You don’t always have to add power, only if you don’t want to slow down during the turn, or if you don’t want to change your vertical speed. All other things be

You don’t always have to add power, only if you don’t want to slow down during the turn, or if you don’t want to change your vertical speed. All other things being equal, making a turn will create additional induced drag, which will either change your rate of acceleration (slowing down, or reducing your forward acceleration if you were accelerating) or the change in drag will cause you to change your vertical speed, trading potential energy for forward impulse. If you want to maintain speed and your rate of climb or decent (including neither climbing nor descending) you will have to add power.