Control reversal: Difference between revisions

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{{Short description|Aeronautical phenomenon}}

'''Control reversal''' is an adverse effect on the controllability of [[aircraft]]. The [[Aircraft flight control system|flight controls]] reverse themselves in a way that is not intuitive, so pilots may not be aware of the situation and therefore provide the wrong inputs; in order to roll to the left, for instance, they have to push the control stick to the right, the opposite of the normal direction.

==Causes==

There are several causes for this problem:, pilot error, effects of [[high-speed flight]], incorrectly connected controls, and various [[Coupling (physics)|coupling forces]] on the aircraft.

===Equipment malfunction===

Equipment failure may cause flight controls to behave unexpectedly, for example the possible rudder reversal experienced on board [[United Airlines Flight 585]].<ref name="AAR01-01 Final Report">{{cite book |url=https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR0101.pdf |title=Aircraft Accident Report: Uncontrolled Descent and Collision With Terrain, United Airlines Flight 585, Boeing 737-200, N999UA, 4 Miles South of Colorado Springs Municipal Airport, Colorado Springs, Colorado, March 3, 1991 |publisher=[[National Transportation Safety Board]] |id=NTSB/AAR-01/01 |date=March 27, 2001 |access-date=January 17, 2016 |archive-date=October 2, 2015 |archive-url=https://web.archive.org/web/20151002041524/http://www.ntsb.gov/investigations/AccidentReports/Reports/AAR0101.pdf }}</ref>{{rp|114}}

Equipment failure may cause flight controls to behave unexpectedly, for example the possible rudder reversal experienced onboard [[United Airlines Flight 585]].

===Pilot error===

[[Pilot error]] is the most common cause of control reversal. In [[unusual attitude]]s it is not uncommon for the pilot to become [[Spatial_disorientationSpatial disorientation#Senses_during_flightSenses during flight|disoriented]] and start feeding inmake incorrect control movements in order to regain level flight. This is particularly common when using helmet[[Helmet-mounted display|helmet-mounted display systems]], which introduce graphics that remain steady in the pilot's view, notably when using a particular form of [[attitude indicator|attitude display]] known as an ''inside-out''.<ref name="jenkins-hmd">{{cite journalthesis |last1=Jenkins |first1=Joseph C. |title=The Effect of Configural Displays on Pilot Situation Awareness in Helmet-Mounted Displays |date=2007 |url=https://corescholar.libraries.wright.edu/cgi/viewcontent.cgi?article=1297&context=etd_all |publisher=Wright State University |bibcode=2007PhDT........44J |access-date= December 19, 2021 |format=pdf |degree=PhD |others=Jennie Gallimore (Advisor)}}</ref> which introduce graphics that remain steady in the pilot's view, notably when using a particular form of attitude display known as an ''inside-out''.<ref name="aviatn-psych-outside-in">{{cite journal |last1=Previc |first1=Fred H. |last2=Ercoline |first2=William R. |title=The `'Outside-In' Attitude Display Concept Revisited |journal=The International Journal of Aviation Psychology |date=October 1999 |volume=9 |issue=4 |pages=377–401 |doi=10.1207/s15327108ijap0904_5 |url=https://www.researchgate.net/publication/232896204_The_Outside-In%27_Attitude_Display_Concept_Revisited232896204 |access-date=December 19, 2021}}</ref>

===Incorrectly connected controls===

Incorrectly connected controls are another common cause of this problem. It is a recurring problem after maintenance on aircraft, notably homebuilthome built designs that are being flown for the first time after some minor work. However itIt is not entirely uncommon on commercial aircraft, and has been the cause of several accidents including the crash of the [[Short Crusader]] before the 1927 [[Schneider Trophy]] and the 1947 death of [[Avro]] designer [[Roy Chadwick]].<ref name="avsafety-G-AGSU-crash">{{cite web |last1=Ranter |first1=Harro |title=ASN Aircraft accident Avro 689 Tudor 2 G-AGSU Woodford Airport (WFD) |url=https://aviation-safety.net/database/record.php?id=19470823-0 |website=aviation-safety.net |publisher=Aviation Safety Network |access-date=19 December 2021 |quote=''PROBABLE CAUSE: Incorrect assembly of the aileron control circuit.''}}</ref>

===Wing twist===

{{main|Aeroelasticity}}

Another manifestation of the problem occurs when the amount of airflow over the [[wing]] becomes so great that the force generated by the [[aileron]]s is enough to twist the wing itself, due to insufficient [[Torsion (mechanics)|torsional]] stiffness of the wing structure. For instance when the aileron is deflected upwards in order to make that wing move down, the wing twists in the opposite direction. The net result is that the airflow is directed down instead of up and the wing moves upward, opposite of what was expected. This form of control reversal is often lumped in with a number of "high speed" effects as [[compressibility]].

==Examples==

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===Supermarine Spitfire===

Due to the high speeds at which the [[Supermarine Spitfire]] could dive, this problem of [[aileron]] reversal became apparent when it was wished to increase the lateral maneuverability (rate of roll) by increasing the aileron area. The aircraft had a wing designed originally for an aileron reversal airspeed of 580&nbsp;mph, and any attempt to increase the aileron area would have resulted in the wing twisting when the larger ailerons were applied at high speed, the aircraft then rolling in the opposite direction to that intended by the pilot. The problem of increasing the rate of roll was temporarily alleviated with the introduction of "clipped" wing tips (to reduce the aerodynamic load on the tip area, allowing larger ailerons to be used) until a new, stiffer wing could be incorporated.

This new wing was introduced in the [[Supermarine Spitfire (Griffon-powered variants)#Mk.C2.A021 .28type 356.29|Mk 21]] and had a theoretical aileron reversal speed of {{convert|825|mi/h|km/h|abbr=on}}.<ref>Jeffrey Quill OBE, AFC, FRAeS ''Spitfire - A Test Pilot’sPilot's Story'' - Arrow Books 1983-89 - {{ISBN|0-09-937020-4}}

</ref>

===Boeing B-47===

The [[Boeing B-47]] was speed limited at low altitudes because the large, flexible wings would cancel out the effect of the control surfaces under some circumstances.<ref name="b-47">{{cite web |title=Boeing B-47E Stratojet USAF six-jet medium bomber |url=http://www.skytamer.com/Boeing_B-47E.html |website=www.skytamer.com |access-date=19 December 2021 |quote=''Wing "twist" limited tree-top speed to 425 knots (787 km/h) to avoid control reversal.'' |url-status=live|archive-url=https://web.archive.org/web/20101126141033/http://www.skytamer.com:80/Boeing_B-47E.html |archive-date=2010-11-26 }} {{unreliable-inline |date= December 2021}}</ref><ref name="nasa-quest-speed">{{cite book |last1=Loftin |first1=Lawrence K., Jr. |title=Quest for Performance: The Evolution of Modern Aircraft (NASA SP-468) |date=1985 |publisher=NASA |chapter=12-2: Jet Bomber and Attack Aircraft |url=https://history.nasa.gov/SP-468/ch12-2.htm |access-date=December 19, 2021 |format=On-line}} ([https://ntrs.nasa.gov/api/citations/19850023776/downloads/19850023776.pdf PDF download])</ref>

===Gossamer Condor===

Control reversal also affected the [[Gossamer Condor]], the [[Kremer prizes|Kremer Prize]]-winning human-powered airplane. When a [[wing warping]] mechanism was tried as a solution to a long-running turning problem, the effect was to turn the airplane in the opposite direction to that expected by conventional airplane knowledge. When the Condor was rigged "conventionally", the inside wing slowed down so much that it settled to the ground. By employing "backwards" wired wing-warping, the inside wingtip [[angle of attack]] was increased so that the added drag slowed that wing while the added lift allowed the airfoil to stay aloft at a slower speed. The tilted canard could then complete the turn.<ref>{{Cite web |url=http://www.humanpoweredflying.propdesigner.co.uk/html/the_gossamers.html |title=The Gossamers (Archived copy) |access-date=December 19, 2021|archive-url=https://web.archive.org/web/20150419063651/http://www.humanpoweredflying.propdesigner.co.uk/html/the_gossamers.html |archive-date=April 2919, 2015 |url-status=dead |website=www.humanpoweredflying.propdesigner.co.uk }}</ref>

==References==