Avro Arrow-An Aviation Chapter In Canadian History pg2
|Wing||Fly-by-Wire||Why was the Arrow Cancelled?|
|Setting the Record Straight: The Designer's View by Margaret McCaffery||"A
Flawed Plane and an Inept Corporation"?
The Historian's View by Margaret McCaffery
AVIONICS ARMAMENT EQUIPMENT ENGINE FUEL
The aircraft was extensively "area ruled." This concept involves aerodynamic shaping of the cross-sectional area of the fuselage along its length, to reduce drag to a minimum. Also called the "Coke bottle" design, the fuselage is characteristically pinched at the waist at the wing joint, although this was not immediately noticeable on the Arrow.
Similarly, the cockpit was designed as an extension of the fuselage rather than as a separate bubble, again for good aerodynamic performance. The cockpit canopy itself was of unusual design, opening and closing in clam-shell fashion due to its size and weight, as well as for case of entry and exit. The canopy was made of a magnesium alloy with partly glazed glass. In back, drag was reduced by trailing the canopy off into a spine running the length of the aircraft to the tail, This also doubled as a conduit for controls and wire cabling. In short, everything possible was done to reduce aerodynamic drag, including the internal carriage of weapons.
The concept of internal weapons carriage has spawned several misguided criticisms about an aircraft that would destroy itself if the weapons package were lowered during supersonic flight. In fact, the weapons package was designed to be lowered and removed only while on the ground. In this way, a fully loaded package could be "snapped" into place, considerably reducing the turnaround time per aircraft. This concept also allowed easy reconfiguration for other roles, including reconnaissance and bomber. The pack was never designed to be lowered in flight; since it was 16 feet long and nine feet across, lowering in flight would have been ludicrous. At no time were any of the completed aircraft fitted with weapons.
Initially, the Arrow was to have carried the Hughes Falcon guided missile. The Falcons were to be re- placed by Sparrow 2D missiles, with a sophisticated weapons control system known as ASTRA. However, Avro engineers judged the Sparrow missiles to be inferior for use in a high performance aircraft without further development.
Each missile was to be mounted on its own hydraulically activated retractable launching mechanism. Be- cause of their large fins, Sparrows would sit partially within and partially outside the belly of the aircraft. (This is similar to the manner in which missiles are carried on the Tornado aircraft: they are recessed into the underbelly; however, no retractable launcher is required.) The smaller Falcon missiles would have been fully internal to the aircraft. Missiles would extend from their own individual bay doors. Aft missiles would be fired first, followed by forward missiles.(2) A sliding bay' door arrangement was being considered for the Sparrows. Door opening or closing was to have been completed in 0.35 seconds; extension was to have taken another 1.25 seconds or less.
It has been argued that no other fighter has duplicated this internal weapons carriage. This is simply not the case. The CF-101 Voodoo aircraft, for example, employed a rotating platform, which carried some of the weapons internally and the remainder externally. The F-106 Delta Dart used an almost identical internal missile system to that of the Arrow. Internal weapons carriage may also become the future norm.
As calculated by Avro engineers, externally mounting four missiles could have increased drag by some 20% at Mach 1.5. Bill Gunston (3) states that the move towards faster, more agile fighters is slowly forcing the removal of externally mounted weapons in order to take every advantage of the resulting reduced drag. He states it will simply no longer be good enough to hang missiles on pylons. One solution is to use the recessed method of missile carriage and the other is to place weapons in an internal bay.
A recent article (4) describes stealth design techniques to reduce radar cross-sectional (RCS) area. These include using aerodynamic shapes such as delta wings, blending cockpit and wings into the fuselage and, of course, carrying weapons internally. Aerodynamic and stealth efficiency appear to be complementary design requirements. The Arrow was not a stealth aircraft, but obviously the concept of a "clean" aircraft could have several inherent advantages.
Avro Arrow design team, left to right. Bob Lindley, chief engineer; Jim Floyd, vice president, engineering;
The requirement for such a large weapons bay necessitated stowage of the main landing
gear in the thin delta wings. This caused a number of engineering difficulties, overcome by
Dowty Engineering Limited. On retraction, the main gear would be shortened, angled
forward and then twisted in order to be accommodated Given the 30-ton weight of the
aircraft and resulting 200,000-Lb compressive load on the main gear on landing,
ultra-high-tensile steel with an ultimate tensile strength of 260,000-280,000 psi was
required. Use of aluminum was obviously precluded, as was the use of butt and gas welding
techniques. Instead, large forgings were made, using a die process. For example, the main
outer leg was the largest forging, weighing 1,000 lb. After machining this would be reduced
to 167 lb. Solutions to the problem put Dowty and Avro engineers at the forefront of
Likewise, the engineers at Jarry Hydraulics were obtaining patents for their steering
mechanism in the nose gear arrangement, among others. In fact, Avro engineers and their
subcontractors made enormous strides in developing high temperature alloys, high pressure
and high temperature systems, fuel technology for supersonic flight and human engineering, in terms of cockpit layout and design. These techniques pushed the world
aircraft industry further ahead. In support of these advances, Avro maintained a huge
metal-to-metal autoclave, a special heat treat furnace, a giant skin mill and a 15,000-ton
rubber pad forming press (then the largest in the world).
Early in the design, it was decided that some form of power assist would be required to help
control and fly the aircraft during supersonic flight. The chosen result was fly-by-wire. In
conventional systems, the pilot's stick and rudder controls are mechanically linked via steel
cables or rods to valves which control high pressure fluid flow to the actuators. These
powerful hydraulic actuators, in turn, operate the aircraft's control surfaces, such as
elevators and ailerons. In military aircraft, automatic flight control systems, gyroscopes and
position sensors are also mechanically linked to the actuators through the control rods.
In the Arrow automatic flight control system (AFCS), in automatic mode, the pilot's stick
and position- ing sensors were linked electrically to electro-hydraulic actuators. Hence,
stability, command and control were effected almost instantaneously in all three axes.
Analogue computers with a mix of vacuum tube and transistor technology were used,
together with autostabilization of the tail fin and artificial feel, to give the pilot some sense of
force on his control stick.
Not until the 1970s did fighter planes use a similar AFCS, although variations had been
employed in ex- perimental aircraft and the SR 71 Blackbird. The F-16 and Panavia Tornado
both used analogue fly-by-wire.The first fighter to replace the analogue system with digital
electronics was the F/A-18 Hornet.
How effective was the Arrow fly-by-wire automatic flight control system? According to test
pilot Spud Potocki, in a 60-degree climb, with full afterburner, he would shut down one
engine and experience no expected sideslip or roll. The AFCS would compensate
instantaneously. Automatic approaches and takeoffs were also successfully completed. The
Arrow was the most modern interceptor in the world, clearly over 20 years ahead of its time.
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