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Why must the guns of the AC-130 be on the side forcing the plane to circle the target. Would it not be more practical to have the guns underneath the plane shooting downwards?

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By putting the guns on the side, the gunner can direct a continuous stream of bullets at the intended target for as long as the pilot wishes to continue circling it. If the guns were belly-mounted, the pilot would have to make multiple passes at the target zone from different directions, with long pauses between them- long enough for the enemy soldiers to reposition themselves and escape.

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    $\begingroup$ Additionally: If the enemy were in a large circle, the pilot could fly a tight circle to aim belly-mounted guns. With side-mounted guns, the pilot can fly whatever size circuits are necessary, from tight to wide, allowing the gunners the luxury of aiming wherever the fire is needed. $\endgroup$ – FreeMan Aug 28 '20 at 17:38
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    $\begingroup$ Not to mention it's kind of hard to land if all your gun barrels are pointing out the bottom. If the pilot also circles the target at the center, it makes the target essentially static for the gunner whereas a belly mounted gun would be basically be a flyby shooting and difficult to aim. $\endgroup$ – DKNguyen Aug 29 '20 at 2:40
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    $\begingroup$ This is not a full explanation: just because the gun is belly mounted does not mean the pilot has to make multiple passes. The pilot can still make circles around the target, the guns are not limited to shooting straight down. See @slebetman’s for some extra explanation $\endgroup$ – Thomas Wagenaar Aug 29 '20 at 7:15
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    $\begingroup$ @ThomasW - In a Turn on a Pylon (Or Pylon Turn), the pitch angle will constantly vary with groundspeed which varies with wind speed and direction. You are keeping a constant throttle/power setting. And, your bank angle should be constant and determined by your distance or proximity to the pylon or center of your circle. This causes the aircraft to keep a constant distance from the pylon. The bank angle can be as little as 10° for a large circle, or as large as 60° for a tight circle. At times, to keep belly-mounted guns on target would require angles of inclination greater than 90°. $\endgroup$ – Dean F. Aug 29 '20 at 8:09
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    $\begingroup$ @ThomasW - This would require a belly mounted turret with plenty of clearance away from the bottom of the aircraft. This clearance would require a redesign of the landing gear to provide ground clearance when not airborne. The C-130 was designed for field expedient loading and unloading with minimal mechanical intervention. You can walk or push something up its low, shallow angle tail ramp with relative ease compared to an aircraft with a floor 6 feet off of the ground. The AC-130 was not a total redesign. It was a refit of an already existing platform. $\endgroup$ – Dean F. Aug 29 '20 at 8:15
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The C130 is a cargo aircraft. The AC130 is the airframe converted for use as a gunship, of which there are several variants.

Guns under an aircraft are usually found on helicopters, mounted on turrets. Purpose built aircraft usually integrate guns into the wings (fighters) or fuselage (A10). WW2 bombers like the Flying Fortress worked more like the AC130, and had gunners in various places including a belly mounted turret.

The low load floor of a cargo aircraft precludes placement of guns there. Additionally, the AC130 carries multiple guns. 7.62 Minigun, 25mm Gatling gun, 40mm Bofors cannon, and 105mm howitzer have all been used. Simultaneous use requires multiple mounting points. Same side placement means they can all be used on one target.

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    $\begingroup$ "Purpose built aircraft usually integrate guns into the […] fuselage (A10)" – I think the Warthog is more the other way around :-D The entire aircraft is built around the gun. $\endgroup$ – Jörg W Mittag Aug 28 '20 at 20:10
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    $\begingroup$ @JörgWMittag - The A-10 is the just the means to get the gun to the fight. 😜 It has the side benefit of protecting the heck out of the gunner. The old saying for mounted ground troops is speed equals security. Combine that with the equivalent of a flying APC, you have a winning combination. If you have ever heard the GAU-8 go off, it is like the fart of God. It will blow away anything it touches from almost any range. Deadly, but not silent. $\endgroup$ – Dean F. Aug 28 '20 at 23:38
  • $\begingroup$ Minor point, the 7.62 miniguns haven't been on any gunship in well over 20 years. The H model gunships got rid of their 20mm cannons in the 1990's, and the 7.62's were out of service well before that. $\endgroup$ – Ralph J Aug 29 '20 at 4:02
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The original concept of the AC-130 gunship was to loiter over an area for an extended period of time in support of ground troops. It provides close air support when precision is needed instead of indiscriminate strafing runs of fast movers like F-4s and other jets. Using a ground reference maneuver called turns on pylons and pivotal altitude keeps one side of the gunship perpetually aligned with a target on the ground as the aircraft circles it.

You can find more detail on the Smithsonian Channel show Air Warriors.

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    $\begingroup$ I am quite sure that a strafing run, with a fighter, but more usually an attack aircraft like A-1 (Sky Raider), A-6 (Intruder) or A-10 (Warthog), is more precise than sideways fire from a gunship. Instead the advantage of a gunship is that it can maintain fire for minutes or even hours where a strafing run lasts just a second or two. $\endgroup$ – Jan Hudec Aug 28 '20 at 21:43
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    $\begingroup$ @JanHudec The gunship has a sophisticated fire control computer integrated with a HUD aiming system. It is extremely precise. Also, the low relative target speed provides time to make small adjustments in aim to avoid collateral damage. The downside to the gunship is it's limited to night ops as it's a big slow target in daylight. The strafing aircraft are much more survivable in daylight, but as you indicate the runs are very short. $\endgroup$ – Gerry Aug 28 '20 at 23:00
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    $\begingroup$ @JanHudec - Considering the computer aided fire controls, they are both very accurate. An attack fast mover like the A-1, 6, and especially 10 are very advantageous against a larger, not very agile, target like a vehicle. Whereas the AC-130 has the advantage over the aforementioned aircraft when engaging enemy personnel in close proximity to friendly personnel. It also has the advantage of being able to move its primary guns in relation to the aircraft to engage a target. The others have to move the entire aircraft. Other than a helicopter, it is the epitome of close air support of troops. $\endgroup$ – Dean F. Aug 28 '20 at 23:31
  • $\begingroup$ @JanHudec - Plus, the AC-130 and it’s predecessor, the AC-47, flew during the Vietnam War. The dedicated close air support A-10 did not fly until after the war ended. There was nothing like it in the sky at the time for that role. $\endgroup$ – Dean F. Aug 28 '20 at 23:51
  • $\begingroup$ seems reasonable precise: youtube.com/watch?v=zbRZwooTYtk gun camera footage starts at 1:33 $\endgroup$ – Jasen Aug 29 '20 at 2:30
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Because the AC-130 is an airplane, not a helicopter, the only way to "hover" over a target is to fly in a circle around the target. This is called "loitering". This is the primary use-case for the AC-130: it's basically a flying artillery position, not a flying tank.

Having guns point downwards will have the guns point away from the target (due to banking) when loitering. Making it unsuitable to be used as a flying artillery position and forcing it to behave like a bomber (flying, reusable "bullet/shell"*).

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    $\begingroup$ "Having guns point downwards will have the guns point away from the target (due to banking) when loitering" Not if they're mounted on a belly-mounted turret, akin to an upside-down tank turret. $\endgroup$ – nick012000 Aug 30 '20 at 10:24
  • $\begingroup$ @nick012000 True, but when the OP said "guns underneath the plane shooting downwards" that pretty much indicates what he has in mind is guns pointing straight down, more or less. $\endgroup$ – DKNguyen Aug 30 '20 at 18:26
  • $\begingroup$ @nick012000 It would be pretty silly to go to all the bother of engineering upside-down guns on the bottom of the aircraft and then, as soon as you go into a banked orbit, point then side-ways at maximum extent. Much simpler to point them out the side to begin with. $\endgroup$ – Oscar Bravo Aug 31 '20 at 6:55
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I think you've got this the wrong way round - you don't fly in circles because the guns are on the side; the guns are on the side because you have to fly in circles.

The only way to get a fixed-wing aircraft to loiter over a particular spot is for it to fly in a circle. To do this you have to bank the aircraft (i.e., tilt it on its side). If you do this, you automatically present the side of the aircraft to the target, so it's the natural place to position your guns. If you put them on the belly, you'd have to then aim them to the side - as far as they would go.

This side-firing strategy grew out of a technique developed by the British Chindits in Burma in WW2. They used small fixed-wing aircraft for supply and evacuation. The planes could not land in the jungle, so flew tight orbits over a clearing and lowered supplies on a rope. They then hauled up casualties the same way. In the Vietnam War, the Americans had the idea of replacing the rope with a line of fire and the gunship was born.

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