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Lateral control limiting with sideslip

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Lateral control limiting with sideslip

Old 25th Feb 2021, 17:49
  #1 (permalink)  
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Lateral control limiting with sideslip

Anyone have experience, flight or theory, of limiting lateral control in a situation involving sideslip from thrust asymmetry. Only able to use aileron for control due to a fixed rudder; e.g. one engine fail, - limited hydraulics, no rudder control.

Not a takeoff situation; climb thrust for max asymmetry, but also need to consider the ability to turn; i.e additional aileron required.

Also, considering a specific situation starting with the autopilot engaged, presumably being more limiting because less aileron control is available than in manual flight.

This is in a non FBW aircraft, conventional manual controls, except rudder with hydraulic boost.
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Old 27th Feb 2021, 20:41
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Join Date: Jun 2002
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Might be an idea to tell ,`what aircraft`...or is it a new design...?
Seems like it needs a bigger rudder/more travel/better hyds/better.larger trimmer.....or better ailerons...
sycamore is offline  
Old 28th Feb 2021, 13:46
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The situation is slightly contrived, but one which reappears in some LoC accidents.
The flight test aspect was many years go involving validation of FAR/JAR for the Russian Federation acting on behalf of a third party.
Their special conditions required evaluation of situations which had previously resulted in accidents, more of interest than requirement. The particular situation related to likely LoC; cf, a Russian operator, and similar to the 747 ‘roll-over’ in the Pacific.

My ‘no rudder’ restriction was chosen to limit any emotive debate of why the crew did not use rudder (or understand the situation), this also enables the situation to be considered more widely and compared with other types.

The test aircraft, Avro RJ, like most others, could achieve and maintain the unbalanced condition with manual control. However, the test point was difficult to set up, requiring co-ordination between yaw (significant thrust split) and applying aileron - need to start with aileron, but yaw had triggered the event. Misjudgement led to disorientating roll-yaw over-bank, and surprise that the situation was not as expected. Typically once the balance was understood the task was easy; a stable condition was achieved and the aircraft could be manoeuvred.

In intervening years, aspects of expectation / disorientation had been identified in 146/RJ autopilot upset incidents, also in accidents with other aircraft types.
The autopilot aspect is a practical starting point; yaw asymmetry (un-alerted thrust split) results in roll, but the AP limited control, and probable disengagement, directs pilot attention to roll and AP failure.
Thence with increasing roll, even if the aircraft is capable of manual recovery, the disorientation, need of instant maximum aileron input, which with continuing yaw might result in an unrecovered LoC accident.

The human aspects involve judging the human contribution in assessing the dynamic situation and then choosing a suitable course of action to recover control. If the pilot gets behind the control requirement - less than full aileron, unaware of thrust split, and with increasing roll, the situation may not be recovered.

Seeking an exchange of views and experiences.
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Old 17th Mar 2021, 16:15
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Join Date: Sep 2011
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First, the RJ was somehow certified, so it complies with the "engine out" requirements.
Second, I heard that when performing SHSS (AEO) on an RJ with 18 degs of flaps you can reach the aileron limit with less rudder.
Maybe the Cnbeta, Cndeltarudder, Cndeltaaileron, beta, deltarudder, deltaaileron plots will give an answer.

TrogAir is offline  
Old 17th Mar 2021, 22:44
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Trog, thanks for the thoughts. The issue is not specific to the RJ.
The background thinking is to explore accident situations, where an aircraft can be shown to meet aerodynamic / control certification requirements - static conditions, but in dynamic accident situations the aircraft is unable to achieve the level of control which might be assumed from certification.
Re the Cn parameters, the issue might be best related to Cn dynamic, but probably not as a classic theoretical problem.
As another example, hopefully, not to confuse the situation further, consider rudder ‘crossover speed’ - a static wings-level test point. By definition this could not be achieved (control limit, constant speed) if the aircraft was already rolling because greater control power is required to first stop the roll, and second return the aircraft to wings level. The limiting roll power is encountered earlier in dynamic ‘failure’ conditions than at the static certification test point.
This issue represents a gap between certification requirements, and the practical control limits encountered in rare accident situations, which could be misinterpreted by investigators.
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