All this controversy emanating from a thread on energy
. Before we digress to much with discussions regarding the flight model here’s my post on the original topic!
How is energy retention defined and how is it determined?[/i] Of course the answer is:
Ps = (T – D) * V / W
For in that equation derived from Newtonian physics lies the secret to the energy puzzle. Thanks to the likes of the infamous Col. John Boyd and Thomas Christie, his partner in stealing computer time for running thrust and drag calculations or the lesser-known E.S. Rutowski, fighter designers and pilots alike now grasp the importance of this simple but elegant equation. Take the following image for instance:
Source:
http://www.tonyrogers.com/news/viper_revolution.htmThe photo compares the turn rate and radius of the F-16 vs. the F-4E. The designers of the F-16 achieved this incredible turning ability as a result of comprehending this elegant equation.
So let’s unravel the mystery behind this equation! Doing so enables even us virtual pilots flying in our pixilated cockpits to extract the most out of the energy of our digital aircraft. To answer how energy retention is defined, we need to first answer the question, “what is energy?”, as well as it’s cousins work and power.
WORK:Let’s first discuss work. Not the work that describes the Dibert-esque existence that some of us know. No, I mean what Sir Isaac called work. In physics work is the action of a force upon an object that displaces it some distance. In our world, when the aerodynamic forces of thrust, drag, lift, and weight act upon an airplane causing it to move through the air, well the airplane literally works (pun absolutely intended)!
ENERGY:Well, for work to be performed, energy is required. Energy is the ability to perform work. It is the capacity to generate aerodynamic forces on an airplane to move it through the air. No energy, then no work. No work, then no airplane moving through the air.
Three sources of energy exist to generate aerodynamic forces that move an airplane: chemical, potential, and kinetic. An aircraft exchanges these energy types to maneuver. The following diagram represents the exchange relationship between these energy types.
Source:
http://www.boeing.com/commercial/aeromagazine/aero_03/textonly/fo01txt.htmlChemical energy (fuel) can be converted into potential (altitude) or kinetic (speed) energy. Potential energy can be traded with kinetic energy and vice versa. Ultimately to maneuver, an aircraft needs kinetic energy to do so.
There’s actually a fourth category of energy that is important to understand for airplanes as well. That is the energy left behind in the air when an airplane passes through it, which displaces the air as well as heats it up. For simplicity we’ll call this drag energy. The following diagram puts it all together:
Source:
http://www.av8n.com/how/img48/energy-con.pngENERGY RETENTION:So what do we mean by energy retention? Energy retention is the aircraft’s ability to preserve its potential (altitude) and kinetic (speed) energy (transferred from its chemical energy) vs. the amount of energy transferred into the air as it moves through it. As depicted in the diagram above the energy imparted into the air by drag cannot be exchanged back into potential or kinetic energy, therefore this transfer of energy “bleeds” the mechanical energy (altitude and speed) of the aircraft. The greater the drag, the greater the mechanical energy bleed.
POWER:Though a pilot can decide to re-arrange an airplane’s energy, the rate at which this energy exchanges remains finite or limited. This brings us to the concept of power. Power is the rate at which work can be done. Stated differently power is the rate of change of energy with respect to time. If we monkey around with the math related to the power concept, for an aircraft we end up with:
SPECIFIC EXCESS POWER:Ps = (T – D) * V / W
Ah, our old friend! This happens to be the equation for the Specific Excess Power (Ps) of an airplane. Simply put specific excess power tells us the rate at which the airplane can exchange energy. It represents the difference between the rate of energy transferred to the plane from the engine (thrust) and the energy transferred (dissipated) to the air from the plane due to drag. In other words specific excess power (Ps) gives us a measure of the rate at which an airplane is gaining or bleeding energy. It is a measure of the energy balance of the combined engine/airframe for a given velocity, altitude and G-load.
When Ps > 0 the aircraft is gaining energy. The excess power can be converted into additional altitude, airspeed, or both. When Ps < 0 the aircraft is losing energy. The negative excess power will result in either loss in altitude, airspeed, or both. The larger the value of Ps above or below 0, the greater the energy gain or loss of the aircraft. Specific excess power is also a measure known as energy maneuverability.
With the right data we can plot energy maneuverability (E-M) diagrams, which tell us Ps for given flight conditions and begin to understand specific aircraft energy maneuverability characteristics. Badboy has put together many E-M charts for AH aircraft. You can do a search for postings by him to find them. He also has an informative article at SimHQ as well posted here:
http://www.simhq.com/_air/air_011a.htmlI won’t belabor the topic that he’s done such a fantastic job covering.
RETAINING ENERGY:Here are a few other tips on retaining energy:
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- Simply put fly your aircraft in the envelope where Ps is equal to or greater than zero. This implies that there is a part of the flight envelope where you can maneuver your aircraft without bleeding any energy. Yes I said maneuver. Here’s where planes like the N1K2-J or Spitfire VIII surprise people sometimes. They assume if the plane is in a sustained turn that it’s automatically bleeding energy. This is simply untrue. If the opposing aircraft has a greater Ps rating then the one I’m flying it’s likely that in a sustained turn it can actually out-turn mine even without losing any energy. Worse yet it may even out-turn me and actually be gaining energy itself in the process.
At an absence of an EM chart to reference, you can do some simple flight tests to figure out where the Ps=0 envelope is for your aircraft. Simply fly a sustained level turn at a given configuration (weight & flaps) for a given velocity and altitude. Apply elevator (stick) input until you either bleed airspeed or lose altitude. At Ps=0 your altitude and airspeed should remain constant for a given g-load. Above or below this point you will gain or lose altitude, airspeed, or both. It should be noted that this is only good for a given altitude. Also you'll need to test this for various airspeeds because the G-load for Ps=0 changes with airspeed.
- Use 2-circle or nose-to-tail turns. This has the effect of creating more distance to travel in a turn before either aircraft is pointed at the other again. If the other plane is hellbent on gaining angles then using a nose-to-tail turn is a good way of inducing the bandit into blowing energy because they are holding as much of a maximum performance turn possible to get those angles but the longer turn results in more energy bleed in the process. On the other hand make that turn at Ps equal to or greater than zero and you’ll be maintaining or gaining energy in the process. Essentially this is one way to trade giving angles to the other guy for gaining a possible energy advantage.
- Make your maneuvers with your nose up vs. nose down. Trade in that speed for higher altitude as much as you can to store energy vs. bleeding it away to drag. You’ll be surprised at how many people don’t do this and you’ll find that you end up more on top of the fight than not.
Of course much more could be said but alas I'm out of energy writing this "novel"!
Tango, XO
412th FS Braunco Mustangs
