Article 11395 of rec.aviation.military:
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From: jfb@usgp1.ih.att.com (Joe Baugher)
Subject: New Fighter Series--McDonnell F-15 Eagle (2 of 26)
Message-ID: <D2xqoJ.EuI@ssbunews.ih.att.com>
Summary: Airframe and flight control system
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Organization: AT&T
Date: Wed, 25 Jan 1995 00:29:07 GMT
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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 an all-metal semi-monocoque 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, 
outboard ailerons, 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 
9.05 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.  

Sources:

   McDonnell Douglas Aircraft Since 1920, Volume II, Rene J.
   Francillon, Naval Institute Press, 1990.

   Combat Aircraft F-15, Michael J. Gething and Paul Crickmore,
   Crescent Books, 1992.
   
   The American Fighter, Enzo Angelucci and Peter Bowers, 
   Orion, 1987.

   The World's Great Interceptor Aircraft, Gallery Books, 1989. 

   F-15 Eagle, Robert F. Dorr, World Airpower Journal, Volume 9,
   Summer 1992.

Joe Baugher			  **************************************
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jfb@uscbu.ih.att.com
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