OEI - Why does the Balance Ball show a slip?

[3 green's]

some more thoughts and questions

morning all

i have some questions related to the post -

1)should we not be refering to a slip string as opposed to a yaw string?

2)for maximum rate of climb (OEI) would we "fly" the aircraft diffrently if there was no requirement to "maintain heading"

3) in a multi-engine piston twin with counterrotating props and both engins operating and developing the same power and in a "balanced level turn" is it correct to say the aircraft is not yawing?

4)given the need to minimise drag / power required for best (OEI)single engine climb performance ( with constant heading) - DOES ZERO SIDSLIP ALWAYS PRODUCE THE MINIMUM DRAG / POWER REQUIRED ?

5)I am starting to write a technical report on this issue for a intructor upgrade...does anybody have an good book titles, websites, articles they can recomend on (OEI) climb performance

thanks