So the sweet spot on a python goes from 31 secs at 0 spd and 100 100% spd is 30 sec to 12 at 50% spd...hmm you actually sure we are using ships that are "balanced" as a reference maybe the older ships are to agile and need to be brought in line with balancing if you look they all get a massive spike when at the optimal..
Still explain how the cutter still out turns the Python at full speed.
I think its more an issue with prior ships then the cutter being "Useless".
Quick disclaimer: I had a little trouble comprehending what you were saying, so if I end up missing the point of your post I apologize.
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I think they are balanced in the environment that FD has made (For example each level of PD not adding a lot of cooling, same weapons/modules for each ship, and a steady decline in maneuverability and top speed as ships get larger - two traits that are very important for dogfighting). Not perfect, (FDL may be a bit OP right now for example) but close.
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I think the reason larger ships with "fast" top speed have such out of the blue penalties is to make them in line with smaller ships. Smaller ships are agile at speed so they can close a gap
and stay on something's 6. Larger ships have more firepower and (usually) defense. But in combat you have to make a choose - close the gap, or turn. Pivoting (FA off turns) are insanely important for such ships as FA off lets you whip your nose around while maintaining speed. However, unlike a normal turn, after the pivot you have to fight your drift (unlike a small ship).
The reason large ships out turn the python at high speed is because their top speed and drift are bad. Very bad. The python's "blue" speed very good for a ship that size, and its boost isn't too far off. As a result it can speed up and get back in the blue very easily, and maintain a much higher average speed (vs the super-heavies). Not as well as small ships, or ships dedicated to agility (FDL for example) but its the trade off for the internal slots and very powerful hard-points.
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The Super-heavy ships however have high drift and very poor top speed. From a logical standpoint I kind of get why they have less penalties, as they do fly at much slower max speeds. From a gameplay one however this is so Super-heavy ships can do more than face tank. Already they are at a disadvantage (as in get hit more) than their smaller counterparts. And gameplay still revolves around dogfighting. With their drift they can't cycle between turning and speed like the python can. They already are the worst dogfighters, but if they couldn't reliably speed up they would be forced to face tank every fight. So they devs threw 'em a bone. Relative to their sweet spot they don't take a hit at max/min speeds like other ships, but they are still very poor and struggle (comparatively) to get back in the blue.
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So to summarize: Super-heavy ships do better at max and min speed relative to the ideal to compensate their already poor pitch and comparative inability to fly in the blue.