COMBATSIM.COM: IL-2: Air-to-Air Gunnery, Part 2

IL-2: Air-to-Air Gunnery, Part 2

by Len "Viking1" Hjalmarson

Article Type: Training
Article Date: February 05, 2002

Product Info

Product Name: IL-2 Sturmovik
Category: WWII Air Combat Simulation
Developer: Maddox Games
Publisher: Ubi Soft
Release Date: Released (Nov. 2001)
Min. Spec: PII 400 (or equiv.), 128 MB RAM, 3D Accelerator
Rec'd. Spec: PIII 600 or better, 256 MB RAM, 32 bit 3D accelerator with 32 MB RAM or better
Files & Links: Click Here

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Target Range and Gun Convergence

I opened fire when the whole windshield was black with the enemy…at minimum range…it doesn’t matter what your angle is to hit or whether you are in a turn or any other maneuver.

—Erich Hartmann
352 victories

When novice pilots first loaded up IL-2 Sturmovik they were surprised to see that they could choose the convergence settings for machine guns and cannon, depending on the type of aircraft they would fly. What is convergence and why is it important?

Convergence Setting

Convergence is the point at which shells fired from guns along a parallel axis meet at that distance in space. While many understand this, not all understand that convergence is set in both the vertical and horizontal planes, correcting for gravity.

Furthermore, convergence settings can be made individually for both cannon and machine guns. The default setting in IL-2 is 300 meters, which means that shells fired from the cannon or machine guns on the FW 190 will meet roughly at a point in space 300 meters ahead of the aircraft. In order to accomplish this, the guns are canted toward the center line of the aircraft and tilted slightly upwards.

Most commonly, guns that are set for air-to-air combat are set at 300m convergence and shorter, while guns that are set for air-to-ground combat are set at 400m and beyond.

The idea is to concentrate a bullet stream on the target at a given distance. Obviously, many forces will affect the flight of a given shell, so it is to the advantage of the shooter to minimize dispersion and concentrate fire power.

Most pilots select a convergence of around 200 meters, but some prefer even less. The novice should start about 220 meters and then work downwards from this base to find the greatest success. The point to remember is that convergence should be set to the range where you prefer to engage your target. Since the novice tends to fire early rather than late, it is best to start with a slightly long convergence setting rather than a short one.

Target at 200m

Target at 100m

Once you have set the convergence on your guns, you should engage when the target is within 20 meters of your preferred setting. Remember that as you close on the target beyond the convergence point the dispersion pattern of your shells will increase.

This raises two further questions: at what range will a given projectile have the greatest effect, and how do I know when my target is within a certain range?

Finding the Range to the Target

We’ll use IL-2 as our simulation of choice for demonstrating the concepts we will discuss here, because IL-2 has the most realistic model of air-to-air gunnery yet seen on the PC. Furthermore, the clarity of graphics will make it easier for you to determine your range to a given target at any time.

Target 210m

The easiest way to determine the range to the target is to select icons ON in the game setup. An icon appears near your target that lists the target type and range. This way you can always tell your exact range to the target.

Icons give the player a great advantage. Furthermore, they allow the player to become familiar with estimating range based on target size. If you fly with icons on for twenty hours or more, when you turn them off you will have a very good idea of the range at which you are engaging.

The other way to determine the range to the target is to measure the target against the gun sight. We’ll use the gunsight in the Bf 109 for all measures from here on.

Target 100m

In this second image you can see that the distance to the target is half that of the first image, being only 100 meters. Notice that the wingspan of the target at this range occupies all the left side of the gunsight plus half the right side, or three quarters of the total gunsight in the Bf 109.

Since the G model of the Bf 109 has a wingspan of 30 feet, we now know that the gunsight in the Bf 109 represents a span of forty feet at 100 meters. We’ll use this knowledge not only for estimating range, but also for estimating lead angles.

He 111 at 100m

Even a fairly small bomber has a wingspan double that of the average fighter. The Ju-88 has a wingspan of 65 feet. At 100 meters it looks VERY large, occupying one and a half gunsights.

Now let’s go back to the original image, where the Bf 109 at 210 meters barely fills the circle in the middle of the gunsight. This will be our standard measure. You should not open fire until the target fills the center circle of the gunsight. The center circle measures roughly forty feet at 200 meters. We’ll use this knowledge to establish appropriate lead angle when firing.

Gunsight circle

Lead Angle and Turning Out of Plane

We mentioned above that in IL-2 a straight tracking shot is not a great solution. With hit bubbles going the way of the Dodo, the hardest hit to get is one with zero deflection. The pilot is far better off to attack from an angle off the tail (AOT) of 15 to 20 degrees, whether horizontally or vertically. From any AOT the profile of the target is much larger, especially the wing profile. Equally important, targets tend to carry less armor on the underside.

All this would seem to take us back to the problem of pulling lead. While this is a good solution in a moderate turn with moderate g’s, it can become a difficult and blind shot under high g conditions. What is the solution, and how do we measure lead angle anyway?

A better technique for providing large amounts of lead is to turn slightly out of plane. This should allow the attacker to maintain sight of the target just to one side of the nose. After the range has decreased substantially, the attacker can roll toward the target and pull the pipper back to the target’s flight path.

—R. Shaw
“Fighter Combat”

Turning out of plane may be part of the solution. Let’s consider the plane of motion and then look at how we estimate lead angle on a manoeuvring target.

Very simply, the plane of motion is the flight path of your aircraft along its fuselage line through the rudder. In this next image you can see two aircraft, each with their own plane of motion, or flight path.

Split Planes

Since these two planes are not aligned we refer to them as split planes. When two aircraft are maneuvering within their own planes of motion they are out of plane with one another. BFM requires you to maneuver out of plane in order to gain turning room and a positional advantage. The basic requirement, as with all BFM, is to manage closure rate and attain a preferred alignment for a guns solution.

Rather than digress into a discussion of fighter maneuvers, we’ll stick with the task of getting our bullet stream into plane with the target. We have two options for accomplishing this: we can fly with our own plane of motion in parallel with the target and our sight in a lead tracking position ahead of the target’s plane of motion.

The alternative is a “snapshot,” flying our aircraft so that our flight path intersects the target’s plane of motion. In this case we simply fire early, so that the rounds we fire intersect the target’s flight path.

Pulling into plane in WB III

Next, we must find a way to estimate our angle off the tail. If we start by imagining the basic structure of WWII aircraft, this isn’t a difficult task.

Angle off the tail is ZERO when you are tracking on the tail of a non-maneuvering target. The rudder is aligned with the nose, and you can’t see the sides of the fuselage or the any of the rudder surface.

From this position angle off the tail varies infinitely. But for the purposes of a firing solution, we can break the angle down into three or four positions. Note that any angle greater than thirty degrees becomes a snapshot.

Estimate Angle off the tail

For the purpose of a firing solution, we’ll break down the angles into three primary segments: 15 degrees, 30 degrees and 45 degrees. Any angle close to zero degrees is a low angle off. Any angle close to fifteen degrees is a medium angle off, and any angle close to thirty degrees is a high angle off.

Low AOT

The angles close to zero degrees are immediately obvious. A fifteen degree angle off looks something like this:

Middle AOT

An angle between 30 and 45 degrees looks like this:

High AOT

When you are tracking a target in flight you will see infinite variations of these angles, with every position in between. But if you begin with these three positions in mind, experience will temper your hand, and you will instantly estimate a firing position relative to these positions.

At this point I am indebted to Robert Shaw and Andy Bush, both of whom have written on lead angles.

In his article, Bush uses the stadiametric principle to demonstrate that a standard size target (the 35-foot wingspan of our Bf 109) can be used to measure angles at a given distance. Bush uses two different target speeds and calculates lead angle based on a .50 caliber shell with its muzzle velocity of 870 m/s.

This results in a chart that shows the lead angle as a function of aircraft size at a given speed. A five degree angle off requires a one half wingspan lead angle at 350 KTAS. A ten degree angle off requires a one wingspan lead angle at that speed, and a fifteen degree angle off requires a one and one half wingspan lead angle.

Angles Chart

Naturally, this is only a starting point. In reality these ideal figures will almost never occur. You are more likely to find yourself at 300 KTAS with 12 degrees. But these numbers work as a guideline, and with practice the interpolation will occur in your head without conscious thought.

In practice this becomes easier than it sounds…easier to recognize, but requiring time to fine tune your own solution. Practice, practice, practice! Let’s look at a few examples.

Low Angle off aimpoint

Medium Angle off aimpoint

The point is minimizing or stopping the relative angular motion between the target and the pipper. In the same way a high-G snapshot is “almost” a tracking shot, and the same procedures apply except that more initial lead is taken.

Aerial gunnery is 90 percent instinct and 10 percent aim.

—FC Libby, RFC
24 victories

Finally, remember that the effect of your guns is proportional to many factors: deflection angle, WHERE the projectile hits, armor of the target at contact point, range to target, projectile size and velocity, convergence settings, and rate of fire.

Download this file to fly a series of gunnery training missions.

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