For the look of scale flight, you need to consider how many fuselage lengths the model moves per unit of time. If the full size airplane moves "x" fuselage lengths per second, then a scale model will give the same appearance if it moves "c" fuselage lengths per second.
Bax, you need to define "x" and "c"? What you meant to say (I think) is that the model should travel the same number of fuselage lengths per second as the full size does.
Example...the 747 is 250 feet long and at 500 mph moves about 3 lengths per second. A 1/20 scale model is 12.5 feet long and should also move 3 lengths per second. For the model this equates to about 25 mph for the model...very slowly indeed. You can see that a model of a 747 must be super light to fly "scale-like".
Another example...A P-51 is 32' long. At 400 mph the Mustang moves 18 fuselage lengths per second. A 1/5 scale Mustang is 6.4' long and should also move 18 lengths per second. For the model this equates to about 80 mph. You can see that a normal Mustang model can come much closer to "scale-like" flight.