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Old 16th Jul 2009, 10:15   #1 (permalink)
 
Join Date: Jan 2006
Location: Between a rock and a hard place
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Hydraulic Nose Wheel Steering

SEP and MEP driver here...

Trying to read up on power steering systems.

Am I right in saying that if you release the tiller the selector valve will provide a by-pass of fluid between the two steering jacks, making the nose gear free to castor? Is this something you can feel in the tiller that it is spring-loaded towards center (or zero deflection) e.g. when you taxi? While as soon as you turn the tiller left or right the selector valve will direct hydraulic pressure to the correct side of the steering jacks and drain the other side?

For towing, does it work to just leave the tiller in neutral or is there any other way you do it? If you tow the aircraft will the tiller move in response to the nose gear?

Hope I didn't confuse you too much.. thanks in advance for any help!
Schematic pictures is very much appreciated as well!
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Old 16th Jul 2009, 10:25   #2 (permalink)
 
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For the 747 the tiller is spring loaded to center the nose will only castor if the steering bypass pin is in. There is feedback to the tiller while towing and the tiller should not be touched. Don't have any diagrams available for the functioning of the steering valve.
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Old 16th Jul 2009, 19:57   #3 (permalink)
 
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Thank you for the info!
I found a text in my book that stated "whenever the control valve is in neutral position fluid is free to flow between the steering jacks, thus allowing the aircraft to be towed"... but if I understand you correctly that is not true on the 747 where you have to insert a pin to enable towing?

I don't know if you are familiar with different learning books (e.g. Oxford), but are they generalising aircraft systems?
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Old 16th Jul 2009, 21:26   #4 (permalink)
 
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large aircraft nose wheel steering systems automatically centre when hydraulics are applied, to enable towing the nosewheel steering has to be deactivated via a bypass pin / switch etc, which effectively joins the hydraulic lines from either end of the steering cylinders together, thus allowing the leg to turn.
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Old 17th Jul 2009, 09:34   #5 (permalink)
 
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So if I understand correctly here, simply centering the tiller (neutralizing the steering valve) will not allow the nose wheel to castor? How does this work during take-off roll where I thought you wanted the nose wheel to castor in response to aerodynamic forces from the rudder. Thanks for your help!

I found this schematic, although it gives no good overview over the actual steering valve.

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Old 17th Jul 2009, 09:42   #6 (permalink)
 
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For Boeing and McD a/cs, the nosewheel is mechanically linked by cables to the nosewheel tiller. So, if there is a movement of the nosewheel, the tiller would move even if the bypass pin is attached. The bypass pin physically deactivates the steering thru the hydraulic lines.


For Airbus, the "FBW" system is used for the nosewheel system.. The bypass pin actually sends an electrical signal to the BSCU which needs certain conditions to be met like eng on before it activates the nosewheel steering thru the tiller and rudder pedals. The rudder pedals can even be deactivated thru the tiller. Turning the tiller actually sends electrical signals to the BSCU which is converted to mechanical action thru thr servo valve.

Hope this helps. Have the pics, but don't know how to paste it here.
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Old 17th Jul 2009, 19:23   #7 (permalink)
 
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When the tiller is centered the fluid is trapped in both lines so the nose wheel is centered, not castoring. You want the nosewheel to go straight down the runway during takeoff. The rudder pedal steering on Boeings has about 1/10 the travel to the tiller. You want most of the crosswind correction to be done with ailerons on the ground then as the nose comes up do what you need with the rudder.
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Old 17th Jul 2009, 19:50   #8 (permalink)
 
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The pieces start to fall in place now, thanks for the info!
So the rudder pedals do modify the position of the steering valve as well, although to a much lesser degree than the tiller?
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Old 17th Jul 2009, 23:33   #9 (permalink)
 
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The steering tiller will never lock out the steering mechanism, it is connected with cables to the steering valve (talking non FBW a/c here).
You need to insert a steering pin in this valve to make sure no hydraulic power will actuate the valve and so the actuators and wheels.
During towing the tiller will follow nose gear angle, even if pin is installed, because of the cable connection.
If the pin is installed you cannot move the tiller by handforce though; you can move it if pin is removed. The wheels will only turn however when hydraulics are on.
Tiller movement will give you about 65 of wheel turning, rudder pedals less than 10, all depending on a/c type. Tiller movement overrides rudder pedals (we taxi using the tiller to remain on a straight line while testing the rudder for free movement)
towing angles are also about 65 (red line on gear doors), so the wheel will not castor free when the steering pin is installed!!
To castor free a mechanic needs to disconnect the scissor links on the strut!
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