Advanced B-26 Bombardier Training

Since you will soon be setting out on your first combat mission, it might be a good idea to get your bombs into the general vicinity of your target,  There are several reasons for this, not least among them minimizing what we call today "collateral damage" and also justifying the personal risk you will be taking flying your aircraft over enemy held territory.

So now I will ask you to go back and review the Phase 2 material about bombing.  This is under SPECIAL OPERATIONS>387th Bomb Group in the left hand menu. I want you to pay particular attention to two charts:  the M117 Bomb Ballistics chart and the Offset Aiming Point Bombing Diagram.  The latter diagram provides a method for determining when to release your bombs in order to hit your target.  Since we don't have a real bombsight in our B-26, this method relies on geometry and what we know about the ballistics of our particular M117 bombs.  It consists in sitting down in the bombardier's seat in the nose of your B-26 and looking down through the unobscured portion of the flat bombardier's sighting window.  We have determined that, for the default seat position (F9, A,A), the  line from your eye position through the center of this window makes an angle of approximately 45 degrees with respect to the longitudinal axis of the aircraft.  If the aircraft is in horizontal flight, this line makes a 45 degree angle with respect to the mean surface of the earth.  Using this fact, simple geometry, and the M117 ballistic properties, we can determine what the chart calls the offset (O) of the current aim point and the impact point of a bomb released at that point.  The distance O is also often referred to as the Trail, for the obvious reason that the impact point trails the aircraft by this distance.  I will use the two terms interchangeably.  So, this method proposes that you chart the position of this "Offset Aiming Point" on the terrain and use that as the effective aiming point.  This note proposes a simpler and better method for doing this which does not depend on the direction of your bomb run, nor require that you sight the offset aiming point in the bombsight window.  However, you must still find the target in the bombsight window.  So, here is that method:.

Instead of calculating the offset distance (O), using that distance to determine the Offset Aiming Point and then finding and sighting that point through the bombsight window, we want to compute the time (Td) takes our aircraft to fly the distance (O) from the point at which the target appears in the bombsight window until the time the offset aiming point would appear in the bombsite window.  Then, when the target appears in the bombsight window, we use a stopwatch to count down Td seconds before releasing our bombs. Easy, right?  Td= O/V , where O is the offset distance in feet and V is the speed the aircraft moves with respect to the ground in feet per second.  The answer comes out in seconds. One, two, three,….bombs away. 

What’s the problem?  If you have GPS, there is no problem.  You just read your ground speed off your Garmin receiver.  It knows for sure. But, if it’s 1943, there is no Garmin, because there are no navigational satellites. So, what did your grandfather’s (my father’s) generation do?  They used what they could measure: indicated airspeed (KIAS), pressure altitude in feet (msl) and outside air temperature (degrees Celsius).  The calculation then goes like this (example for 10000’ msl, 0 deg. C, and zero windspeed Vw):
         
          210 KIAS →~ 210 KCAS →(A,T) 245 KTAS →(Vw)245 KTGS

The variables in the parentheses are there to indicate that these are the quantities that must be taken into account to convert the quantity on the left of the arrow into the quantity on the right. The speed (KIAS) is the indicated airspeed, determined by a pitot tube and your airspeed indicator.  It measures the dynamic pressure of the oncoming airstream and depends on the speed of your aircraft relative to the air, the pressure altitude and outside air temperature (OAT) – basically the density of the air.  The number KCAS is the calibrated airspeed, which is the same as KIAS, but calibrated for the details of the instrument.  The KTAS is your “true” speed with respect to the air around your aircraft and is calculated using the measured values of OAT and pressure altitude.  The KTGS is your speed with respect to the ground.  It will be the same as KTAS if the airmass around your aircraft is motionless with respect to the ground underneath your aircraft.  Usually this is not the case and you have to calculate your KTGS (speed and direction) by vector addition using the speed and direction of the wind (Vw).  If this is non-zero, you will have to adjust your heading to track the line of your bomb run and you will also have to offset your bomb run track upwind to allow the wind to carry your bombs cross track into the target.  You will also have to compute Td  based on the in-track component of your KTGS.  Perhaps you're starting to see what the Norden bombsight was doing.

So, how are you, a 22 y.o. bombardier, supposed to do that?  You don’t have a digital calculator or a digital computer, or your iPad.  But you do have an analog computer, your E6B.  Remember that Lieutenant?  You did pick one up from base supply back at Eglin, right?

By the way, this description ignores the difficulty of knowing the velocity of the airmass surrounding your aircraft and the problem of wind shear, different wind speeds and directions at different altitudes. But, we’re sweeping those complexities under the rug.

So, you’ll do the KIAS to KTAS correction using the front of the E6B and the vector wind addition using the back.  You should have learned this when you navigated across the North Atlantic by dead reckoning to get to England.  By the way, this will be on the exam. So, completing the calculation for our example with still air.  We get the trail (O) for our altitude and indicated airspeed by subtracting the horizontal range (R) – determined from your M117 bomb ballistics chart - from our altitude (A).  Let’s say the answer for R is 3000’.  So, our trail is (10000’ – 3000’) = 7000’.  Our KTGS is 245 kts so:

Td = (7000 ft.)/((245 nm/hr)*(6076 ft/nm)*(1 hr/3600 sec.)) = 16.9 sec.

Td = 16.9 sec  So, we wait for the target to appear in the center of our bombsite window and start our stopwatch.  At 17 seconds it’s bombs away. Believe me, it’s going to feel like you’ve waited way too long, but trust me.

You can do this Lieutenant.