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I know gatling gun is fixed mounted to the airframe of the plane, thus cannot be rotated left/ right or up/down. However watching DCS pilots dogfighting with opponents they sometimes mention their radar is broken thus have no precision aim on the main gun.

I do not understand how can radar assist you to aim main gun, since only possible gun aim is to aim the plane's nose to the target or offset of the target. I know that fighter jets has a gun tunnel shown on a HUD and that it's up to a pilot capabilities to correctly use this feature. But if I understand the HUD gun tunnel it is just the speed, distance and G force calculation where the bullets will actually fly.

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    $\begingroup$ The complexity is that you're also moving, and aiming the gun will affect the trajectory of your own plane and the bullets. The bullets will also travel with inertia from your own plane, so if you were doing a steep bank, the bullets will not travel straight like in video games. If anything, I wouldn't expect a human to properly aim within any reasonable distance. $\endgroup$
    – Nelson
    Sep 8, 2021 at 2:08
  • $\begingroup$ @Nelson Why would not bullets in video games follow the same rules (if simulated) of physics? $\endgroup$
    – Vladimir F Героям слава
    Sep 8, 2021 at 11:28
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    $\begingroup$ @Vladimir because it is a massive amount of work, and nobody really cares, because most people are ignorant about real firearm behaviors. $\endgroup$
    – Nelson
    Sep 8, 2021 at 11:29
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    $\begingroup$ @Nelson People absolutely do care and integrating the EOMs of a single solid body is not that hard. The aerodynamics will be simplified but can be close enough. $\endgroup$
    – Vladimir F Героям слава
    Sep 8, 2021 at 11:31
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    $\begingroup$ @VladimirF You're free to add the physics simulation as a mod. Plenty of video games support mods. $\endgroup$
    – Nelson
    Sep 8, 2021 at 11:39

5 Answers 5

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F-16 engineer here.

This is from a great article about fire-control radar in fighter jets:

For guns kills, if the aircraft has a radar lock on a target, it can accurately gauge range to the target, and provide the pilot with the appropriate corrections for lead and gravity drop, to get an accurate gun kill. Without the radar, the pilot simply has to rely on his or her own judgement.

(migflug.com)

Without radar lock, you won't get all the nice information to help get the best shot possible.

Such as the offset target on your HUD that you mention.

You can compare it to fire-control systems on anti-air ship cannons. It's far easier to hit a moving target far away if you know its direction and speed, and way quicker if you let a computer do all the calculation.

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    $\begingroup$ Welcome to Av.SE! $\endgroup$
    – Ralph J ♦
    Sep 7, 2021 at 10:32
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    $\begingroup$ Welcome to Aviation.stackexchange! $\endgroup$
    – DeltaLima ♦
    Sep 7, 2021 at 10:56
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    $\begingroup$ You mean bullets don't travel in straight lines at infinite speed like in video games?! $\endgroup$
    – DKNguyen
    Sep 7, 2021 at 21:05
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    $\begingroup$ @DKNguyen If you turn up the bullet speed knob they will eventually behave just like in video games! Only problem is they'll be flying so fast that impact won't be registrered, and then promptly ignored! /s $\endgroup$
    – Noddle
    Sep 8, 2021 at 7:10
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    $\begingroup$ @DKNguyen Which video game in particular has such bullets? I am not aware of any. (But stay in this century please, if not this decade.) $\endgroup$
    – Vladimir F Героям слава
    Sep 8, 2021 at 11:30
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There are three components to an air-to-air gun solution, that must be taken into consideration by the engineers designing the gunsight mechanics, and well understood by the pilot utilizing the system.

  1. Lead for Target Motion or Velocity Lead (Lm). This is the same thing as is utilized by a skeet shooter. You must aim the barrel of the gun where the target will be when the bullets get there, not where it is now. You cannot just point the gun at the target. You must point it some angle in front of the target in the direction the target is moving in your field of view. Lm always lies in the target's plane of motion. This requires that the system know the time of flight (TOF) of the bullets, and the Line-of-sight rate (LOS) of the target (how fast is it drifting across the inertial background in your field of view. If the target is moving at 20 degrees per second, and, based on the range, the TOF is 1.5 seconds, then you must aim 30 degrees in front pf the target, along it's projected velocity vector. Fighter pilots use a radar lock on to determine the range and TOF, and when there is no radar lock, gunsights assume a default range (in the F-4 it was 1000 feet) . The target LOS is determined by the gunsight by assuming that if the pilot is "tracking" the target, the shooter aircraft turn rate must be identical to the target's LOS. When the pilot is not tracking the target (as in a highly dynamic close range, high angle shot (called a snap shot), the pilot must understand this effect in order to turn on the gun with an additional lead angle to compensate for the target's relative motion in the windscreen. Effectively he pulls the trigger when the target, moving towards the gunsight aiming reference, appears to be about to cross that aiming reference about a second or two in the future.

  2. Lead for Target Acceleration or Acceleration Lead (La). If the target was at zero Gs, (in ballistic free fall), there would be no need for La. Only Lead for target motion and Drag shift would be required. La is required when the target is turning away from the ballistic path that the gunsight would be predicting without La. La always lies in the target's plane of acceleration (the plane the target aircraft is accelerating in (normally defined by where the lift vector or vertical stabilizer points). The gunsight assumes that the target Plane of motion is the same as the shooter plane of motion, and that the the target G-load is the same as the shooter G-Load. The solution is based on the assumption that the target is accelerating or pulling Gs towards the shooter, and that the shooter, to match the target's turn rate, is pulling the same amount of Gs in the same direction. This is normally close to accurate when the target is at low angle off relative to the shooter, but is not accurate at higher angle off.
    enter image description here

  3. The last component is the correction for what is called Drag Shift. Bullets slow down after they leave the muzzle. This slowing causes the bullets to drift aft of their original line of fire (Line of departure or LOD). Imagine the top turret on a B-17 firing forward and upwards at a head-on approaching target. If the bullets maintained a constant velocity after they left the muzzle, all that would be necessary s to point the guns where the target will be one TOF from trigger squeeze (La), and correct for 1 G target acceleration (La). But the bullets slow down. That means it will take a slight bit longer to get out there, and the target will drift further aft in that small amount of time. Therefore, the gunner must aim slightly lower in elevation (less lead), to compensate for that. Imagine a high pressure water hose, forwards and upwards out of the sunroof of your SUV at 80 mph. The stream of water will appear to curve and bend backwards relative to the vehicle, the further it is from the nozzle. If you were trying to hit a drone flying 15 feet above you and 15 feet in front of you, you would have to aim in front of him to compensate for the apparent rearwards drift of the stream of water. This correction is required whenever the gun firing line is not aligned with the shooter aircraft velocity vector through the air. For a fighter, since guns are must be in lead, this means it reduces the the lead required by a small amount in the plane of target motion. enter image description here

The relative contribution to the total gunsight solution from these three components, generally, is as follows:

  • Lead for target motion: 70-85%

  • Lead for target acceleration: 10-25%

  • Drag Shift: 5-10%

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On the JA37D, it would auto-steer

enter image description here

On the D-version of the fighter variant of the Saab 37 Viggen, it would help in steering

The last version of the fighter variant of Viggen had an auto-steer mode, where the "steering automat" would give yaw and pitch corrections to aid in firing the cannon. This was — of course — dependent on data from the radar.

NOTE: Since the Saab Gripen is even more advanced than the Viggen, it would surprise me greatly if the descendant did not get the same thing.

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  • $\begingroup$ Imagine your radar suddenly locks a bird or wingman. Hope the system isn't very .. abrupt. $\endgroup$
    – Olaf
    Sep 9, 2021 at 8:57
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    $\begingroup$ @Devilscomrade It is not all hands off, so — no — that is not how it happens. I have the flight manual in Swedish but it is hell trying to translate it. Just rest assured that this is not any kind of Rabid Terminator Mode. :) $\endgroup$
    – MichaelK
    Sep 9, 2021 at 9:17
  • $\begingroup$ A lot of Swedish kit was designed for operation by reservists, I wonder if this is an example (do/did they let reservists fly fast jets, I wonder). $\endgroup$
    – Rich
    Sep 10, 2021 at 3:18
  • $\begingroup$ @Rich You are thinking of conscripts. Pilots/aviators are commissioned officers. There are reservist pilots too (which was part reason of the 2005 JAS-39 Gripen crash). $\endgroup$
    – MichaelK
    Sep 10, 2021 at 7:32
  • $\begingroup$ Ah, do they not have conscript officers in Sweden, like in Switzerland? I was really thinking of conscript/reservist as meaning not a fulltime career officer. $\endgroup$
    – Rich
    Sep 10, 2021 at 23:08
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In reviewing the book titled "Fighter Pilot Gunnery" from 1943, it is critical for a fighter pilot to know his own air speed, the bullet velocity, and the speed, direction, and range to the target. With this information, the pilot can aim the plane to where the bullet and the target will intersect. With radar, the target information is essentially a known vice a guestimate and a fire control computer can calculate the aim point and display it on the HUD.

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Most aircraft guns are not radar guided (except [CIWS systems] on naval vessels1). Fighters use HUD or illuminated visual reference, because the range of typical cannon rounds is insufficient compared to a missile; radar makes little practical use.

However in the 60's/70's B-52 bombers had a tail gun with an M61 cannon and radar aperature.

A 20x102mm round has an effective firing range of 2000 feet. With deviations of a moving target, filling the sky with lead in a general direction is more important than dead on sniper like accuracy.

Visual synchronicity is achieved by having the gun sights to the pilot offset to the position of the gun (Often below the pilot) so it's angled at a set of degrees past a given range, a fighter pilot knows the gun is useless past "X" amount of feet thus wouldn't even fire. Radar is used to determine general threats before engagement and seeking of potential targets as well as navigation.

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    $\begingroup$ "filling the sky with lead in a general direction is more important than dead on sniper like accuracy." Yeah... but also the rate of fire is so high that ammo must be used wisely because it doesn't last long. Once you're at gun range it can be harder to safely disengage, also. I would err on the side of being in a control position and waiting for the target to lose speed and therefore turn rate to try to have an easier firing solution versus "filling the sky with lead" and hoping something hits. Total firing time of an F-16 gun is five seconds worth of ammo. $\endgroup$
    – Todd Wilcox
    Sep 9, 2021 at 9:01
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    $\begingroup$ And the dispersion of the M-61 A1 Gatling gun (on almost all U.S. Fighter aircraft), is about 6-8 milliradians. (that's about half of one degree). That means that 1000 feet in front of the gun, 80% of the bullets lie within a 6-8 foot circle. The wingspan of a typical adversary fighter aircraft is 45 - 60 feet. If you fire a 50 round burst, the bullet density will be about 1 bullet in every 3 square feet of that 7 foot circle. So yes "fill the sky with lead"? $\endgroup$
    – Charles Bretana
    Sep 15, 2021 at 11:46

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