back, yawing the aircraft toward it. This yawing effect produced by rolling motion is known as adverse yaw. This has to be countered precisely by application of rudder in the same direction as the aileron control (left stick, left rudder – right stick, right rudder). This is known as synchronised controls when done properly, and is difficult to learn and apply well. The correct amount of rudder to apply with aileron is different for each aircraft.

Name

The origin of the name Dutch roll is uncertain. However, it is likely that this term, describing a lateral asymmetric motion of an airplane, was borrowed from a reference to similar-appearing motion in ice skating. In 1916, aeronautical engineer Jerome C. Hunsaker published: "Dutch roll – the third element in the [lateral] motion [of an airplane] is a yawing to the right and left, combined with rolling. The motion is oscillatory of period for 7 to 12 seconds, which may or may not be damped. The analogy to 'Dutch Roll' or 'Outer Edge' in ice skating is obvious." [3] In 1916, Dutch Roll was the term used for skating repetitively to right and left (by analogy to the motion described for the aircraft) on the outer edge of one's skates. By 1916, the term had been imported from skating to aeronautical engineering, perhaps by Hunsaker himself. 1916 was only five years after G. H. Bryan did the first mathematical analysis of lateral motion of aircraft in 1911. [4]

Accidents

See also

Related Research Articles

<span class="mw-page-title-main">Aileron</span> Aircraft control surface used to induce roll

An aileron is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.

<span class="mw-page-title-main">Aircraft flight dynamics</span> Science of air vehicle orientation and control in three dimensions

Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of gravity (cg), known as pitch, roll and yaw. These are collectively known as aircraft attitude, often principally relative to the atmospheric frame in normal flight, but also relative to terrain during takeoff or landing, or when operating at low elevation. The concept of attitude is not specific to fixed-wing aircraft, but also extends to rotary aircraft such as helicopters, and dirigibles, where the flight dynamics involved in establishing and controlling attitude are entirely different.

<span class="mw-page-title-main">Autopilot</span> System to maintain vehicle trajectory in lieu of direct operator command

An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator's control of the vehicle, allowing the operator to focus on broader aspects of operations.

<span class="mw-page-title-main">Flight control surfaces</span> Surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

<span class="mw-page-title-main">Dihedral (aeronautics)</span> Angle between each wing or tail surface within a pair

In aeronautics, dihedral is the angle between the left and right wings of an aircraft. "Dihedral" is also used to describe the effect of sideslip on the rolling of the aircraft.

<span class="mw-page-title-main">Stall turn</span> Aerobatics turn-around maneuver

The hammerhead turn, stall turn, or Fieseler is an aerobatics turn-around maneuver.

<span class="mw-page-title-main">Slip (aerodynamics)</span> Aerobatic maneuver

A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow or relative wind. In other words, for a conventional aircraft, the nose will be pointing in the opposite direction to the bank of the wing(s). The aircraft is not in coordinated flight and therefore is flying inefficiently.

Aircraft flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".

<span class="mw-page-title-main">Vertical stabilizer</span> Aircraft component

A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, stability and trim in yaw. It is part of the aircraft empennage, specifically of its stabilizers.

<span class="mw-page-title-main">Stabilizer (aeronautics)</span> Aircraft component

An aircraft stabilizer is an aerodynamic surface, typically including one or more movable control surfaces, that provides longitudinal (pitch) and/or directional (yaw) stability and control. A stabilizer can feature a fixed or adjustable structure on which any movable control surfaces are hinged, or it can itself be a fully movable surface such as a stabilator. Depending on the context, "stabilizer" may sometimes describe only the front part of the overall surface.

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in lift and drag of each wing. The effect can be greatly minimized with ailerons deliberately designed to create drag when deflected upward and/or mechanisms which automatically apply some amount of coordinated rudder. As the major causes of adverse yaw vary with lift, any fixed-ratio mechanism will fail to fully solve the problem across all flight conditions and thus any manually operated aircraft will require some amount of rudder input from the pilot in order to maintain coordinated flight.

<span class="mw-page-title-main">Barrel roll</span> Aerial maneuver

A barrel roll is an aerial maneuver in which an airplane makes a complete rotation on both its longitudinal and lateral axes, causing it to follow a helical path, approximately maintaining its original direction. It is sometimes described as a "combination of a loop and a roll". The g-force is kept positive on the object throughout the maneuver, commonly between 2 and 3g, and no less than 0.5g. The barrel roll is commonly confused with an aileron roll.

The dynamic stability of an aircraft refers to how the aircraft behaves after it has been disturbed following steady non-oscillating flight.

<span class="mw-page-title-main">Coordinated flight</span> Flight of an aircraft without sideslip

In aviation, coordinated flight of an aircraft is flight without sideslip.

A yaw damper is a system used to reduce the undesirable tendencies of an aircraft to oscillate in a repetitive rolling and yawing motion, a phenomenon known as the Dutch roll. A large number of modern aircraft, both jet-powered and propeller-driven, have been furnished with such systems.

<span class="mw-page-title-main">Crosswind landing</span> Going from flight to surface amid strong air currents angled from the runway

In aviation, a crosswind landing is a landing maneuver in which a significant component of the prevailing wind is perpendicular to the runway center line.

<span class="mw-page-title-main">Aircraft principal axes</span> Principal directions in aviation

An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; pitch, nose up or down about an axis running from wing to wing; and roll, rotation about an axis running from nose to tail. The axes are alternatively designated as vertical, lateral, and longitudinal respectively. These axes move with the vehicle and rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first crewed spacecraft were designed in the late 1950s.

<span class="mw-page-title-main">Radio-controlled aerobatics</span>

Radio-controlled aerobatics is the practice of flying radio-controlled aircraft in maneuvers involving aircraft attitudes that are not used in normal flight.

In signal processing, a washout filter is a stable high pass filter with zero static gain. This leads to the filtering of lower frequency inputs signals, leaving the steady state output unaffected by unwanted low frequency inputs.

The minimum control speed (VMC) of a multi-engine aircraft is a V-speed that specifies the calibrated airspeed below which directional or lateral control of the aircraft can no longer be maintained, after the failure of one or more engines. The VMC only applies if at least one engine is still operative, and will depend on the stage of flight. Indeed, multiple VMCs have to be calculated for landing, air travel, and ground travel, and there are more still for aircraft with four or more engines. These are all included in the aircraft flight manual of all multi-engine aircraft. When design engineers are sizing an airplane's vertical tail and flight control surfaces, they have to take into account the effect this will have on the airplane's minimum control speeds.

References

  1. Davies, David P. (1971). Handling the Big Jets: An Explanation of the Significant Differences in Flying Qualities Between Jet Transport Aeroplanes and Piston Engined Transport Aeroplanes, Together with Some Other Aspects of Jet Transport Handling (3rd ed.). Air Registration Board. p. 100. ISBN   9780903083010.
  2. Johnston, A.M. "Tex" (1992). Tex Johnston: Jet-Age Test Pilot . New York: Bantam. p. 140. ISBN   9780553295870.
  3. Hunsaker, Jerome C. (1916). "Dynamical Stability of Aeroplanes". Proceedings of the National Academy of Sciences of the United States of America . National Academy of Sciences. 2 (5): 282. Bibcode:1916PNAS....2..278H. doi: 10.1073/pnas.2.5.278 . PMC   1091005 . PMID   16576144.
  4. Bryan, G. H. (1911). Stability in Aviation. p.  123.
  5. Accident description at the Aviation Safety Network
  6. HistoryLink, posted 7/23/2017. Boeing 707 jetliner crashes in Snohomish County, October 19, 1959. Archived 31 October 2020 at the Wayback Machine
  7. Accident descriptionfor 63-8877 at the Aviation Safety Network . Retrieved on 21 October 2014.
  8. Humphrey, Jeff (20 June 2013). "Cellphone video may have captured deadly KC-135 crash". Spokane, Washington. Archived from the original on 15 March 2016. Retrieved 21 October 2014.
  9. "Investigation board determines cause of KC-135 crash in May". 14 March 2014. Archived from the original on 16 March 2017. Retrieved 21 October 2014.
  10. Davis, Kristin (13 March 2014). "Malfunction, pilot error caused May KC-135 crash". Air Force Times. Springfield, Virginia. Retrieved 21 October 2014.
  11. Camden, Jim (13 March 2014). "Tanker's tail separated in flight before Kyrgyzstan crash". Spokesman-Review. Spokane, Washington. Archived from the original on 28 January 2021. Retrieved 21 October 2014.
  12. U.S. Air Force Aircraft Accident Investigation Board Report; KC-135R, T/N 63-8877; 22nd Air Refueling Wing McConnell AFB, Kansas; Location: 6 miles S. of Chaldovar, Kyrgyz Republic (PDF) (Report). 31 December 2013. Archived (PDF) from the original on 21 October 2014. Retrieved 21 October 2014.
  13. Johnson, Oliver. "AgustaWestland: AW609 was performing high-speed tests on day of crash". Archived from the original on 19 November 2021. Retrieved 28 December 2019.
  14. Interim Report Archived 19 November 2021 at the Wayback Machine ANSV

Articles

Videos