A Broad Brush Look at...

The F-15 Hydro-Mechanical Control System

By B.P. "PERRY" HOFFMAN/ Senior Engineer. Flight Control Section, Avionics Engineering Laboratories
At the beginning of any aircraft design program, the customer specifies his requirements and desires. In the case of the F-15. handling qualities were rigidly spelled out by the USAF: "The aircraft must meet or exceed Level II requirements throughout its operational envelope without the aid of electronic augmentation. " Military Specification M1L-F-8785B(ASG) defines all the details of flying qualities sought in an aircraft. For the sake of this article, the following brief definitions should suffice:
  • Level I - Flying qualities clearly
    adequate for the mission Flight Phase.
  • Level II - Flying qualities ade-
    quate to accomplish the mission Flight
    Phase, but some increase in pilot work-
    load or degradation in mission effec-
    tiveness, or both, exists.
  • Level III - Flying qualities such
    that an aircraft car. be controlled safely, but pilot workload is excessive or mission effectiveness is inadequate, or both. In short, this means that the basic hydro-mechanical control system must be such that a pilot can complete an air-superiority mission without a bunch of electronic boxes doing it for him.

    To better explain Level II handling, an F-4 Phantom (in contrast with the F-15) is incapable of meeting Level II requirements throughout its maneuvering envelope with SAS (or Stab Aug) operating.

Within this article we'll explain how the controls of the F-15 Eagle satisfy this requirement. In later issues of the DIGEST we'll go a little deeper into the system to shed some light on the role of electronics in increasing control capabilities to Level I handling qualities.


Fig. #1: Flight Control System Hydraulic Diagram

Since the Eagle's flight controls are designed with a fighter pilot's needs in mind, the end result is a blend of specification requirements and pilot desires. Any reference to the similarity between a conventional century series fighter control system would be difficult. It's obvious that both contain control sticks and control surfaces: however, in the F-15 it's what's in between that makes the difference.

The part that's "in between" is what we call the "CSBPC," or Control Stick Boost/Pitch Compensator. This device is the "brains" of the F-15 mechanical control system and contains two major assemblies known as the "Pitch/Roll Channel Assembly" (PRCA), and the Aileron Rudder Interconnect (ARI).

Since any aircraft responds differently to a given control surface input, depending upon the flight condition and extent of maneuvering, considerable sophistication must be employed within the mechanical control system to assure uniform response to pilot commands. The PRCA and ARI units help the basic hydro-mechanical controls provide the maneuvering capabilities and handling qualities required to satisfy the Level II specifications.

Since the applications of the PRCA and ARI in the Eagle are quite involved, we won't discuss them in detail at this time. Instead, we'd like to consider the total hydro-mechanical control system now with coverage of individual axis and electronic portions in forthcoming issues of the DIGEST.

The F-15 control system is powered by three separate hydraulic systems: Power Control One (PC-1) driven by the left engine. Power Control Two (PC-2) driven by the right engine, and a Utility system which contains two pumps, one on each engine. Each system is provided with a switchover valve which senses system return pressure. If pressure falls below a pre-selected value, required pressure is regained through a switch to another system.

Referring to the hydraulic system block diagram (Figure 1), you can see which hydraulic system powers which control system actuator. The PC-1 system powers the left side of the aircraft plus both stabilator actuators. The PC-2 system powers the right side of the aircraft plus redundant power to both stabilator actuators. The Utility hydraulic system is a backup system and can provide power to the entire control system. The PRCA and ARI receive their hydraulic power from the Utility system with PC-2 as a backup. What this all adds up to is a system that can be safely flown and landed after a total loss of any two of the three hydraulic systems.

At first glance the Longitudinal Control System (Figure 2) seems to be a conventional system, but as you look at component locations some interesting and important differences become evident.

The feel trim actuator, located in the aft fuselage of most aircraft, is located below the control stick in the F-15. This reduces the amount of linkage, thus reducing control stick dead-band, and lessens overall applied stick force.

Added safety is also obtained should there be a linkage separation downstream of the PRCA. If a separation does occur, a "fly-by-wire" capability is provided by the electronics and the pilot will still have positive feel at the stick. With a manual system such as installed in the F-15, a pilot may not even realize he has a linkage separation since the aircraft will fly and feel the same with or without the problem.

The Pitch/Roll Channel Assembly (PRCA) provides variable mechanical advantage of the pitch control system as a function of airspeed system data.

It also aids in controlling stabilalor de-flection to eliminate the difference between commanded and actual load factors. This feature compensates for trim changes due to such things as speedbrake or flap extensions, external store separations, and aircraft speed changes. The combination of feel trim, variable mechanical advantage, and series trimming gives the pilot, as near as possible, a constant stick force per G and keeps the stick pretty well in the same place in the cockpit throughout the flight. The linkage friction within the PRCA is carefully controlled to reduce control stick breakouts. The feel trim actuator location and shortened linkages to the PRCA and its low linkage friction provide the pilot with smooth, light control stick breakout forces. The PRCA output is hydrauli-cally boosted, eliminating any feeling by the pilot ot excessive frictions downstream of the PRCA. In addition, the hydraulic boost provides a shear force for chips and other foreign objects.

Outside of the PRCA the pitch linkage is fed to a "mixer" linkage where it is combined with roll inputs. These give the stabilator inputs reflective of either pitch or roll. The F-15 stabila-tors arc used collectively tor pitch and differentially for roll.

Fig. #2: Flight Control System - Longitudinal Controls
As you review the design of the Lateral Control System (Figure 3) you'll find some similarities to what we've just covered in the longitudinal system. The feel trim actuator is located below the control stick, and it is there for the same reasons mentioned for the pitch trim actuator.

Variable mechanical advantage of the Lateral Control System is provided by the Roll Channel of the PRCA as a function of airspeed data. The stick-to-aileron ratio is also reduced as a function of longitudinal stick position. As angle of attack is increased, deflections are decreased for a given stick deflection. This eliminates the need for a pilot to remember to roll only with rudder during high angle air combat maneuvers. As the stick-to-aileron ratio is decreasing, the ARI is supplying information to increase rudder deflection.

Roll output of the PRCA is hydraulically boosted for the same reasons as is pitch output. The mixer linkage, referred to earlier, receives a lateral input which is transmitted to the ailerons and as differential signals to the stabilators.

A safety spring is provided, allowing continued roll control operation should one side become totally jammed. The aircraft can be safely flown and landed with one aileron and differential stabilator control. Aileron surface power is supplied by conventional hydraulic actuators.

Fig. #3: Flight Control System - Lateral Controls
The Directional Control System (Figure 4) is equipped with a feel trim actuator which is located forward and between the rudder pedals. A safety spring cartridge permits continued aircraft control and nosewheel steering in the event the rudder linkage jams. Should a linkage jam occur, mechanical control is no longer possible: however, pedal forces can be sent to the CAS electrically which will give "fly-by-wire" control of the rudders.

Fig. #4: Flight Control System - Directional Control

Mechanical pedal inputs are supplied to the ARI box. scheduling rudder control as a function of lateral and longitudinal inputs. The output of the Aileron Rudder Interconnect repositions a flexible ribbon which moves two rotary actuators, deflecting the rudder control surfaces.

Should hydraulic power to the PRCA be lost, or if the pilot elects to select emergency modes of either roll or pitch through cockpit switching, the PRCA positions itself to a preset ratio, locks up. and allows adequate control for sate flight and landing. In this configuration, the functions of the PCRA and ARI packages could literally be replaced by simple bellcranks.

In addition, there is dual trim mechanization which prevents runaway trim. Takeoff trim for pitch, roll, and yaw can be achieved through a single switch setting.

All control surfaces, including the stabilators, are balanced. Should control surface power be lost, or a mechanical disconnect occur, the surface will go to a trail position, permitting continued trim flight.
Fig. #5: Flight Control System Linkage Illustration

Putting it all together...
We'll wrap up this introductory look at the Eagle hydro-mechanical control system by saying that the F-15 doesn't do anything by magic: you still have to pull on the pole to make the stabilator move. However, in the Eagle the distance the stabilator moves for a given input depends upon the PRCA. The same applies to the ailerons and rudders. If everything is operating normally you won't know just why, but you'll find that "it just feels good."

In future issues of the DIGEST, we'll get into the basic control system in more detail. In addition, we'll take a look at the Control Augmentation System (CAS). Stability Augmentation System (SAS). and Automatic Flight Control System (AFCS). and how they enhance and parallel the basic system. We believe that the Eagle has a good flight control system, and we hope these articles will help you understand why we feel this way.