Yaw dampers

A SYS­TEM THAT SMOOTHS OUT FISHTAILING ON SMALL AIR­CRAFT AND HELPS SQUELCH DUTCH ROLL TEN­DEN­CIES ON LARGER AIR­CRAFT

Flying - - Contents - By Rob Mark

In its most ba­sic form, a yaw damper in­hibits move­ment of an air­craft around its ver­ti­cal axis, per­form­ing like an automated set of feet on the rud­der ped­als.

A yaw damper pulls air­craft move­ment in­for­ma­tion from a se­ries of ac­celerom­e­ters or rate sen­sors in the rud­der and trans­lates it into the proper amount of calm­ing rud­der in­puts. In a sin­gle-en­gine air­plane, the yaw damper smooths out the left-right move­ments of the ver­ti­cal sta­bi­lizer, of­ten re­ferred to as fishtailing, cre­at­ing a more com­fort­able ride for pas­sen­gers.

The yaw damper on a sweptwing air­craft, es­pe­cially one with a T-tail, also in­hibits the Dutch roll ten­dency, a wal­low­ing com­bi­na­tion of yaw­ing and rolling mo­tions of the wing. When a Dutch roll oc­curs on an air­craft with­out a damper, any yaw­ing mo­tion can cre­ate corkscrew-like os­cil­la­tions that con­tinue un­til they ei­ther die out nat­u­rally or es­ca­late.

In older straight-wing air­craft, yaw damper func­tions can be se­lected on or off by the pilot, while in more re­cent air­planes, such as

the lat­est model Cir­rus SR22, the yaw damper en­gages au­to­mat­i­cally once the air­craft climbs above 200 feet agl. The damper sys­tem au­to­mat­i­cally dis­en­gages when the air­plane de­scends be­low 200 feet agl on ap­proach to land­ing.

Cir­rus yaw damper ser­vos in the tail of the air­craft are in con­stant com­mu­ni­ca­tion with most of the avion­ics on board, in­clud­ing the air-data at­ti­tude head­ing ref­er­ence sys­tem. The ADAHRS is, in fact, con­stantly mon­i­tor­ing ev­ery pitch, roll and yaw move­ment, and the Cir­rus pro­vides en­ve­lope pro­tec­tion whether the au­topi­lot is en­gaged or not.

If the com­put­ers con­nected to the rud­der sense a yaw move­ment be­yond their pre­set limit, the yaw damper sends a sig­nal to the rud­der servo in­di­cat­ing the proper amount, di­rec­tion and fre­quency of rud­der pres­sure that should be added in or­der to calm the event. Land­ing an SR22 with the yaw damper on means the pilot must over­come the au­to­ma­tion-in­duced con­trol forces.

On many swept-wing air­planes, the yaw damper is switched on or off from the cock­pit, of­ten due to trans­port cat­e­gory cer­ti­fi­ca­tion re­quire­ments.

At­tempt­ing a take­off in a large air­craft with the yaw damper en­gaged could lead to the air­plane cor­rect­ing on its own for ad­verse yaw in the event of a power-plant fail­ure. That makes iden­ti­fi­ca­tion of the failed power plant more dif­fi­cult.

Land­ing a swept-wing air­craft with the yaw damper switched on, es­pe­cially in a strong cross­wind, could limit the pilot’s avail­able con­trol au­thor­ity at time of touch­down.

Us­ing the yaw sen­sors in the tail of the air­craft, a yaw damper will add just the right amount of rud­der in a turn for the an­gle of bank to en­sure co­or­di­na­tion. Yaw damper au­to­ma­tion might ex­plain why so many pilots never touch the rud­der ped­als when mak­ing turns, whether they’re fly­ing a jet or a Cir­rus.

A down­side for pilots used to fly­ing air­craft with yaw dampers oc­curs when they tran­si­tion back to an air­plane with­out a yaw damper or one that’s in­op­er­a­tive.

The first few hours of watch­ing an air­plane skid or slip through turns is nor­mally all that’s needed to reac­quaint a pilot with the need to de­cide on their own how much rud­der to add to pro­duce co­or­di­nated turns.

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