A very common 1vs1 fight goes like this. Two planes from opposing sides see each other, and turn towards each other. As the two planes pass each other (guns blazing usually), one plane goes into a hard horizontal turn. The other plane does an immelman (pulls vertical until inverted, rolls out wings level). The plane that uses the vertical maneuvering, has he actually lost any energy? He's traded his kinetic energy into potential, right?
It really depends on how the planes turn, but in most cases, the plane going up will end up with more energy after completing a 180 degree change of direction.
The reason is that the plane going upwards is converting its speed into alt and pulling its turn at a slower speed. Slower speeds means less drag and a higher prop efficiency. The plane doing a flat turn is doing all of it at a higher speed, so if he wants to complete the 180 at about the same time as the plane going up, he has to pull some significant G's at a higher speed which means burn more E.
The other advantage of going up is that if you started at a high speed and your plane is G limited (i.e. pulling into a blackout), a slower speed means a smaller turn radius, so in terms of the geometry, the plane pulling up to reverse will complete the move inside the turning circles of the flat turning plane (as seen from above) and above it, which allows the high plane to point its nose down and and use roll to pull ahead of the turning plane - if the other player is stupid enough to keep going round and round, so don't count on it.
Some planes in AH have low induced drag and can get away with a high G 180 degree turn and keep most of their E. Others (190's, P47s for example) will lose a lot of speed in a prolonged flat turn. Some players will choose to start with a flat turn in order to suck the other player into a knife fight, especially if the suspect the other player will keep a lazy/safe BnZ routine or just run. Generally, a high wing load plane with a good power/weight ration is best served by going upwards, so expect every 109K4 that you see to pull up immediately after the merge. Now, is that the right move to pull? in a duel it is efficient and what most players will do. In the MA it will often get you killed because trading speed for alt leaves you a stationary and less maneuverable target. In a furball I always keep my eyes on the 109's waiting to see them pull this vertical move on someone - then, immediately as I see their nose pointing up I leave my current target and swoop in to pick their cherry at the top of their vertical turn. Gives one almost as much satisfaction as blasting an F4U with its flaps and gears out (
Furballer's cherry-picking manual, Chapter 1).
Every move has its value and the right time to use it - and also the wrong time. In a furball, speed is more important than alt, or in energy terms, a Joule of real kinetic energy is more useful than a potential one.
