How can something moving at a constant speed have an acceleration?

Acceleration means that there is a change in speed or direction. Since it is spinning, it is constantly changing direction.

if your pedal is to the floor in a car, is it going as fast as it can? Or is the engine at it's highest rpms?
Acceleration in the case of my example is relative to distance traveled, no the rpms.

it is kinda like terminal velocity. gravity is 9.8 m;s squared which is your acceleration. It never stops acting on you. but there is a point at which the air restistance is stronger than gravity.

so there is a limit to how fast the flywheel can rotate because of resistance.

First off, "speed" is relative to something else.  (Relativity, right?)

In the case of rotary speed, you are constantly changing direction and that constant change of direction is brought about by a force.  In the case of an orbit, its gravity pulling the satellite towards it.  In the Earth's case, at 9.8m/s^2.

However, with a fixed object, the force is caused because the rim of the wheel is connected to the rest of the wheel.  Never-the-less, any point on that rim, by virtue of the fact that the wheel is turning, is accelerating away from its straight-line course ...

Acceleration is a change in velocity.  Velocity is a particles direction and speed.  An particle moving in a circular path at a constant tangential speed has a constantly changing velocity and therefore constant acceleration, which requires a steady force.

centripetal force = m(v^2 / r);  where m = mass, v = tangential speed, and r = radius