by Richard Sheffield
Air Combat-Related Aerodynamics
This section serves as a refresher course in aerodynamics as they apply to flying your aircraft in combat. It isn't intended to be a complete course, just an overview of information that will enable you to understand the physics behind some of the maneuvers used in air combat, and how to use them to your advantage.
The concept of energy awareness during air combat is fairly new. Wise use and conservation of energy during combat will increase your chances of victory.
Your aircraft has two kinds of energy: kinetic and potential. Kinetic energy is related to airspeed. High levels of kinetic energy, or speed, are needed to perform many combat maneuvers. Potential energy is related to altitude and the force of gravity on your aircraft. If you have low speed (kinetic energy) but high altitude (potential energy), you can dive and pick up speed needed to perform a series of combat maneuvers. Conversely, if you have high speed but low altitude, you can convert this speed (kinetic energy) into altitude (potential energy) by climbing.
This combination of airspeed and altitude is often referred to as your energy state. The aircraft's ability to climb, dive, and accelerate to change this state is called energy maneuverability. So what does this mean to you in combat situations? To illustrate this concept, consider the following examples.
Example 1. You're at low altitude, down in the weeds, approaching an enemy aircraft at the same altitude head-on. You're flying considerably faster than your opponent. In a tight turning fight, the slower plane will have the advantage, but you're thinking in three dimensions. So as you approach, you pull up into a steep climb and your opponent pulls up after you. This is called a zoom maneuver.
Since you possess more kinetic energy (you're flying faster), you're able to climb higher and gain the advantage. He'll run out of airspeed first and be forced to dive to regain it. You can then pitch back and dive to get on his tail.
Example 2. You're following an enemy aircraft flying at the same speed as you, but you're at a higher altitude. As your enemy twists and turns in an effort to escape, he'll lose speed (kinetic energy). If you follow him through those turns, you'll lose speed, too. But, because you're at a higher altitude, you'll have more potential energy, so you can dive to pick up speed, catch the enemy, and maneuver into a good firing position.
What these examples mean to you, a fighter pilot, is that you must constantly keep an eye on your speed and altitude during combat. A heavy-handed pilot who twists and turns the aircraft around without paying attention to energy losses will soon be unable to maneuver. Once lost, energy is hard to regain; then your only hope is to dive hard and regain some speed (assuming, of course, you've left yourself enough altitude to perform this maneuver.) The lesson here is to avoid low altitude and low speed conditions. If you don't, you're a sitting duck for air-to-air or surface-to-air fire. Also, in this type of fight, it's easy to depart the flight envelope and stall. At low altitude, a stall generally translates into a smoking hole in the ground.
Always keep an eye on your airspeed in low altitude situations. Remember, the ground has a very high kill ratio.
A good rule of thumb is to maintain a high cruise speed (Mach .9 or so) and a good cushion of altitude (35,000 feet or so) when entering a combat situation. With this speed and altitude, you'll have all the options of climbing or diving at will. Just remember, this altitude can put you in a bad situation tactically on some missions, so the optimum situation isn't always appropriate.
Your aircraft's ability to gain altitude, or climb, is hampered by weight and drag. The easiest way to improve your climb performance is to get rid of any unnecessary equipment. If you're going to concentrate on air-to-air combat, get rid of any bombs you're carrying. All that air-to-mud equipment only slows you down, making you more vulnerable.
If you have external fuel tanks that are empty, drop them, too (in the F-15 Strike Eagle, for instance, your external tanks are empty when fuel remaining is less than 13,500 pounds). Fuel tanks increase drag, hurting your ability to climb.
The F-15 has excellent climb performance when not loaded down with bombs and fuel. In this configuration, the F-15 is considered ballistic because it can produce more pounds of thrust than the plane weighs and therefore can accelerate straight up at full power.
For each aircraft, there's an optimum climb rate at which trading speed for altitude and altitude for speed is minimized. When performing a sustained climb, keep your airspeed in mind. If you climb too steeply, you'll use fuel faster and lose air speed, which will take some time to recover once you reach your desired altitude. If your climb is too shallow, it will take longer to reach your desired altitude.
Acceleration Performance and Unloading
Acceleration is primarily affected by weight and thrust. Kicking in the afterburner increases your speed somewhat, but the best way to pick up a lot of speed in a short amount of time is to dive.
Figure 3-1. Ballistic Climb
An F-15 in an air-to-air configuration (AIM-7 Sparrow and AIM-9 Sidewinder missiles) in a ballistic climb.
A gentle, unloaded dive will produce the best acceleration performance in most situations.
The trick here isn't to push hard on the stick in an attempt to dive straight down, but to perform a maneuver called unloading. In unloading, you're removing the weight of the aircraft that slows acceleration.
To do this, push forward on the stick slightly to obtain a gentle dive. This starts a gradual dive and produces a zero G condition. This is similar to going over a small hill with a car or bicycle—as you go over the crest, you're momentarily weightless as you come out of your seat. The same thing happens when you unload an aircraft; however, an aircraft can continue to lose altitude and thus remain unloaded for quite some time.
Without the weight of the airplane holding it back, the speed of the plane can be increased rapidly. This can happen so quickly, in fact, that you need to keep your eye on the air-speed indicator to make sure you don't exceed the structural design limits of the aircraft, or Vmax. The Vmax of an aircraft is the maximum airspeed it can attain without ripping its wings off.
The F-15 Strike Eagle simulator gives a visual warning when you're approaching Vmax.
When you see the Vmax warning, you should immediately cut power, extend your airbrakes, or pull up. (Make sure you already know which key operates the airbrakes, because you won't have time to look it up.)
Another method of increasing acceleration is to use a very steep dive. During a steep or ballistic dive, gravity greatly increases your aircraft's acceleration. In this case, if two planes are equal in all aspects but weight, the heavier plane will accelerate faster and achieve a higher terminal velocity.
If two planes are equal in weight but one has lower drag (because its shape is more efficient, or it's carrying fewer externals such as drop tanks or bombs), the one with the lower drag will have the acceleration advantage.
Even in this kind of steep or ballistic dive, it's best to perform the unloading maneuver first, and then progress to steeper dive angles.
One of the most important performance characteristics of a modern fighter plane is its ability to turn sharply and to maintain a tight turn for an extended period of time. In most contexts, maneuverability and turn performance are synonymous. The better an aircraft's turn performance, the better it maneuvers. That, of course, translates into a better chance of winning a fighter/fighter contest.
Turn performance is generally divided into two types: instantaneous turn performance and sustained turn performance.
Instantaneous turn performance is an aircraft's ability to turn at any given point in time. This is a function of the aircraft's speed and altitude. As the term implies, the turn doesn't have to be sustained for more than an instant. Something called maximum instantaneous turn performance is achieved at very high speeds. Altitude is also a factor here since as you get higher, the density of air is reduced. The reduced amount of air passing over the wings reduces lift capability, which then reduces the turning performance.
Sustained turn performance is the aircraft's ability to maintain a turn for an extended period of time. Turn performance is measured three ways:
Load factor or G's (gravity units) pulled during the turn. In a 5-G turn, the pilot weighs five times as much as normal.
Turn radius, which is the area it takes to accomplish a complete turn. This is normally expressed in feet or miles.
Turn rate, which is how fast the aircraft is changing course during a level turn. This is expressed in degrees of change per second.
The maximum G force an aircraft can handle is set by the manufacturer and normally allows for a significant safety margin. High-G turns can be performed at low and high speeds, but keep in mind that any time you're performing a maximum-G turn, all available lift is used just to maintain the current altitude. If you must climb, you'll have to reduce the angle and severity of the turn.
However, the aircraft's maximum-G turning ability isn't the most important factor to a fighter pilot. Turn rate and turn radius are more important because they determine the ability of the aircraft to turn inside another plane, either to escape or to obtain the necessary lead angle for a shot.
Maximum turn rate and minimum turn radius can best be obtained in high-G low-speed turns. Normally the speed in these turns is just slightly higher than the stall speed for that aircraft. When turning this slow at maximum G, it becomes imperative that you watch your airspeed. A stall will send you rapidly toward the ground and disrupt any maneuver you're attempting. Once your airspeed begins to bleed off in these turns, you must decrease the G load in order to increase airspeed and prevent a stall.
The most frequently misused control in a dogfight is the throttle. To maintain a high-G turn for a sustained period will often require you to gradually increase the throttle all the way to afterburner.
Care should also be taken when you're increasing thrust or decreasing G's during a turning fight. This can easily cause you to overshoot your opponent and quickly change your posture from an offensive one to a defensive one.
Roll performance is the ability of the aircraft to change its plane (geometrically speaking) of operation—its ability to go from level flight to inverted flight or into a steep bank maneuver.
Roll acceleration determines how fast an aircraft can get into a steep banking maneuver or a continuous roll, and is a good measure of the aircraft's “agility.” The aircraft that rolls the fastest has the advantage during a close turning fight.
Roll performance and roll acceleration are basically determined by the design of the aircraft, although roll acceleration can be increased by unloading the aircraft before performing the roll. Be sure to level out the plane after the roll to prevent an excessive loss of altitude.
Inverted Flight Performance
The F-15 has excellent inverted (upside down) flight performance characteristics. It's very steady and can even climb while inverted. Sustained inverted flight is rarely necessary in combat situations, but brief periods of inverted flight are necessary to perform many of the offensive and defensive maneuvers used in air combat.
Developing good inverted flight skills takes time and practice. All controls are reversed when you're flying upside down. To fly toward the ground, for instance, you pull back on the stick; to go up, you push forward. To execute a bank to the right, you must push the stick to the left. These skills should be practiced and perfected so you don't make the mistake of flying into trouble rather than away from it.
Jet aircraft wings are very efficient and provide a great amount of lift. When you need to dive in a hurry, this lift capability can be used to pull you down toward the ground by flying inverted and pulling back on the stick. This is an oftenused escape maneuver. You not only dive quickly, but when you pull out, you're heading 180 degrees from your original course.