SimHQ.com - Air Combat Zone - Feature: Falcon Flight Model Evolution

In this article we will consider the evolution of the Falcon series flight model and explore some of the implications of changes in the performance of the default F-16, in terms of the resulting influence on dissimilar air combat, and in particular, with the F-16 v MiG-29 engagement.

Some History

Firstly though, if you cast your mind back to the beginning of 1995, you may remember a utility called Falcon Test Pilot (FTP) and how it gave a new lease of life to Falcon3. At a time when we had all been flying Spectrum Holobyte's Falcon 3.0 for four years already, suddenly it was possible for players to fly all of the aircraft in the game. Dissimilar air combat, against real opponents, was a refreshing change for anyone who had been flying F-16 v F-16 in online competition for years. However, the different characteristics of so many new aircraft presented a formidable challenge! That inspired me to prepare an analysis of the performance of the Falcon3 aircraft, both the Complex and High Fidelity flight models, and many of the other aircraft that could then be flown. In the September '95 issue of "Enemy Lock On" magazine I published an eight page special feature that included this analysis in the form of about twenty Energy Maneuverability (EM) diagrams, with accompanying explanation. Now, nine years since then, and about twenty years since Falcon's birth (If we count from F-16 Fighting Falcon released in 1984), it is still possible to fly all of the aircraft in the modern version, and thanks to a small group of highly skilled and enthusiastic volunteers, the flight model of each of them is still evolving, and getting better with each generation!

Back then, Falcon 3.0 had three different flight models, a simple flight model, a complex flight model and the high fidelity flight model. It was possible, only with the aid of FTP to edit the complex flight model, today in Falcon 4.0, something similar is possible because a lot of the aero and engine data is contained in look up tables that are very easy to interpret and edit. In this article I hope to provide you with an insight into the development of the Falcon 4.0 series flight model, and its evolution through some of the versions, with some discussion on the implications this has had to the resulting air combat.

But first, for those of you who may be unfamiliar with the method of comparison I'm going to be using, I'll just say a few words about Energy Maneuverability diagrams. The Concept used in these diagrams was originally a British invention, and has been used to analyze the turning performance of fighters since the beginning of World War II. A more recent adaptation of the method was developed by Boyd and Christie who coined the phrase "Energy Maneuverability" to describe the duel purpose of such diagrams. A previous article describing the application of these diagrams can be seen here, and for those who need to brush up on what the lines and curves mean, there is a slightly more in-depth description in the article here, and although the latter article is more concerned with prop' fighters, the EM theory is essentially the same.

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The Default SP4 F-16 Flight Model

So, finally, here is the EM diagram for the default SP4 F-16 (note: click the image below for a larger version).

Notice that there are two curves on the left of this diagram, one is marked as the Max AoA (Angle of Attack) limit of 25.2 and the other is marked up as the CAT 1 limiter curve. So, this is a good time for an important disclaimer… Firstly, the CAT 1 limiter shown on this diagram uses the control laws used by the real F-16, and this may or may not be what is being used in Falcon 4.0's flight model, but in view of the excellent work normally carried out by the flight model development team, I strongly suspect that it is. In this diagram I've shown both limits as a comparison between the maximum angle of attack limit allowed in the data tables of 25.2 and the real CAT I limiter for the F-16. The CAT I limiter is important because it informs us of the true constraints on the flight envelope and thus the location for the corner plateau, which in this case is between 340kts and 430kts with a maximum turn rate of twenty three degrees per second. However, in subsequent diagrams I will omit the CAT 1 limiter curve, and use the maximum AoA limit, simply because that is the limit used by most of the adversary aircraft, and because the diagrams should be thought of as a comparison of the data used by the flight model, and not a comparison of the flight model itself. That distinction is important, because when the tool used to produce these diagrams was developed, even though I had a copy of the flight model, it was intended for the analysis of real aircraft and was not based directly upon the Falcon 4.0 code, even though the methods used in the simulation are fairly standard, and quite typical.

So the natural question for readers is, how accurate are these diagrams? There are a number of considerations here, firstly, due differences in the Falcon 4.0 code (including errors) and the code I use in my analysis tools, such as the methods used to solve the equations of motion, for example, there are some small discrepancies between these diagrams, and the performance that can be ascertained by flight testing in the simulation. As an example of what I mean, the equations of motion in a flight model require that a set of simultaneous differential equations be solved, and it is typical for flight simulations to use lower order numerical methods and solve at relatively low frequency to conserve processor cycles for other important code, such as the artificial intelligence, the campaign engine, the flight models for computer controlled aircraft, weapons models such as the various types of missiles and guns, damage modeling, graphics and… well you get the idea. In contrast to that, when producing diagrams such as the ones used in this article there is the luxury of using more accurate methods and solving at higher frequency, so differences are bound to exist. However these discrepancies have been found to be small, and most of the important comparisons between the aircraft, such as their instantaneous and sustained turn performance across the envelope, and comparisons between their relative turn rates and radii, can all be verified by flight testing, and correspond to a degree that makes no practical difference to the air combat.

Now let's just talk about the Ps = 0 curve for a moment, you can see it clearly marked on the diagram above. That curve is important because it represents the turning performance that can be sustained in a level turn. Don't worry that this curve isn't smooth, it is an accurate representation of the data used by the Falcon 4.0's flight model and because that data is contained in look-up tables that have discrete values, the steps in the curve are therefore unavoidable. Indeed, this is one of the many areas in which the flight model has improved, because the number of data points has increased and so the Ps = 0 curve is smoother now than it has been in the past, and is therefore able, potentially, to match the real curves more closely. However, the important thing to remember here is that if you fly at a point above that curve your aircraft will be losing energy, either speed or altitude. If you fly at a point below that curve, you will be gaining energy, you will either gain speed, or have the ability to climb in the turn.

That's why, when we compare the EM diagrams for different fighters by overlaying them, the fighter with the higher Ps = 0 curve has a sustained turn rate advantage, and is most likely to win a sustained turning engagement. So, one of the first things to notice when you look at an overlay of this type, is to compare the Ps curves across the envelope, to see where you can gain energy, or lose it less quickly, while out turning your opponent. But that's if the fight ever gets to a sustained turning engagement! A high sustained turn rate isn't everything, fast transients are also important. For example, an aircraft with a high pitch rate or high instantaneous turn rate could end the fight early by getting the first shot. That ability is defined by the highest point on the envelope, either the one defined by the 25.2 angle of attack limit, or the one defined by the corner plateau caused by the CAT 1 Limiter. Naturally, the Russian aircraft that have relatively high angle of attack limits have an advantage in that department, and can reach maximum instantaneous turn rate values far in excess of the CAT 1 limited F-16, albeit a brief advantage due to the resulting rapid loss of energy. Now, I know you want to see a concrete example of this, and in a moment we will look at the F-16 v MiG-29 engagement, but firstly let's compare the diagram above for the current default F-16, with previous versions of the F-16 flight model.

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Original, SP3 and SP4 F-16 Comparison

Here is a diagram that shows the evolution of the default F-16 from the original, through SP3 to SP4:

In this set of curves, I haven't shown the CAT 1 limit for the F-16 for the reasons explained earlier and also because the CAT 1 limiter hasn't been employed consistently through each version. Even so, this overlay does give an insight into the way the data has changed, and the effect that has had on the sustained and instantaneous turning ability of each of the default F-16s. The important thing to notice from this set of overlays is that the SP3 F16's zero Ps curve (shown in brown) was a major departure from the original (green) curve. Notice that both the original green and current blue zero Ps curves fall off dramatically below corner velocity, while the SP3 zero Ps curve did not. That of course resulted in dramatically different handling characteristics. The original Ps = 0 curve behavior was, when compared with the real F-16 EM diagram, closer to that of the real F-16's zero Ps curve and so the SP3 flight model was, I believe, a step backwards.

Hardly surprising then, that the SP4 zero Ps curve (shown in blue) is much closer now to the original green curve, particularly below corner speed... You might be getting the feeling that we have been riding our flight models around in circles, if you can forgive the pun. The point is that, in terms of the low speed sustained turning ability, the current default flight model has now taken positive steps back to where it started. The shape of the current Ps = 0 curve, as with the original, now demands proper energy management. Remember, I mentioned the dramatic effect on aircraft handling characteristics? Well, you can now no longer simply hold full aft stick, as you could with the SP3 flight model, and expect to get good results. In SP3 you could simply hold full aft stick to achieve the best sustained turn rate, and while that certainly made it easier to fly, it removed the skill required in learning to hold just the right amount of back pressure to achieve the flight conditions for the best sustained turn rate. With SP3, everyone became a good stick overnight. So fortunately, the SP4 flight model has corrected this behavior and it is now as it should be, once again. There is of course, room here for more detailed discussion of the development of the various Falcon 4.0 flight models, and possibly some EM comparisons to the real F-16, but that would be more appropriate for another article. The point to take away from all of this is that the Falcon 4.0 flight models are evolving, and they are getting better. The work of the guys involved in developing the flight models is difficult, and thankless, and generally the good work they are doing, in my opinion does not get the recognition or appreciation it deserves, but I digress, let's get back to the fun stuff.

MiG-29 v F-16

So what about the MiG-29 v F-16 within visual range engagement? Here is a Sea Level EM diagram overlay from the original game...

These two aircraft were clearly closely matched, with just enough of an edge to the F-16, in terms of its low speed sustained turning ability, to make it a tough fight, but close enough so that if the player made a mistake, the Artificial Intelligence could take advantage and clean up. It was tough, because you can see from this diagram that the MiG-29 still had an instantaneous turn rate advantage, but the fight was always a lot of fun, because you could win.

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MiG-29 v F-16

Now compare that with what happened in SP3, the diagram is shown below. Here you notice that the MiG-29 has improved, relatively speaking, and is now superior to the F-16 over the entire envelope, and has a smaller turn radius, but only marginally. The biggest difference, is of course, the fact that the F-16's Ps = 0 curve can be seen to climb to the left, increasing as the speed decreased. That meant that the slower you got, the higher your sustained turn rate became, and that simply allows the pilot to hold full aft stick for best performance, a dramatic change from the original version that demanded proper energy management. Of course, just holding full aft stick for best results is not the way the aircraft is or should be flown! This reinforced the bad habits simulation pilots may have acquired flying other simulations with similarly flawed flight models, examples of such simulations abound. Despite that step in the wrong direction, the relative performance between the MiG-29 and the F-16 was still close, and even though the MiG-29 was more difficult to beat, it was, due to weaknesses in the Artificial Intelligence (AI), quite beatable.

What happened next, is that the default F-16 flight model changed to that seen in the first diagram, while the MiG-29 flight model remained the same as it was in SP3. The result is shown below, and notice the significant loss in sustained turning ability, of more than five degrees per second relative to the MiG-29:

Now, because the MiG-29 has remained the same in both SP3 and SP4 we can overlay it with both the SP3 and SP4 F-16 to see the difference, here it is:

By retaining the MiG-29 flight model from SP3 the default SP4 F-16 is now outclassed in terms of its manoeuvrability, conceding more than five degrees/second at low speed. That's a lot to give away, when you consider that as little as two degrees per second is considered to be a decisive advantage! This is the biggest disparity in turning performance between the MiG-29 and F-16 in Falcon4 to date, and makes any protracted sustained turning engagement extremely difficult for the F-16 driver, even though the SP4 F-16 is slightly faster.

Just that comparison alone and the differences it makes to the MiG-29 engagement, aircraft handling and energy management are such that pilots converting from the SP3 to SP4 flight model would notice that they are no longer able to do what the F-16 has always been good at… turning!

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Now, energy tactics become more appropriate. Updating on the new aircraft handling and energy management requirements for SP4, based on this diagram, suggests that due to the F-16's higher energy bleed rate and lower sustained speed, he should choose the one circle fight and early high aspect shots, scissors type maneuvers, followed by extensions if guns only, due to the higher top speed... then rinse and repeat. Let's take a look at that in diagram form. Below you see the sustained turning performance of both aircraft shown in a way that illustrates their relative turn radius and turn rate, it shows the relative position of both aircraft during a turn lasting six seconds.

You can see that while the turn radius of both aircraft is very similar, the MiG-29 has gained a significant angular advantage. In order to counter that, I mentioned previously that the F-16 driver should choose the one circle fight and early high aspect shots, and scissors type maneuvers, the diagram below shows how that should work.

Here you see that even though the MiG-29's sustained turn rate is superior, the F-16 gets the first shot in this situation. If you miss the shot, reverse again into a scissors, and try again, or if the situation (other enemy fighters approaching or low ammunition or fuel) dictates, take the shot and use the time required for the MiG to reverse, to execute a zero G extension, but only providing the MiG doesn't have a missile still on the rails.

The real difficulty for the humble flight sim pilot is that the appropriate tactics have changed along with the flight model, so the big question is, now that we have the new High fidelity flight models how have they influenced the air combat? In the second part, we will look at the performance of the F-16 and consider the Dissimilar Air Combat implications for the F-16 versus MiG-29, F-16 versus Su-27 engagements. Lastly, as pointed out earlier, there is a small group of highly skilled and enthusiastic volunteers working constantly on the flight models and much of their work has been directed towards accurate modeling of the different F-16 blocks and variants including the different engine and airframe combinations, so in part two we can also take a look at the outcome of their efforts, as they strive to ensure that Falcon4 remains the most accurate simulation of modern air combat currently available.

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