Plane of Motion
The plane of motion is the direction our aircraft is going. You may ask if the plane of motion and the plane of symmetry are the same. The answer can be yes…or no. The issue is gravity. Anytime that gravity is exactly in alignment with the plane of symmetry (wings level, inverted or upright), the two planes are the same. But add a little bank, and gravity now becomes a force that takes the plane of motion away from the plane of symmetry. Why? Because our aircraft is affected by gravity, and the gravity force (or vector) has to be added to our lift line to get our actual plane of motion. It looks like this.
Let’s shift our attention to the gun line. As we turn our aircraft, our gun line follows our nose across the sky. If our gun line was a pen, it would draw a line that would represent our actual plane of motion. As seen through the HUD, it would look like this.
Now let’s bring gravity drop back into the discussion…this time we’ll apply it to the bullet stream. Gravity starts acting upon the round as soon as it comes out of the barrel…the further the round flies, the further it drops. Looking at Figure 4 again, I’ll add a nominal gravity drop value to the end of the bullet stream. By connecting the two lines, we get a simplified representation of the bullet stream.
This is what it would look like from the cockpit. But this is a two dimensional view of the situation. To get a three dimensional view, we need to take a ‘God’s eye’ view from above. The next figure is a very exaggerated view of the situation. For the sake of illustration, we’ll say our gun fires five rounds as we turn. We open fire at position A and cease fire at position B. The five lines represent the paths of rounds 1 through 5. Please note the lines are straight. Rounds fired in a turn DO NOT curve or bend because we are in a turn. They fly straight and true as this figure shows.
But gravity does alter their flight path. That is why the HUD view shows a slight ‘drop’ if we could visually see the bullet stream. And we can. Our sims always show the bullet stream as a tracer path…by firing a long burst in a hard turn, you can easily see the effect of gravity on the bullet stream. Just keep in mind that the HUD view can be misleading since it is a two-dimensional picture. Take Figure 6 and file it away for safekeeping. You’ll need it as we get into the next section.
As a final comment, please recognize that the ‘spread out’ nature of the bullet stream is caused by the shooter’s turn rate, ie G load. As turn rate increases, the bullet stream ‘thins out.’