Air To Air Gunnery - Theory and Application,
Ill never forget a piece of
F-4 gun film from Vietnam. The F-4 pilot has gotten himself
nicely situated at the six of this MiG-21. Hes well
maybe 1000 or less. Hes at low
angle off, with his closure under control, and in plane with
the MiG. Hes got a good radar lock on so his gunsight
has accurate ranging. But the pilot has two little problems.
One is that the pipper is behind the MiG as the pilot chases
the bandit in a descending turn. The other is that the pilot
is pulling the trigger. Hes got a full load of 20mm
tracer, and, as you watch the film, you see the pilot spewing
those beautiful cannon shells out into nowhere as he tries
to work the pipper up to the MiG.
BRAAAP! BRAAAP! The tracers continue
to fall just behind his target as the F-4 pilot moves the
pipper ever so closer to the MiG. Finally the pipper is there!
Smack dead center on the fuselage of the bandit
as the Gatling Gun runs dry. Oh, the heartbreak!
Well, no doubt emotions run high in
combat, but this day, our F-4 pilot could have used a bit
more discipline. His Silver Star was there for the taking,
but he blew the opportunity. Was it impatience or lack of
skill? Well never know. In this article, Im going
to talk about the latter
the skill part.
The gun is not magic, nor is it an I wish you were dead!
weapon system. It may well be the most difficult of all air
to air weapons systems to use, and its successful use is seldom
a matter of luck. Skill in aerial gunnery is based on two
a solid foundation in the academics of the subject,
and the opportunity to train realistically. Fortunately for
us simmers, most of our simulations offer a realistic representation
of a gun weapon system
so we can practice as much as
we want. Its the academic part that has been missing.
Few manuals explain the gun system or offer tips on how to
use it. Thats what this article is going to do. Explain
the gun system, and then pass along a few pointers on how
to employ it.
Heres the overview. The available
literature on gunsights and their use is fairly limited. To
help make up for that, Im going to make this article
an in depth review of the subject. Because of its length,
the article will be in three parts. Use this overview to jump
to the parts that you are interested in.
Part One - Basic Theory.
Gun line concept.
Lead angle problem.
- Lead for target motion.
- Gravity drop.
Part Two - Gunsight Types.
Lead Computing Optical Sight Systems
- Disturbed reticle Pipper
- Director Systems.
Part Three - Attack Types and Techniques.
- Low aspect.
- High Aspect.
I know you all want to jump right
into the part where we gun the bandits brains out, but,
sadly, well have to set that part aside for a time.
The simple fact is that the more you understand gun employment
theory, the better you are going to be at it. Sounds about
right, you say. And it is
but this understanding comes
at a price. And that price is your willingness to put a little
study time into the various principles involved. My role in
all of this is to take this academic mumbo-jumbo and make
it as digestible as possible
and make it relevant to
our sims at the same time.
Well start off with some definitions.
Most of these are terms that you may have run across at one
time or another. The definitions that Im going to use
are intended to get the idea across without getting too deep
into rocket science. Gun employment theory can get real complex,
I dont want to do that, so Ill
keep it as simple as I can.
Gun versus Cannon. As far as fighter aircraft go, these
terms are roughly related to the caliber of the gun. Anything
up to .50 caliber is a machine gun
anything over that
is a cannon.
Caliber. The caliber of a gun is the size of the round as measured
by its diameter. The units may be inches or millimeters. A
.50 caliber is about one half inch in diameter. A 20mm is
about one inch in diameter. A 37mm round is about one and
one half inches in diameter. The following figures show the
But diameter is only half the picture.
As diameter increases, so does projectile length
consequently, projectile weight. The next figure makes this
Kill mechanism. Projectile types include ball, armor piercing, incendiary,
high explosive, and combinations of these.
Ball - Typical rifle round, not used in modern fighters. More
suited for WW1 and early WW2.
piercing - The round has a hardened steel core for
penetration of aircraft structures.
Incendiary - The round contains a chemical that ignites upon impact.
Good for setting fuel and hydraulics on fire.
explosive - Similar to the incendiary round, except
the chemical has more destructive power.
Combination The above round types can be combined
explosive, incendiary), API (armor piercing, incendiary)
for extra hitting power.
fire. Also known as cyclic rate. This is the number
of rounds fired in a given amount of time, usually rounds
per minute (rpm). May be expressed for a single gun or multiples.
Some modern guns, such as the Gatling 20mm, may have cockpit
selectable rates of fire. For example, the F/A-18E pilot can
select a 4000 or 6000 rpm setting. The A-10 pilot can select
either 2000 or 4000 rpm. Korean War and earlier gun types
did not offer this feature.
In Figure 4 and 7, the colored horizontal
lines represent values for typical WW2 guns. You can compare
these lines to the modern day guns shown in the figures.
velocity. The speed that the round leaves the barrel.
Usually expressed in feet per second (fps) or meters per second
(mps). Most modern gun types have muzzle velocities around
3000 fps. After leaving the barrel, the projectile will decelerate
as a function of its unique ballistic characteristics. The
modern US 20mm round has improvements that have greatly increased
its ability to retain a high velocity. Other rounds, such
as the Soviet 37mm in the MiG-15, have a low muzzle velocity
to begin with that is further degraded due to high drag during
its time of flight.
flight (TOF). The time it takes the round after leaving
the barrel to reach the target. Usually measured in seconds.
Dispersion. A target shooter will fire his rifle a number of times to
establish a group. The smaller the group, the
more accurate the shooter is. A modern aircraft gun has a
similar characteristic. Technicians will fire the gun at a
target and then count the projectile impacts and measure their
pattern from the center aim point. Typically, this calculation
will be expressed as a percentage of rounds fired within a
certain area, usually a circle with the aim point at its center,
and is called the gun dispersion. A typical modern gun dispersion
results in about 80% of the rounds being grouped in a five
foot diameter circle at a range of 1000 feet.
of fire. A concept used to describe hitting power.
More often used in Korean War and earlier gun types. Literally
expressed as x amount of weight in a given period
of time. A good example is the Spitfire with its eight .303
caliber guns. Even though the gun had a small caliber, it
had a relatively high rate of fire, and when all eight fired
in unison, the amount of lead being thrown was considerable.
Because of the lethality of the modern aircraft cannon round,
this term is seldom used anymore.
Angle off. This refers to the relative headings of the fighter and its
target. Angle off is simply the difference in the direction
the two aircraft are pointing. If they are pointing in the
same direction, the angle off is zero
if they are approaching
head on, then the angle off is 180 degrees. Angle off is a
measurement of heading.
angle. This term is a measurement of position. The
heading of the attacker relative to the target is irrelevant.
Aspect angle refers to the attacker and is measured using
the target as the reference. This measurement originates at
the targets six oclock. This is the zero aspect
position. The twelve oclock position off the targets
nose is the 180 degree aspect position. From the six oclock
position to the twelve oclock, aspect angles are referred
to as either right or left. This is
in reference to what side of the target you as the attacker
are looking at. If you are looking at the target from its
3 oclock position, you have a 90 Right aspect. And if
you are looking at the target from its 7:30 position, you
have a 45 Left aspect. Remember, your heading is not included
in this term. Aspect angle is only a way of defining your
position relative to the target.
Note: Aspect and angle off tend to
be used in the same manner when we talk about gun attacks.
This is a unique situation and occurs because we are usually
thinking of the attacker being pointed at the target. When
the attacker has his nose on the target, then his angle off
and aspect are basically the same. In this discussion, Ill
use the term angle off with this in mind.
apparent size. We all recognize the significance of
target size. Big targets are easier to hit than small ones!!
Target apparent size refers to a single target and how it
looks from various angles. If we shoot at a target from its
dead six, we have a much smaller target size than if we were
to fire at it from directly above. Planform is a term that
refers to target apparent size. Planform is greatest when
looking down on the target. The greater the planform, the
better chance of hitting the target. Planform and aspect angle
have much in common since they both refer to how the target
appears from the shooters perspective.
computer. This device takes into account range, closure,
and altitude values to arrive at a TOF computation. The TOF
value is used to then compute a gravity drop correction.
computer. Ill use this term in the broad sense
to refer to the device that takes into account target position
(range, aspect, angle off, closure, rate of turn, etc) to
compute the lead for target motion value.
sight rate (LOS). The LOS is the speed that the target
is crossing your gun line. This value is zero in a head on
or tail aspect, and is maximum when the target is at 90 degrees
The Gun Line Concept
If you were to look through the barrel
of the gun out to infinity, you would be looking along the
gun line. The gun line establishes the initial vector of the
round as it leaves the barrel (also known as the line of departure,
the LOD). The gun line is an important part of the process
of matching up the gun sight to the gun in an aircraft. It
is the basis upon which all other calculations are made. If
an aircraft has multiple guns, therefore it has multiple gun
In our modern aircraft HUDs, the gun
line is often represented by a small cross. This cross is
fixed, meaning it doesnt move. You can think
of it as being similar to the sight on the end of a rifle.
It is one way of visualizing where the gun is aimed.
The Sight Line Concept
The sight line is similar to the gun
line. It too is a line from the eye to infinity, but this
time, we are talking about the pilots eye as seen from
the cockpit. Since few guns are co-located in the cockpit,
there is a difference in the physical location of the gun
line and the sight line. The following figure illustrates
Once the round leaves the barrel,
it becomes a falling object subject to the laws of gravity.
A round will drop approximately 16 feet in its first second
of flight. The next figure will give you an appreciation of
this factor. In this figure, notice that the pipper in the
reticle is below the gun cross. The aircraft is in wings level
flight at one G. The pipper position represents the gravity
drop of the round over the range that the sight is computing
for. As you can see, gravity drop is not an insignificant
value as TOF increases.
In discussing harmonization, we will
use the concepts of gun line, sight line, and gravity drop.
Harmonization is the process of lining up the gun line so
that it intersects the sight line at some point in front of
the aircraft. The TOF for the round to cover that distance
will be computed and used to calculate a gravity drop value.
That value will be added to the gun line. Then the gun(s)
will be adjusted so that the resulting projectile path (including
gravity drop) will intersect the sight line.
In older fighters that had guns installed
in the wings and nose, harmonization was much more of a factor
to be considered. The basic idea is to adjust the guns so
that all the gun lines converge at a predetermined distance.
Why, you ask? Some might think it would be better to have
the guns adjusted to spread out the gun lines...that way the
pilot might have a better chance of hitting something. Now,
there is a smidgen of logic to that idea, but only a smidgen.
The better idea is to have the gun lines come together. That
way the pilot has a highly concentrated area of fire that
will deliver a killing blow to whatever it hits. Certainly,
that area may be relatively small, but the issue is not the
size of the projectile impact area. Instead, the issue is
accuracy in aiming. Well get to that eventually. For
now, we just want to establish the idea that harmonization
is the process of converging gun lines so that they intersect
the sight line at a predetermined distance.
During WW2, harmonization was a hot
topic among pilots. The debate raged back and forth over what
range the guns should be harmonized at. Some liked a short
short being in around 300 feet. Others wanted the
range a bit further out
as much as 1000 feet. In many
fighter units, the matter was left up to individual preference.
In simple terms, projectile density
refers to how many bullets we can expect to have in a given
amount of space at a particular point in front of our aircraft.
We all immediately recognize that the denser the bullet pattern,
the greater chance we have of hitting our target.
We have all seen the WW1 movies of
the Red Baron blasting away at his opponent. Rat-tat-tat-tat!
One, maybe two small caliber machine guns. A moderate rate
of fire for the time. But nothing like the modern guns of
todays fighters. Today, the common perception is that
a fighters gun fire is like a red hot laser beam. Well,
Lets try to interject a reality
check to the matter of projectile density. What you want to
take away from this part of the discussion is the understanding
of how angle off and aspect angle affect your chances of hitting
Weve all heard it before. "Man!!
That Gatling spits out 100 rounds a second! Nothing can escape
that kind of firepower." If only it were so. Too often,
the typical person visualizes those 100 rounds all in the
same spot. Not true. A little math will make this clear.
Lets fire a one second burst
from our M61. 100 rounds, just for arguments sake. Now
lets picture what the bullet stream looks like. For
starters, its 3000 feet long
remember muzzle velocity.
As the last round is coming out of the barrel, the first round
is one half mile away! Spread those rounds out over that distance,
and we end up with one round every 30 feet. Then we have to
remember dispersion. The bullet stream is not a frozen
rope. Instead, it is a cone that is about 15-20 feet
in diameter at 3000.
One round every 30 feet! Not exactly
the blizzard of fire that some might think. And then those
rounds get spread around due to dispersion. That doesnt
help things much. But there is one more parameter that we
need to look at, and that is the relationship of the targets
flight path to our bullet stream.
If we are at the targets dead
six and are shooting at it, then many of the rounds have a
chance of getting a hit. This is because the target remains
in the general area of the bullet stream during the entire
burst length. But what happens if the target is crossing the
bullet stream? Whoa!! Our neat little picture of instant target
obliteration takes a big hit (no pun intended!). Lets
use a little math again to make the point. Lets have
the target cross the bullet stream at 90 degrees. Well
say the target is doing 500 knots
that will give it a
speed of about 850 feet per second. The target is a typical
modern fighter with a length of about 60 feet. How long does
it take the target to cross the bullet stream? About one tenth
of a second! We remember our rate of fire was 6000rpm or about
100 rounds per second
so, in 1/10 of a second, only about
10 rounds have a chance of hitting the target. Now, we throw
dispersion into the equation and our chances of hitting the
target become even less.
It is very important to visualize
the bullet stream as three dimensional. In the next screenshot,
the bullet stream is represented by a funnel display. The
funnel extends below the gun line and appears to run through
the target. Because of this fact, this may look like a valid
aiming solution. But it is not. In fact the rounds that are
at target range are in front of the target
solution has too much lead.
Figure 16 is a drawing of what the
situation in Figure 15 would look like from a side view. This
drawing when combined with the screenshot gives you the complete
a three dimensional visualization that makes
the concept of the bullet stream much more meaningful.
In seeing the bullet stream in this
manner, significance of target angle off and apparent size
becomes all too clear. The faster the target moves through
the bullet stream, the less chance it has to be hit. If we
as the shooter can do something to keep the target in the
bullet stream longer, then we increase our chances of success.
This is an aiming problem, and since this article is ultimately
about aiming the gun, well now move on to looking at
that problem. Well call that problem the lead
The Lead Angle Problem
There are two variables to solve for
when we look at the lead angle problem. First, lets
identify that problem. We are in a gun platform that is moving.
We are trying to hit a target that is also moving. We intend
to shoot rounds at the target
this will take a certain
amount of time (TOF) and this in turn will result in some
The problem then is to fire our gun
having taken into consideration two things
lead for target
motion, and gravity drop. The next figure is a common illustration
of the lead angle problem found in many sim manuals. Both
the attacker and target are flying straight. The situation
is similar to a skeet shooting problem. Well use this
figure to discuss the problems in computing lead for target
motion and gravity drop.
the lead angle. In a gun attack, the firing geometry
can range from a pure tail chase to a head on set up. Clearly,
the lead for target motion is greatest when the target angle
off is 90 degrees and is essentially zero when the angle off
is zero or 180 degrees. The gunsight computer must solve for
this value, and the first question that always came to my
mind was how does the computer know where and what the
target is doing? Believe me
when it comes to gunsight
computations, that is the $64,000 question!!
Our illustration shows a target that
is not turning. Throw in a turning target, and the problem
becomes very difficult to solve. In fact, it has only been
in recent years that radar technology and computer improvements
have been able to come close to an accurate answer. Prior
to these new systems, gunsight computers used a number of
assumptions about both the attackers and targets
flight behavior to arrive at a solution. As you might expect,
life seldom matched these assumptions, and the resulting lead
angle solutions were only approximate at best. Some of these
assumptions included the following: the two aircraft were
the aircraft true air speed was a certain value
range was fixed
the altitude was a constant
target was turning, it was turning at the same rate as the
attacker. It was a lucky day for the attacking pilot when
these assumptions matched the actual firing situation. More
often than not, this was not the case, and the pilot had to
fall back upon prior experience to make up for errors in his
gravity drop. We have already shown that the gravity
drop value is a function of TOF. Many of the assumptions mentioned
above also have a negative impact on the gravity drop calculation.
Incorrect closure, range, and altitude values all result in
while the gravity drop part of the total lead
angle is usually much smaller than the lead angle part, the
value is still significant to the overall gunnery solution.
it all together. The next figure shows a hypothetical
(and simplistic) view of the lead angle solution. In Part
Two, we will go into each gunsight type in detail and explain
how each type either does or does not replicate this view.
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