Almost every flight sim' pilot with Internet access will have been aware of the multitude of message threads over past weeks generated by the Jane's F-15E simulation. Those discussions have focussed on comparisons between the real F-15E and the Jane's simulator model of the same aircraft. It is only natural that so many serious flight sim' pilots are interested in making real world comparisons, because that can lead to correct strategic and tactical decisions within the simulation. Unfortunately there is much confusion regarding the F15, its variants and their respective strategic roles and capabilities. So, before introducing the simulation, let's put this discussion on a firm foundation.

The F-15 was primarily designed to fulfil the offensive and defensive roles within a counter-air campaign, the strategic objective of which is simply to permit friendly ground or sea operations to proceed without prohibitive enemy interference, which in a nutshell, is air superiority. As an air superiority fighter the F-15 has reigned supreme!

Enhancing the roll of the F-15 to that of combat support and anti surface operations has involved dramatic modifications to what was an already significant air to ground capability and has led to the F-15E. It was intended that by enhancing the F-15 with new air to ground systems and avionics, including structural changes, extra pylons and conformal fuel tanks, the F-15E would also have all weather long-range interdiction capabilities.

It was further intended that by retaining its air to air systems, none of the F-15's already awesome air superiority and Beyond Visual Range (BVR) capabilities would be sacrificed. Jane's F-15 simulation has done a remarkable job in modelling those avionics and weapon systems. In being faithful to the strengths of the aircraft, it has brought the art of Air to Ground combat to the flight sim' pilot with a new level of realism. However, in this article I intend to focus on aspects of the Jane's F-15 sim' dominated by the Within Visual Range (WVR) Air-to-Air engagement.

REAL WORLD COMPARISONS

Despite the admirable efforts of developers, simulations of modern air combat will always suffer from the lack of important classified data required for truly accurate modelling. Of course some degree of fidelity to the real world aircraft is possible because a lot of information can be easily obtained, but for aircraft still in service, where lives may depend on the security of classified data, what is available is unlikely to be complete, and may possibly be inaccurate.

Furthermore, the most publicly available data is not always typical. For instance in 1974 the USAF awarded McDonnell Douglas a two million dollar contract to support attempts to break the time-to-climb records previously held by the F-4 Phantom and the MiG-25 Foxbat. The aircraft used for this attempt was the F-15A (Serial No 72-0119) known as the "Streak Eagle". It was able to out climb the Apollo moon rocket to 60,000ft and its performance smashed all previous records and was highly publicised.

However, that aircraft was highly modified by removing all non-essential combat systems in order to save 1800lb in weight. Right down to a saving of 50lb of paint, which of course meant flying the Eagle naked, which is how it came by its name! The performance of the Streak Eagle is very well known, yet it can not be seriously compared with any production model F15.

The Jane's F15 flight model data has been obtained from the aircraft manufacturers and other official USAF sources, and as such, is as accurate as any non-classified data can be. It's quite simply as good as we can ever expect. But what do we expect? In simulations of modern air combat where at least some basic aircraft data is almost always highly classified, the wise flight sim' pilot won't expect the modelled aircraft to compare to the real thing with extreme fidelity.

As important as comparison with the real world may be, the competitive flight sim' pilot is more concerned with the performance of the aircraft as represented by its flight model. In any case, if the non-classified data is good, the aircraft will match some of the more public numbers and remain within the realms of credibility.

As an example consider the data available for the maximum speed of the F15. Published data that is readily available quotes the maximum speed of the F-15A as 1650mph at an altitude of 36,000ft, that's Mach 2.5 or two and a half times the speed of sound. At sea level the maximum speed is quoted as 915mph or Mach 1.2 which is comfortably supersonic and compares favourably with the sea level performance of Jane's F15.

Similar figures are also quoted for all later models of F15 regardless of the differences that exist between them and some of that data is listed in the following table for reference. While a top speed of Mach 2.5 remains the only publicly available data, there is little doubt that the performance of production F15's fall significantly below that figure. Indeed, the specification for Mach 2.5 was controversial right from the beginning because of the degrading effect that such a capability would have on the performance in the transonic region of the envelope, which is much more critical in combat.

While such considerations remain contentious, what really matters to most is how the in-sim' aircraft performance compares with that of their artificially intelligent or online opponents. They expect the physics of the flight modelling to be sophisticated enough that real world tactics can be successfully employed against their opponents.

They also hope that the relative performance of dissimilar aircraft is such that it will dictate similar tactics to those used in the real world. That generally means that energy and angles tactics when appropriately employed bring correspondingly predictable outcomes. This article will explain how modern methods of aircraft performance comparison can be applied to the F-15E as modelled in the Jane's simulation. In doing so I will frequently refer to the real F-15 by way of validation and in the course of that discussion you will learn how to get the most from your aircraft when competing against others.

In order to do that we will use a method of comparison that allows a pilot to visualise the contrast between the performance of different fighters and determine which one is superior to the other at each point in the envelope. So for example a pilot might compare the flying characteristics of an F-16 to those of a MiG-29 and thus identify what regions of the flight envelope are most advantageous or dangerous to him. The method was originated and developed in the early sixties and revolutionised the way the USAF looked at tactics and designed fighters.

It is particularly relevant in this case because it was used extensively to validate the design of the F-15. It was also largely responsible for the rejection of the early 60,000lb variable geometry aircraft proposed in the Air Force FX concept formulation package, in favour of the 30,000lb fixed wing, high performance fighter we know today. The method is based upon the theory of energy manoeuvrability and can be translated into simple graphs called EM Diagrams or Doghouse Plots. The advantage in using them is that they are very easy to read and permit aircraft comparison at a glance.

I will use Energy Manoeuverability theory to explain how you can fly the F-15E to its full potential, and squeeze every drop of performance from the flight model in order to win against less informed pilots, but first let's compare the Jane's F-15E to the real F-15A. That exercise will bring the comparison issue into perspective and provide us with a feel for the Strike Eagle's performance relative to a more familiar aircraft.

Download Dan Waldrep's latest F15 Mission: Bridge Busters ATC: 1 meg This mission includes a large number of custom voice files... = ) STRENGTHS AND WEAKNESSES

While the BVR capability has been retained, the enhanced capabilities of the F-15E have not come without some cost in performance. A huge increase in weight, (The maximum gross take-off weight has increased by more than 60% from that of the F-15A) along with other aerodynamic penalties, have resulted in significant degradation of performance for those aircraft still powered by the 220 turbofans. For example, early FAST (Fuel And Sensor Tactical) packs, that were basically conformal (blended to contour smoothly with the fuselage) fuel tanks that could hold almost 10,000lbs of fuel along with ECM (Electronic Counter Measures) and Reconnaissance sensors. They only increased the drag factor by around 25% of the penalty imposed by mounting external tanks for a joint capacity of 12,000lbs of fuel.

However the type-4 conformal fuel tanks (CFT's) used on the F-15E, even without more than four tons of air to ground weapons that can be carried on the twelve weapon stations mounted on them, bring a significant drag penalty. To put that into perspective, the drag caused by the type-4 CFT's has been compared with that of a little more than two external tanks, due largely to the effect of the weapon stubs. That large drag penalty is considered worthwhile because the CFT's provide an increase in range and the six weapon stations on each (while also leaving pylons free that would have otherwise been required for external tanks) provide a large increase in payload.

Consider also the large number of structural changes (very few of which are visible) and additional equipment that make the E more than 5000lbs heavier than the A when empty, and then factor in double the amount of fuel and you have an aircraft that is significantly less manoeuvrable. The F-15E simply can't match the performance of its more agile stable mates in turns, climbs, acceleration or speed. This is a direct result of its higher wing loading, lower thrust to weight ratio and higher drag, and can be illustrated by comparison with the F-15A.

One method of comparing the Jane's F-15E with the real world A model would be to overlay their EM diagrams. While the Doghouse plots for real world aircraft are classified and therefore not available, it is possible to use published data in order to speculate, and produce best guess curves. So for example, widely published information on the F-15A compares it to the F4 Phantom. Namely that, while the F4 holds a 4G turn at Mach 0.9 and 20,000ft, the F-15A can match the radius while gaining 7100ft in altitude. That's not only often quoted, but very impressive!

Fortunately for us, that information, along with other available data can be reverse engineered and converted into the plot shown below. This EM diagram has turn rates in degrees per second on the vertical axis and true airspeed in knots on the base axis. Lines of constant G load and turn radius are also shown. Most importantly though the diagram shows the instantaneous and sustained turn rates for both aircraft, overlayed for comparison. The instantaneous curve for Jane's F15E is the higher of two blue curves as shown, and the zero Ps curve represents its sustained turning performance. The curves shown in green are those correspsonding to the F-15A, based on the above data.

You will notice that the Ps=0 curve for the F-15E only passes slightly above the indicated sustained turning point shown for the Phantom. Also, that this prediction of the F-15A envelope suggests that the real F-15A is able to sustain just over 7G at 20,000ft and Mach 1.34. You can see this on the graph by comparing the zero Ps curve of the F-15A with the lines of constant G load. A reality check on that can be made by yet further comparison with the F-16A which can actually sustain 7G upto 24600ft at Mach 1.4. So the F-16A has slightly better energy performance than the F-15A, which in turn is better than the F-15E by the extent shown in the diagram above. All of which sounds perfectly reasonable.

The conclusion is that having the ability to acquit itself in a long-range fight, the F-15E should avoid its main weakness by declining to join manoeuvring engagements when ever possible. However, one thing that real pilots and flight sim' pilots have in common is that it doesn't matter very much what aircraft you give them to fly, they are going to want to dogfight in it. If I assume that regardless of your desire to employ the F-15E for within visual range engagements, at some point it will happen anyway. The important thing is to provide you with the information you need to make the best of a bad situation. You will thus be able to fly in that part of the envelope where you have the advantage, if that place exists!

Before moving on, the EM overlay above only showed the zero Ps curve, the others were omitted for clarity. The diagram below shows more of the Ps curves at Sea level.

The EM diagram above shows the performance of the Jane's F15 at Sea level using the default stick settings shown below.

Let's just consider the main features. Firstly you will notice a corner speed of around 380kts, the top speed at sea level of 824kts (M1.25) and a stall speed of 108kts. There is a kink in the curve at Mach one, which corresponds to a 1G reduction that occurs in the sim above that speed. The blue curves are the Ps lines and represent your sustained performance. You can sustain energy if you fly at any point below the Ps=0 line, which means you will be able to accelerate or climb. At any point above that line you will need to lose altitude or bleed speed. For example, at the configuration shown you can sustain 400kts at 6G. You will also notice that the minimum turn radius for this configuration is a little over 1000ft. ENERGY MANAGEMENT

The information on the chart above is of no value unless you can make it work for you. But what exactly does it tell you to do? You might look at the chart above and see the corner speed at 380kts and go all misty-eyed thinking that you just need to stay at corner, turn at your maximum rate, and good things will happen. Unfortunately life isn't that simple!

The red curve on the EM diagram represents your instantaneous turn rate. The highest point on that curve is well known to flight sim' pilots as the corner velocity and it is easy to see how the turn rate at that point makes it very attractive. Unfortunately you simply will not be able to stay at that point without out losing energy very rapidly. For example, at 400kts in the above diagram you will have a negative excess power of around 1250 feet per second.

That means that if you want to stay at that speed, pulling 12.2G you will need to trade in your altitude at a rate of 1250ft every second. That's the catch, the price is simply impossible to pay! You can't lose altitude that quickly because 1250 ft/s is over 700kts and you are only at 400kts. Even if it were possible to lose altitude at that rate, placing yourself 6000ft below an opponent is tactically unsound.

So in order to answer the question, we need to reflect on something known as energy management. Energy management is difficult to explain because while turn rate is easy to see in flight, it's a solid concept that most flight sim' pilots find easy to visualise, energy is much more difficult to grasp. That's why inexperienced pilots take the turn rate now, and worry about the energy later. That's a mistake that can be illustrated with an example.

Starting at an altitude of 10,000ft with a clean configuration at 400kts, execute a break turn by banking and pulling full aft stick and maintain it for five seconds. What will happen is shown in the diagram below. If you hold your altitude during that five seconds your speed will bleed from 400kts to 200kts, because with a negative Ps of 1250ft/s you will decelerate at a rate of 38 knots every second. That situation is shown in the diagram below.

So far we have considered two possibilities. Firstly we considered trying to trade altitude to maintain airspeed, but we would need to lose around 6000ft, which we know is impossible. The second alternative was to allow our speed to bleed at the rate of 38kts every second. However there is a third option, you could maintain your speed by using less back pressure on the stick, and maintain just that G load that will allow you to keep your speed constant! That is the correct option!

That's the hard part to grasp, most pilots won't ease the backpressure for fear of being out turned. They can't believe that maintaining their energy could be as important as maintaining their turn rate because the effect of turn rate is so much easier to see. And few of us would deny that the sight of a bandit gaining angles is enough to inspire an urge to pull harder on the stick. But let's see what happens if we do pull less G and maintain our energy, that means maintaining both our 10,000ft altitude and our 400kts airspeed.

Here you see that by sustaining your 400kts you have given up sixty degrees to the pilot who pulled full aft stick. However, if he elected to stay co-alt he will now be at 200kts and will suddenly realise that those angles were only on loan. Had he elected to trade some altitude to help mitigate his loss in airspeed, he would still be both lower and slower, faced with the prospect of losing his angles and his airspeed to get the altitude back.

You, on the other hand, have only been using your energy at the same rate at which your turbofans have been producing it, you will therefore be able to continue to turn at your sustained turn rate indefinitely. Your opponent has sacrificed his energy permanently because that extra turn rate required a big energy overhead that was non-refundable! It's easy to see in this example that being low or slow is a bad thing.

Generally speaking, spending all of your energy for a gain that does not prove decisive is unwise. It is better to save that energy for when you really need it, an emergency, or a sure kill. It is better to make small gains and retain the energy even if faced with the prospect of a shot. However if you are as close to getting a kill as shown in the screen shot below… Take it.

In this screen shot you can see the pilot holding a sustained turn at just over 300kts at 4.8G while holding the Bandit in the funnel. You see that the last extra pull for lead and the shot cause another 40kts to bleed away, and this is an extremely vulnerable position to be in. If the bandit had a wingman, or if the fight had attracted other enemy aircraft, 260kts is not a good place to be! Regardless of how good your situational awareness may be, dogfights have a tendancy to attract other aircraft like mothes to a flame, while any degree of target fixation will ensure that you remain unaware of your peril. Your only safe guard in this, is training. You must develop the habit of never geting slow with out very good cause.

The prospect of a kill, may have justified the shot above, however don't be like the pilot in the previous example who used all of his energy to gain 60 degrees, with a further 60 yet to go, it wasn't worth it. Wallowing at 200kts with poor control and a bandit pulling away from you is not going to give you the sort of shot that will compensate you for the energy loss. What is worse, your opponent will own the vertical, because when he recognises your low Es he will be able to make use of the vertical and win back those angles and more, since you will be unable to follow him.

Only spend all of your energy if you are sure of a kill or your life depends on it! If you detect a missile launch, or a bandit making weapons parameters, then spend it all. There are no prizes for dying with an extra 100kts on board. However if you are tempted to spend all of your energy just for a better position, remember that even with the prospect of a shot, it may only be a worthwhile risk in a clear one versus one environment. Once your energy is gone it's not easy to get back, and another bandit or a missile launch warning while you have low Es will spoil your day.

The point of this example is simply that when the pilot in the above example used G for brains and thought he was winning the angles fight, he was actually losing an energy fight he didn't even know he was in, the old rope-a-dope in action! Referring to the next and last EM diagram then, we are now in a position to state what it is really telling us. Basically, don't take the turn rate now, and worry about the energy later. Hold your energy, fly intelligent BFM and the angles will take care of themselves!

So to recap, if you try to fly the red line with full aft stick, you will gain angles for a short time, but your speed or altitude will be consumed so quickly that your ability to continue turning will ebb with your energy. You will become low or slow with no where to go! Think of the red line as being an indication of where you could go in an emergency! A wise opponent, who flies closer to the blue line and his sustained turn rate, will maintain his energy and thus retain the ability to continue turning indefinitely.

So, the answer to the original question is that the diagram shows you to fly to the blue line! Stay in that part of the envelope where you have positive or zero Ps, try and stay out of the negative Ps region when ever possible. FURTHER COMPARISONS

An important point has been made regarding the wisdom of pulling such high loads, you have seen how expensive that is in terms of lost energy, and that it is simply not worth the cost! Pilots who fly that way are said to have G for brains and they don't live long! But let's make further comparisons with the real world for a moment, you may be able to pull 12.2G in the sim' and you may bleed energy very rapidly if you do so, but 12.2G, that's got to hurt!

It has been demonstrated in a centrifuge that real pilots can endure loads of up to 10G for as long as two minutes. However there is little doubt that excursions into areas of the envelope involving 12G loads may occur, but they would need to be extremely brief in order to have anything other than disastrous results. The F-15A and B had a 7.33G restriction applied, while an overload warning system in the C and D models permitted the pilots of those versions to reach 9G, and you have to love those "Over G" warnings.

Even so, greater loads are possible by ignoring the warnings and applying greater stick force or flying CAS off. The F-15 has a dual flight control system, in effect a conventional hydromechanical system complete with push rods and hydraulic actuators that move the control surfaces, and a separate Control Augmentation System (CAS). The CAS system uses electrical wire to transmit signals to servomotors that operate the same hydraulic actuators as the mechanical system. The CAS system takes data from sensors that feed it all relevant information about the aircraft load and motion. That information, along with stick force sensors allows the control laws of the CAS fly-by-wire system to compute the optimum control surface deflections and modify them appropriately.

Stick forces greater than 3lb per G require the pilot to apply a stick force of around 20lb in a 6G turn and even greater forces are required to initiate a response to a control demand at higher load factors. Even though I too have 20lb springs in my flight control stick (FCS), in fairness, these things are absent from most simulations, along with typically weak physiological modelling that allows you to stay at higher loads for longer periods than is realistically possible.

That is slightly unfortunate because the ability to produce such high loads, with none of the real world constraints is no help to flight sim' pilots who in the heat of virtual air combat will already have difficulty controlling the urge to pull all the way back on the stick. Although you lack that physiological feedback, there is still something that can be done to assist energy management in Jane's F15. The answer lies in the joystick calibration. Basically, you can de-tune the calibration in order that you never receive more than a 9G load from full aft stick at any point across the entire envelope. You might think of this as being able to customise the control laws of the CAS. Doing that makes the EM diagram look quite different, as shown below.

What you notice immediately when you examine this new EM diagram is that the blue Ps=0 curve has not moved. That is as you would expect because altering the joystick calibration doesn't and shouldn't have an effect on either the thrust or drag on which that curve depends. What it has done however, is restricted the amount of G that can be achieved with full aft stick and thus reduced the rate at which you will lose energy in that situation. With the default calibration there was a 1250 feet per second negative Ps at 400kts, now that has reduced to 450 feet per second, less than 40% the rate of energy loss. You will also notice the lower corner speed at 325kts.

Before we move on, let's just look at the implications of flying at our best sustained turn rate. You will notice that the slope of the Ps=0 curve is very shallow. The turn rate between 200kts and 400kts changes by less than two degrees per second. However, the turn radius varies by more than 1000ft over the same speed range. So it seems from this curve that sustaining lower speeds is more advantageous due to the reduction in turn radius, than the small increase in turn rate.

However flying at lower speed has two distinct disadvantages. Firstly at speeds close to 300kts any slight increase in G can result in a stall with the sudden drop in turn rate associated with the sudden loss of lift. Also at such speeds, you are particularly vulnerable to other threats. So it would appear that the choice of an ideal combat speed is more difficult when energy considerations are brought into the equation, and that staying as fast as the situation permits against the computer pilots keeps your options open and reduces the risk from other threats.

Good advice would be to pull only as hard as required to make small gains, holding your energy just as preciously as your angles. In any event you should try to never allow your speed to drop below 300kts. At that speed you can sustain 5G (see screen shot 1) at 100% throttle, a little less if you are caught with a full fuel load, and a little more if you are light. Flight sim' pilots with experience in Su-27 will find these aspects of energy management are already familiar, because that is another simulation in which the flight modelling is such that good energy management is rewarded.

PUTTING IT INTO PRACTICE

An important point to make in closing is that having the information presented in the EM diagrams is valuable because it provides you with the information you need to fly in the best tactical region of the envelope under the circumstances. However, during a dogfight it is difficult enough, while flying in the padlock view, to maintain your grasp on the BFM requirements, let alone monitor your flight parameters. However there is a way you can do both at the same time.

Firstly though, pay attention to the overload warning, and unless you have a bandit pulling lead, ease up when you hear it. Secondly, and more importantly, you can maintain better situational awareness, and remain conscious of your flight parameters by making frequent use of the "Glance Forward" view (zero on the numeric keypad). That view works very nicely in padlock because it allows you to keep a tally on the bandit while providing you with a reference to where your nose is pointing and allowing you to check the Head Up Display (HUD).

While there has been no attempt in this article to discuss BFM as such, it is hoped that the advice and data provided will allow you to fly all of your BFM more successfully! In that I wish you good luck, and happy hunting!

Leon "Badboy" Smith