McDonnell F-15 Eagle - Airframe and Flight Control Systems

The F-15 is a large, twin-engined aircraft with high, shoulder-mounted fixed-geometry swept wings and twin tails. It is somewhat lighter in weight than the F-4E Phantom and since it is much more powerful, it has a much more spectacular climbing performance.

The aircraft has a fuselage of 58 feet 3 inch in length. The fuselage is of conventional semi-monocoque construction, and has a central pod and lateral twin-boom configuration. The F-15 airframe contains 25.8 percent titanium by weight, most of it concentrated around the engines and in the inboard sections of the wings. The three main wing spars and the bulkheads connecting them and the frames of the engine pods are of titanium. Aft of the forward main wing spar, the fuselage skin is also of titanium. The cantilever booms outboard of each engine which carry the twin fins and horizontal stabilators are made of titanium, as are the stabilator attachments and the spars of the fins. There is a titanium firewall between the two engines to prevent a fire in one engine from spreading to the other.

The heart of the aircraft structure is a set of four carry-through frames which run across the central fuselage, each with holes cut into them to allow the engine air intake ducts to pass through. At each end, they form the main attachment points for the wings, the three aft frames being attached to the three wing spars, and the forward point attaching to a leading-edge member. Machined titanium frames in the rear fuselage maintain structural integrity and provide the main mountings for the engine installation.

Eight individual fuel tanks are located in the main inter-spar areas of the wing and in the center section of the fuselage between the intakes, for a total of 1790 US gallons of internal fuel. Three 610-gallon drop tanks can be carried, one on the fuselage centerline and one on each of the underwing pylons.

There are three separate hydraulic systems which can detect and isolate leaks in their associated subsystems and each of which can sustain the flight control systems on its own. There are two electrical systems powered by 40/50 KVA AC generators. They can operate independently.

A fire suppression system is installed, the Eagle being one of the few fighter aircraft to be so fitted. This consists of a pressurized bottle containing a non-corrosive fire retarding agent located between the engine bay firewalls. There are three nozzles that can release the agent into either engine or into the space between them.

The main landing gear legs retract into the fuselage, the legs turning 90 degrees as they retract forward to lie flat in wells underneath the fuselage. The undercarriage track is rather narrow (only 9 feet), but to consider another undercarriage configuration with a wider track would have incurred an unacceptable weight penalty. The narrow track caused some problems during crosswind landings, where the upwind wing would tend to come up, causing the aircraft to weathervane into the wind and again drift downwind. The nosewheel retracts forward into a well underneath the pilot's cockpit. It is steerable through plus or minus 15 degrees. The space between the jetpipes is occupied by a retractable field arrester hook which is used in emergency situations to stop the aircraft when the brakes have failed.

The F-15A has a very large, cantilever, shoulder-mounted fixed-geometry wing, swept back at a 45 degree-angle. The wing area is 608 square feat, offering a low wing loading and providing excellent combat agility. The wing is set at zero incidence, and has a slight amount (one degree) of anhedral in order to reduce stability in the rolling plane. The wing is a multi-celled, three spar structure with multi-stiffened skins. In contrast to some other modern fighters, the F-15A has conventional outboard ailerons and conventional flaps, and no other control surfaces. In particular, no spoilers or leading-edge extendible slats are fitted. The wing area is sufficiently large that no slotting or blowing is needed to achieve acceptably low landing speeds. The use of variable camber with movable surfaces on both the leading and trailing edges of the wing was ultimately rejected by the design team, since a design with a fixed leading edge employing conical camber offered only slightly higher supersonic drag and only marginally reduced subsonic performance, both of which were more than offset by increased advantages in terms of reduced weight, simplicity of manufacture, and ease of maintenance.

Early in the development program, the builder removed three square feet of area from the trailing edge of wing tip on each side beginning with the 4th aircraft in order to cure a problem encountered with severe buffet experienced above 30,000 feet at speeds between Mach 0.9 and 0.95 and at 6g or more. This created the characteristic raked wingtips of the F-15.

The tail unit of the F-15A is an all-metal structure consisting of twin fins and rudders made of boron composite skin over honeycomb material. The twin fins are positioned so that they make maximum use of vortices generated by the wing roots. The height of the fins ensures that they maintain full authority at high angles of attack. Having twin vertical fins rather than just one large one sacrifices weight for good high-alfa performance and better survivability. All-moving horizontal tail surfaces are mounted outboard of the vertical fins. These all-moving tailplanes are mounted somewhat below the line of the wing in order that they receive an undisturbed airflow and maintain effectiveness at high angles of attack. Stabilizers and rudders are interchangeable from side to side. The all-moving tailplanes act in unison for pitch control and differentially for roll control acting in conjunction with the ailerons.

During the flight test program, the tailplane leading edge was given a sharp dogtooth notch to generate vortices and increase its effectiveness, while curing flutter problems and eliminating buffet.

One of the characteristic features of the F-15A is the use of a large spine-mounted dorsal airbrake. This airbrake can be deployed without pitch change at any speed. During flight testing, an unacceptable amount of buffeting was produced when the speed brake was deployed to its full extension, so the extension angle was reduced and the area was increased from 20 to 31.5 square feet.

The pilot's cockpit is mounted high on the forward fuselage central pod behind a one-piece windshield. The canopy itself is a single transparency with only one transverse frame. It is hinged at the rear and opens in a clamshell-type fashion. The cockpit canopy offers excellent all-round visibility.

The aircraft is provided with a McDonnell Douglas ACES II ejection seat, with zero-zero capability. At zero airspeed, the catapult fires within 0.3 seconds, followed by the rocket sustainer in 0.45 seconds, separation of the pilot from the seat after 1.3 seconds, and opening of the parachute pack in 2.3 seconds.

The F-15A has a dual flight control system. The first of these is a conventional hydraulic system that operates through push rod linkages acting on the valves of hydraulic actuators which deflect the control surfaces. The pitch-roll control assembly is a mechanical system which modifies the response of the system and the aileron-rudder interconnect couples the rudders and stabilators so that the rudders operate automatically in conjunction with the stabilators, allowing maneuvers to be carried out using the stick alone. However, this system is made to disconnect automatically upon touchdown to eliminate the accentuation of weathervaning during landing, a problem which turned up during early flight testing. The other flight control system is an automatic control augmentation system (CAS) which is fly-by-wire. It uses electrical signals and servo motors to operate the hydraulic actuators. The CAS system includes pitch and yaw rate, angle of attack, dynamic pressure sensors, and accelerometers which continuously monitor vertical and lateral accelerations. The system computes the correct settings for the control surfaces at any combination of speed and g forces. The CAS also senses the stick forces applied by the pilot and converts them into electrical signals to apply the correct amount of deflection to the control surface activators. The CAS is a dual system in which the signals generated by each channel are compared with each other. If a difference greater than a preset amount is detected, this is interpreted as a malfunction and the CAS automatically disengages, the conventional mechanical hydraulic system taking over.