4 weeks ago

“It Depends”

“It Depends”

Trevor “Boat” Boswell
U.S. Air Force

After posting our episode 71 video to YouTube, a viewer commented that during the listener question segment I did not answer caller Kevin’s voicemail question regarding whether instantaneous or sustained turn rate is the more important aspect in fighter design.

My answer, “it depends”—a common answer in this line of work, frankly—did not sit well with the viewer. I doubled down, emphasizing that you would not find any fighter pilot who would say that one is always better than the other. The viewer was still not convinced…but I stick by my answer, and here’s why:

Each type of turning rate has its advantages and disadvantages. On the whole, like so many things in life, neither is better or worse all the time—just at certain times. Let’s build a scenario to show why one right answer is not necessarily possible, nor really a smart way of doing business.

First, Assumptions

Like all good hypotheses, you must build the “control” set from which all variables can be measured and considered.

Suppose we have two aircraft, an F-16C and F/A-18C, in an air-to-air engagement at 20,000 feet MSL where the sun is overhead and prevailing weather conditions are insignificant. The Viper and Hornet are at a merge, separated by 1,000 feet, each at 450 knots, with full airframe-designed G-loading and AoA performance capability available.

Both have the loadout of 3x AIM-120C7, 1x AIM-9X, and a full gun load. Each pilot has a full-up aircraft with Helmet Mounted Cueing System (HMCS) and Link 16.

As most are probably aware, the Viper design resulted in a very high sustained turn rate, but the plane is instantaneous-AoA limited. The Hornet, on the other hand, enjoys a high instantaneous turn rate with no AoA limiting; however, its lower thrust-to-weight ratio limits its sustained turning rate capability.

So, given these starting parameters, the weapons and systems available, and knowing where each has an advantage: which one wins?

Old School

Each aircraft has an ideal gameplan based on a multitude of factors—the size of its turn radius at certain altitudes, airspeeds, configurations, etc. Additionally, the instant the crew is able to identify the adversary type (via AWACS, GCI or visually), their gameplan may change. I’ll label the two traditional gameplan types ‘Classic’ and ‘New’ to more-clearly define the differences.

Classic BFM, would be the traditional horizontal-based fight where each aircraft maneuvers no more than within about 20 degrees above or below the horizon—much like two cars harassing each other in a parking lot. In this type of fight, there are two ways to turn at the merge: towards the adversary (“2-circle”) or away from the adversary (“1-circle”). Sure, you can also just “blow through” the merge without turning, but given current weapons capabilities, that’s just going to get you shot in the rear end…so let’s not go there.

Legend

Flight Path Level
Slight Nose Up/Down (~10 Degrees)
Moderate Nose Up/Down (~20-40 Degrees)
Aggressive Nose Up/Down (~45+ Degrees)

2-circle BFM is so called because when viewed from above, each aircraft scribes its own circle away from the other aircraft, one to the east and one to the west (think figure 8).

2-Circle Classic BFM

1-circle BFM, however, is when both aircraft turn the same cardinal direction (perhaps east), scribing what appears to be a single circle through the sky.

1-Circle Classic BFM

In either example, on paper at least, the F/A-18 appears to have an immediate advantage because of its superior instantaneous AoA advantage. However, if both aircraft have HMCS and the off boresight angle of the weapons is the same, does the superior AoA-pulling ability really matter? (Spoiler: not as much as you might think.)

I won’t get in to the specific math because it’s classified, but the general reason I say “not really” is because the amount of time it would take for both aircraft to enter a weapons employment zone (WEZ) is nearly identical and the amount of time in the WEZ would be fleeting (i.e. limited time available before reaching a WEZ boundary; time and/or minimum distance), and so for both aircraft to attempt to “point first” (i.e. maneuver their aircraft to get the AIM-9X seeker within its gimbal limits the fastest), you’re talking fractions of a second advantage. If both pilots are equal, a mutual kill results as both will have fired their respective weapons before their adversary’s weapon impacts them.

So, let’s move on to “New” BFM…it’s got to be better, because it’s “new”, right?

BFM, the Next Generation

New BFM takes the fight more vertical to an almost pure nose-down (90 degrees perpendicular) to the horizon sight picture, because we’re using the Earth’s gravitational pull to accelerate our turn rate around the circle, shrink our turn radius, and help us get our nose (i.e. missile seeker) within its field of regard (FOR) first, and hopefully fast enough that our missile impacts the other aircraft before they launch theirs.

2-Circle New BFM

Guess what…”New” BFM but same result—the Hornet is just not going to be fast enough to be able to get a missile off the rail before the Viper gets a shot off toward the Hornet. Another mutual kill. The math simply does not support a clear winner, regardless of what kind of fight they engage in or what kind of missiles they have on board (assuming our previous assumptions remain the same for both aircraft).

“But,…”

…why did you say it depends in the Air to Air Mission Planning episode for every answer?”

Well, I’m glad you asked, because it always depends. Always. The scenario laid out above was simply an artificial training scenario under ideal conditions. A perfect clear day, two perfectly performing pilots, perfectly working aircraft/systems/ weapons all being pushed to their designed limits, and gameplans being executed exactly as the USN’s TOPGUN and the USAF’s Weapons School designed them.

So what if…

…one pilot had a lower G-tolerance than the other (can’t pull the full G limit)?

Low G Tolerance

…one pilot didn’t have sight at the merge (didn’t know when to start turning)?

No Joy / Blow Through BFM

…one pilot used “Classic” BFM when the other used “New” BFM (too much turning room allowed)?

2-Circle Old Vs New BFM

…one pilot decided to shoot an AIM-120 (needs a FCR lock)?

2-Circle AIM-120 Shot Focused

…the AIM-9X misses (no more HMCS advantage)?

…the Viper is almost out of gas (can’t keep fighting for very much longer)?

2-Circle Low On Fuel

…or…or….or….this list goes on and on and on, with every second that passes.

This is why “it depends” is the only answer that truly covers any and everything a pilot would see and do throughout a fight. And it’s not just happening at the merge—everything is being assessed continually, hundreds, perhaps thousands of times in a 30-40-second BFM engagement. Each pilot is going to perceive an action by an adversary and their aircraft differently and continuing to only act based on how your aircraft was designed is foolish, at best.

The Final Turn

If I knew that every MiG-21 pilot I was going to fly against was going to try for a 2-circle fight every single time, then I would always turn my F-16 across their tail and use my superior sustained turn rate advantage to eventually kill them from an offensive position—very simple.

But I don’t know that, which is why only having a “superior sustained turn rate” as my decision-making criteria for how to fight BFM is not only foolish, but also probably going to cost me my life, shortly after passing that MiG-21 for the first and only time.

I hope this helps clear up why I answered the way I did, and why answering in ultimatums is very difficult to accept in something as fluid and unforgiving as BFM. But if we ever meet at the merge in DCS, I highly recommend the blow through…you’ll definitely win every time! 😎


*Disclaimer – This musing is exclusively for non-thrust vectoring aircraft; thrust-vectoring capable aircraft, utilizing thrust-vectoring capabilities, do not fly with the same set of assumptions with respect to turn radius because their thrust is not aerodynamically oriented out the tail of the aircraft at all times.