Here is my very basic explanation of energy maneuverability. The idea is important though, so it is worth getting a firm grasp of it.
Energy is a concept used to describe the condition of objects. It can sometimes make life very much easier to describe and understand why things happen if we refer to energy instead of the things that cause it to have the energy in the first place. Energy is a very broad notion, we say that an object has energy for various reasons, because it is moving, because it is high, or because it is hot or will burn well. Those are very different types of energy, and so they can to be broken down and named, you are probably already familiar with several types. It can sound very technical to refer to something like thermal or electromagnetic energy, if you are not technically minded, those descriptions can be daunting. The important thing to remember is that they are just labels, nothing more than names. So when a pilot talks about energy, he is talking about two things, Kinetic and Potential energy. Kinetic energy is the name given to the energy possessed by an object in motion. An aircraft that has some speed is said to posses some Kinetic energy. It’s as simple as that! The faster the aircraft travels, the more energy it is said to possess. Potential energy is the name used to describe stored energy. One way of storing energy is to raise an object above the ground. When the object is lifted, energy is required to raise it. When the object falls, it gains speed and the energy is recovered. while it was in its high position it had the potential to fall, it had potential energy. Similarly an aircraft that has altitude is said to possess potential energy. The higher the aircraft is, the more energy it is said to possess, because it will be able to gain more speed when it falls. At this point you may be thinking that things have hardly become any simpler. Initially we had speed and altitude, and now we have simply replaced them with two types of energy. However thinking in terms of energy allows us to perform a trick that does simplify things a great deal.
Normally If you want to add quantities together it is important to ensure that they have the same units. However two apples plus two oranges makes sense, only if you consider using a new system of units called fruits, when the answer can then be four fruits. In the same way, adding speed and altitude together is fine, providing you think in terms of the energy. That this is a reasonable thing to do can easily be demonstrated. For example, if you hold an apple 6ft above the ground it will have potential energy because of its altitude, but it will have no kinetic energy because it is stationary. However when you drop it, it will lose altitude and gain speed. As it falls lower, it will become faster as it converts potential energy to kinetic energy on the way. The moment before it reaches the ground, all of the potential energy will have been converted to kinetic energy. However the amount of energy that existed in the beginning as potential energy will be the same as the amount remaining at the end in the form of kinetic energy or speed. Originally it was high, and then became faster as it fell, however the total energy was always the same. If you consider the energy when the object is half way down, it will have gained some kinetic energy and lost some potential energy, and you can now add the kinetic energy due to the speed, to the potential energy due to the height and obtain a figure that represents the combination of speed and altitude. What we have done is added the speed and altitude together in terms of the amount of energy in each. What we have done is forged a link between speed and altitude using a tool called energy. We have discovered that two seemingly separate quantities, are infact intimately related. The ways in which this relationship is so vital in air combat can now be considered.
So for a fighter pilot, the concept of kinetic and potential energy, allow him to describe the combined effect of speed and altitude together. When you fly your aircraft the overall energy status is the sum of its kinetic and potential energy and that will depend upon its speed and altitude. Just like the apple in the example, an aircraft can trade altitude for speed by diving. The higher it is, the more potential energy it can convert to kinetic energy and the more speed it can gain. If the pilot climbs he will lose speed but gain altitude, which can be converted back to speed by diving once again.
The way in which speed and altitude are converted back and forth in this way is called “Energy Management”. The combination of an aircraft’s speed and altitude is referred to as its “Energy Status”. It is possible for two aircraft to have the same energy status even though their speed and altitude may be different.
For example, an aircraft at 6000ft and 350mph has exactly the same total energy as another at 8000ft and 250mph, they are at different altitudes and speeds but their total energy status is the same. They are said to be Co E. Being Co E means that in this example if we ignore drag for a moment, if the low fighter climbed to 8000ft his speed would bleed to 250mph and if the high fighter dived to 6000ft his speed would increase to 350mph. That means that they have the same amount of total energy.
Now you will begin to see the advantage of talking in terms of energy instead of speed and altitude. In this example, comparing two aircraft would require both the speed and altitude for each aircraft, that is four quantities, and it would in any case be difficult to interpret. However by combing those quantities and thinking in terms of the energy, it was easy to see that the two aircraft were infact identical in that respect.
So, we have arrived at a way of describing speed and altitude in terms of a new quantity called energy. That allows us to combine the speed and altitude together into a single quantity that expresses the sum of both, quite a neat trick. But what does that energy tell us about the manoeuvrability of an aircraft? Well total energy is not yet the end of the story because two aircraft at the same altitude and speed won’t necessarily have the same total energy, because that depends on their weight. That this is true can be seen if you consider the amount of work you would need to do to stop two objects moving at the same altitude and speed, but one much heavier than the other. You will appreciate that the heavier object would be much more difficult to stop, it would require more energy. So for example a 150 ton B-2 at 20,000ft and 400kts would have a great deal more energy than a 12 ton F-16 at the same altitude and speed, but the B-2 would be a good deal less manoeuvrable.
So the heavier aircraft in this example has more energy, yet is less manoeuvrable. The reason for this is that if you apply the same aerodynamic force to two aircraft, the heavy one will accelerate less than the light one. We need to take another important step. Instead of considering the total energy, we need to look at the specific energy for an aircraft. That is just the aircraft’s total energy divided by its weight. That provides a measure of the energy per pound and is much more useful because it removes the effect of weight or inertia that caused the problem with the B-2 and F-16 comparison. The B-2 had a much lower specific energy, that’s why it was less manoeuvrable. The F-16 had a high specific energy because it was much lighter, that explains its greater manoeuvrability. Specific energy is called Es (pronounced “Ease”) by fighter pilots who can refer to charts for comparison.
So in terms of being able to climb to a greater altitude, or to the same altitude at a higher speed, more Es can already be seen to endow a fighter with more manoeuvrability. You might say that an aircraft with more Es has more options for converting between its potential and kinetic energy. It will be able to trade airspeed to altitude more readily, you might say that it was more energy manoeuvrable. The concept of energy manoeuvrability is an important one that can be applied at all altitudes. For similar aircraft an Es advantage would also translate to manoeuvrability in the sense of turning ability and that can be explained by taking the next step and combining the idea of energy manoeuvrability and angular manoeuvrability… But that’s another story.
Hope that helps...
Badboy
