737doctor From United States of America, joined Mar 2001, 1332 posts, RR: 41 Reply 1, posted (9 years 6 months 1 week 5 days 2 hours ago) and read 9368 times:
This is the way it works for the 737:
Hydraulic power from system A is used to turn the nose wheels to either side from zero to 78 degrees. An alternate nose steering switch on the captain's panel allows power from hydraulic system B to turn the nose wheels if power from hydraulic system A is lost. Steering is controlled by a tiller wheel by the captain's left knee and by an interconnect mechanism from the rudder pedals. The tiller gives you full steering authority up to 78 degrees. Rudder pedal steering is available during takeoff, landing, and taxiing when small directional changes are required. Full deflection of the rudder pedals produces about 7 degrees of nose wheel steering. A squat switch on the nose landing gear ensures that rudder pedal steering is only available while the a/c is on the ground.
RduBE90Pilot From United States of America, joined Jun 2004, 63 posts, RR: 0 Reply 2, posted (9 years 6 months 1 week 5 days 1 hour ago) and read 9313 times:
On smaller aircraft, such as the King Air:
Ground steering is accomplished by direct linkage between the rudder pedals and the nose wheel steering linkage. After take-off, a straightener roller centers the nose wheel as the gear retracts into the wheel well.
Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 3, posted (9 years 6 months 1 week 4 days 23 hours ago) and read 9238 times:
Is the deflection of the nose gear proportional to the deflection of the tiller as in a car or does the tiller directly control the valves of the steering actuators ("more left" / "more right").
If it´s proportional, how exactly does the servo mechanism work in the older (mechanical) models to achieve that proportional control? (In the newer ones, I´d expect an angle sensor and a digital valve controller with analog or digital input from the tiller.)
Is the tiller self-centering?
Is there an indication of the current nose gear deflection in the cockpit?
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 4, posted (9 years 6 months 1 week 4 days 22 hours ago) and read 9229 times:
Actualy it is usualy just a mechanical system with a valve controled by the tiller and the rudder pedals through cables and a feedback, which closes the valve again once the desired angle is reached.
The control input from the rudder pedals is smaller.
On the 737, once the nose gear becomes airborne, the squat switch will give a signal to a small electrical actuator, which will disengage the rudder pedal input from the nose wheel steering mechanism, so that the fully extended gear can center itself through a centering cam inside the shock strut.
On the MD-11 this disconnection is done purely mechanically.
Klaus From Germany, joined Jul 2001, 21346 posts, RR: 54 Reply 6, posted (9 years 6 months 1 week 4 days 21 hours ago) and read 9187 times:
MD11Engineer: Actualy it is usualy just a mechanical system with a valve controled by the tiller and the rudder pedals through cables and a feedback, which closes the valve again once the desired angle is reached.
Thanks... I guess the same was/is true for the flap positioning servos.
Do the newest models still use mechanical servos everywhere or have they moved towards electronic/digital ones in the meantime? (It´s obvious for primary flight control on FBW types, but what about flaps and nose gear steering?)
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 7, posted (9 years 6 months 1 week 4 days 19 hours ago) and read 9148 times:
On the A320 and later the flaps and slats are controled electrically, same as the nose wheel steering. The A320 has 3 cable systems, if I remember correctly, it´s been a while, rudder, stabilizer trim and emergency landing gear extension.
320tech From Turks and Caicos Islands, joined May 2004, 489 posts, RR: 5 Reply 12, posted (9 years 6 months 1 week 4 days 10 hours ago) and read 8994 times:
On the A320 and Dash-8, at least, there is something called a LVDT or RVDT (Linear or Rotary Variable Differential Transducer) that measures the position of the nosewheel. This information is sent to the BSCU (Brake and Steering Control Unit - A320) or ECU (Electronic Control Unit - Dash), where it is compared to the tiller or rudder input. The BSCU/ECU then commands the hydraulic valve to port more or less pressure to the steering cylinder, and the wheels move as commanded.
Is the tiller self-centring? Yes.
Is there indication in the cockpit? No.
[Edited 2004-06-03 07:26:18]
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
MD11Engineer From Germany, joined Oct 2003, 13602 posts, RR: 63 Reply 13, posted (9 years 6 months 1 week 4 days 9 hours ago) and read 8989 times:
In myold company (LH Technik SXF) we had a short, skinny young guy to do the rigging of the 737 nose wheel steering cables. He wassmallenough to fit comfortably through those panels into that tight space beside the wheel well. Only problem was, he was singing the whole time while he was working .
If I remember correctly from ERJ School, the yoke only gives you 5 to 8 degs of turning radius, and only really used during the final moments of take off and landings. The real steering is done by the tiller which gives you significantly more turning radius.
[Edited 2004-06-03 22:56:54]
"It's not the size of the dog in the fight, but the size of the fight in the dog"
Mender From United Kingdom, joined Feb 2004, 234 posts, RR: 0 Reply 18, posted (9 years 6 months 1 week 3 days 6 hours ago) and read 8785 times:
"how exactly does the servo mechanism work in the older (mechanical) models to achieve that proportional control? "
This works in exactly the same way as any other cable controlled hydraulic actuator such as an aileron or rudder actuator. This would be very easy to explain using a pen and paper. I'll try my best with text.
If you imagine the actuator has a hydraulic control valve mounted on the body of the actuator. Bolt the body of the actuator with the control valve to the wing and the moveable end of the actuator to the control surface.
If you apply a minute input to the control valve the actuator will slowly extend to maximum extention. A larger input will make the actuator extend to max quicker. With the actuator connected like this you do not have any "feedback" to the control valve.
Now imagine the same setup as above but turn the actuator around and connect the moveable end of the actuator to the wing and the body and control valve to the control surface.
Now what happens is that when you put a small input into the control valve via your control cable the actuator and control valve extends away from the control cable pulley and "nulls" the input so stops extending. Move the cable more and the actuator extends a little move then nulls. This is a feedback system.
I hope you can understand this. It'd be very easy to understand is you could see it.
Starlionblue From Greenland, joined Feb 2004, 16345 posts, RR: 66 Reply 20, posted (9 years 5 months 3 weeks 2 days 8 hours ago) and read 8544 times:
Question to those holding the tiller/wheel...how do you stay on the yellow line when you are turning or lining up at a gate? You are driving semi blind...no?
Actually a lot of pilots prefer to taxi with the nose gear to one side of lines. Since these tend to have lights inset in the middle, riding directly on the line will yíeld a bump-bump-bump-bump to the ride. No big deal but I guess it either wears out the tires or is aesthetically irritating.
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo