[Graham Boak]

Sorry Jukka, but I sincerely hope that no-one relies on this advice. All aircraft have a minimum drag speed. This is also the optimum speed for maximum endurance, because more power is needed to fly either faster (because of an increase in zero-lift drag) or slower (because of an increase in lift-induced drag). I suggest looking up the term "drag polar".

I'm afraid my official technical education in performance was restricted to jet engines. In this case the optimum cruise speed is indeed faster than the optimum endurance speed, because of the behaviour of the turbine engine with velocity. (As I understand it.) Many years after first being gainfully employed in the business, I had the opportunity to study the notes of a fellow engineer from another college, and was surprised to learn that for a piston engined aircraft the speed for best endurance and the speed for best range are identical - the minimum drag speed.

So once at the optimum cruise speed, any Ju.88 pilot could only fly at a lower speed by increasing power. This is because of the need to fly at a greater angle of attack to obtain enough lift, hence increasing the lift-induced drag, which would increase the total drag more than the reduction in zero-lift drag gained by reducing speed. More power = higher fuel flow = less endurance. And less range.