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TECHNICAL

Stabilizer Trim for the Sim


Methods of Control:

In the 737NG aircraft, there are three methods for control of the horizontal stabilizer position:
Manual - The pilots use the stabilizer trim wheels for manual pitch trim control.
Electric - The pilots operate the electric stabilizer trim control switches for main electric pitch trim control.
Autopilot - The Autopilot operates the electric stabilizer trim control for pitch trim control.

Manual: Pitch Trim Control

When the pilot manually turns the trim wheels:
The trim wheels are directly connected to the forward cable drum by a chain and sprockets.
The forward cable drum is connected by cables to the aft cable drum.
As the aft cable drum rotates it turns the stabilizer gearbox which turns the stabilizer jackscrew.
The horizontal stabilizer moves up or down depending on the rotational direction of the stabilizer jackscrew.
Note: Anytime the stabilizer jackscrew rotates the trim wheels will rotate.

Manual Trim Authority: -0.20 to 16.9 Trim Units

Manual Trim Wheel Rotating Force: >= 22 pound-inches and <= 62 pound-inches

Forward Cable Drum and Chain Drive
Aft Cable Drum, Stabilizer Gearbox, Electric Trim Actuator and Horzontal Stabilizer Jackscrew
Photos provided by Sjap's 737 maintenance experience exchange

Electric: Pitch Trim Control

When the pilot operates the electric stabilizer trim control switches:
The trim control switches control logic for the electric stabilizer trim actuator.
The electric stabilizer trim actuator turns the stabilizer gearbox which turns the stabilizer jackscrew.
The horizontal stabilizer moves up or down depending on the rotational direction of the stabilizer jackscrew.
The stabilizer gearbox also backdrives the aft cable drum which rotates the forward cable drum.
The rotation of the forward cable drum turns the manual trim wheels.

Power Required:
The stabilizer trim actuator requires 115 volt 400 Hz AC three-phase power.
The control column stabilizer trim control switches and logic require 28v dc power

Electric Trim Authority:
Flaps Retracted = 3.95 to 14.5 Trim Units.
Flaps Extended = 0.05 to 14.5 Trim Units.

Electric Trim Speeds Based on Flap Position:
Electric Pitch Trim Control:
Flaps Up Low Speed Electric Stabilizer Trim at 0.2 Trim units per second.
Flaps Down High Speed Electric Stabilizer Trim at 0.4 Trim units per second.
Autopilot Electric Pitch Trim Control:
Flaps Up Low Speed AutoPilot Stabilizer Trim at 0.09 Trim units per second.
Flaps Down High Speed AutoPilot Stabilizer Trim at 0.27 Trim units per second.

Horizontal Stabilizer Position Indicator:

Show's the position of the horizontal stabilizer and the indicators are driven by the forward cable drum.
The Indicator range is -0.20 to 16.9 Trim Units with a full scale travel of 17.1 Trim Units.
Horizontal stabilizer neutral position is at 4 units of trim.
The green band show's the allowable range of trim for takeoff.

Horizontal Stabilizer Travel:

The horizontal stabilizer can move a total of 17.1 degrees in these directions:
4.2 degrees stabilizer leading edge up (airplane nose down trim).
12.9 degrees stabilizer leading edge down (airplane nose up trim).
The elevator is downrigged 4 degrees when the stabilizer is at the 4 units of trim position.
Note:1 degree of Stabilizer travel equals 1 Trim Unit.

Parameters that need to be calculated:

Trim Wheel RPM for each Electric Trim Speed.
Total number of Trim Wheel turns from full nose down to full nose up (-0.20 to 16.9).

Stabilizer Trim for the Sim - Goals:

1. Manually turning the trim wheels controls the horizontal stabilizer position.
2. When manually turning the trim wheels they will have the proper rotating resistance.
3. When using the Electric Trim Control Switches the Manual Trim wheel turn at the proper RPM.
4. When releasing the Electric Trim Control Switches the Trim wheels stop rotating in the required time.
5. The Electric Trim Control will only opperate with the proper AC and DC buses powered.

Stabilizer Trim for the Sim - How it works: Part 1

Manual Pitch Trim Control:
In the 737NG the trim wheels, are connected to a chain sprocket which in turn is connected by a chain to the forward cable drum. Since this simulator only has the throttle quadrant, a Kollmorgen U9M4HT motor will used in place of the 737NG's forward cable drum.
The trim wheel sprocket in N9070Uís throttle quadrant has 44 teeth, so in order to keep the math simple I selected a 11 tooth sprocket for the Kollmorgen U9M4HT motor. This made the ratio between the trim wheels and the motor 1:4.
One gray area is the total number of turns of the trim wheel required to move the horizontal stabilizer from full nose down to full nose up (-0.20 to 16.9 trim units). Based on some pilot reports I used 256 as the total number of turns. Using the 1: 4 ratio between the trim wheels and the motor, the motor will turn a total of 1,024 (256 x 4) revolutions to move the horizontal stabilizer from full nose down to full nose up.
A 4 Counts Per Reveloution (CPR) incremental encoder is fitted to the rear of the Kollmorgen U9M4HT motor. Moving the horizontal stabilizer trim wheel from full nose down to full nose up the motor encoder will generate 4096 counts (4 x 1,024).
The 4 CPR encoder A and B channel outputs are connected to a US Digital EDAC2 Encoder Digital to Analog Converter. The EDAC2 converts the incremental encoder into an analog position sensor with 12 bit analog resolution. The output of the EDAC2 is a proportional DC voltage based on the pulse count. The output range for this application is 0 to 4.095 Volts.
The proportional DC voltage drives the target input value for the Pololu Jrk 12v3 USB Motor Controller with Feedback. The Pololu Jrk 12v3 uses the target value to position the horizontal stabilizer position indicators leadscrew via a small gear reduction dc motor. A linear travel variable resistor provides closed loop feedback to the Pololu Jrk 12v3 of the horizontal stabilizer position indicators position.
A second linear travel variable resistor provides an analog input to a Ultra Precision Joystick Controller BU0836A with 12-bit (4096 step) analog inputs. This Joystick Controller will be used to set the horizontal stabilizer trim position FSUIPC offset value.

MANUAL PITCH TRIM CONTROL DIAGRAM

Stabilizer Trim for the Sim - How it works: Part 2

Electric Pitch Trim Control:

ELECTRIC PITCH TRIM CONTROL DIAGRAM

KOLLMORGEN U9M4HT MOTOR
ILLUSTRATED 3/4 VIEW WITH ROTARY ENCODER

KOLLMORGEN U9M4HT MOTOR
ILLUSTRATED REAR VIEW WITH ROTARY ENCODER

ENCODER SUPPORT

OMRON EE-SX672 & ENCODER SUPPORT

OMRON EE-SX672 SLOT-TYPE PHOTOMICROSENSOR

OPTICAL INTERRUPTER

ROTARY ENCODER MOUNT

TEST ROTARY ENCODER
KOLLMORGEN U9M4HT MOTOR

WIRING THE TEST ROTARY ENCODER
KOLLMORGEN U9M4HT MOTOR

ORIGINAL STABILIZER TRIM INDICATOR LEAD SCREW - UPPER LEFT
MODIFIED STABILIZER TRIM INDICATOR LEAD SCREW - LOWER RIGHT

STABILIZER TRIM INDICATOR POSITION TRANDUCERS

POLOLU JRK 12V12 USB MOTOR CONTROLLER TO PROVIDE POSITIONAL FEEDBACK

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Page last updated on October 31, 2010
Copyright © 2001, 2002 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 David C. Allen