F-15 Flight Control System
Part II - Directional Control

By B.P. "PERRY" HOFFMAN/ Senior Engineer. Flight Control Section. Avionics Engineering Laboratories

Last issue we took a "Broad-brush" look at the overall F-15 hydro-mechanical control system; now let's get into the specifics of the directional control system. In future issues we'll review the longitudinal and lateral control systems as well as the impact of various electronic functions. Be on the lookout for each bi-monthly issue of the DIGEST so that you'll be able to get the full flight control story.

Directional control of the Eagle conies from two vertical tins and two synchronized rudder control surfaces (Figure 1). Conventional rudder pedals position the rudder control surfaces. All rudder pedal inputs go through the Aileron Rudder Interconnect (ARI) box. a part of the Control Stick Boost/ Pitch Compensator. The ARI combines rudder pedal signals with functions of roll and pitch, providing turn coordination over a wide range of pitch and roll maneuvers.

Input authority to the rudder control surfaces in production F-15 aircraft is 15 degrees maximum. Lateral control slick inputs are scheduled within the ARl box for a maximum surface deflection between zero and 30 degrees depending upon longitudinal slick position.

The ARI output is fed via flexible push-pull shafts to each of the rudder control surface actuators. The F-15 rudder power actuator is a part of the rudder hinge, allowing a smooth, streamlined surface with no linkage to wear or jam.

Fig. #1: Flight Control System - Directional Controls

The F-15 feel trim actuator, forward and between the rudder pedals, receives its basic position signals from the rudder pedals through a common bellcrank and torque tube. The feel trim actuator establishes the neutral or zero force position of the rudder pedals by electrically extending or retracting the overall length of the actuator. Aircrew operation of the feel actuator is accomplished through actuation of the Yaw Trim switch, located just aft of the right-hand throttle. All trim circuitry is dual so that no single failure can result in runaway trim.

The Yaw Trim switch signal goes to the CAS roll/yaw computer where the switch commands are amplified by transistor relay drivers. This output is supplied to the yaw feel trim actuator, picking up relays within the actuator, powering its motor, and driving the actuator to a new position. Simultaneous with actuator travel, electrical signals are generated by a pair of Linear Voltage Differential Transformers (LVDT). These signals are fed back to the CAS roll/yaw computer and are used for three distinct functions. The signals:

Establish actuator neutral when the takeoff-trim button is held depressed.

Limit actuator travel through use of voltage level detectors within the CAS roll/yaw computer to prevent driving the trim actuator into its mechanical stops.

Advise the yaw CAS of a change in trim command so that the CAS doesn't defeat the pilot-inserted trim.

In addition to the trim LVDT's. another pair of LVDT's within the feel trim actuator measure deflection of the "feel" springs, and supply pedal force commands to the yaw CAS. Rudder deflections commanded by the CAS can add 15 degrees with respect to the position held by the mechanical system, up to a combined maximum of 30 degrees of rudder. The mechanical input pedal force per degree of rudder deflection amounts to 9.75 pounds on the pedal for each degree of rudder deflection. Twice as much rudder per pound force can be commanded when yaw CAS is engaged.


A safety spring cartridge is provided so that, in the event of jammed linkage, pedal forces can still be applied allowing CAS control of rudder operation. This provides an excellent "fly-by-wire" rudder system allowing safe return to the Eagle's nest. This also permits continued use of the nose-wheel steering system with a jammed rudder link. The same applies in the event of a linkage separation: CAS can again supply the pilot pedal commands to the rudder.


A rudder pedal limiter has recently been added to the rudder pedal torque tube. At Mach 1.5 or greater, a discrete signal from the left-hand air inlet controller actuates the pedal limiter actuator, physically restricting movement of the torque tube and pedals thus limiting rudder surface deflection to no more than 5 degrees. This prevents excessive rudder-induced rolls in a flight regime where roll/yaw coupling is a potential hazard. If the right-hand air inlet controller has not attained Mach 1.5 and the discrete signal has not been developed, or the limiter actuator has not extended to close the Maximum Extend Limit switch, a warning light illuminates to advise the pilot to use caution when operating the rudders.


The Aileron-Rudder Interconnect (ARI) box is the heart of the F-15 directional control system and is shown in simplified block diagram form in Figure 2.

Fig. #2: ARI Block Diagram

Starting at the upper left corner of the diagram, yaw input from the pilot's pedals is fed directly into a summing linkage and out to the rudders. Since this function is a straight-through linkage arrangement with no hydraulic boost at the output, rudder linkage friction downstream of the ARI can adversely affect the ability of the rudder control surfaces to return to neutral when pedal forces are relaxed. This means that the maintenance man needs to eliminate all possible sources of friction within flex cables and bell-cranks during any maintenance action. Roll input from the pilot's stick and pitch output from the Pitch/Roll Channel Assembly (PRCA) harmonize the rudder output through the Yaw Ratio Controller, the Plus/Minus Ratio Changer linkage, and the summing linkage.


The Flaps Down shift valve modifies the schedule allowing more rudder, sooner, with flaps down than is available through the flaps up schedule. Looking at the graph (Figure 3), note that with flaps up and 10 degrees nose-up stabilator. you can expect 6 degrees of rudder per inch of lateral stick (two inches of left stick equals 12 degrees of left rudder). Likewise, with flaps down and 8 degrees of nose-down stabilator, you'll get 3 degrees of rudder per inch of lateral stick (two inches of left stick equals 6 degrees of right rudder).

The Booster Servo at the roll input prevents rudder pedal commands from being fed back into the lateral control system. Coupled to the Booster Servo is the Full Stroke Pressure Limit valve which keeps the Booster from physically overloading the ARI structure as the ram reaches full stroke and simultaneous pedal inputs are applied.

Fig. #3: Mechanical Lateral Control - Rudder Interconnect (Production Aircraft)

Since no ARI functions are required with the aircraft supersonic, a hydraulic shutoff valve located in the PRCA Pitch Ratio Controller turns off the supply pressure to the ARI unit when the aircraft reaches Mach 1. Rudder pedal commands are still available, as are the 15 degree CAS commands.

The Rapid Shutoff valve is actuated by the anti-skid wheel spin-up signal. Since we don't want the rudder to be controlled by lateral stick during cross-wind landings, lateral stick inputs to the rudder are turned off at ground-roll speeds of 50 knots or greater. The maintenance technician can duplicate this during preflights. While holding lateral and longitudinal stick inputs, note the rudder deflections, place the Anti-Skid switch to OFF. and the rudder should rapidly return to the trim position. Reselecting Anti-Skid should return the rudder to its deflected position within 25 to 35 seconds. You can get the same results by turning the Roll or Pitch Ratio switches to EMERGENCY. ARI will shut down, neutralizing the rudder, and the rudder will return to its deflected position when the Ratio switch is returned to the AUTO position.

A recent addition to the ARI is the Rapid Warm-up valve contained in the -17 ARI box installed in all production F-15 aircraft. The need for a reduction in the time required to attain ARI operation became apparent during the first winter operation of the system in St. Louis. On several occasions aborts and near-aborts were blamed on poor or missing ARI response to stick inputs. A number of corrective schemes were devised, including cycling of the stick several times and cycling of the ramps to get Utility hydraulic oil moving and heated up, but none of these could be relied upon to be effective. The manufacturer of the ARI box. Moog, Inc.. then devised a thermo valve to bypass the hydraulic supply until the oil temperature reached about 140F and installed it within the ARI unit. Once past 140F, the valve opens fully.

An additional thermo bypass valve has been installed in the aircraft Utility hydraulic supply just prior to entering the PRCA to help speed the warming of oil during aircraft engine operation (this thermo bypass valve does not apply during external power operation). Though pilots or maintenance personnel will see little change in PRCA or ARI operation, there will be some increase in ARI turn-on time, as well as a definite increase in noise levels. A rather loud, low-pitched noise may be heard due to the input reducing pressure valve slowly building up to rated pressure. Though this may take up to 30 to 35 seconds, don't worry about it; it's normal, and the noise will go away.

At the output of the ARI. mechanical push-rod linkages are replaced with a flat steel ribbon riding on steel balls within a flexible housing. You may be familiar with similar cables used in some throttle systems. This unique system of linkage connects the single output of the ARI individually to each of the rudder actuator control valves.

The control valves, operating from Utility hydraulic power, provide a rotary motion rather than the conventional extension or retraction of a hydraulic ram. Because of this design, the actuator requires no additional bell-cranks or other linkages to change motion direction. Each actuator forms an integral part of the rudder hinge: the actuator is a self-contained unit and positions the rudder control surface in response to pilot pedal commands

through the ARI. and electrical commands from the automatic flight control system (CAS).

If input linkage separation should occur, a pair of centering springs will return the input control valve to detent but yaw CAS commands of up to 15 degrees may be initiated through pilot pedal inputs allowing safe flight home. Electrical commands from the yaw CAS are received by an electro-hydraulic servo valve which in turn ports hydraulic pressure to the CAS piston. The CAS piston repositions the actuator control valve sleeve with resultant main actuator motion. Electrical feedback signals are generated by the CAS actuator Linear Voltage Differential Transformer, establishing a position authority on the surface. If a fault occurs anywhere in the CAS system that results in one rudder moving 3 degrees more than the other, yaw (and roll) CAS will automatically disengage.

Conventional maintenance procedures apply to the directional control system, these are covered in the technical order. However, flexible shafts and cables require special care.

Be especially careful when working with the flex shaft transmitting pitch information between the PRCA and ARI. Kinks or bends in this shaft cannot be tolerated and are cause for cable replacement.

Be sure that the rod-end (bearing) is connected on the top of the ARI input arm; if it is connected to the bottom of the arm. interference with the ARI is likely and cable kinks will occur (see Figure 4).

Be careful when handling flexible cabling; don't bend it too much or twist it any more than necessary. When connecting cables to bellcranks, strive for the best alignment possible by juggling clamps where necessary. On later delivered airplanes (beginning at about airplane 51), special adjustable support brackets will be available to assist in careful cable alignment. With cautious handling and installation linkage friction and rudder surface hangup will be minimized.

Fig. #4: Flex shaft rigging on ARI