The concept of firing a projectile from the side of an aircraft was patterned after a technique used by bush pilots to deliver mail in the Amazon Basin. The aircraft would be flown in a pylon turn while a rope with a bucket on the end would be let out until it reached the ground at the center of the orbit. Incoming mail could now be retrieved from the bucket, and outgoing mail could be placed into the bucket and pulled back into the aircraft. The gunship concept is the same; but, of course, it is not mail that is to be delivered but ordnance. In any case, the whole idea was to keep the target stationary in relation to the aircraft; so the target could be destroyed without losing sight of it as would be the case in an aircraft making a conventional straight ahead pass.
The first real fixed-wing gunship was the AC-47, a military version of the venerable DC-3. This gunship's computer was in the pilot's head. There was a so-called iron sight (because the symbols in the sight did not move) mounted perpendicular to the lateral axis of the airplane and situated so the pilot could look out the left side of the airplane. The sight was adjustable in azimuth and elevation to compensate for different employment altitudes and airspeeds. Once the gunsight was set, the pilot was required to fly at a predetermined (nominal) altitude, airspeed, and bank angle.
Assuming that the gunsight and guns were set properly and that the pilot could fly on the nominals, he could then hit the target. He could hit the target, that is, if he could see the target to shoot and if there were no wind acting on the projectile. Wind in those days was taken care of by the pilot firing a few bursts to determine where the projectiles hit the ground, adjusting his aim point upwind for the amount he missed the target, then firing again.
This scheme worked quite well at low altitudes employing high velocity projectiles from a rapid fire weapon. At night, flares usually had to be dropped from the gunship so that the pilot could see the target; this not only illuminated the target but it in many cases highlighted the gunship as well. Highlighting the gunship at night when people were shooting back was not conducive to good health.
The AC-130 was developed to provide a gunship that could fly higher, carry bigger guns, and see at night. To see at night, two sensors were placed on the gunship: A low-light level television and an infrared detection set. To tie the sensors and the gunsight together required a computer.
The first computer was an analog device built to solve the ballistic problem only at 30 degrees of bank and at various altitudes and airspeeds. This computer could also compensate for the wind acting on the projectile. The computer would gather look-angle information from the selected sensor and position a moveable symbol, the target, in the pilot's gunsight. A second symbol, the aircraft aimline, would be computer positioned upwind to take care of the wind error. The pilot still had to fly on nominals and superimpose the symbols in the gunsight, but he did not have to see the target to shoot at it.
The development of the analog computer was a big step forward, but it was still a so-called dumb computer in that the pilot was forced to fly the nominals and superimpose the symbols in order to hit the target. Since it is not easy to fly an airplane in a pylon turn at a particular bank angle, airspeed, and altitude (especially in wind conditions of over 12 knots), a newer smart computer was developed.
This computer was an all-purpose digital with 16K memory in a ferrite core. The computer could not only take care of the ballistic problem but with its navigation sensors it could navigate the airplane as well. The flight program presented the gunsight two symbols: The Computed Impact Point (CIP) which corrected for all off-nominal conditions and the wind and the Primary Aim Line (PAL) which was positioned by the selected sensor look-angles. Now, all that was required to hit the target was to superimpose the CIP and the PAL and fly the airplane close to the nominal conditions; i.e., + five degrees of bank, + five knots of airspeed, and + twenty-five feet of altitude.
Up to this time, the guns had been fixed mounted to the airframe, so the pilot aimed the guns by positioning the airplane. At this point, the big guns were individually mounted on hydraulic platforms. Now, the guns were computer-controlled and could move within a twelve degree box centered on the computed nominal. All that was required to hit the target was to fly the aircraft so the PAL, the target, was within the twelve degree box with the CIP at its center. In this trainable condition, the gun is, in effect, slaved to the sensors look-angle corrected for wind and ballistics.
This small 16K computer required imaginative programming to cram all of the gunship requirements for ballistics and navigation into it. There was no room for growth. A new 356K computer that operates much faster has been developed and, even as we speak, is being tested on board a gunship. Not only is the computer much more capable but the panels that operate the computer are smart in that they have their own microprocessor and 128K of memory.
The computer in the gunship has evolved from a grease pencil mark on the pilot's side-window (the computer being in the pilot's head) to a state-of-the-art digital computer which can display ballistically-corrected symbols to the gunsight, hydraulically position the guns, correct boresight and alignment errors, determine and correct for the wind, and navigate the airplane. A brand new airplane is now being developed which will require two 1.3 meg computers to run its flight program. This new computer set-up will allow simultaneous attack on two separate targets and, with its new radar sensor, will be able to attack through the clouds without visually sighting the target.
Ballard, Jack S. Development and Employment of Fixed-Wing Gunships 1962-1972. Office of Air Force History, United States Air Force, Washington, D.C., 1982
Information from the author's personal experiences operating various models of fixed-wing gunships from June 1970 to August 1989
U. S. Air Force Systems Command. Part 1 Specifications for Pave Spectre Software Performance Specifications. Wright-Patterson AFB, OH, June 15, 1971
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Last edited 09/11/2004.