SimHQ.com - Air Combat Zone - Prop Pilots Always Get Their Man: Winning The Co-E Chase

Our newest writer, Leon "Badboy" Smith, takes a quick look at a common problem in sim combat..... you're on the bandit's six, and he's running scared. But you're juuuuust out of guns range, so what can you do? Badboy takes a common jet tactic and applies it to the world of prop sims. This article was prompted by a thread on the Aces High message board, when someone posted a similar situation.

So check six, and remember, that guy behind you who's diving may be getting ready to eat you for lunch. Badboy explains more about winning the Co-E Chase.

The technique described below is one that I’ve been aware of for some considerable time. I've been using it myself and I use a variation of it at higher altitude. The question being asked is how can it be correct. However, before I offer the explanation, I would like to say that when I first used the method and noticed that it worked, I wasn't at all surprised. I am more familiar with modern aircraft performance and tactical doctrine because most of my writing has been concerned with simulations of modern fighters. The method is well known and used for jet fighters, and so when it worked in the prop sims, I was curious about the differences in execution, and not with the right or wrong of it.

The diagram below is compressed due to lack of space and shows the method for closing to within guns range from a Co-E start. It involves diving at Zero G for maximum acceleration for closure, followed by a zoom climb to within guns range.

When I noticed that this worked in a prop sim for the first time, I was actually doing what I would have done had I been flying a jet. I never gave a second thought to the question of the technique surviving the difference in characteristics between jet and prop driven aircraft. I used the technique and found it to be successful without a thought...I just assumed it should be so because it worked for jet-engined aircraft. Interestingly, I discovered that the technique is not very well known, and after consistently catching the same pilots in the online arenas, and being repeatedly accused of cheating, or hacking the code, I thought it would be a good idea to share.

Before I explain how such a thing could be correct, I'd like to show you how a real fighter pilot would approach the problem in a modern jet, because the solution is not just surprising but useful as a datum. Picture yourself in the cockpit of an F-15 at 20,000ft and M0.7 making radar contact with a distant target. At that point a real fighter pilot would have a great deal of busy work, but high on his priority list will be the desire to enter the fight with the highest possible energy state in the shortest possible time. You would achieve that by selecting afterburner, unloading to zero G, and accelerating in a dive to M0.92, losing several thousand feet in the process. You would then begin a climb to an altitude above the enemy if there were time.

What surprises most folk about this story is the initial dive to a higher speed. That is the correct procedure in a jet regardless of altitude, providing you are not at sea level.

To understand why this works, take a look at the diagram below. This diagram shows lines of constant specific Energy (Es) in green and lines of constant specific excess power (Ps) shown in blue. If you want to achieve the highest energy state at the greatest rate of energy increase, you need to maximize your Es and Ps at the same time. That can be seen to occur where the P's curves are tangent to the E's curves. The red line is drawn through all such points and represents the best energy transfer line. For the F-15 that happens at M0.92 and for most jet fighters is somewhere close to M0.9. In the diagram below, our pilot started at A, accelerated to B than climbed to C.

The amazing thing is that in order to get to the best possible energy advantage for the fight, you start by diving! You increase speed to a point well above that for minimum drag. The objective, after all, was not to minimize drag, it was to gain the most energy in the least time. Drag is not the only factor that needs to be considered. Jet engines generally produce more thrust at higher speeds, and so a higher speed can result in a greater specific excess power...that's more Ps and that’s always good! The diagram below is typical of real jet fighters.

So the modern fighter pilot will always begin an intercept (or a tail chase) by increasing his speed to maximum energy transfer rate as quickly as possible, using a zero G maneuver. How does that contrast with the aircraft of W.W.II? Propeller and turbocharged engine combinations were generally designed to give maximum efficiency close to top speed at the critical altitude. So both altitude and airspeed have an influence on the best energy transfer, just as it does for the jets. That this technique survives the differences between jet and prop driven aircraft is both surprising and advantageous to those who know.

So assuming a realistic drag polar, and properly modeled prop thrust, the energy transfer diagram will appear similar to the one above. A wise pilot will therefore dive to get to his maximum energy transfer speed as quickly as possible before starting the climb that allowed him to maintain that speed. If that placed him at a faster speed than his opponent, he would not only be closing but also building an overall energy advantage prior to the zoom climb.

Just as for jets, drag is not the only concern in maximizing rate of energy transfer. If you start a chase above the critical altitude for your supercharged engine, diving towards that altitude would increase the available power, resulting in higher specific excess power. That this works in simulations like Aces High and Warbirds can be confirmed by chasing an aircraft at high altitude. If you deliberately stay below it, you will have more engine power and a slightly higher air speed, resulting in closure and better energy transfer.

This is interesting enough to do properly so I will revisit the topic and plot some genuine curves for Aces High that will show the best energy transfer speed for some of the aircraft. I will do that quantitative analysis when time permits and it will be interesting to see how the figures clock out. Meanwhile I feel very confident that the flight models of most of the current W.W.II simulations are close enough to reality that what happens in the sim will agree with what real world aerodynamics predicts.

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