737FLIGHTSIM.COM

TECHNICAL

ACTIVE CONTROL LOADING / FEEL

Building an Active Elevator Control Loader

In the 737NG there is only one axis with active control loading, that is the Elevator. The Aileron's and Rudder are just a simple Cam and Follower for centering. The image below is a 737-200 Elevator Centering Unit. Attached to this unit is the Mach Trim actuator (Red circle) and the Feel Pressure Hydraulic Actuator (Black circle).

Here is a diagram of the Elevator Centering Unit. In the 737 Feel Pressure Hydraulic Actuator is a dual chambered device, which keeps the A & B system control pressures isolated but allows each system pressure to provide independent control of the actuator.

Here is another image of the Elevator Centering unit.

The next step was to position Elevator Centering Unit input control arm through a range of angular motion and measuring the force required to hold that deflection from neutral.

This is then plotted to develop a graph of the force in pounds verse angular position of the Elevator Centering Unit input control arm. This measurement is done without any pressure applied to the Feel Pressure Hydraulic Actuator. This is the resulting graph from the measurements.

Based on the graph a cam profile was developed for the cam and follower centering unit.

Here is a simple Elevator Centering unit based on the cam profile shown as the green line in the graph above.

In this image the cam is rotated 90 degrees to provide more room so additional parts can be added.

Alan Dyer moved the formula derived from the measurements into a application that could be used to check the force on the control column based on position and Stabilizer Trim position.

The formula needs to be modified based on the aircraft airspeed. The airspeed is measured at the Vertical stabilizer. There are two Pitots one for the A system section of the feel computer and a second for the B system section of the feel computer. The Feel Computer modulates the A & B system pressure before it is applied to the Feel Pressure Hydraulic Actuator. This actuator is circled in black in the image below.

In the image below you can clearly see the mounting points for the Feel Pressure Hydraulic Actuator. The two spring that are visible are for the Cam and Follower for centering.

Elevator Feel Computer Pitot tube located on the vertical Stabilizer.

This image shows the Elevator Pitot heat Korry indicators.

As the airspeed increases the Feel Computer increases A & B system pressure applied to the Feel Pressure Hydraulic Actuator. This results in an increase in force applied to centering unit which increases the centering force on the control column. The base centering force (no airspeed correction) is shown in green. The curve takes on a more vertical angle as seen in the orange and red curves with a increase in airspeed.

In this image the spring attachment point is replaced with an electric linear actuator. The linear actuator shaft is shown in the extended position, this provides the base centering force (no airspeed correction) which is shown as the green line in the graph above.

The linear actuator shaft is positioned (extended/retracted) based on aircraft airspeed. In the image below the linear actuator shaft is retracted which provides the "Maximum" centering feel airspeed bias which is shown as the red line in the graph above. The linear actuator shaft positioning is just like the "Feel Computer" modulating the A & B system pressure applied to the Feel Pressure Hydraulic Actuator.

A higher level of fidelity would be to implement a electrical/mechanical control loader.

Here is a image of both the 737-200 Elevator Centering Unit and the electrical/mechanical control loader

You need a place to start

Sometimes one must take things apart to fully understand how they work.

Copyright © 2009, 2008, 2007, 2006, 2005, 2004, 2003, 2002, 2001 David C. Allen