RUDDER TRIMMED AT AIRPORT

[mourgo]

Can somebody help me with this please.

Sometimes I see an aircraft parked at the airport, engines off and rudder trimmed fully right or left. Emirates planes are usually parked like this. Is there any reason why the rudder is left like that?

[IFixPlanes]

With Hydraulic OFF the Rudder act like a (damped) Flag.

[mourgo]

I dont understand what you mean.


Why is it only on some aircraft and not others?

[IFixPlanes]

I dont understand what you mean.
Why is it only on some aircraft and not others?
It depends on the internal resistance of the mechanic.

[ROSCO328]

I think what your meaning is the rudder is fully deflected either full left or right? If so this is gererally done in windy conditions and acts to reduce the surface area of the tail in order to minimise the aircraft's natural tendency to weather cock which have cause aircraft in the past to spin 180 degrees on stand.Hope this helps.

[FullWings]

With the hydraulic systems unpressurised, most of the flying controls are (relatively) free to move. Ailerons, elevator, slats, etc. can 'droop' and the rudder gets blown to one side or the other by the wind...

[Piltdown Man]

Is it the rudder or one/some of the rudder trim tab/s that is fully deflected?

[mourgo]

sometimes its the bottom half of the rudder (usually on the 777).

[mourgo]

Look at this pic.

http://www.airliners.net/open.file/1026001/L/

[Clandestino]

It's not "trimmed" at all. When hydraulic actuators are not powered, rudder is free to move with the wind. On picture you've posted wind is coming from the right side. As soon as pressure gets to actuator, rudder moves to its commanded and trimmed position and it's fairly close to neutral 99% of the time.

Why some a/c and not others? Depends on trapped hydro pressure when pumps are off, rudder geometry and wind direction/velocity.

[Bolty McBolt]

mourgo

I read you want to become a pilot.
With your mechanical aptitude I am glad you do not wish to become an engineer and I doubt you will make pilot either....!:ok:

[cwatters]

That might be a bit harsh. Perhaps he's not that old?

[TotalBeginner]

With your mechanical aptitude I am glad you do not wish to become an engineer and I doubt you will make pilot either....!

Woa there Bolty! We all have to learn about our interests somehow.

[On-MarkBob]

On some aircraft, it might not necessarily have anything to do with hydraulics. The DC9 / MD80 the control surfaces are moved by way of a servo tab (like a trim tab). The only exception is that they have a hydraulic supply to the elevator for down only (to recover from a super stall) and the rudder (for full rudder control in the event of an engine failure on take off) - ( the rudder supply is taken from the rear stair accumulator, so if the rear stairs don't work you can't go!). It is quite common to see an MD80 taxiing out with one elevator up and one down, blown that way by the wind. The control wheel only operates the servo tabs and not the main control suface. For this they need airflow. The ailerons are balanced together and sprung loaded centre. If you turn the wheel without airflow over the wings the ailerons don't move, only the tabs. Likewise the elevator except that if you push the control column forward the hydraulic supply (if on) will push both elevators down.
Incidently this system of control was used as a back-up system for the Spuce Goose, by Howard Hughes.

Bob

[mapper]

(SLF speaking) I notice on the Cessna 172 the rudder is externally clamped to (I guess) prevent damage to rudder or mechanical linkages back to the pedals. Does this mean any more sophisticated type with hydraulics relies on this residual pressure (as described by Clandestino) to damp the movement? Do these systems retain sufficient pressure after sitting idle for an extended period?
thnks.

[Bolty McBolt]

It was harsh but fair...
Read "mourgo" other posts :ok:

[Clandestino]

Trapping of hyd pressure in lines is purely incidental and is never relied upon for anything. I've noticed that on some planes I fly (same type and series) pressure goes down to zero half a minute after engine shutdown and on some it can linger for half an hour. Both are considered normal and don't affect normal operation of hydraulics.

Transport category airplanes rely on beefy rudder structure, to withstand gusts, rather than on external gust locks.

[nomorecatering]

Along the lines of one of the other posters, are the rudder stops, and the rudder structure designed to take the force of flapping from stop to stop. Many times have i have been on the ramp listening to the rudders banging against the stops (left and right) in gusty and stormy conditions. engineers seemed un concerned.

[Escape_Slide]

Quite apart from cleats, some aircraft, like the later F27s had mechanical locks operaterate by a handle in the cockpit. When this was opererated the flight controls were faired.

Some might remember the Ansett F27 that was spiralling out of the sky when a procedures manual pulled from behind that capt seat inadvertantly engaged the lock!

An engineer on the ground, his name escapes me, asked the crew to check if the lock had engaged and sure enough........another catastrophe avoided. :rolleyes: (Rebecca will love this Smiley).

[singleseater]

Mourgo,
Another thing to consider is that the 777 is fly-by-wire. It does not actually have a rudder trim as you mean it. Everthing is done by the PFC's and ACE's inputs to the power units. In the photo, the tab you can see deflected in the sane direction as the rudder is a flight tab, not a trim tab. Basically, it's smaller size means that it can be deflected more in to the slipstream for the same hyd power input. Also it is at the back of the rudder, so the moment arm is greater and it has more effect for the same deflection. At higher speeds, the main rudder may not move, only the tab. A trim tab would be deflected the opposite way, so providing a force to pust the rudder.
The 777 does not have a yaw damper as such, this function is carried out by the PFS'c/ACE's. On the ground it relies on internal friction to stop the rudder flopping around and breaking something.
If you look at the 777 on the ground, you will often see the flaperons drooping unequally, same reason, no Hyds., rely on internal friction to hold them up.
Bolty, It must be assumed by the fact that you made no attempt to answer the question that you had no idea either.