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On most warbirds, guns are placed some distance apart. Wing-mounted guns are several meters apart. If they fired straight ahead, parallel to the aircraft centerline, the bullet trajectories would never cross or come together. This is no way to hit a target.
Instead, the alignment of each gun barrel must be adjusted to aim slightly inwards so that the bullet trajectories converge at a single point some distance ahead of the aircraft.
Imagine a target set 1,000 feet in front of the aircraft. The aircraft fuselage is leveled and pointed directly at that target. An ordnance technician, using a special tool, peers through each machine gun barrel, or bore, and adjusts the alignment of each weapon so that the target is sighted through the gun bore ... "boresighted."
When the guns are all properly adjusted and fired, the bullet trajectories from each wing-mounted gun will converge at the target, 1,000 feet in front of the plane.
There is no "correct" number for this convergence distance, since it's really a matter of personal taste and shooting style. For most WWII fighter aircraft, it ranges between 500 and 1,200 feet.
To boresight wing-mounted guns on your model, start by numbering your guns, so you can tell which gun you're sighting.
Left Wing: 0=Outboard, 1=Middle, 2=Inboard
Right Wing: 3=Inboard, 4=Middle, 5=Outboard
Next, create a table listing the X, Y, Z muzzle coordinates (meters) for each gun. Here's a sample table used to set the gunstations for Flerk's Corsair.
| Gun |
Muzzle Coordinates |
| |
X |
Y |
Z |
| Gun 0 |
-2.826 m |
-0.653 m |
0.696 m |
| Gun 1 |
-2.669 m |
-0.677 m |
0.705 m |
| Gun 2 |
-2.514 m |
-0.700 m |
0.712 m |
| Gun 3 |
2.826 m |
-0.700 m |
0.712 m |
| Gun 4 |
2.669 m |
-0.677 m |
0.705 m |
| Gun 5 |
2.826 m |
-0.653 m |
0.696 m |
Once you have the data table listing the muzzle coordinates of your wing guns, you have all you need to "boresight" your guns to any convergence point you like as a matter of personal choice, flying style, or fighting technique.
Setting the convergence point is simply a matter of applying a little trigonometry to the situation.
Let's say you want your bullets to converge at some distance "d" ahead of your guns.
To do this, the centerline of each wing gun barrel must be turned inward slightly (azimuth angle) and adjusted upward or downward (elevation angle) in order to hit the extended aircraft centerline at the convergence distance (d).
The formulas for calculating these angles comes from trigonometry. For each wing gun,
The azimuth angle (degrees) is the angle whose tangent is ( X/d ).
The elevation angle (degrees) is the angle whose tangent is ( Y/d * tan( X/d) ).
Finally, the bank angle (degrees) is the angle whose tangent is ( X/Y ).
These are the pitch, bank and yaw angles set in the gunstation entry for the weapon.
NOTE
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The sign convention for these angles follows the left-hand rule. Laying the thumb of your left hand along the axis, your fingers will point in the positive direction.
Azimuth ... thumb along the Y axis; positive yaw is to the right.
Elevation ... thumb along the X axis; positive pitch is nose down.
Bank ... thumb along the Z axis; positive bank is to the left.
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There you have it. The X, Y, Z coordinates for each gun are fixed. By changing the value for the convergence distance (d), you can "boresight" your guns to any distance you like.
You can hand calculate these values using a calculator or, if you prefer, you can use the spreadsheet CFboresighting.xls, which will do the calculations for you! All you need to do is input the muzzle coordinates for each gun, select a convergence distance and copy the pitch,bank,yaw data into your gunstation entries.
NOTE
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To give yourself more flexibility, create a set of gunstations in your DP file, each using a different convergence distance between 500 and 1200 feet. Put a semi-colon at the head of each line in each gunstation "set" you aren't using. If you want to change the convergence distance later, simply remove the semi-colons from gunstation set you want to use and put a semi-colon at the start of each gunstation you don't need.
Here's a brief example, using a convergence distance of 225m ...
[GUNSTATIONS]
; Convergence 150m (492 ft)
;gunstation.0=0,-2,1, ... ,1.60
;gunstation.1=0,-2,1, ... ,1.60
;gunstation.2=0,-2,1, ... ,1.60
;gunstation.3=0,-2,1, ... ,1.60
;gunstation.4=0,-2,1, ... ,1.60
;gunstation.5=0,-2,1, ... ,1.60
;
; Convergence 225m (686 ft)
gunstation.0=0,-2,1, ... ,1.60
gunstation.1=0,-2,1, ... ,1.60
gunstation.2=0,-2,1, ... ,1.60
gunstation.3=0,-2,1, ... ,1.60
gunstation.4=0,-2,1, ... ,1.60
gunstation.5=0,-2,1, ... ,1.60
;
; Convergence 300m (914 ft)
;gunstation.0=0,-2,1, ... ,1.60
;gunstation.1=0,-2,1, ... ,1.60
;gunstation.2=0,-2,1, ... ,1.60
;gunstation.3=0,-2,1, ... ,1.60
;gunstation.4=0,-2,1, ... ,1.60
;gunstation.5=0,-2,1, ... ,1.60
; Convergence 400m (1219 ft)
;gunstation.0=0,-2,1, ... ,1.60
;gunstation.1=0,-2,1, ... ,1.60
;gunstation.2=0,-2,1, ... ,1.60
;gunstation.3=0,-2,1, ... ,1.60
;gunstation.4=0,-2,1, ... ,1.60
;gunstation.5=0,-2,1, ... ,1.60
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The "Flerk Spread"
There is no rule, regulation, or other edict that states all guns must converge at a single point. If you want a real devastating set of guns, set your aircraft up to fire the deadly "Flerk Spread."
This configuration requires a minimum of four wing guns and is very effective on aircraft with six wing guns.
The "Flerk Spread" is the result of setting different convergence distances for each pair of guns. The six .50 cal guns on Flerk's Corsair converge along a distance of 20 m (61 ft) starting at a range of 280 m (853 ft).
The two outboard guns converge at 300 meters (914 feet). The two middle guns converge at 290 meters (884 feet). The two inboard guns converge at 280 meters (853 feet).
All six rounds enter the aft end of this "kill box" within 12 inches of each other and crossover in pairs, exiting the forward end of the box within a 12 inch spread. Moving the box closer to your aircraft widens the spread.
Happy hunting!
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