The only real trouble I had during BFM-1 was with the 9,000 foot setup. After drawing it out on the grease board back in the squadron room, I understood why. You cannot stay in a constant high-G turn against a continuously turning bandit when starting from 9,000 feet. If you do, you’ll end up with a very high-angle gun shot at endgame. At some point in the fight, you must ease off the G, drive back farther into lag, and get closer to the bandit before going to lead pursuit. Chapter 2, “Offensive BFM,” will reveal several such important air combat tenets. You’ll learn not just what to do (like easing off the G after your initial turn on a 9,000-foot perch), but why these techniques work.
Introduction to Offensive BFM
The ultimate goal of offensive BFM is to kill the bandit in the minimum amount of time. In order to accomplish this goal, the fighter pilot must understand basic offensive maneuvering. It is helpful to think of offensive BFM as a series of fluid rolls, turns and accelerations. Some of the maneuvers in offensive BFM have names, but the modern day fighter pilot thinks in terms of driving his jet into the control position from an offensive setup, rather than in terms of executing a series of named “moves” to counter the bandit’s defensive maneuvering. The sustained maneuverability of a modern fighter has made a “move-counter-move” discussion of offensive BFM obsolete. This study guide reflects current offensive BFM thinking.
It may seem obvious, but the primary reason that you need offensive BFM techniques is to counter a bandit’s turn. When you are behind a bandit who is flying straight and level, it is a simple matter to control your airspeed with the throttle and fly around behind him. When the bandit turns, however, things change dramatically. A turning bandit will immediately create BFM problems as shown in Figure 2-1.
In order to stay in weapons parameters and in control of the bandit, you must stay at his 6 o’clock. To do this, you must maintain control of angle-off, range and aspect angle. Remember from Chapter 1 that these terms defined the angular relationship between two aircraft. Figure 2-1 shows how a bandit’s turn will change the angular relationship between the offensive and defensive fighter. To control the “angles” and stay at 6 o’clock, the offensive fighter must also turn his jet. Figure 2-2 shows why an immediate turn by the offensive fighter will not work. If the offensive fighter goes into a turn to match the defensive fighter, he will just end up out in front because the center of their turn circles are offset.
An immediate turn will not work, and driving straight will not work. A turn of some sort is the solution to solving the BFM problems of angle-off, aspect angle and range caused by the bandit’s defensive turn. The problem is twofold — how to turn and when to turn. Let’s look first at the mechanics of turns.
BFM and Turns
BFM has a lot to do with turns. It is important to understand several concepts about turns in order to be successful at BFM. These include the concepts of positional energy, turn radius, turn rate, corner velocity and vertical turns.
Power for Position
“PS (specific power) for position” is a concept that is an integral part of BFM. Fighters have two types of energy: kinetic and potential. Kinetic energy is simply the velocity or speed at which the jet is traveling. Potential energy is “stored” energy that can be converted to kinetic energy. Potential energy is directly related to aircraft altitude. If a jet is at high altitude, its potential energy is high. If the same jet is flying at low altitude, its potential energy is low. Always remember that you can trade altitude (potential energy) for speed. Likewise, you can convert aircraft speed back into altitude or potential energy.
You can also exchange energy for nose position. Anytime you maneuver or turn a fighter, it “costs” energy. When you turn a jet at high G, you “spend” or lose energy. That’s the bad news. The good news is that the defensive fighter also gives up energy to turn and defend himself.
Turn Radius and Turn Rate
The first two characteristics of turns are turn radius and turn rate. Turn radius is simply a measure of how tight your jet is turning. If you are looking down on the aircraft as it turns, the turn radius is the distance from the center of your turn circle to the aircraft, measured in feet.[15]
It is not important that you understand how to compute turn radius. Just realize that velocity is squared in the turn radius equation, meaning that turn radius will grow exponentially based on velocity. The equation also includes aircraft Gs. The more Gs that you pull, the tighter the turn. Still, velocity is squared, so airspeed has a greater effect on turn radius than G.
Turn rate[16] is the second important factor for turning the jet. Turn rate indicates how fast the aircraft moves around the turn radius or circle we just talked about. It is also described as how fast an aircraft can change its nose position. Turn rate is measured in degrees per second and is also dependent on Gs and airspeed.
The higher the G in the above equation, the faster the turn rate. Velocity still remains an important factor. Notice that G is divided by velocity. If G remains at maximum, a higher velocity will cause turn rate to decrease. The reverse is true: a lower velocity will yield a higher turn rate.
Corner Velocity
You may think that slowing down to minimum airspeed and pulling as hard as you can is the best course of action in order to achieve a high turn rate. Not so fast. There is a relationship between airspeed and Gs. At lower airspeeds, you have less G available or, in other words, you can’t pull as many Gs as you get slow. Less lift is produced by the wings of an aircraft at slower speeds, and as a result, there is less force available to turn the aircraft. If you get going really fast (above Mach 1, for example), you also lose G availability. For every fighter, there is an optimum airspeed for achieving the highest turn rate. The airspeed where the jet has the quickest turn rate with the smallest turn radius is called corner velocity. In most modern fighters, it is between 400 to 500 KCAS.[17] The F-16 has a corner velocity of about 450 KCAS.
Figure 2-3 shows the relationship between airspeed (labeled as a Mach number), turn rate and turn radius. The top of the figure shows turn rate and turn radius broken out individually, while the bottom of the graph shows them combined. These graphs in Figure 2-3 are generic turn rate and radius charts. The bottom chart represents the approximate turn performance of an F-16.
Note that at 0.6 Mach, the jet can pull 9 Gs and turn at a rate of 24° per second. At 0.6 Mach, the jet can also turn in a radius of 1,500 feet. This is the best (tightest) radius the jet can achieve at the highest turn rate possible. The jet can turn this same radius at slower airspeeds, but turn rate will go down significantly. At 0.4 Mach, for example, the jet can turn with a radius of 1,500 feet, but the turn rate falls from 24° to 16° a second. Just to put this figure in perspective, a 2° per second turn rate advantage will allow you to dominate an adversary.
15
The math involved to calculate turn radius is: TR (turn radius) = V2/gG
V is the aircraft’s velocity in feet per second. Little g is gravity, and big G is the G the aircraft is pulling.
16
Turn rate = K G/V
K is a constant and big G and V are the same as in the equation for turn radius.