Originally Posted by
Eddie Dean
Is this float plane "on the step" or some other sort?
Yeah, it was those rare float equipped metroliners.
If that's a serious question, there is a myth that you can get an airplane in flight "On the step" in a similar way that a boat or floatplane will get "on the step" and that once "on the step" the airplane will cruise at a higher airspeed with the same power. Yeah, it is complete nonsense. In order to do that, there necessarily must be a reversal in the drag vs. airspeed curve. Yet in 70 something years of pretty extensive empirical testing in wind tunnels and such, nobody has ever observed this reversal of the drag curve. So, the question is how is it that this "step" can be reliably detected by the hundred dollar hamburger crew with the low resolution analog airspeed indicators in the panel of their bugsmasher, but all the aerodynamics and all the aeronautical engineers in all the research institutions and air-frame manufacturers around the world have not been able to detect this "step"? It just seems unlikely, doesn't it?
Yes, a boat or a floatplane will "get on the step" and once on the step it is in fact possible to cruise at a higher speed with less power than at a speed below "the step". The reason for this is that the boat is operating within two fluid mediums, sir and water, one of whcih (water) has far greater drag. when a boat is traveling slowly, it is in displacement mode, that is to move ahead, it must move all the water displaced by the hull aside or beneath the hull. That takes power, because compared to air, water is dense and viscous and hard to move. As a boat hull reaches a certain speed the boat begins "planing" where hydrodynamic forces will lift the hull out of the water, so that the boat is being supported primarily by the hydrodynamic forces rather than the buoyancy of the hull. With a float-plane, obviously, some of the weight is being supported by lift on the wings. This is easy to observe in a hydroplane; virtually none of the hull is in the water, it's skimming across the surface. This reduces drag in two ways: first, the hull is no longer displacing much water as it moves, and second because less of the hull is in the water, there is less skin friction from the water. The portion of the hull that has been lifted out of the water is now moving through air, whcih creates far less form drag and skin friction.
In short, a boat gets "on the step" because in planing mode, most of it is lifted out of the water which is creating the vast majority of the total drag.
There is no equivalent effect with an airplane in flight. The airplane is moving through a single fluid medium, air. It is not lifted out of anything as a boat is when planing, the entire aircraft remains submerged completely in that single medium.