Rudder trim... What for?

OkieAviator

OkieAviator

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OkieAviator
My plane has a rudder trim, not to be confused with a trim tab which it also has on the actual rudder. Labels on the trim seem to indicate that you put it neutral for takeoffs and landings, so assuming you only use it in flight when do you use it? Is it just to relieve fatigue so you're not apply rudder pressure during a long cross country?
 
Used so that you don't have to keep constant foot pressure on the rudder pedals.
 
Ever do a long duration climb? That rudder trim sure does relieve the pressure on that right leg.
 
BillTIZ said:
Ever do a long duration climb? That rudder trim sure does relieve the pressure on that right leg.
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Makes sense. Haven't encountered a climb long than about 5 mins yet. I'll mess with it next flight. Straight and level flight you neutral it back out or just put it to whatever you need to hold your bearing?
 
To keep the ball centered in climb/cruise/descent. You want it in the neutral position for takeoff and landing.

The actual mechanism doesn't move anything on the rudder specifically, it, like most small GA aircraft, readjusts the "center" position for the rudder pedals.

Find an open area or go to an uncontrolled field. Set the tab for full left/right deflection. Try to taxi. Note the differences. Be sure to reset before takeoff.
 
OkieAviator said:
Makes sense. Haven't encountered a climb long than about 5 mins yet. I'll mess with it next flight. Straight and level flight you neutral it back out or just put it to whatever you need to hold your bearing?
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No, your crab angle will hold your bearing.

The rudder trim is used to relieve pressure or keep the ball centered. Some aircraft will cruise slightly nose-left of center.
 
I noticed that the plane I fly cruises with the ball left of center. Never thought much about it because it flies straight and true so it's just a "quirk" I guess.
 
jspilot said:
I noticed that the plane I fly cruises with the ball left of center. Never thought much about it because it flies straight and true so it's just a "quirk" I guess.
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Well, unless the inclinometer is improperly mounted, there's an obvious contradiction there.

If the ball is not in the center, the plane is slipping. It must not be level, so the angle of bank, whatever it is, is too great for the rate of turn, which is zero.

Hence a slip, and not "straight and true".

Right?
 
I learned the hard way that when flying uncoordinated (out of trim ball not centered) the fuel gauges do not read accurately. The fuel is forced the out and inboard side of the fuel tanks not reading accurately and giving you a false reading.
 
FastEddieB said:
Well, unless the inclinometer is improperly mounted, there's an obvious contradiction there.

If the ball is not in the center, the plane is slipping. It must not be level, so the angle of bank, whatever it is, is too great for the rate of turn, which is zero.

Hence a slip, and not "straight and true".

Right?
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I've flown a lot of airplanes with improperly-mounted inclinometers...you center up the ball, and the airplane flies in a slight bank.

The amazing part to me is how many pilots are willing to let one inclinometer override 3 attitude indicators and the second inclinometer.:yikes:
 
MauleSkinner said:
I've flown a lot of airplanes with improperly-mounted inclinometers...you center up the ball, and the airplane flies in a slight bank.

The amazing part to me is how many pilots are willing to let one inclinometer override 3 attitude indicators and the second inclinometer.:yikes:
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Good news is it's pretty trivially easy to confirm - pretty much all planes have instructions on how to level them laterally, and most inclinometers, or the bank instruments containing them, should be adjustable.
 
OkieAviator said:
Makes sense. Haven't encountered a climb long than about 5 mins yet. I'll mess with it next flight. Straight and level flight you neutral it back out or just put it to whatever you need to hold your bearing?
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I have a bird with a heavy rudder (206). Climbing from sea level to 25,000 WILL wear our your right leg, I don't care who you are. Rudder trim is a necessity IMO.
 
I'm grateful that my airplane has three axis trim. Having an airplane fly slightly out of rudder trim is more annoying to me than having it out of pitch trim.

I rarely use the rudder trim in a climb or descent, but I adjust it to fine tune neutral cruise trim often.
 
A drooping flap, a slightly heavy wing, a twisted fin, etc will make the ball drift from center while the pilot thinks he's straight and level. Have a good mechanic check the rigging.

I use rudder trim all the time and I never climb higher than 1200' or so. No reason to fight a crossing wind when you have an easy adjustment to ease the pressure.
 
OkieAviator said:
Makes sense. Haven't encountered a climb long than about 5 mins yet. I'll mess with it next flight. Straight and level flight you neutral it back out or just put it to whatever you need to hold your bearing?
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Cruise, put it where you need to keep the ball centered with your feet off the rudders.

Bearing? My bearing (military) is just fine.
 
jspilot said:
I noticed that the plane I fly cruises with the ball left of center. Never thought much about it because it flies straight and true so it's just a "quirk" I guess.
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It's creating drag and not very efficient. Center the ball!
 
OkieAviator said:
...Is it just to relieve fatigue so you're not apply rudder pressure during a long cross country?
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Partially but in cruise you are normally going to have it neutral or close to it.

I'm sure your question has already been answered but in airplanes that are a bit larger with heavier loads, more power and bigger props you are going to have to adjust rudder input much more for both climbs and descents to compensate for the effects the engine and prop are going to impart on the airframe. Your'e also most likely going to be flying higher with longer climbs as a result and any trim system is simply there so you don't have to physically maintain those inputs and can basically let the plane fly hands off.

Think of it like cruise control.
 
If you have to ask, its a sign that your plane needs a bigger engine.
 
JHW said:
If you have to ask, its a sign that your plane needs a bigger engine.
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Well there is that.

I have elevator, rudder and aileron trim, in cruise rudder ain't neutral.
 
FastEddieB said:
Well, unless the inclinometer is improperly mounted, there's an obvious contradiction there.

If the ball is not in the center, the plane is slipping. It must not be level, so the angle of bank, whatever it is, is too great for the rate of turn, which is zero.

Hence a slip, and not "straight and true".

Right?
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I was talking about straight and level flight during the cruise. I'm not sure why this is the case but when I add left rudder( step on the ball) the plane becomes decidedly more slipped than if I just let it fly. I've never heard of applying left rudder in cruise( never taught that either.) Next time I fly with a CFI I'll be sure to ask.
 
jspilot said:
I was talking about straight and level flight during the cruise. I'm not sure why this is the case but when I add left rudder( step on the ball) the plane becomes decidedly more slipped than if I just let it fly. I've never heard of applying left rudder in cruise( never taught that either.) Next time I fly with a CFI I'll be sure to ask.
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You're lucky to have a whole gaggle of CFI's right here!

Certain of the "left turning tendencies" are still present in cruise. Think torque and spiralling slipstream. Precession and p-factor should NOT be present, and if you review them you'll see why.

But if two of the tendencies are there, why do we not need right rudder (to counter the spiralling slipstream) or right aileron (to fight the torque) in normal cruise?

Because the designers have designed out the need. On many planes you can see the leading edge of the vertical stabilizer is slightly offset to the left for that purpose. The engine may also be offset in its mounts, and/or each wing may have a slightly different angle of incidence.

If done just right, the plane should be "balanced" at normal cruise. Wings level, ball in the center.

Slower than that, you need to correct for left turning tendencies. We tend to notice that mostly in climb and slow flight.

But as aerobatic pilots know, dive much faster than cruise and eventually a pilot finds the need for left rudder to keep the ball in the center, since the built-in corrections become too great.

Clear?

More on the need for trim in GA aircraft to follow.
 
jspilot said:
I was talking about straight and level flight during the cruise. I'm not sure why this is the case but when I add left rudder( step on the ball) the plane becomes decidedly more slipped than if I just let it fly. I've never heard of applying left rudder in cruise( never taught that either.) Next time I fly with a CFI I'll be sure to ask.
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I use left rudder or trim on every descent.

If you have rudder trim then you are not necessarily applying left rudder in cruise. A little left rudder or left rudder trim might actually be less right rudder depending on how the trim is set. Most often when you reach cruise you will be trimmed for right rudder from the climb.
 
Nothing is more annoying in cruise flight than an aircraft with rudder out of trim. As stated above, in level cruise in a small GA aircraft, you should not require rudder input for straight and level flight. I spent 18 hours (Round trip 9hrs each way) in a 172 that needed constant left rudder in straight/level flight. We traded off who got to hold the pedal 20 minutes at a time. Eventually I think we propped something against the pedal that could be easily knocked out of the way.
 
FastEddieB said:
Good news is it's pretty trivially easy to confirm - pretty much all planes have instructions on how to level them laterally, and most inclinometers, or the bank instruments containing them, should be adjustable.
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Yep in the 172's you place the level on the seat rails and if the inclinometer doesn't match, you've got issues.

I've flown a 182 that had rudder trim, but it's more common in twins (where it's really handy in single engine ops).
 
Pedals2Paddles said:
Nothing is more annoying in cruise flight than an aircraft with rudder out of trim. As stated above, in level cruise in a small GA aircraft, you should not require rudder input for straight and level flight. I spent 18 hours (Round trip 9hrs each way) in a 172 that needed constant left rudder in straight/level flight. We traded off who got to hold the pedal 20 minutes at a time. Eventually I think we propped something against the pedal that could be easily knocked out of the way.
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Maybe one of the pedal springs was broken?
 
Most of the planes I've flown have had only pitch trim.

Of course a handful had rudder trim as well, especially as the planes got bigger and/or more powerful.

For the average GA plane, I'll put it down as a "nice to have" option - but each extra option is not without its own drawbacks - weight, complexity, maintenance, add'l failure modes, etc.

Interestingly, the very first Cirrus SR22's had rudder trim, sourced to a small company*. They were very problematical, and later SR22's did not come equipped with them, and I think many early versions had the removed.

With 310 hp, the Cirrus certainly could use rudder trim in a long climb. But it is not a huge deal, especially at faster cruise climb speeds where its just a matter of resting your right foot on the rudder pedal. Newer models may have a yaw damper, which pretty much eliminates the need entirely.



*Owned by Norm Smith of Mooney fame? My memory is fuzzy here.
 
This thread makes my head hurt. I think using rudder trim to hold bearing is what put me over the edge. Where do you start with that?
 
FastEddieB said:
I know that modern Flight Design CT's have trim in all 3 axis.

The owners seem to like that, though it seems excessive to me.
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My CT needs all 3. Pitch I use constantly, rudder I use for climbs and descents and aileron tends to be a one time adjustment for solo vs passenger.

Without all 3 I would be holding pressure or flying wierd.

Edit: Just look at my avatar, obviously I just let my passenger off and have not yet set the aileron trim. ;)
 
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Captain said:
This thread makes my head hurt. I think using rudder trim to hold bearing is what put me over the edge. Where do you start with that?
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I'm sincerely hoping it is just poorly worded.
 
But even in level flight, at different airspeeds you will sometimes want rudder trim. The reason is, the trim tab on the rudder is less effective at lower airspeed (plane is also at different angle of attack). I don't have onboard rudder trim. In an ideal airplane, you would have onboard rudder and aileron trim. Most planes don't fly in perfect wing level when ball is centered, due to a number of factors like prop effect etc. Remember the down stroke of the prop is more powerful than the upstroke when the plane has positive angle of attack. YMMV. On a long flight in still air with a flat horizon, you tend to notice things lie this....
 
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Captain said:
This thread makes my head hurt. I think using rudder trim to hold bearing is what put me over the edge. Where do you start with that?
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I 'hold bearing' by tracking it. For that I use flaperon not rudder or rudder trim, I trim my rudder in cruise to keep from slipping which has nothing to do with my track.
 
Captain said:
This thread makes my head hurt. I think using rudder trim to hold bearing is what put me over the edge. Where do you start with that?
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Well lets start with what I was trying to convey. In an example I'm tracking a course of 360, but because of wind my airplane heading is pointed to 355. To maintain the bearing of 360 (Am I using this term wrong?) I would have to apply rudder to keep the ball centered and the plane crabbed. My question was do I use rudder trim in this case to relieve the pressure. I believe the answer was that I could.
 
OkieAviator said:
To maintain the bearing of 360 (Am I using this term wrong?) I would have to apply rudder to keep the ball centered and the plane crabbed. My question was do I use rudder trim in this case to relieve the pressure.
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As Fast Eddie would say, I think you are having a stick and rudder moment.

You would not have to apply rudder pressure to keep the ball centered and the plane crabbed. If the rudder is not trimmed you would need rudder pressure to keep coordinated but that is about flying efficiently without unwanted drag. You keep the plane crabbed by changing your heading, usually done with a little aileron input, until your ground track matches your bearing. Rudder and rudder trim have nothing to do with your crosswind crab unless that is how you change your heading.
 
OkieAviator said:
Well lets start with what I was trying to convey. In an example I'm tracking a course of 360, but because of wind my airplane heading is pointed to 355. To maintain the bearing of 360 (Am I using this term wrong?) I would have to apply rudder to keep the ball centered and the plane crabbed. My question was do I use rudder trim in this case to relieve the pressure. I believe the answer was that I could.
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Yes.

Once an aircraft is in the air, it feels no crosswind. The only reason you recognize there is one when is when you compare your heading to your ground track.
 
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