737NG flight controls

[172_driver]

Regarding the flight control system in the old lady.. the FCOM is not very detailed in its description and the schematics leave a few things to be desired.

As I understand it there are cables from the control column to the Feel- and Centering unit - I have a rough idea how this one works, with the cam and spring-loaded roller. So the feel and centering unit is moved (rotated) purely mechanically from the control column? Further linkage from the feel- and centering unit moves the control valve in the PCU. Am I right so far?

Then there is the feel Elevator Feel Computer.. this one receives A and B pressure (uses the higher of them two), modulates this pressure based on airspeed from the elevator feel pitot probes and then sends the pressure to a hydraulic actuator trying to center the Feel- and Centering unit? I.e. assists the spring in returning the roller to the "notch" of the cam? This feedback is sensed in the yoke.

How does the autopilot control pitch? Does it use the same cable linkages as manual flight does?

Lots of questions, i know.. and words can't describe it as well as pics. So if anyone managed to figure out my mental model of the system.. i am impressed. If you're able to answer the questions, even better.. :ok:

Thanks a lot!

[DozyWannabe]

Does this help any?

Boeing 737 Primary Flight Controls - DutchOps.com powered... (http://www.dutchops.com/Portfolio_Marcel/Articles/Flight%20Controls/B737_Flight_Controls/B737_Primary_Flight_Controls.html)

[172_driver]

Thanks.. have seen that one, partly answered what i am looking for. But thought the i'd use the expertise of pprune for the rest ;)

[172_driver]

Piezoresponse force microscopy (http://en.wikipedia.org/wiki/Piezoresponse_force_microscopy)

Naah.. that sounds too easy! I think you're off

[DozyWannabe]

Pressure and Flow Modulation. :ok:

[172_driver]

Google is not always your friend..

[rmm]

As I understand it there are cables from the control column to the Feel- and Centering unit - I have a rough idea how this one works, with the cam and spring-loaded roller. So the feel and centering unit is moved (rotated) purely mechanically from the control column? Further linkage from the feel- and centering unit moves the control valve in the PCU. Am I right so far?

The cables connect to the top of vertical shaft in the picture below. The feel & centering unit is at the bottom. You can see one of the springs that keep tension on the roller, a second spring is obscured. There are 4 input rods that extend to the right. These are the PCU input rods. The PCU and it's reaction link is between the input rods. (Upper two blue arrows) There is a smaller diameter input rod on the LH side. This is the auto pilot input. (mid way between the two upper blue arrows)

Not many good pics of the elevator mechanism around, but essentially the same setup in theory, just looks a lot different. Pitch is controlled by an output rod from the elevator autopilot PCU. This rod drives the torque tube that the cables connect to.

http://www.sjap.nl/lhailinputng.JPG

Elevator mechanism

http://www.sjap.nl/elevfeelspring.JPG

[172_driver]

rmm, thanks a lot! :ok:

On the picture of the feel- and centering unit there is a hydraulic actuator at the bottom, is that pressure from the elevator feel computer? Or is it the mach trim? I understand both of them act on the feel- and centering unit.

[rmm]

Yes, that's the feel actuator. The mach trim actuator sits on top of the unit and is out of view. Have a look at the link below. It's a simulator site but has alot of diagrams in relation to the questions you are asking.

737FlightSim.com Flight Control Centering / Feel Technical (http://www.737flightsim.com/737center.html)

[172_driver]

Have been away for a few days, but thanks again rmm!
The link was quite helpful.

Regarding the aileron feel and centering unit, i understand there is only the springs, one visible in the picture above, that centers the torque tube? Even though we've got 3000 psi powering the aileron PCUs, do we retain some aerodynamic loads via manual reversion cables? That would explain the increase in forces with altitude, i.e. the ailerons are power-assisted rather than fully power controlled? The FCOM does not touch upon that.

[putz]

Regarding the aileron feel and centering unit, i understand there is only the springs, one visible in the picture above, that centers the torque tube?The centering springs hold the roller in the valley of a cam that is attached to the torque tube. When the control wheel moves, the cam turns with the torque tube and moves the roller up on the cam. This extends the springs and supplies the feel force to the pilot. When the control wheel input is released the springs force the torque tube to rotate back to the neutral position.
Even though we've got 3000 psi powering the aileron PCUs, do we retain some aerodynamic loads via manual reversion cables?
Nope, the ailerons are fully powered during normal operation.

[Aluminium shuffler]

You couldn't have manual reversion if they were power controls rather than power assisted. It's one of few medium category aircraft to have such a system rather than full hydraulic controls with a complete tertiary hydraulic system to power the controls.

[172_driver]

Thanks for your inputs! But I am with Aluminum Shuffler that there must be a degree of aerodynamic feedback that changes the feel depending on IAS.

For those interested I did find a rather good diagram of the aileron flight control system


.http://i307.photobucket.com/albums/nn287/patrikcertain/737aileronFCS.png


I have marked an input rod between the RH torque tube and the cable quadrant. I understand this one is what transmits forces from the control wheel to the ailerons when all the hydraulics is caput (i.e. manual reversion)?

I have heard something about a spring cartridge between the input rod from the R/H torque tube and spoiler mixer as well? I presume this is to allow some form of lateral control if any other part of the system jams. E.g. I could see for instance a jammed spoiler mixer prohibiting aileron movement because of the mechanical connections.

[putz]

I have marked an input rod between the RH torque tube and the cable quadrant. I understand this one is what transmits forces from the control wheel to the ailerons when all the hydraulics is caput (i.e. manual reversion)?
What you have indicated is the Aileron spring cartridge. It doesn't serve any purpose during manual reversion.


I have heard something about a spring cartridge between the input rod from the R/H torque tube and spoiler mixer as well? I presume this is to allow some form of lateral control if any other part of the system jams. E.g. I could see for instance a jammed spoiler mixer prohibiting aileron movement because of the mechanical connections.Correct (except for location), it and the transfer mechanism (override mechanism in the diagram) isolate the ailerons from the flight spoilers during certain override conditions. There are other springs, shear rivets and such that cover other override scenarios.

Normal operation: Turn the control wheel, the left body cables rotate the input tower shaft which provides input to the pcu's through the pogo's and input cranks. The body quadrant's are rotated by movement of the pcu bodies and operate the ailerons via the wing cables. The body quadrant also operate the spoilers through the aileron spring cartridge, spoiler tower shaft, ratio changer, mixer and spoiler cables.

Manual reversion: The only difference is at the pcu's. Instead of the pcu input cranks porting hyd. pressure to move the pcu body, the cranks travel until they hit mechanical stops on the pcu body and move the body mechanically.

[172_driver]

putz :ok:

Straightens out the question marks..