eli5 why does the ac-130 have propellers instead of jet engines?
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Basically, when you move in fluid you have to adapt your propulsion to speed.
Let’s say you want to move a boat, if you take a row, with a very wide tip, you can exert a lot of force to propel yourself. But as soon as you go faster, you can’t keep up moving a big row. You need a smaller row that you can move as fast as the boat.
Aircraft works the same way:
If your target top speed is low, you can get a very big propeller “grab” a very wide airmass and push it a bit, this gives you the best thrust for the power you have.
To move faster, you have to concentrate your power onto a smaller air mass so you can still shoot that airmass behind you faster than how fast you are going. If you try to use a big propeller, you won’t have the power to spin it that fast.
It’s overly simplified, but that’s the concept. It’s like torque vs speed on a car, but applied to fluids, where the “torque” is the mass of air you use.
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