Hi moot:
Your picture doesn’t quite describe what I explained. 1) The boundary layer doesn’t usually separate & re-attach like you’ve drawn it (though it can in certain situations but that’s another story!). 2) Turbulent flow in the boundary is different from boundary layer flow separation from the surface.
Let’s sharpen our understanding.
Adverse Pressure Gradients:When air flows around a cambered object it will accelerate & decelerate in relationship to the objects shape. Here’s a diagram to illustrate:
In the above diagram we can see where air accelerates & decelerates around a shape. Where the airflow decelerates there must be a region of increasing pressure. This region is known as an adverse pressure gradient.
Adverse pressure gradients are greatly influenced by an object’s shape. The shape determines the amount of fluid velocity increase & decrease and where the peak pressure or transition from decreasing to increasing pressure begins. Here’s another diagram to demonstrate:
We have 3 airfoils compared. The right-hand graph is a plot of pressure (Cp) vs. % of chord length (x/c) for all 3 airfoils. Annotated on the graph is also where the transition point between laminar to turbulent flow occurs.
We see the Cp curve goes up and then comes down toward the trailing edge for each airfoil. Where the curve descends & all the way to the trailing edge is the adverse pressure gradient. The downward slope of the curve is the degree of adverse pressure gradient; the steeper it is the greater the adverse pressure gradient.
The blue airfoil shape is thicker & more curved near the leading edge. Note on the Cp chart this results in a long, steep adverse pressure gradient compared to the others. Also note that the laminar to turbulent flow transition point is much further up toward the leading edge as well. These are all direct result of the airfoil shape. Why is all this important?
Parasite Drag:Parasite drag consists of two fundamental components:
1) skin friction drag &
2) pressure drag
Skin friction drag is strongly related to boundary layer turbulent flow. Pressure drag results from boundary layer separation. Both turbulent flow formation (skin friction increase) & boundary layer separation (pressure drag) are strong functions of the adverse pressure gradient.
The Me410 has a similar conceptual shape like the blue airfoil above. The forward section of the Me410 fuselage has a large radius of curvature around the nose & cockpit & then tapers off. Like the blue airfoil it creates an adverse pressure gradient which moves the turbulent flow transition point closer toward the leading edge / nose of the aircraft. This results in more turbulent flow along the length of the fuselage which increases skin friction drag.
The adverse pressure gradient is also steep & long meaning the boundary layer will tend to separate earlier along the fuselage. Earlier separation of the boundary layer results in increased pressure drag.
I speculate these factors conspire to increase the parasite drag due to the shape of the adverse pressure gradient. Understand this is just an educated guess on my part, but a guess no less on why we are seeing the Me410 with a calculated drag coefficient we are from published performance numbers. I’d have to do some fancy maths with panel codes & the like to better estimate.
Loose Ends:A couple of loose ends to tie up you asked about in your post: a) pressure peak placement, b) motorcycle rider analogy.
Yes pressure peak placement toward the trailing edge will help reduce parasite drag but it comes with a big trade off with other key variables such as degraded lift, etc. There's no free lunch in aero
.
As for the motorcycle analogy with the rider tucking, it's similar but not the same. In that instance the tucking of the rider is drastically reducing pressure drag for having a blunt shape in the oncoming wind. We're talking about smooth bodies here and how the pressure gradient is affected by the shape of the smooth body.
