Hello Ibrahim,

Due to the interruption of the bottom line, this step makes it easier for the seaplane to rotate and bitch on the water in order to get the highest lift for take offs. Further, it reduces drag at take-offs inside the water.

After take-off, it probably increases the pressure drag of the seaplane, due to separation.

Best regards, Johannes

See also:

http://images.google.de/imgres?imgurl=http://avstop.com/ac/flighttrainghandbook/imagebnu.jpg&imgrefurl=http://avstop.com/ac/flighttrainghandbook/characteristicsofseaplanes.html&h=236&w=467&tbnid=af0a__PmfwL-5M:&tbnh=90&tbnw=178&docid=6d5mVarpGJU89M&client=ubuntu&usg=__LCsF_0rNF1VnJZ8dJMkbp4mmHho=&sa=X&ved=0ahUKEwjFvOnzpO7PAhXFhywKHTFTDG8Q9QEIMDAD

Hello Ibrahim,

Due to the interruption of the bottom line, this step makes it easier for the seaplane to rotate and bitch on the water in order to get the highest lift for take offs. Further, it reduces drag at take-offs inside the water.

After take-off, it probably increases the pressure drag of the seaplane, due to separation.

Best regards, Johannes

See also:

http://images.google.de/imgres?imgurl=http://avstop.com/ac/flighttrainghandbook/imagebnu.jpg&imgrefurl=http://avstop.com/ac/flighttrainghandbook/characteristicsofseaplanes.html&h=236&w=467&tbnid=af0a__PmfwL-5M:&tbnh=90&tbnw=178&docid=6d5mVarpGJU89M&client=ubuntu&usg=__LCsF_0rNF1VnJZ8dJMkbp4mmHho=&sa=X&ved=0ahUKEwjFvOnzpO7PAhXFhywKHTFTDG8Q9QEIMDAD

I do not think this step will create a big difference on drag. Before the step, this blade-shape structure is used to increase directional stability.  That is my understanding.

Hi Ibrahim,

The step is needed to break the water's surface tension and allow the plane to ease onto the surface (i.e. skim on top rather than dragging through the water). During takeoff, positive pilot action (i.e. slight forward pitch) is required at a certain speed to allow the plane to lift onto the step. Without being on the step, most planes will lack sufficient power to further accelerate the aircraft to takeoff speed. You will find the step not only on the hull of amphibious aircraft but also on the floats of float planes. For a detailed description of seaplane operations in the water see also: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/seaplane_handbook/media/faa-h-8083-23-3.pdf (pages 4-4ff)

As for aerodynamics once airborne, I'm pretty sure there is some difference in drag caused by the step as compared to a hypothetical smooth surface; nevertheless, since the step was always an integral part of the seaplane I flew, I can't give any account of how much difference it would make without it...

Stefan

Dear Ibrahim,

While Stefan Kleine provides very interesting information, it is misleading to state that "surface tension" is involved. "Surface tension" at the interface of a liquid with gas or solids is a force due to the cohesion of molecules in the liquid. It is only significant at small length scales. As the length scale L decreases the ratio of bulk cohesion energy to surface cohesion energy decreases linearly with L. Hence at some length scale surface energy becomes dominant. This is for water typically in the length scales below a millimetres. Insects walking on water make use of "surface tension" effects. An aircraft with length scale of the order of 10 m is much to large for any significant "surface tension" (surface energy) effects.  I guess that the "surface tension" Stefan Kleine mentions is a reference to "wave drag". This is the resistance to motion induced by the loss of energy by emission of surface waves. Yours sincerely,

Mico Hirschberg

Hi Avraham,

Thanks for clarifying - I loosely used the term in the way it is applied by the FAA for pilots. Nevertheless, local surface tension is the root cause for surface friction (i.e. shear stress distribution in the boundary layer) and adverse pressure gradient (i.e. pressure distribution along the surface) at the interface between the hull/float and the water. By 'cutting away' the rear half of the hull/float (thus, greatly reducing surface area in contact with the water), both skin friction and pressure drag are reduced for the hull/float to water interface. This idea is similar to riding a surfboard on the crest of a wave (i.e. with reduced contact area) rather than in the water...

Stefan