"Downwards lift" on a stab occurs when the foil is traveling in a straight line. But that same foil section can produce upwards lift at a different AOA.
Yes,but asymetrical foil sections are very ineficient creating lift on their "wrong" side.Lots of drag ,stall at lowish AOA etc..Matteo V wrote: ↑Mon Apr 12, 2021 4:45 pm"Downwards lift" on a stab occurs when the foil is traveling in a straight line. But that same foil section can produce upwards lift at a different AOA.
Think of a jet fighter that rolls inverted, and trims its AOA so that its upside down wings produce lift in the opposite direction compared to when they are operating in the normal orientation. This demonstrates that both upwards and downwards force can be produced by almost every foil section if AOA is adjusted accordingly. It is useful to note that while symmetrical foil sections (50/50) are equally efficient in generating upwards and downwards force, asymetrical foil sections are not.
But this is not necessarily the main idea. Forward propulsion is the main goal of pumping. Thus reversing the direction of force on the stab is not done to lift upwards, but rather to provide the horizontal component of a mostly upwards force to forward motion.
If you want to simplify the situation......
Do longer or shorter dive fins produce more efficient thrust? Definitely an over simplification that may not necessarily scale infinitely, but thats much easier to conceptualize.
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