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Rudder trim control on your aircraft?

Started by Hardy Heinlin, Fri, 1 Jun 2012 03:07

Hardy Heinlin

Quote from: John H Watson on Tue, 20 Mar 2018 02:09
The tiller is a different kettle of fish. At the gate, I haven't noticed any movement of the tiller with rudder trim use. It may be a gearing ratio issue or something else (trim motor strength?). Perhaps the tiller wants to move. We probably need pilot or steering specialist input on this issue. Does rudder pedal input (with feet) move the tiller? In my engineering career, I've only operated the rudder pedals a few times with hydraulics operating and my focus wasn't on the tillers.

From a plain logical point of view, there must be a mechanical link between tiller and rudder. Otherwise there would be no possibility whatsoever to keep the gear steering centered during rudder check. Also the manuals say the pilot should hold the tiller centered during rudder check. I've never seen an aircraft taxi in S-curves during rudder check.

Perhaps the tiller and the gear will turn better when the aircraft is rolling, so you can't see any big effect when the aircraft is stopped?


|-|ardy

mark744

Quote from: Hardy Heinlin on Tue, 20 Mar 2018 13:05
Also the manuals say the pilot should hold the tiller centered during rudder check.
|-|ardy

.....Probably refers to checking the movement of the rudder before starting to taxi.
An SOP in some companies. Holding the tiller is necessary to stop it moving

Regards

farrokh747

#22
QuoteAs far as I've learned it, on the 747 the rudder trim actuator is driven by a DC motor. The trim actuator varies the rudder's neutral angle; the rudder is at this neutral angle when the rudder hydraulics are relaxed or symmetric.

morning - i dont think the rudder trim actuator will 'hold' the rudder/s at a new neutral position if there is no hyd psi... as far as the rudder power control actuators are concerned, they cannot tell the diff between an input from the pedals or the trim actuator - all rudder inputs are summed up in the aft quadrant and  feel and centering unit, which passes it on to the actuators (via i guess the rudder ratio unit)  - of course, with any hyd psi present, i guess the rudder would be at the trimmed position..... 

However, since the trim actuator backdrives the pedals, they would now be stuck in the off center position without elec power

https://image.slidesharecdn.com/fc744flightcontrol-090325224309-phpapp02/95/fc744-flightcontrol-11-728.jpg?cb=1238021025

In the front, the rudder/triller system will disconnect the link when airborne, and vice versa

http://farrokhchothia.com/slists/ruddertrim/

the second pair of slightly smaller and thicker springs is the AP rollout override mechanism ...

I have the acft feel and centering unit mounted in the front, tied to the fwd quadrant on the FO side....  works perfectly...

cheers...

fc


John H Watson

QuoteAs for taxiing with trim. I would assume that full rudder trim would be equal to, or less than, full rudder for purposes of nosewheel deflection on taxi. But I have never tried to taxi using rudder trim.

As you probably shouldn't. I just found a brave/crazy soul willing to check the effect on a real 767 for the purposes of sim modelling (and his own curiosity). The only 744 pilot who volunteered to do this didn't have much taxiway for the purposes of the check and couldn't discern any noticeable effect during this time.

Hardy Heinlin

Hi Farrokh,

my comment you quoted is 6 years old, 2 years before PSX came out :-)

This old thread here was pulled up again by Blake yesterday, with a different question.


Cheers,

|-|ardy

farrokh747

Quote2 years before PSX came out :-)

Ha... apologies, senior moment....   interesting discussion tho...

cheers,

fc

John H Watson

#26
QuoteFrom a plain logical point of view, there must be a mechanical link between tiller and rudder. Otherwise there would be no possibility whatsoever to keep the gear steering centered during rudder check. Also the manuals say the pilot should hold the tiller centered during rudder check. I've never seen an aircraft taxi in S-curves during rudder check.

Yes, there must be a mechanical link, but can't be rigid (otherwise the rudder pedals wouldn't move if you kept the tiller still during flight control checks). There might be a breakout mechanism like the ailerons. i.e if one control wheel is jammed, you can still use the other control wheel. The steering mechanism shows a spring, but I can't figure out what it does. (EDIT: This is labelled as a centering spring)

The tiller has priority to prevent the nosewheel steering operating during the rudder check, but hasn't it been mentioned here or elsewhere that the airplane steering gives a slight kick when the rudder pedals are used (even when the tiller is held)?

John H Watson

From a purely mechanical ratio point of view, the rudder pedals should move the tiller through 1/10th of the tiller's arc. Since the tiller moves 150 degrees left/right, the tiller should move a rather noticeable 15 degrees with max rudder pedal movement and an almost equally noticeable 12 degrees for rudder trim.

Question for the pilots: Does the tiller move the rudder pedals full deflection during large turns?

Britjet

The tiller doesn't move the rudder pedals at all.
Peter.

John H Watson


kevmac86





The steering tiller connects to the captain's side ruder controls, it is operated do disengage in-flight. There are two actuators for back-up, either actuator can engage the rudder to the steering on the ground.
On the ground the primary actuator retracts, and the secondary actuator extends, this allows a spring to engage the steering mechanism to the rudder steering rod. to detect for failures a switch S1349, is monitored for full range operation of the actuators.

Kev

Hardy Heinlin

In the electronic world, when forces may be vectored from A to B and back to A, we can use a diode to block the opposite direction.

In the hydraulic and pneumatic world, to block the opposite direction, we can use a valve.

In the mechanical world with rods and springs, how can we block the opposite direction there? Is there anything else aside from a ratchet-like tooth system?

For the nose wheel steering, which requires a symmetric left-right control, I can imagine a dual ratchet system (even if there is just one ratchet tooth in use on each side, it's a ratchet principle). However, as the videos and the pictures show (see green part), the core component looks asymmetric. That's hard to understand.

emerydc8

QuoteThe tiller doesn't move the rudder pedals at all.
Peter.

Thanks, Peter. Do you recall if the rudders move the tiller at all?

Britjet

Yes, they do. Previous comments about the need to hold the tiller while doing a control check are correct.
Peter

John H Watson

QuoteThere are two actuators for back-up, either actuator can engage the rudder to the steering on the ground.

This is modelled in PSX, including the electrical power for the actuators (and, I recall, the proper operating logic for when only one actuator moves).

As Hardy says, it's the mechanical stuff which is hard to understand.  There are no clear diagrams in any of my books. I recall in our training centre we had working models of various mechanical devices, such as the outboard aileron lockout mechanism (pure genius). We really need one for this.

emerydc8

QuoteYes, they do. Previous comments about the need to hold the tiller while doing a control check are correct.
Peter

Thanks!

IefCooreman

Did some checking on my last flight. Not sure what info is needed.

The answer to the question "do rudder pedals move with rudder trim application", the answer is yes, both in the air and on the ground, moving or standing still, although the big movements only start from 5 units and beyond.

Does rudder trim move the steering tiller? The problem here is that a large trim deflection will only generate a very tiny amount of tiller deflection, so tiny it is hard to "feel" because the rudder trim moves fairly slow so the tiller moves very very very slow over a very small deflection. Steering tillers have a much bigger range compared to the pedals and almost no dead zone. Which is why sudden steering tiller movement can create some pretty uncomfortable reactions when trying to go straight, and as many instructors will say "don't try to move it, but squeeze it".

I do have to add, our fairly old 737s are not really straight rolling birdies, they will go slightly left or right depending on how they feel like it. And since the tiller movements are so tiny the movement is hard to distinguish from the natural corrections you apply trying to go stay straight. The tiller movement equalising a full rudder pedal movement is about the same as going from the upper edge of the steering tiller _centerline_ to the lower edge of the centerline (picture below to make it more clear). But my impression is yes, we tried moving the trim during taxi and the tiller did feel like reacting to it.

https://www.flightdeck737.be/wp-content/uploads/2016/11/DSC02037.jpg

John H Watson

 
QuoteThe problem here is that a large trim deflection will only generate a very tiny amount of tiller deflection, so tiny it is hard to "feel" because the rudder trim moves fairly slow so the tiller moves very slow over a very small deflection.

Based on pure numbers, I think we have to be careful about making comparisons between 737's and 744's. The rudder moves 29 degrees left and right on a 737, trim is 18 degrees (62% of full deflection versus 80% on a 744). The tiller only seems to move 90 degrees (?) on a 737 Vs 150 degrees on a 744. Unfortunately, I don't have any data on dead zones for the 744 and trimming speeds for the 737. Does the 737 have dual or triple trim speeds?


kevmac86


IefCooreman

Single speed indeed...

From the 777 (which I believe to have the same style of tiller as the 747?) I remember the steering sensitivity to be rather the same although it had the "larger" rotation angles and no dead zone as well. But I also admit, because of the placement and the design, it was more difficult to "squeeze" the tiller.

The effect could also be 777 related, as I was told that compared to other widebodies, the nose gear does not carry a lot of weight. Wet aprons and taxiways could be really tricky, any knot extra and the bird would drag it's nose wheel straight forward...