Purpose of rudder trim if you have yaw damper?

kicktireslightfires

kicktireslightfires

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kicktireslightfires
What's the purpose of rudder trim on an airplane that also has a yaw damper? The yaw damper keeps the rudder coordinated automatically, right? So when would you use rudder trim?
 
Ah, that explains it! Thank you! Because where I saw this was on a multi-engine plane. So then would it be correct to assume you'll only find a rudder trim on an airplane that has a yaw damper if it's multi-engine? I'm not multi-engine rated. If you have an engine failure in a twin, do you have to disengage the autopilot and then adjust the rudder trim? Can the autopilot still function with one running engine on a twin, and if so, how does the A/P yaw control handle the unbalanced thrust? I almost feel like Garmin would have a mode for multi-engine aircraft with an autopilot that allows for single engine autopilot operations?
 
kicktireslightfires said:
So then would it be correct to assume you'll only find a rudder trim on an airplane that has a yaw damper if it's multi-engine?
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No, that’s not correct. There are plenty of multi-engine aircraft without yaw dampers but still have rudder trim, just like some single engine aircraft have them and others don’t. Generally the higher end, high performance singles will have a y/d, but it may or may not be an aftermarket installation.

Yaw dampers are generally coupled to an autopilot, unlike rudder trim, which is manually adjusted by the pilot like any pitch trim. They use accelerometers and rate sensors (like an autopilot would for roll and pitch) to determine motion and then algorithms determine what rudder inputs are needed to keep a turn coordinated and reduce Dutch roll tendencies.
 
Yaw dampers typically work to damp out yaw *rate* like that of Dutch roll oscillations or adverse yaw to some extent, and doesn't do anything for a static offset in yaw trim. Yaw trim is used to trim for balanced flight, i.e. ball centered in non-maneuvering flight. Yaw trim in and of itself does nothing for yaw oscillations and adverse yaw, but is useful for changes in rudder required over a wide range of airspeeds and power settings - single or multi-engine.

Nauga,
and his second-order model
 
They serve different purposes, although on a light plane maybe it’s not a big deal for the YD to handle both purposes. You wouldn’t want the YD to have to fight an untrimmed rudder in the climb in something like a PC-12 with all that torque.

It’s funny that you mention Garmin maybe having an autopilot mode for flying a twin with a dead engine. I think they recently announced just that feature on one of their higher-end autopilots.
 
RyanB said:
No, that’s not correct. There are plenty of multi-engine aircraft without yaw dampers but still have rudder trim, just like some single engine aircraft have them and others don’t. Generally the higher end, high performance singles will have a y/d, but it may or may not be an aftermarket installation.

Yaw dampers are generally coupled to an autopilot, unlike rudder trim, which is manually adjusted by the pilot like any pitch trim. They use accelerometers and rate sensors (like an autopilot would for roll and pitch) to determine motion and then algorithms determine what rudder inputs are needed to keep a turn coordinated and reduce Dutch roll tendencies.
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Hmm I guess back to my original question then. If a single engine airplane has an always-on yaw damper, what's the point of rudder trim? Sorry if I'm not yet understanding what you're saying.
 
iamtheari said:
It’s funny that you mention Garmin maybe having an autopilot mode for flying a twin with a dead engine. I think they recently announced just that feature on one of their higher-end autopilots.
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I recall hearing about that as well. If you lose an engine, the autopilot detects the rapid change in yaw and compensates for it to reduce pilot workload. Although I don’t remember what suite it’s included with or what it was called. Cool stuff!
 
iamtheari said:
It’s funny that you mention Garmin maybe having an autopilot mode for flying a twin with a dead engine. I think they recently announced just that feature on one of their higher-end autopilots.
Click to expand...
Interesting. I'd like to hear more about that. In highly-augmented airplanes we have had the ability to make engine failure completely transparent to the the crew (in terms of handling, not performance) for years. We found that performance was greatly improved by having *some* 'traditional' cues still noticeable to make it easier to determine which engine had failed. So here's an example where the maximum authority of 'automation' is not used to improve crew performance.

Nauga,
swapping ends
 
nauga said:
Interesting. I'd like to hear more about that. In highly-augmented airplanes we have had the ability to make engine failure completely transparent to the the crew (in terms of handling, not performance) for years. We found that performance was greatly improved by having *some* 'traditional' cues still noticeable to make it easier to determine which engine had failed.

Nauga,
swapping ends
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Found it!

https://www.garmin.com/en-US/blog/a...ology-for-select-twin-engine-piston-aircraft/
 
kicktireslightfires said:
Hmm I guess back to my original question then. If a single engine airplane has an always-on yaw damper, what's the point of rudder trim? Sorry if I'm not yet understanding what you're saying.
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A yaw damper cancels out yaw rate, not sideslip angle. Just because you have a yaw damper doesn't mean the aircraft will fly coordinated.
 
Lot of times i'll torque out on an engine before the other. The 4 conquest I fly seem to like a bit of left trim near vmo mmo and a bit of right on approach. On the 145 those things had 20k plus hours and needed rudder trim left or right to stay coordinated. YD doesn't make the plane coordinated just dampens the yawing motions.
 
kicktireslightfires said:
Hmm I guess back to my original question then. If a single engine airplane has an always-on yaw damper, what's the point of rudder trim? Sorry if I'm not yet understanding what you're saying.
Click to expand...
If you’re climbing in calm air a yaw dampener will not help you center the ball whereas rudder trim will allow you to do it.

yaw dampener stops yaw oscillations, it’s dynamic. Rudder trim is static.
 
nauga said:
Interesting. I'd like to hear more about that. In highly-augmented airplanes we have had the ability to make engine failure completely transparent to the the crew (in terms of handling, not performance) for years. We found that performance was greatly improved by having *some* 'traditional' cues still noticeable to make it easier to determine which engine had failed. So here's an example where the maximum authority of 'automation' is not used to improve crew performance.

Nauga,
swapping ends
Click to expand...

My brother got typed in the Gulfstream G500 recently, which is a fancy bizjet with full fly by wire controls. There is zero help with rudder input during OEI operations. He said that it required significant rudder pressure to stay coordinated, much more so than the Global Express he used to fly. I am surprised that they didn't provide at least a little help, probably wouldn't have been that hard to do.
 
The question is a bit like asking why an airplane has yokes when there's an autopilot installed. Rudder, elevator, or aileron trim, if you have them, are parts of the primary physical control system (all intended to relieve control pressures); an autopilot (including the yaw damper) is part of your avionics.
 
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