6
$\begingroup$

I flew an SR22T the other day, the first high-performance airplane I've flown and I was incredibly surprised by the amount of yaw induced during a touch-and-go, nearly causing a ground loop.

After the flight I went and read about the yaw damper feature on the aircraft and learned that it doesn't engage until 200ft AGL. Why is the damper not available for use at takeoff considering the large torque-induced yaw produced in aircraft like this?

$\endgroup$
2
  • $\begingroup$ A yaw damper on a single suggests the plane is "under-finned" so the yaw static stability is only mildly positive. The result is stronger torque reactions and an annoying tail wagging tendency in bumpy air. Rather than enlarge the fin, which would mean revising the entire fuselage mold, easier to add a YD. The other benefit is you don't have to use your feet at all while hand flying the thing, which is also the case with jets. $\endgroup$
    – John K
    Jun 15, 2023 at 12:44
  • $\begingroup$ @JohnK Interesting, I guess it's entirely possible considering how they've been upping the HP on these things without airframe changes. Considering the target market, I would definitely also assume that relieving the pilot of turn coordination (among other things) is a big selling point. $\endgroup$
    – joshperry
    Jun 15, 2023 at 19:15

1 Answer 1

7
$\begingroup$

As pointed out in this answer, the term yaw damper is a bit wide. This autopilot function is typically used to

  1. Dampen unwanted oscillations in the yaw axis such as the dutch roll mode
  2. Helps to keep coordinated turns by counteracting sideslip
  3. Can be part of an Automatic Flight Control System (AFCS)

This is also described in the according manual (page 414).

However the seconds point becomes important for the take-off or landing of a small general aviation aircraft such as the Cirrus: In a cross-wind landing, the pilot tries to achieve a sideslip angle, to be able to land on the runway. The yaw damper would try to counter-act this sideslip. For this reason it has to be deactivated for the landing. The same holds true for take-offs: In order to be able to fully control the aircraft e.g. in a crosswind, without having unwanted inputs by the yaw damper, this system is fully deactivated.

In pretty much all other flight states (e.g. 200 ft above ground), you most likely would never fly with a sideslip angle.

According to the flying magazine, a second reason (although only for bigger aicrafts) can be:

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

$\endgroup$
8
  • $\begingroup$ @Jamiec you are right, I reformulated my answer to make it a bit more sound and point this out. $\endgroup$
    – U_flow
    Jun 15, 2023 at 12:00
  • 2
    $\begingroup$ I would disagree with FLYING's answer. Airliner YD systems are always running in the background. When an engine fails on an airliner, the YD doesn't disengage itself. Its authority is so limited, about 1/4 of rudder travel, it has little noticeable effect. With a swept wing jet, if you don't input a LOT of rudder right now when an engine quits at takeoff thrust, you will be on your back shortly. Whether YD is engaged for not matters little. In the case of the Cirrus, resistance to cross control inputs when you really need them is probably the best reason. $\endgroup$
    – John K
    Jun 15, 2023 at 12:36
  • $\begingroup$ @JohnK Good point on the pilot authority to input cross-control, in-fact that may have been what allowed me to keep the wing off the ground. Had it input right rudder at that point, things probably would have ended up much worse. $\endgroup$
    – joshperry
    Jun 15, 2023 at 19:10
  • $\begingroup$ @U_flow I appreciate the answer! These are all great points, but I'm most interested in the takeoff as that's where torque and p-factor induced yaw is the biggest problem if you have any other info on that phase (maybe we should roll-up John's point about the importance of pilot authority). $\endgroup$
    – joshperry
    Jun 15, 2023 at 19:18
  • $\begingroup$ @joshperry when the Cirrus's YD is engaged, do you feel the rudder pedals moving under your feet? On airliners with cable/hydraulic controls, the YD is often just a servo in the linkage ahead of the rudder that lengthens and shortens the linkage, and you don't have any movement of the pedals. $\endgroup$
    – John K
    Jun 15, 2023 at 22:11

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .