An airplane is flying with constant speed along a horizontal circle. is the direction of its acceleration constant?

No.

Explanation:

Any object moving at constant speed whose trajectory is a circle is experimenting an acceleration that points towards the center of this circle. This is what is called the centripetal acceleration. Since the object is moving along this circle, the direction to which this acceleration points (the center of this circle) changes with time.

r = ρ x length / area  

ρ = r x area / length  

ρ is in ohm-meters  

in basic units, it is m³kg/s³a²  

i think this is it.