Centering the Ball
Thread Starter
Centering the Ball
I suppose everyone has noticed that the Turn Coordinator Ball, takes up different positions at different stages of flight. For instance, during Take-Off it is 1 ball to the right, so you push with the right foot.
Then during cruise our C172 flies 'hands off' but with 1 ball to the left. If I centralise the ball with the rudder trim, I am then required to maintain about 200 grams of pressure on the yoke to stay on course.
I just wonder; Is the aircraft designed to ever be Neutral Ball and Hands-Off.?
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Then during cruise our C172 flies 'hands off' but with 1 ball to the left. If I centralise the ball with the rudder trim, I am then required to maintain about 200 grams of pressure on the yoke to stay on course.
I just wonder; Is the aircraft designed to ever be Neutral Ball and Hands-Off.?
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Because there is a engine thrustline offset a cockpit adjustable rudder trim tab will assist in 'centering the ball'. Then there are also aircraft with an offset vertical stabiliser as well.
Refer POH/Flight Manual.
CC
Refer POH/Flight Manual.
CC
If the aircraft doesn't have a cockpit adjustable rudder trim, and just a ground adjustable tab, then the tab is usually adjusted so that no rudder pressure is needed during cruise flight power & IAS.
For clockwise prop as viewed from the cockpit:
If more power is applied while maintaining the same speed then some right rudder will be needed. Similarly, if the speed is reduced for the same power then R rudder will be needed.
If power is reduced with the same speed then L rudder will be needed. If IAS is increased with the same power then L rudder will be needed.
This means that during typical climbs or descents, some additional rudder pressure will be needed. If the plane has a small speed range and low power the change isn't much and the pilot just has to bear with it while it is needed. For aircraft that have a large speed range or significant power it becomes a bit of a pain so a cockpit adjustable trim is usually fitted. Any time that the plane isn't in balance (ie longitudinal axis aligned with the airflow) then the rudder trim should be adjusted to relieve the pilot from holding rudder pedal pressure.
For clockwise prop as viewed from the cockpit:
If more power is applied while maintaining the same speed then some right rudder will be needed. Similarly, if the speed is reduced for the same power then R rudder will be needed.
If power is reduced with the same speed then L rudder will be needed. If IAS is increased with the same power then L rudder will be needed.
This means that during typical climbs or descents, some additional rudder pressure will be needed. If the plane has a small speed range and low power the change isn't much and the pilot just has to bear with it while it is needed. For aircraft that have a large speed range or significant power it becomes a bit of a pain so a cockpit adjustable trim is usually fitted. Any time that the plane isn't in balance (ie longitudinal axis aligned with the airflow) then the rudder trim should be adjusted to relieve the pilot from holding rudder pedal pressure.
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The simple answer is yes. And why not yours? That is probably due to age accumulated wear and tear. So you need to see your friendly engineer, explain your problem and get him/her to tweak your rudder and/or rudder trim. Having to apply trim pressure for hours at a time is both a pain and totally unnecessary.
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One little thing to consider though is the lateral balance. Aircraft that are normally flown two-up (student + instructor) are trimmed to fly balanced in that configuration. If you then fly such aircraft solo, the lateral imbalance may require a bit of constant pressure on the yoke.
You can alleviate that pressure with the rudder (secondary effect) but then the aircraft won't fly with the ball in the middle. (And for the same reason, tweaking the rudder trim tab may not work either.)
The only proper way to address this would be to have trim in the roll axis. However, light aircraft such as the C172 are manufactured to a budget, both in monetary terms and in mass terms. So this is normally not installed: They only have trim in the pitch axis.
The rudder typically has a fixed trim tab, but there may be a fixed trim tab on an aileron somewhere as well, or the ailerons will have a different way to be trimmed permanently. But in all cases, as said before, this requires consultation with an engineer and possibly a series of test flights to get things setup properly. DO NOT tweak the trim tabs yourself if this is not your aircraft. And do not try to solve a roll problem by introducing a yaw imbalance.
You can alleviate that pressure with the rudder (secondary effect) but then the aircraft won't fly with the ball in the middle. (And for the same reason, tweaking the rudder trim tab may not work either.)
The only proper way to address this would be to have trim in the roll axis. However, light aircraft such as the C172 are manufactured to a budget, both in monetary terms and in mass terms. So this is normally not installed: They only have trim in the pitch axis.
The rudder typically has a fixed trim tab, but there may be a fixed trim tab on an aileron somewhere as well, or the ailerons will have a different way to be trimmed permanently. But in all cases, as said before, this requires consultation with an engineer and possibly a series of test flights to get things setup properly. DO NOT tweak the trim tabs yourself if this is not your aircraft. And do not try to solve a roll problem by introducing a yaw imbalance.
Last edited by BackPacker; 13th Jul 2017 at 09:23.
Thread Starter
Thanks Dash6, I think the TC is Ok as it is centralised when parked on level ground.
And yes our 172 has got the rudder trimmer on the consul.
It's just that we can either have one or the other in trim, but not both. I may Velcro a removable small lead weight to the right hand side of the Yoke... just a couple of ounces should do the trick..!
I also have access to a glider that has a novel pitch trim arrangement; It uses a spring on the stick, which can be 'zeroed' once you have held the stick in the desired position.
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And yes our 172 has got the rudder trimmer on the consul.
It's just that we can either have one or the other in trim, but not both. I may Velcro a removable small lead weight to the right hand side of the Yoke... just a couple of ounces should do the trick..!
I also have access to a glider that has a novel pitch trim arrangement; It uses a spring on the stick, which can be 'zeroed' once you have held the stick in the desired position.
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The other issue with aircraft with 2 wing tanks is that if you are using fuel from one side or the other your aileron trim will change as you consume fuel.
Personally I've never noticed aileron trim to be an issue, whereas sitting with constant - even light - pressure from your foot is annoying.
Personally I've never noticed aileron trim to be an issue, whereas sitting with constant - even light - pressure from your foot is annoying.
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My PA28-181 has an adjustable rudder trim so I do tend to tweak it while in the cruise (and if doing a steady climb or descent).
Years ago my brother was in a Robin Aiglon group - it had two small plain aluminium taps one riveted on the port aileron and one on the rudder - these could be tweaked with pliers, once set they were left for years, put the bubble in the middle in the cruise.
Not too sure if they were 'standard' for the Aiglon or someone's unapproved tweak but they did work.
Years ago my brother was in a Robin Aiglon group - it had two small plain aluminium taps one riveted on the port aileron and one on the rudder - these could be tweaked with pliers, once set they were left for years, put the bubble in the middle in the cruise.
Not too sure if they were 'standard' for the Aiglon or someone's unapproved tweak but they did work.
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As mentioned before get your maintainers to do it properly for you. If as you describe, you end up with the control wheel offset or out of trim ask them to check the flap settings.
On my Warrior when the rudder trim was correctly set, the yoke was offset.
The maintainers adjusted the zero flap setting slightly (not the ailerons) and all was well.
