FROM:    STEPHEN AMES   (FMVB73A)

B17 BOMBING TUTORIAL
Part One: Problems of the Norden Bombsight

The B17 carrying bombs may be compared with a gun platform.
But it is an unstable platform that never stays in one
place,  moving rapidly in 3 dimensions and about 3 axis.
During the bomb's flight to earth 3 forces act upon it:
   1) While hanging in its rack it is the forward motion of
the plane, and when released, it has that same forward
speed. In a vacuum it would continue to move at the same
speed in the same direction as the B17. It would in fact be
directly under the plane at the moment of impact.
   2) Gravity, the 2nd force, affects the bomb from the
moment of release. It's effect is easily calculated in terms
of speed or distance fallen at any instant after release.

Gravity works straight down. The bomb's initial velocity
works horizontally. By combining the two we get the
fundamental problem of a bombsight: How far DOWN and how far
FORWARD will the bomb be after a certain amount of TIME?

The 3rd force is air resistance, acting to resist the other
two forces. Air resistance acts more on horizontal motion
than on gravity: a bomb falls in an ever steepening path. If
the bombs are made consistently, it is only necessary to
time the fall from all altitudes in tests.

Example: a B17 travelling at 200mph drops a bomb. The actual
time of fall from 1000' is 8 secs. But in that time it loses
enough horizontal velocity to make it travel 67' short of
the 2,350' it would have gone @200mph. This lag is called
TRAIL. This lag was measured for various speeds and
altitudes to create tables for bombardiers. But there are
other factors involved...

Their is wind speed. Suppose the wind is a constant 30mph on
the nose. The B17 going 200mph will move 2,350' through the
air in that 8 seconds. But the air block will move 350' in
the opposite direction. Instead of traveling 2,350' over the
ground, it will travel only 2,000, and the bomb will lag
that 67' further, for a distance of only 1,933'. This
horizontal distance moved by the bomb is called RANGE.

If the wind is on the tail, the effect is opposite: the
RANGE now being 2,633' while the B17 travels 2,700' over the
ground. Data is therefor based on true airspeed and time of
fall, while the Norden must work on groundspeed.

This is the basic function of a bombsight: a groundspeed
meter. But there are still more variations...
Consider a 30mph wind blowing across the flightpath. The B17
must "crab" by heading along the airpath, rather than
pointing directly at the target. (Among sailors it would be
said to be following the "apparent wind"). It makes good
that path with respect to the air, and the bomb still lags
the same amount.

However, during that time the block of air has moved to the
side over the ground, and here is the problem: the bomb does
not fall along the flight path of the plane.
At no time after release, in fact, is the bomb along that
flight path of the B17. Yet this "cross-trail" can be
determined as a function of TRAIL & DRIFT ANGLE. In the
Norden this factor is compensated automatically as a result
of the other entered data.
The wind is determined from the difference between airspeed
and ground covered, as well as from observations of trees on
the edges of woods, and from waves at bends of rivers.

The last major factor is that the platform, the B17 itself,
is not stable. It is free to move in 3 directions and rotate
about 3 axis. Changes in the reference of 3-4 degrees is
normal, and 10-15 is not unusual (especially in combat).
One degree = apx 17 mils. 1 mil covers 25' on the ground
when sighted from 25,000'.
An oscillation of 3 degrees error in direction, maintained
for 40 secs, produces an error of 750' at the release point.
And 25,000' below, the bomb impacts, after another 42 secs,
1,500' to the side of the target.

Thus the reason to favor stability over maneuverability in
the design of the B17. Innovations in electric rotors for
gyroscopes also minimized this problem.

To know the moment to release the bombs, the Norden must:
   1) know the length of time to fall from the altitude and
      the horizontal speed relative to target;
   2) then multiply that speed by time of fall, resulting in
      the horizontal distance.
   3) the TRAIL (lag) is subtracted  from that, and the
      Norden produces a RANGE ANGLE: the angle formed
      between the line of sight to target and a verticle
      line from the B17 to the ground.

This is the Norden: an automatic speed and distance
calculator that interprets its findings as an angle.
What does this have to do with the Norden in Swotl?
Not much, to tell the truth.

The Swotl Norden is greatly simplified. For one thing, there
is no wind at all, ever. Just imagine the chores if we had a
wind factor like in Red Baron!
But it is useful to know the problems of the real
bombardiers, and what they're up to while we're back there
shooting all the guns and swearing at the Jerries. To know
what they had to go through in the midst of flack, bullets,
and rockets can't help but fuel the imagination as we peer
through the sight.

     Authors Note: In considering the way "minor" deviations can
     induce great errors, consider the problems at Schweinfurt
     and Regensburg: At Schweinfurt, there were actually three
     targets, located within the midst of the city. And at
     Regensburg, while the airplane factory was located outside
     of town, there was a two wing hospital located only 600'
     away. Our targets in swotl don't have these problems.
 

B-17 BOMBING TUTORIAL  Part Two: The Swotl Norden Bombsight

After flying many, many missions to research and practice
targeting, I can now share with you the ability to take out
even small builings with one bomb from altitudes up to
29,000 ft!  Here's how...
The Norden Bombsight as modeled in Swotl is very accurate.
Unlike the problems of the real life Bombardiers, you do not
have to calculate speed over the ground or wind drift. The
bombsight works by using a mirror to reflect the view from
forward of the flight path, vertically into the sight. The
Angle of View (AV) is expressed in degrees. 90 degrees is
straight down, while 15 degrees is well forward of the
current position. From this we can learn the following:
    1) The lower the degree, the greater the angle of view
(AV), the further away the target, the longer the countdown
time, and the greater the induced error in targeting.
   2: The higher the degree, the closer the target is,
the shorter the countdown time, and the lower the induced
error in targeting.

Most errors in targeting are caused by the variable use of
the magnification power. The reason has to do with the
change in the area of view in the sight. For instance, you
fly a mission and your bombs were late, so you release next
time with one of the concentric rings as your trigger point.
But what magnification are you using? If you are setting the
timer as the smallest ring passes the target, that position
will be different if you change the magnification. At 10x
the ring may be 100 yds away from the point under the center
pip, while at 25x it may be only 10'.

This tells us: As you zoom in & out, only the point under
the center pip remains in the same position.

Consistency, always using the center pip as your targeting
point, and awareness of magnification are the keys to
precision bombing.  Here is what I consistently score with:

Altitude  Speed  Engine   Mag   Angle  Time   Distance
 23,000    160     53%    15x    50     :42     3 miles
 23,000    160     53%    15x    60     :17     2   "
 25,000    160     53%    15x    50     :47     3   "
 25,000    160     53%    15x    60     :20     2   "
 27,000    155     52%    20x    50     :52     4   "
 27,000    155     52%    20x    60     :23     3   "
 29,000    155     52%    25x    50     :60     4   "
 29,000    155     52%    25x    60     :28     3   "

NOTE:  "Distance" in the table refers to distance from the
       target at the time the timer is set.

I hit "S" for the timer as soon as the pip touches the
leading edge of the target. To destroy a small building with
one bomb, I wait until the pip is partway up the wall on
that side. It's THAT precise. On the last test, 29k, 25x, AV
of 60, the building filled the sight! I set the pip in the
middle of the angled pasnel between the wall & roof - and
that's exactly where the single bomb hit.
  RULES: Always use the pip as the reference.
         Use the highest angle possible
         Be conscious of the magnification factor.
         If the time is over :60, reset it when you can.
                                           BOMBS AWAY!!!!!
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