Blunt noses subsonic drag - why a complete ellipse?

Question

At subsonic speeds, blunt noses - both fuselage and airfoil - are good for two reasons:

1. the need to maintain attached flow at varying angles of attack.
2. it has less surface area for the volume, reducing skin friction.

TOP: 0 AoA/Cruise | BOTTOM: extreme AoA, 20-30degrees, takeoff/landing

If the airflow had to flow over a ridge/point in scenario 2, instead of the smooth curve, flow separation would be more likely.

But things don't seem to add up.

Firstly, it's not like most airplanes - particularly airliners - fly at AoA beyond 20-30 degrees. At least not without high-lift devices. Am I correct? But on the rounded nose, there are surfaces smoothly curving all the way from 90 degrees! This seems to me like a useless increase in cruise drag.

Secondly, why push a blunt face ahead of you just to increase the volume a little, whem you can use a curve terminating at a point/ridge, as will be demonstrated below? If the surface area to volume ratio becomes so important, we can just sacrifice the fineness ratio and still have less drag, as the new nose creates less drag, right?

Would limiting the curvature to the max AoA limit for the aircraft, then terminating at a point, as in the image below, not keep the flow attached throughout the expected AoA range, while producing less drag? At the maximum AoA, the flow over the top of the shape encounters no "ridge" or "point", as it is parallel to the half-angle at the nose.

TOP: less drag is created at cruise | BOTTOM: Flow remains attached at extreme AoA conditions.

What am I missing about the sharp nose? How and how much does it hurt performance? Why, after all this seemingly shown in the question, is it - aside from the radar - that big planes have round noses?

In short: I am claiming that an elliptical nose tipped with a triangle or cone of 20-30 degrees half-angle should be better than a pure ellipse for subsonic aircraft. Why am I wrong?

Answer 1

What you are missing is suction on that round nose which adds some thrust.

Granted, around that stagnation point there is a pressure region which also acts on a forward-facing surface, thus creating drag. But that region is small. Thanks to the curvature of the elliptic nose shape, air accelerates quickly out of this high pressure region. In order to follow the curved surface, the pressure has to fall below ambient on a still somewhat forward-facing area so the air is actively pulling the nose forward. No such suction can be expected on the straight contour of a pointed, cone-shaped nose.

The plot below only shows an airfoil, but flow around an elliptic nose is quite similar.

CPV plot from XFOIL, showing local pressure as arrows. Suction is pointing out, pressure into the contour. Length denotes difference to ambient pressure.

Answer 2

There is a stagnation point at the nose, where the airflow separates around it. As the aircraft attitude changes, it reduces drag if this stagnation point can move. Technically the explanation for this is a problem of three-dimensional flow, which is difficult to analyze. Intuitively the air "wants" to flow the most efficient way and fixing the stagnation point prevents that. There is an increase of pressure on the side of the point facing the incoming flow; the air has to turn harder to avoid it on that side, and that increases the local pressure. This is also destabilizing, so tail surfaces have to be larger. A round nose allows the stagnation point to move, in turn letting the air find the lowest-drag route.

Also, extending the nose uses more material, which increases skin drag, weight and cost.

Having said all that, these effects are small to trivial and many planes have had pointed noses - or pointed prop spinners - and not suffered unduly.