Diego From Italy, joined Apr 2001, 135 posts, RR: 0 Posted (11 years 2 months 2 weeks 1 day 9 hours ago) and read 6924 times:
Hi folks got a couple of questions about the B737 flight control system:
The aileron control system as far as I know consists of the two control columns, of an aileron trim, of two aileron power control units and of a single dual powered aileron attached to the outboard portion of each wing. Movement of the yokes, which are interconnected through a cable system and a bus control drum, is transferred through a set of wires and push pull rods to the two power control units which then, that's where it gets cloudy, forward the inputs to the aileron. If I am not mistaken each power control unit consists of a set of pistons horizontally placed and basically transfers the hydraulic pressure from the associated hydraulic system to the aileron. Now my question is: how? Is it connected directly to the aileron through a hydraulic actuator or it operates a sort of pully, cable, push-pull rod sytem in turn connected to the aileron? the only thing that I know is that the piston is secured to the structure of the wing while the housing, as a pressure is routed to either side of the piston depending on the direction of the yoke movement, slides back and forth moving the aileron. Somewhere in the wiring between the control columns and the PCU there is a Feel control unit which I assume, please correct me if I am wrong, consists of a spring system that creates a sort of artificially induced resistance to the displacement of the yoke, giving the pilot the feeling of aerodynamic load. The trim, I believe somehow increases or decreases the tension of the spring. Is that correct? besides, what is the function of the balance panel, where is it located, how does it interact with the aileron, I know it is normally held down by the pressure over the wing acting upon it, but somehow that pressure is disrupted when the aileron is deflected, still wondering which way the balance panel moves? One last question: in the event of a dual hydraulic system failure the system automatically shifts into a manual mode and the aileron is operated manually through the balance tab, mechanically connected to the yokes, is the reversion process initiated automatically by a sort of pressure switch in the PCU or there is a switch?
Thanks for your willingness
LMP737 From United States of America, joined May 2002, 4731 posts, RR: 22
Reply 1, posted (11 years 2 months 2 weeks 1 day 8 hours ago) and read 6895 times:
The the two aileron PCU are connected to the control surfaces via cable to a wing quadrant. At this point the wing quadrant is attached to the aileron by a control arm. There are two spring mechanisms that isolate the aileron system from the flight spoilers.
In the event of hydraulic failure the pilot provides the power necessary to move the ailerons. There must be three degrees of control wheel movement at the aileron PCU before the input crank hits the mechanical stop. To answer you're last question if there is a hydraulic failure manual reversion occurs by default. There is however two flight control switches that the pilot can select off to remove pressure from the system.
MD11Engineer From Philippines, joined Oct 2003, 14463 posts, RR: 62
Reply 2, posted (11 years 2 months 2 weeks 1 day 7 hours ago) and read 6879 times:
You have two seperate cable trains from the control column to two torquie tubes, coming down from the roof of the main wheel well, close to the fwd bulkhead. The L/H cables come from the captain´s control wheel. The L/H torque tube is attached to the feel and centering mechanism. It is essentialy a heart shaped cam with a spring loasded roller riding on the edge. The roller tries to push the cam in a position so that it will sit in the notch of the cam, giving it the center position. Trimming moves the cam to the left or the right and the roller follows it to the new center position. If you move the torque tube with the roller, the forcepushing back on the roller increases with the angle of distortion. Next are the input rods to the two aileron PCU´s, both are big servo cylinders, attached to the forward bulkhead of the main wheel well. Moving the rods will cause a valve to open ineach, moving the pistons, with a feedback, to close the valve again. These two servos operate two quadrants, from which big cables run out to the wing. In the wing there is another quadrant and a tension regulator. The wing quadrants operate the ailerons through push rods. There is another rod from the input torque tube through a spring cartrdge to the ratio changer and spoiler mixer, controling the flight spoilers. The R/H (F/O) cable system is directly connected to the ratio changer and can indirectly control the ailerons.
HAWK21M From India, joined Jan 2001, 31754 posts, RR: 55
Reply 3, posted (11 years 2 months 2 weeks 19 hours ago) and read 6834 times:
The PCUs move the Aileron bus drum which is in a sense Two Cable operated Large Pulleys connected together by a Lug & fork attachment,enabling both Drums to move together irrespective of Both PCUs pressurized or only one.In case of no hydraulic pressure The Aileron PCUs have stops on the housing that mechanically move the Aileron PCUs manually to transmit mvmt to the Aileron Bus drums.In case any one PCU is Jammed which prevents Aileron mvmt,then after 12 deg of control wheel mvmt the Flight spoilers can be operated to enable control around the Longitunal axis.
From each Bus drum cables move behind the Rear spar seperately to each Aileron control mechanism.The Lower bus drum supplies mvmt to the Left Aileron & upper Bus drum to the Rh Aileron.
The cable moves around a run around pulley at the Wing end near the aileron,this pulley is spring loaded to take into account wing flexing does not vary the cable tension.From there its connected to a cable with a push rod which operates a mechanism which drives the Aileron.
The feel & centering unit is present in the Wheel well Lh side which is a Cam & Spring unit which provides Trim & feel.
The Balance panel is hinged between the Rear spar of the wing & the LE of the Aileron.It uses Differential Air pressure on either side of the panel to help the aileron move.The balance panel moves in a direction opposite to the Tab
Manual mvmt is possible thru mechanically moving the PCUs thru stops on the PCU housing when there is no hydraulic pressure present.
Diego From Italy, joined Apr 2001, 135 posts, RR: 0
Reply 4, posted (11 years 2 months 1 week 6 days 13 hours ago) and read 6765 times:
Thanks to everybody for the explaination which cleared a lot of doubts, however I have got to additional questions:
Is the quadrant another way to name the aileron bus drum? and
About the balance panel HAWK says that it is operated by a differntial air pressure acting upon it. Exactly to which pressure do you refer to, and does it move in the opposite direction with respect to the balance tab therefore in the same direction of the control surface? The mechanical of the balance panel is what puzzles me the most, the way it interacts with the flight control, in this case the aileron.
Thanks again for your willingness