Basic ACM and Merge Tactics
(by Murdr)


One thing we want to keep in mind when discussing Air Combat Maneuvers (ACM) is that the Basic Fighter Maneuvers (BFM) involved take on a new objective when used against an opposing plane. When we perform BFM absent an opponent, we are able to complete the maneuver within 100% of its technical aerobatics definition. Being able to "hit the marks" during a maneuver to achieve aerobatic perfection is an important skill to develop. However in that situation, our objective is to perform an aerobatic maneuver correctly. In a sense we are performing it just for the sake of performance.

When we add an opposing plane to the situation, our objective becomes gaining gun angles on the "enemy". So at this point we are now performing BFM with consideration of the opponents position, not in spite of it. As an example lets look at the Immelman (Basic Manuvers) when starting it from a due South heading. At the half way point of our Immelman (nose pointing true vertical), our Lift Vector (Flight Controls) should be pointing due North, and as we come over the top we should be able to un-invert to wings level heading due North. When performing the same Immelman against an opposing plane, at the half way point we want our lift vector pointing in the general direction of our opponent (wherever that may be). As we come over the top, we want to orient our lift vector toward our next turn, and our next turn is entirely dependent on the "enemy's" position.

BFM are the building blocks that we string together for ACM. With all of this in mind, in later sections we may refer to BFMs on occasions where only the majority of the BFM is performed to its aerobatic definition. An excellent example of this is the standard loop. We could break down the loop into two 180° maneuvers, and consider them an Immelman, and a Split-S. The 180° turn performance on each halves of the loop are identical to those two maneuvers respectively. The exit and entrance rolls of the individual BFM may be missing, but their combat effectiveness are exactly the same.

The Merge

39 line 2A merge happens when two planes cross through each others best turn radius from approximate opposite headings. More specifically “the merge” usually refers to the point at which they cross through each others 3-9 line. In air combat it is often the case that after a merge both planes will be attempting to be the first to gain a guns solution on the other. In a way, you can consider the merge point a starting line for ACM. There are ways to “jump the start” to gain an advantage on your opponent, which we will get into a bit later. Something to keep in mind is that “the merge” isn't just the first time you and your opponent cross, but every time you cross. Several merges can take place in a fight. The initial merge can set the pace of the rest of the fight, but advantages can be gained or lost just as easily during subsequent merges.

Now let's take a look at some things in general we would like to try to do during a merge:

  1. Avoid the opponents guns:
    A well placed head on shot can quickly end the fight. However, a missed head on attempt by your opponent can be used against them. If you are maneuvering into your post merge turn while your opponent is trying to line up a shot, you've just gained an angles advantage by starting your turn before them.

  2. Enter the merge point with your lift vector pointed toward your next turn:
    The sooner you have your lift vector oriented for your next turn, the sooner you can start your turn. However, this is trumped by 1. Your opponents position may force you to avoid their guns before you can orient your lift vector.

  3. Try to get your opponent to pass through your lift vector at the merge:
    This allows you to turn into them, rather than around to them. This also can allow you to start turning before the merge point. This is trumped by 1. and 2. It's not worth attempting if it will commit your lift vector into a turn that you don't want to make, or if you're exposing yourself to guns in the process.

Post merge turns

If we want to try to enter the merge point with our lift vector oriented on our next turn, then we need to have an idea of what our next turn will be. For that, let's look at a few possible choices of post merge turns, and how they interact with each other.

Immelman vs Flat Turn

An Immelman spends a majority of its turn thrusting directly against gravity. That means we are slowing down at a quicker rate than with many other maneuvers. This can be useful for reaching a desired cornering speed quickly. We also are exchanging energy states. We are trading speed for altitude. From this higher position, we could thrust with gravity (dive) and gain much of our speed back if needed. With a Flat Turn, all of our speed loss is due to drag. We are not slowing down as quickly as with an Immelman, and we are not exchanging energy states. The only way we can gain speed is by going lower than our starting altitude. Let's look at an illustration of how we can expect them to stack up against each other after a merge.

lift vectorWith the exact same plane type and load out, the plane that did the Immelman clearly beats the flat turning plane. Notice how the turn radius of the Immelman tightens as its speed decreases. Also notice its position on top of the other plane, where it has the force of gravity at its back. In short we can expect:

Immelman beats a Flat Turn

Flat Turn vs Split-S

With a Split-S, we are doing the opposite of an Immelman. Gravity is working with our thrust to accelerate us even faster. This can be a problem if we are already at or near our desired cornering speed. flat versus spit

This turn match up usually happens two or more merges into a fight, in this case it's the 2nd merge. What happens is that while the plane performing the Split-S is accelerating through his turn, the opponent, delays his own acceleration while still reversing his heading with a Flat Turn. Once the Split-S planes 3-9 line crosses underneath him, he can then nose down and orient his lift vector to follow. In short:

Flat Turn beats a Split-S

Immelman vs Split-S

This turn match up also usually happens two or more merges into a fight. In the other examples, the planes are roughly staying within reach of each others turn radius. In this case, the turns head in opposite directions, and take place outside the radius of each other, so the turn radius and rates are usually splitsirrelevant because they separate enough for both planes to complete a turn and re-merge.

Because of excessive speed, a Split-S on an initial merge is a different situation, and is very rarely countered with an Immelman. This may be demonstrated in the future, but for now I'll say “Don't Split-S into an initial merge. It's a bad idea.”

Immelman vs Split-S is a draw

Initiating a Merge

Now that we've determined than an Immelman is often a good turn choice after a merge, let's look at how to initiate merge to try to gain an advantage at the merge with a lead turn for an Immelman.


This figure shows a number of components needed for a lead turning merge. Schatzi plans on doing an Immelman after the merge. For an Immelman she wants to have her lift vector pointing up. So to have the opponent pass through her lift vector at the merge, she wants them to pass over top of her at the merge. To accomplish this, on approach to the merge, she dives to a point under the merge. She is trying to achieve “vertical separation” at the merge. This has a number of beneficial side effects. It forces the opponent to chase her down to the merge. As they close, the opponent will have to increasingly angle the nose down even more. The problem for the opponent is that his nose is being drawn in the opposite direction of his initial turn. If the opponent wants to attempt a head on shot, this effect will be even worse. Not only that, but it forces them to either go negative G, or invert to make a shot. This fulfills the point of “Avoid the opponents guns”. Looking at the figure, notice that the higher plane's flight path leaves it slightly nose down at the merge point even after giving up the race to get under the merge.

Now look at Schatzi's flight path at the merge point. It is more than 25º nose up. This is part of the reason we are seeking to have our opponent pass through our lift vector at the merge. Under this condition, as soon as the other planes flight path crosses through her “imaginary” projected best turn arc, she can start her turn. As you can see in the figure, this can mean starting your turn well before the merge point. Using the Merge Point as our reference, Schatzi has nearly a 40º angles advantage at the merge! If the opponent on the other hand tries to turn before the merge, they will place themselves in front of Schatzi's potential flight path, making matters worse. This forces the opponent to actually reach the merge point before they can start maneuvering.

But that's not all!

If we removed the angles advantage from the lead turn, the plane “under the merge” still has a positional advantage. Looking at the next figure we see the two opponents performing nearly identical turns. However, since “vertical separation” has offset their starting altitudes of their respective turns, it results in the plane starting under the merge also being under the 2nd merge. At the 2nd merge that means they are inside their opponents turn radius, and have gun solutions.



As a baseline, the merge and post turn merge examples were done with same plane types and same fuel load. Meaning with equal plane capabilities, that is how much advantage can be gained with merge tactics and turn/counter turn choices. With similar planes we can expect the same results, but it is up to the less maneuverable of the two planes to try to successfully use these tactics before the fight devolves into a sustained turn fight. Sustained turns are where any differences between two planes turn performances will be most pronounced. Even between exact same plane types, we can see the effects of weight differences (mainly fuel load) on their turn performance during sustained turns.

Between dissimilar planes, we can still often apply the same tactics. However, it becomes even more imperative for the less maneuverable plane to win the merge if they intend on out turning their opponent. In that situation, not only does the less maneuverable plane want to avoid sustained turns, but they also want to gain an advantage almost immediately because they will likely lose ground with every merge and reversal. If they gain trail position on their opponent early, it will be more difficult for the opponent to use their turn performance to recover. There are limits with dissimilar plane types though. If we look at an A6M2 vs an FW-190a8 as an example. We'll find that that for every turn the FW performs, the A6M will complete a turn and a quarter, and it will do it well inside the turn of the FW. What that means is if we compare those two models in post merge turns, the A6M could choose all the "wrong" turns, and still likely get gun solutions first simply due to its turn performance.

So when considering ACM choices, an important part of the decision should be base on your knowledge of your, and your opponent's plane performance. This does not mean that you need to memorize specific stats for every plane type available in AH, but you should have an idea of how the opponent's plane compares to yours for relative turning performance, speed, and acceleration. Another factor to consider is the strengths and weaknesses of your respective planes. Playing to the strengths of your plane can be obvious, but it can also take some very detailed knowledge. A plane that should lose a fight at slow speeds in a specific match-up may very well have the upper hand at a higher speed range. A plane that should lose a sustained flat turn fight in a match-up, may match, or even better their opponent in vertical turns. These kinds of situation specific details, as you discover them during fights, are worth remembering for the next time.

So while this is intended to give you a framework or point of reference for understanding ACM, it takes practical experience to actually do it, and know what to expect in a specific situation. As demonstrated, a merge can be a key turning point at any time in a fight. Consider this, at a closing speed of 750 mph on a merge, in 1 second the separation between planes will change 366 yards. If you start your turn 1 second late during merge, that 366 yards could mean the difference between a winning, neutral, or losing position. Add to that heading into a merge, we are making a best guess at where a “merge point” will be. All this takes experience to judge timing and position, even if you understand what you should do and why. Don't become discouraged if it takes awhile to develop those skills. Just by reading this article, you have knowledge that may take the unaided player months to figure out on their own.

One or all of the post merge turn examples can be applicable to almost any fight at some point. There are certainly more named maneuvers, and more variations on merges and turn match ups that could be studied. However, this is a good point to suggest you practice, practice, and practice some more with the information here. Concepts such as Energy Management, Situational Awareness, Gunnery Technique and Practice, and using Lead, Pure and Lag Pursuit to manage separation and angles, are all important aspects of ACM, but they go hand in hand with knowing what to do next, and what possible out comes will result. The ACM discussed here and those concepts are all things you can practice with a buddy in the Training Arena or Dueling Arena anytime.

Good Luck!