Rudder trim question

alfadog

alfadog

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alfadog
I know some of this is RTFM and I will do that but I have a question about rudder trim and would appreciate some input.

My Arrow II has rudder trim and I would like to know more about using it.

First, I did not realize that it might have a takeoff trim setting. Does it? I never noticed one but I never looked closely either. Should mention that I don't have a ton of time in my Arrow.

Second, how is this for proper technique for trimming in cruise? If I hold the ailerons neutral (assuming I can do that but I can compare them to the flaps) and then raise the low wing with the rudder then I should be compensating for a lateral imbalance, no? I might be flying a bit sideways but that is unavoidable until I can balance out a bit better with fuel burn?

What am I missing here (I feel like I am missing something)?

Thanks
 
Rudder trim is just there to do for your feet what elevator or pitch trim does for you on the yoke or stick. Use enough trim to reduce your workload on the rudder pedals.

Don't try to raise or lower the wing with rudder trim.

Level the wings with the ailerons, and hold them level See where the inclinometer (skid ball) sits. Apply enough rudder to center the ball. Trim to take out any pressure you're holding on the rudder. Done.
 
DouglasBader said:
Rudder trim is just there to do for your feet what elevator or pitch trim does for you on the yoke or stick. Use enough trim to reduce your workload on the rudder pedals.

Don't try to raise or lower the wing with rudder trim.

Level the wings with the ailerons, and hold them level See where the inclinometer (skid ball) sits. Apply enough rudder to center the ball. Trim to take out any pressure you're holding on the rudder. Done.
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This speaks to what I think I am missing here. I can hold the wings level and move the tail left and right. I can adjust the ailerons to keep the wings level and the airplane flying a constant heading (I think). I know that the ball moves left and right but I do not see why? I mean if the wings are level and we are flying slightly sideways, how does the ball "know" that?
 
Skylane81E said:
Then if the weight is about equal between the sides of the plane (fuel etc) you need to be rerigged
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I am already assuming that there is unbalance. Perhaps I have equal fuel and am flying alone.
 
alfadog said:
This speaks to what I think I am missing here. I can hold the wings level and move the tail left and right. I can adjust the ailerons to keep the wings level and the airplane flying a constant heading (I think). I know that the ball moves left and right but I do not see why? I mean if the wings are level and we are flying slightly sideways, how does the ball "know" that?
Click to expand...

Lateral acceleration...
 
alfadog said:
This speaks to what I think I am missing here. I can hold the wings level and move the tail left and right. I can adjust the ailerons to keep the wings level and the airplane flying a constant heading (I think). I know that the ball moves left and right but I do not see why? I mean if the wings are level and we are flying slightly sideways, how does the ball "know" that?
Click to expand...

I noticed this when using the autopilot. Set for a constant course heading, if I adjust rudder trim to the right, the autopilot will bank left. course will remain the same, but there will be more drag and the ball will shift as it flies sideways slightly, as you say. But, the wings are not level. If the wings remained level and the ball was not centered, then the heading would change (turn?)
 
Skylane81E said:
Hold wings level with aileron and burn fuel from heavy wing
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Yes, that was my original premise: "I might be flying a bit sideways but that is unavoidable until I can balance out a bit better with fuel burn?"

So if I am unbalanced and force the down wing up with aileron, what does that do to the ball?
 
alfadog said:
Yes, that was my original premise: "I might be flying a bit sideways but that is unavoidable until I can balance out a bit better with fuel burn?"

So if I am unbalanced and force the down wing up with aileron, what does that do to the ball?
Click to expand...

Adverse yaw. Likely small, but there.
 
Clark1961 said:
Ball centered and wings not level = turning
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Of course. My point being that you want to get the wings level. Rudder or aileron? What about if you have a wing leveler AP like in my Arrow? Have to use the rudder trim to level the wings if the AP is on? That is how it seemed to me.
 
alfadog said:
Of course. My point being that you want to get the wings level. Rudder or aileron? What about if you have a wing leveler AP like in my Arrow? Have to use the rudder trim to level the wings if the AP is on? That is how it seemed to me.
Click to expand...

Rudder for yaw, ailerons for roll and elevator for pitch
 
I mean if the wings are level and we are flying slightly sideways, how does the ball "know" that?
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The ball doesn't know anything; it only reacts to the forces acting upon it.

At rest on the ground, the only force acting on the ball is gravity. This is why the ball will be out of center if the instrument is tilted, or if the airplane is parked on an incline, or if one gear is low.

In flight, if the wings are level and the ball sits centered on the ground with wings level, then the ball being out of center shows that the airplane is not properly trimmed about the vertical axis. The ball is sometimes referred to as a "skid" ball, and we refer to it's being out of center sometimes as the airplane not being "coordinated."

If your wings are level and the ball is not centered, you need rudder application to center the ball. If you need to hold the rudder application, then you can use rudder trim to do that. You're already familiar with P factor and asymmetrical forces in the climb because you normally need rudder at high angles of attack at high power settings and low airspeeds (a climb). This is one such time when one might see an application for rudder trim. One might also see it in cruise when forces causing the airplane to yaw are uneven.

Adverse yaw is most often felt in a turn with a strong aileron deflection, but it can be felt in cruise, too. If you have a significant imbalance, such as one wing completely full of fuel and the other empty, you're going to be carrying a fair amount of aileron to keep the wings level. As you do this, your "down" aileron creases considerable induced drag on that wing, causing the airplane to want to yaw in that direction; you need rudder to counteract it. Slight amounts of aileron displacement require lesser amounts of rudder use and trim.

Sometimes mechanics use an old trick to rig an airplane to fly straight. If the airplane has a "turn" in it or a tendency to not remain neutral in roll, then it's sometimes easier for the mechanic to tweak the flaps a little rather than re-rig the airplane (especially if it's an incidence issue between the two wings). Some manufacturer maintenance procedures actually call for this.

It's possible, over time, to get an airplane that isn't properly rigged or in trim to fly wings level. I actually encountered one extreme case in which the IA who did the last annual had rigged the airplane to not be able to turn left at all; full left deflection was a wings-level aileron condition, and the airplane could only be turned right in flight. He defended his actions, incredibly enough, and the owner flew the airplane that way, commercially, for over 10 hours. I've never seen anything quite like it.

If you hold the wings level and then center the ball with your feet, and then trim out the rudder pressure so the airplane holds itself there, you've done your part. If you let go of the yoke or stick and the airplane tries to bank, you've got a lateral imbalance which either needs to be held, or trimmed out (if your'e in an aircraft that's got aileron trim).
 
DouglasBader said:
The ball doesn't know anything; it only reacts to the forces acting upon it.
Click to expand...

Understood. That is what I mean by "know", i.e. what forces are acting on it.

In flight, if the wings are level and the ball sits centered on the ground with wings level, then the ball being out of center shows that the airplane is not properly trimmed about the vertical axis. The ball is sometimes referred to as a "skid" ball, and we refer to it's being out of center sometimes as the airplane not being "coordinated."

If your wings are level and the ball is not centered, you need rudder application to center the ball. If you need to hold the rudder application, then you can use rudder trim to do that. You're already familiar with P factor and asymmetrical forces in the climb because you normally need rudder at high angles of attack at high power settings and low airspeeds (a climb). This is one such time when one might see an application for rudder trim. One might also see it in cruise when forces causing the airplane to yaw are uneven.
Click to expand...

To break my apparent confusion down, let's take a balanced airplane. If I hold the wings level and hit the rudder, the airplane will turn, correct? This is what I am taught to do on the ILS and it seems to work.

Can I fly a balanced airplane on a constant heading with the wings level and the ball out of center? It would seem not. It would enter a skidding turn and that is consistent with where the ball is, to the outside of the turn.

Adverse yaw is most often felt in a turn with a strong aileron deflection, but it can be felt in cruise, too. If you have a significant imbalance, such as one wing completely full of fuel and the other empty, you're going to be carrying a fair amount of aileron to keep the wings level. As you do this, your "down" aileron creases considerable induced drag on that wing, causing the airplane to want to yaw in that direction; you need rudder to counteract it. Slight amounts of aileron displacement require lesser amounts of rudder use and trim.
Click to expand...

OK, I get that. So let us say the right wing is low due to imbalance or rigging issue. I add left aileron. Now the wing is up level. But the ball will be on the left, yes? Do I add left rudder (or rudder trim, same difference)? It seems one condition would make me turn and the other would not. Which one?

If you hold the wings level and then center the ball with your feet, and then trim out the rudder pressure so the airplane holds itself there, you've done your part. If you let go of the yoke or stick and the airplane tries to bank, you've got a lateral imbalance which either needs to be held, or trimmed out (if your'e in an aircraft that's got aileron trim).
Click to expand...

Guess you have answered the question I asked above. It seems counterintuitive to me that I could fly straight with an aileron imbalance (holding a heavy wing up) without holding a rudder imbalance.

Anyone know how a wing leveler would factor in? It seemed mine wanted to hold a wing low so I "fixed" it with the rudder trim.
 
It's only counter intuitive if you forget that you have trim for only one of the controls. You are holding the aileron imbalance and the plane is holding the rudder imbalance for you.

The wing leveler wanted to hold a wing low because the rudder was off and it needed to slip the plane to maintain the desired heading.
 
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Skylane81E said:
The wing leveler wanted to hold a wing low because the rudder was off and it needed to slip the plane to maintain the desired heading.
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I will have to do some more experimenting but, IIRC, when I trimmed the rudder to bring the low wing up that the AP was holding down, I then had the ball out-of-center. Assuming there was left/right imbalance, that seems to make sense to me though I have not worked out why but you say it doesn't?
 
Ah, if only physics teachers would have just expained Isaac Newton's theory of coordinated airplane flight. Please bear with me, because it's a little like Ohm's law. Rule number 1 - If the wings are level AND the heading is constant, the aircraft is in coordinated flight. Rule 2 - If the the heading is constant but the aircraft is in a bank, (no matter how slight) the aircraft is uncoordinated. Rule 3 - If the wings are level, but the airplane is yawing, (no matter how slowly) you're uncoordinated. Wings banked, airplane must be turning, wings level, airplane must be NOT turning. With me so far?

Once you teach yourself to see coordinated flight, you can start to relax and feel coordinated flight. If you already fly a Luscombe, you're on the road to coordinated nirvana.

In an airplane with an autopilot, in cruise you can demonstrate the theory thusly: Trim the airplane for cruise, turn on autopilot in heading or tracking mode, ie, heading 360 or track your present VOR radial to or from a station. So, we've tasked the autopilot with maintaining a constant heading...are the wings level? If you're in a slight left bank, the autopilot is telling you "Hey, I can fly heading 360, but if you insist on holding this much right rudder, I can only do it by flying in a left bank." Unless you're an Asian guy named Won WingLow you will need to trim the rudder towards the bank until the wings are level. Does that help?
 
Sounds like the TC might be crooked. Level the airplane (W&B section of the POH might tell you how but a level across the seat tracks is probably a good start) and see if the ball is centered sitting on the ground.

See also "garbage in garbage out"


Be sure only a rated mechanic does the task of loosening the TC and turning it till the ball is dead nuts centered then retightening the screws if needed:wink2:
 
Skylane81E said:
Sounds like the TC might be crooked. Level the airplane (W&B section of the POH might tell you how but a level across the seat tracks is probably a good start) and see if the ball is centered sitting on the ground.

See also "garbage in garbage out"


Be sure only a rated mechanic does the task of loosening the TC and turning it till the ball is dead nuts centered then retightening the screws if needed:wink2:
Click to expand...

Well, I know from checking carpenter's levels (did I mention that I used to build houses?) that if I park the Arrow, check the "bubble" then rotate the airplane 180 degrees, park it on the same spots (bit of chalk on the ramp), and check the "bubble" again, I will know if there is error in the TC. I will do that this weekend.
 
alfadog said:
Well, I know from checking carpenter's levels (did I mention that I used to build houses?) that if I park the Arrow, check the "bubble" then rotate the airplane 180 degrees, park it on the same spots (bit of chalk on the ramp), and check the "bubble" again, I will know if there is error in the TC. I will do that this weekend.
Click to expand...

Go forth and educate yourself about your planes condition.

Then let us know what you find. I'm intrigued my own self.
 
j1b3h0 said:
Rule number 1 - If the wings are level AND the heading is constant, the aircraft is in coordinated flight. Rule 2 - If the the heading is constant but the aircraft is in a bank, (no matter how slight) the aircraft is uncoordinated. Rule 3 - If the wings are level, but the airplane is yawing, (no matter how slowly) you're uncoordinated. Wings banked, airplane must be turning, wings level, airplane must be NOT turning. With me so far?
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Not really with you. Help me puzzle this out, if you will.

You say "Wings banked, airplane must be turning, wings level, airplane must be NOT turning" yet I am taught on the ILS final to hold the wings level and make slight heading corrections (turns) with the rudder. Yes, they are not coordinated turns but they are turns nonetheless. If I kick the thrust line off the direction of flight, I introduce a turning tendency just as surely as if I had banked, n'est-ce pas?

Now, on your Rule 1. What if the aircraft is out of balance left/right or out of rig and you are holding aileron to hold the wings level. Where will the ball be if you are holding a constant heading? I cannot see it centered as you have unequal drag on the wings that you are offsetting with rudder.

Once you teach yourself to see coordinated flight, you can start to relax and feel coordinated flight. If you already fly a Luscombe, you're on the road to coordinated nirvana.
Click to expand...

Thanks, it certainly helped a lot.
 
alfadog said:
Not really with you. Help me puzzle this out, if you will.

You say "Wings banked, airplane must be turning, wings level, airplane must be NOT turning" yet I am taught on the ILS final to hold the wings level and make slight heading corrections (turns) with the rudder. Yes, they are not coordinated turns but they are turns nonetheless. If I kick the thrust line off the direction of flight, I introduce a turning tendency just as surely as if I had banked, n'est-ce pas?

Now, on your Rule 1. What if the aircraft is out of balance left/right or out of rig and you are holding aileron to hold the wings level. Where will the ball be if you are holding a constant heading? I cannot see it centered as you have unequal drag on the wings that you are offsetting with rudder.



Thanks, it certainly helped a lot.
Click to expand...
Those rudder only turns arent coordinated...

As he said, if the wings are level and the plane is turning you aren't coordinated.;)
 
The problem in my aircraft is un-even fuel burn, which of course Cessnas are notorious for... and you can chase forever, if you feel like it. CPA's Tech Tip on the matter has grown to match the ridiculousness of the design itself, using one vent to handle two big long-range bladder tanks, and so many cross-vents internally that it really doesn't matter where you place the Left/Right fuel valve until you're well below half tanks. Even then, it might feed from the other side. :)
 
denverpilot said:
The problem in my aircraft is un-even fuel burn, which of course Cessnas are notorious for... and you can chase forever, if you feel like it. CPA's Tech Tip on the matter has grown to match the ridiculousness of the design itself, using one vent to handle two big long-range bladder tanks, and so many cross-vents internally that it really doesn't matter where you place the Left/Right fuel valve until you're well below half tanks. Even then, it might feed from the other side. :)
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Shoulda got a wet wing year:D:lol:
 
denverpilot said:
The problem in my aircraft is un-even fuel burn, which of course Cessnas are notorious for... and you can chase forever, if you feel like it. CPA's Tech Tip on the matter has grown to match the ridiculousness of the design itself, using one vent to handle two big long-range bladder tanks, and so many cross-vents internally that it really doesn't matter where you place the Left/Right fuel valve until you're well below half tanks. Even then, it might feed from the other side. :)
Click to expand...

LOL. I got started again on my IR on Sat in a club 172. After 1.2 hours we landed and fueled up. Right tank 1.6 gal/ left tank 6.8 gal; fuel on Both. And that is with a guy even heavier than me in the right seat as safety.
 
Skylane81E said:
Those rudder only turns arent coordinated...

As he said, if the wings are level and the plane is turning you aren't coordinated.;)
Click to expand...

Yeah, that line is prolly only true if coordinated (definitely) and balanced/rigged (my gut feeling).
 
alfadog said:
LOL. I got started again on my IR on Sat in a club 172. After 1.2 hours we landed and fueled up. Right tank 1.6 gal/ left tank 6.8 gal; fuel on Both. And that is with a guy even heavier than me in the right seat as safety.
Click to expand...

Amazing what only putting the vent on the left does
 
The reason some instructors suggest using rudder only to make heading corrections on an ILS is because corrections should be limited to 2 degrees. It can certainly be done by making shallow coordinated turns to track the localizer, but often results in 10 degrees right, 8 degrees left. Also, by keeping the wings level on approach, the heading doesn't vary much (again, assuming the aircraft is in coordinated flight).

The airplane can be in coordinated flight in ANY attitude...watch the Master, Bob Hoover conduct a barrel roll while someone pours a glass of water. But when in cruise, think of aiming the nose of the airplane like a rifle, with the wings level. Look waaay out at some distant spot on the horizon, then lock the heading on it. While you're holding it, and the wings level, trim the rudder.
 
j1b3h0 said:
The reason some instructors suggest using rudder only to make heading corrections on an ILS is because corrections should be limited to 2 degrees. It can certainly be done by making shallow coordinated turns to track the localizer, but often results in 10 degrees right, 8 degrees left. Also, by keeping the wings level on approach, the heading doesn't vary much (again, assuming the aircraft is in coordinated flight).

The airplane can be in coordinated flight in ANY attitude...watch the Master, Bob Hoover conduct a barrel roll while someone pours a glass of water. But when in cruise, think of aiming the nose of the airplane like a rifle, with the wings level. Look waaay out at some distant spot on the horizon, then lock the heading on it. While you're holding it, and the wings level, trim the rudder.
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Actually, Hoover pours the iced tea, himself. Backhanded. :D

I flew Sat with a instrument pilot that is doing his currency. I had not flown under the hood since last year. He was correcting with aileron and all over the place chasing the needle. My turn, I kept the wings level and was much more "on rails". Made me a believer although certainly individual technique is the biggest factor. He was, by his own admission, rusty.

Flying the Luscombe has gotten my head outside a lot more than it used to be and it was not so bad previously.
 
Flew a Turbo Lance PA32R300T with a 300hp turbo out front for several years. If you didn't dial in rudder trim during climb to altitude your leg would cramp up something fierce. I quickly figured out a good "take off" position based on rotation from in-flight neutral (about 1/2 of full-power climb). I'd then dial in the rest during the climb.
 
You say "Wings banked, airplane must be turning, wings level, airplane must be NOT turning" yet I am taught on the ILS final to hold the wings level and make slight heading corrections (turns) with the rudder.
Click to expand...

This is poor technique, and a crutch.

Even an autopilot doesn't do that.

Yes, they are not coordinated turns but they are turns nonetheless.
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No, they are skids.

If I kick the thrust line off the direction of flight, I introduce a turning tendency just as surely as if I had banked, n'est-ce pas?
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No, you skid the airplane around the sky, roughly, and manhandle the aircraft when it ought to be flown.

I will have to do some more experimenting but, IIRC, when I trimmed the rudder to bring the low wing up that the AP was holding down, I then had the ball out-of-center.
Click to expand...

If you trim in pitch only, you'll cause a turn as well; increase speed or decrease speed, change angle of attack, and you've affected the aircraft in other ways. This does not mean that trimming in pitch is the proper way to trim out a turn. Likewise, trimming the rudder is not the proper way to trim out a turn.

Fly the airplane, first and foremost. Put the airplane where you want it. In this case, level the wings. You do that. Then use the rudder to center the ball. Trim the rudder such that you don't need to hold the rudder. You've now completed all the rudder trim you need to do.

Yes, if you push a rudder, you'll ultimately cause bank, and you'll also ultimately affect pitch, because you've disturbed a trimmed condition. However, we don't use the rudder to turn the airplane; we use it to adjust the quality of the turn by correcting for adverse yaw; we streamline the turn, bank varies or changes. The rudder isn't used to bank or turn. You might as well try opening doors to turn. That's not what the doors are for, and it's not what the rudder is for.

Don't try to trim out bank or turn using the rudder trim. Use your ailerons to correct for a turn and bank issue; use your rudder to correct for the ball not being in the center. It's very simple.
 
A method for checking rudder trim on an airplane with a sophisticated autopilot...a 3 axis for example, is to just take note of any bank angle (with the autopilot in a heading mode) while the aircraft is on a constant heading. If there is zero bank, well, the rudder is properly trimmed. It's quite common though to see 1 or 2 degrees of bank. To correct the rudder trim, trim it towards the bank - just a little, then wait for 30 seconds for it to take effect. It will eventually settle into somewhat less bank...just repeat until the bank is zero. I suspect this technique won't work on an airplane with a simple wing leveler-type auto pilot. Tried it recently in a V35B Bonanza and it didn't work very well, so YMMV.
 
alfadog said:
LOL. I got started again on my IR on Sat in a club 172. After 1.2 hours we landed and fueled up. Right tank 1.6 gal/ left tank 6.8 gal; fuel on Both. And that is with a guy even heavier than me in the right seat as safety.
Click to expand...


That there is a classic symptom of a leaking RH fuel cap. Either its gasket is cracked or not seating well enough, or its vent check valve is leaking. The low pressure atop the wing will draw air out of the RH tank, lowering its pressure and holding the fuel back. The LH tank will feed more.

Flying right wing low will do the same thing. It increases the head pressure on the left tank over the right.

Both of the above will cause fuel transfer from the left tank through the selector valve (when on Both) to the right side.

Dan
 
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