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Pursuit Curves or, The Why of Turning
By
--)-Rapier
--, Fighter Ace
Content Manager
| "To stab at the heart means, when fighting and there are obstructions above or to the sides, and whenever it is difficult to cut, to thrust at the enemy. You must stab the enemy's breast without letting the point of your long sword waver . . ." |
| - Miyamoto Musashi Go Rin No Sho 60+ victories in hand-to-hand combat |
What are pursuit curves and why the heck should I care?
A natural question. Ever wonder how that Yak caught your P-51 in a tail chase or how that FW190A-8 caught you as you went over the top in a loop in your Spitfire XIV? Air combat is a constantly changing maelstrom of pursuit curves. Awareness of what they are and how they affect the geometries of the fight can radically change how you are maneuvering. Armed with a little knowledge, we fly much smarter instead of harder.
Three Types of Pursuit
While combat in Fighter Ace
may seem straightforward, you will find that many of the Generals are not merely chasing their enemies and shooting at them till they die. They are making intelligent use of the capabilities of their airplanes and using pursuit curves to maximize kills. You can do this too.
First be aware that there are three types of pursuit: lead, pure, and lag pursuit. Each is useful but you must utilize it correctly in the right time and place to ensure your success.
The three types of pursuit are defined by whether we are placing the velocity vector behind, on, or ahead of the plane we are pursuing. Since we (OK, I) don't want to get into a long and incredibly boring discussion of velocity vectors, we will assume that the velocity vector of your plane is represented by the direction its nose is going. From the cockpit, the direction your plane is headed right this second is represented by the boresight cross hairs.
Lead Pursuit
In lead pursuit, we point the nose of our plane forward of the target's present position on its flight path. It can also be called a collision course (something to keep in mind as you approach).
All of air combat is based on tradeoffs.
Lead pursuit offers us several advantages and disadvantages. It generates the greatest closure rate, which is useful when you are trying to chase down a rapidly moving opponent (remember the Yak and the P51?). The farther ahead of the target's present position you place the nose, the greater the lead and closure rate, provided that the target maintains its present flight path. It also means that you will be approaching from a high angle of deflection. Keep in mind that if the target changes its flight path, you will also have to adjust yours. If you have selected a lot of lead, you will be farther out of position if the target turns in the opposite direction. All of air combat is based on tradeoffs, and the better pilots are those who can make the best guesses (usually based on experience and knowledge) at what the enemy is going to do. You will need to balance lead against the possibility of change of course and you can minimize loss of ground gained if you keep close watch on that other plane. An indication of optimum pursuit angle is the bogey's appearing to be still against the background. If they are floating back toward your boresight, you need to crank up some more lead. The drift is an indication that you are moving toward their six position (keep this in mind for when you get ready to shoot).
Lead pursuit has some down sides to it also. There is the aforementioned high angle of deflection, which makes for difficult shots. Additionally, high angles of lead at close range may mean that the enemy is below your nose and invisible if you are in cockpit-view mode. This invisibility can result in losing the target if it changes direction (bad), missing firing opportunities (also bad), or colliding with the target (very bad). You can minimize this loss of view if you take up a flight path that is parallel to the target but at a lower or higher altitude, so that you can see it off of your nose. Lead pursuit also tends to generate high-G situations, which means that it requires a greater expenditure of energy to maintain. You can be burning more energy than you really need to if you always stay in lead pursuit.
Knowledge of pursuit angles can also be handy if you are being pursued. If the enemy is attempting to run you down, you can make their job more difficult by turning constantly to keep them at your dead-six position (of course, make sure they are out of range first). You can deny them the ability to use lead pursuit, and unless they have a speed advantage, they can't catch you. Keep in mind that pursuit angles work in any plane of maneuver, the vertical as well horizontal, and a plane with a good climb rate can surprise you. Climb rate is somewhat a measure of a plane's ability to defy gravity. If your plane is a poor climber and an opponent with an excellent climbing plane pulls lead pursuit of you in the vertical, you may be unpleasantly surprised more quickly than you anticipated.
Pure Pursuit
Pointing your boresight directly at the target's present position generates pure pursuit. It doesn't make you close as fast as lead pursuit does, but it generally doesn't have its drawbacks either. Visibility is usually not a problem and the deflection angle doesn't change. This is usually a good choice where the enemy is not likely to run away from you and you are setting up your shot. Additionally, since you are pointed directly at them, just the front view of your plane is presented to them, which may make you harder to see.
Lag Pursuit
Lag pursuit is (guess what?) placing the nose of your plane behind the target's present position, but on their flight path.
Once they've slowed, you own them.
At first glance, this does not seem to be very useful. You can't shoot from a lag position (lead is necessary for a shot), and your closure rate really drops. While lag is more useful for jet fighters that use missiles (since they can fire off their boresight) it still has uses in the Fighter Ace
arena. Lag pursuit reduces the deflection angle and reduces or stops the rate of closure. This can be handy when the following apply: you have used lead pursuit to close with the enemy, you now want the closure rate to slow, and you're moving to their six position for shooting. Say you overshoot a fighter in a turn. After you slew past them, if you slow and pull in tighter, you may find yourself directly on their six. What has happened is that you have shifted from lead to lag to pure pursuit.
A lag pursuit can also be useful if you want to accelerate in relation to a bandit. In shifting from lead to lag pursuit, you relax the stick, which reduces your G load and allows you to accelerate your plane forward or maintain its speed relative to the target. Offensively, this can be handy if you want to stay close to corner speed (maximum turn-rate speed, beyond which burning energy will not move your plane through more degrees of arc per second). When your opponent looks back as you ease off the stick, they cannot tell at first that you aren't lining up for a firing opportunity. They will generally turn tight and hard enough in response that they fall below their corner-speed threshold. This is useful when you have the better-turning -- or at least comparably turning -- plane and want them to burn energy to the point where they can no longer hang with your turns. Once they've slowed, you own them.
Defensively, you can use a temporary lag pursuit to set up your bug out. The same parameters are in effect, except that you are not directly behind them but across the turn circle from them. You are accelerating while they are slowing by pulling a tight turn to get on your tail. Once you pass somewhere close to head-to-head, go nose low and bug out at full throttle. Your acceleration has given you an extra speed boost, while their tight turn has reduced their speed and they will need time to accelerate.
So now you have three additional tools in your arsenal. Use them and use them wisely, and you'll be rewarded with an increasing kill total. Check six!
Musashi, Miyamoto. Go Rin No Sho (A Book of Five Rings
). Translated by Victor Harris. Woodstock, New York: The Overlook Press, 1982; p. 64
Shaw, Robert. Fighter Combat: Tactics and Maneuvering
. Annapolis, MD: United States Naval Institute Press, 1985; pp. 62-67
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