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Compared to a sharper conical nose, a blunter conical nose is aerodynamically superior in subsonic flight. My question is why. I've read a lot about this and allegedly:

  • A blunter nose accepts a wider range of angles of attack without sideways airflow across it, which would cause additional drag that is avoided.

It would appear that airliners have blunter noses because they fly at a wider range of AOAs and gliders have sharper noses because they fly at a narrower range of AOAs. However...

  • At optimal AOA, the form and skin friction drag is also lower. I don't understand why!

According to this, a blunt nose is aerodynamically superior even when flying at the optimal AOA, but then why do gliders, which obviously value aerodynamics above all, always have relatively sharp noses? According to some descriptions, a hemispherical cap is the optimal shape for decreasing form drag and skin friction drag, but I haven’t found any description of why the first is true and the second is often motivated with hemispherical caps having the lowest surface area to volume ratio, but a conical cap of equal height has a lower surface area relative to the radius! In other words, if we were to design a new airplane, why wouldn't we sharpen the nose closer to a point instead of blunting it closer to a hemisphere, only considering aerodynamics?

Edit: yes, this is a unique question because I’m asking why and specifically why form and skin friction drag of a blunter nose is smaller than for a sharper nose, challenging intuition.

Edit 2: basically, why does No 2 in the following have less drag than No 5? Horner According to the image (I’ve yet to read the source, I’m on it!) there’s less pressure drag, but according to an answer by aeroalias to a related question, there’s less friction drag: Why/when is the blunt nose better? Which one is it? Both? Which one is most relevant? Or are both irrelevant in reality? Peter Kämpf’s answer makes no mention of either alternative, only arguing about the AOA and stagnation point.

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  • $\begingroup$ Not sure if it's a dupe, but aviation.stackexchange.com/questions/24414/… provides a pretty good explanation. $\endgroup$ – Jihyun May 26 at 21:31
  • $\begingroup$ The first answer mentions my first point above and the second answer mentions the second point, but writes in text that the skin friction drag is lower while supporting it with an image that says the form drag is lower. The former is supported by the erroneously wetted area argument and the latter is stated as fact without reasoning (I’ve yet to read Hoerner). Why is the form drag and skin friction drag of a blunter (maybe even hemispherical) nose smaller than for a sharper nose (even at optimal AOA) and why do gliders ignore it? $\endgroup$ – Aerocurios May 27 at 1:48
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    $\begingroup$ Not a duplicate. This question states awareness of the difference between subsonic and supersonic. It is a well stated question and does not deserve to be downvoted. $\endgroup$ – Koyovis May 27 at 2:51
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    $\begingroup$ I didn't downvote, to be clear. I'm just saying that the answer I linked provides a very good explanation that partially clears up some conceptual questions. Any extra answers on this page should at least add to what @PeterKampf explains in his answer. $\endgroup$ – Jihyun May 27 at 16:26
  • $\begingroup$ @Aerocurious great job getting that Figure 20! Look at the first one! Here we see the advantages of a smooth curve over sharp edges seen in #3 and #4. All advantages of the point are lost in #4 due to the sharp break in the curve. $\endgroup$ – Robert DiGiovanni May 27 at 23:31
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Nose shape on fuselages

Fuselage noses of airliners hold the antenna of the weather radar, and a blunter nose will put the least amount of material in the way of the electromagnetic waves. On a three-dimensional body the shape of the nose is of less importance and sometimes used to express a brand image. In subsonic flow, sharp changes in the contour must be avoided: Here, air would have to change direction abruptly which requires strong pressure gradients. Therefore, curvature along any flow path should change gradually. This favors an elliptic body shape. If you want to have no jump even in the second derivative, use a lemniscatic function.

A hemispherical nose would have constant curvature followed by no curvature along the cylindrical portion of the fuselage. The sudden change in curvature at the transition between the rounded nose and the cylinder would require a sudden jump in pressure which causes more drag than the gradual reduction of curvature (causing a gradual increase of pressure) of an elliptic nose.

Edit 2: basically, why does No 2 in the following have less drag than No 5?

Because the sharp corner at the base of the cone in No. 5 will cause a suction peak which in turn will cause a jump in boundary layer thickness. The hemispherical nose of No. 2 is better, but best is the elliptical nose in No. 1.

A blunter ellipse will have lower overall surface but a steeper change in pressure - here, as so often, the optimum is a compromise which needs to include the Mach and Reynolds numbers as well as structural considerations. Since the optimum is flat, there is considerable space for individual solutions.

What if No 5 had a smoother “ridge” (cone to cylinder transition) but was still pointy? Say, if No 1 ended in a point?

The pointy tip would not "fix" the stagnation point to that point - rather, it would still move with the angle of attack, but the part of the flow negotiating this tip would exhibit a strong suction peak and "smooth out" the contour by means of a local separation. This would place more strain on the boundary layer and increase its thickness downstream. Any hope for laminar flow over this nose past such a tip would be destroyed by this, of course.

The smoother ridge would be better, so the elliptical nose with a pointy tip would improve things like No. 4 does over No. 5. On No. 4 the sharp ridge will most likely produce a separation on the ridge which will make the whole body look larger to the oncoming flow. The bisection of the drag coefficient in No. 4 indicates to me that the separation is much smaller. Note that this is all only valid in subsonic flow! In supersonic flow No. 4 would look best.

Nose shape on wings

On wings, however, the nose shape is extremely important - witness the jumps im performance that sometimes were achieved by optimizing the nose shape. To get to the bottom of the wing nose shape, we need to talk about boundary layers, Mach effects and much more, so be prepared for a long answer.

Glider airfoils can afford to use small nose radii because they fly at low Mach numbers. Airliners, on the other hand, need to keep local Mach numbers low which favors a larger nose radius. If you are happy with this answer, better stop reading now.

At the Symposium Transsonicum of the International Union of Theoretical and Applied Mechanics (IUTAM) in 1964, E.V. Laitone presented something like a magic number for transsonic flow: $$Ma = \sqrt{\frac{1}{\gamma - 1}} = 1.581$$ with $\gamma$ the ratio of specific heats of a fluid. Once the local suction peak at the nose of an airfoil reaches such a speed, lift stops growing any further. With stall speeds of around 120 knots, the early jet airliners routinely hit this limit using the 6-series NACA airfoils of that time.

Below you see the pressure distribution on a flapped airfoil near maximum lift (picture source). The specifics of the airfoil don't matter much; important is the suction peak at the nose which is made possible by a large angle of attack and a flap with a ventilated gap between wing and flap.

Pressure distribution on a flapped airfoil near maximum lift

In such a case, a small nose radius creates a very strong but narrow suction peak because a very strong pressure gradient is needed to force the flow around the tight nose contour. If the nose is blunter, the suction peak can spread out lengthwise and becomes less peaky, simply because the trajectory change of the flow around the nose is becoming more gradual. Since there is a direct relationship between local suction and local Mach number, those suction peaks must be spread out and flatter in order to allow higher angles of attack. That is the main reason why supercritical airfoils have blunter noses - they will tolerate higher lift coefficients made possible with powerful flaps.

Initial glider airfoils used rather blunt noses and high camber. With composite technology the smoothness of wings could be improved and early numerical codes helped to shape the pressure distribution such that a fairly wide angle of attack range can be covered without incurring sharp suction peaks on either surface. The Eppler code of Richard Eppler was the first such tool and by prescribing pressure levels over sections of the airfoil at specified angles of attack it made laminar airfoils easy to design on the low-power computers of that time. The result are contours with a small nose radius which would produce a sharp suction peak once the specified angle of attack range was exceeded. By suppressing the suction peak over that specified angle of attack range, the laminar bucket could be maximized in a way that a blunter nose would not allow.

Now for that promised excursion into boundary layer theory: A laminar boundary layer is stabilized by a positive speed gradient (accelerating flow) but a negative speed gradient will trip the transition to turbulent flow rather quickly, especially at higher Reynolds numbers. The back side of a suction peak has just such a negative gradient; therefore suction peaks must be avoided in order to keep the flow laminar over much of the wing chord of gliders. In addition, using camber flaps allows to shift the laminar bucket up and down on the lift coefficient scale, so the combination of a small nose radius and a camber flap will allow laminar flow over a large lift coefficient range.

Airliners cannot do the same: The Reynolds number on their wings is so large that turbulent transition happens near the nose regardless of the local pressure distribution. Therefore, they are not concerned with laminar buckets and will happily tolerate the suction peaks that come with higher angles of attack. In cruise, when managing the pressure distribution is essential to reduce shocks, the angle of attack range is extremely narrow, so the blunt nose produces no disadvantage. However, this transsonic pressure distribution demands very little camber in the forward part of the airfoil, so in order to tolerate high angles of attack during approach and landing, they need a blunt nose. The larger nose radius also helps to integrate leading edge devices like slats and Krüger flaps, which is another advantage at low speed.

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    $\begingroup$ I’ve been praying for an answer from you :o) Written many good answers! I’ll dig into this... $\endgroup$ – Aerocurios May 27 at 18:45
  • $\begingroup$ Hold on, my question is about the nose of the fuselage and the answer appears to be about the nose of the airfoil, unless the answer is the same then? Basically, internet answers seem to suggest that blunter noses are aerodynamically superior to sharper noses for, say, an airliner based on 1) acceptance of greater range of AOAs, 2) smaller friction drag, 3) smaller pressure drag. I don’t understand why smoothing a shape decreases the friction and pressure drag, if that’s valid. And whether/why a hemispherical cap is aerodynamically ideal. $\endgroup$ – Aerocurios May 27 at 19:31
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    $\begingroup$ Well, it’s important enough for no one to fly with cylindrical noses. If it really wasn’t important I would expect some airplane to have a trunk and tusks to amuse the passengers. Instead Airbus and Boeing airliners recently appear to have converged on a common almost exact contour. According to my interpretation, Boeing planes used to have a distinctly more two-stage nose (on top of it) and based on what I’ve read this was related to the cost of window curvature at more than one angle necessary to make them flusher, check 707 with 777 etc. However, maybe it was for other construction reasons. $\endgroup$ – Aerocurios May 27 at 22:28
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    $\begingroup$ Typo: "The hemispherical nose of No. 2 is better, but best is the elliptical nose in No. 2." #2 isn't both hemispherical and elliptical, is it? ;) $\endgroup$ – FreeMan May 28 at 12:20
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    $\begingroup$ @PeterKämpf - you type more, better, in a hurry answers than I could ever come up with! A simple typo or two is more than excusable! $\endgroup$ – FreeMan May 28 at 15:56
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Blunt noses are best at subsonic speeds because they provide the best shape for the air to get out of the way. From this site:

The speed of "sound" is actually the speed of transmission of a small disturbance through a medium.

From an old uni book

The picture shows a single point traveling at a constant speed V, emitting small disturbances :). The air in front of the travelling point is forewarned, and is pushed out of the way isentropically, without losses. It is this part of the subsonic airflow that is key to the optimal shape: a parabolical one. The point pushes air out of the way spherically, while travelling at a constant speed.

If our travelling disturbance is not an infinitesimally small point but an actual 3D body, its optimal shape is the same: parabolic, then rounded off at where the cylindrical body starts creating an elliptical shape. With this shape, the air in front of the body can get out of the way in the most orderly fashion.

enter image description here

The streamlines in the picture above are at equal distances. The air moves out of the way, creating a lower static pressure which sucks the nose into the airstream. With a hemispherical shape, the streamlines are closer together at some places, creating a pressure increase that negates the initial lower pressure.

Notice that in your figure 20 the first shape is elliptical and actually has a negative $C_D$ for the nose only: it sucks itself into the airflow. None of the other shapes do, not even the hemispherical shape 2. With subsonic incompressible flow, it's what happens in front of the nose that creates lower drag, not at or beyond the nose.

That's all valid when the shape travels at a certain speed, at zero Angle of Attack.

  • At any other speed, the optimal parabolic shape is different, still a parabola though for an infinitely wide body, or an ellipse for a body with finite dimensions.
  • At any other AoA, it is very difficult to create a 3D elliptical body shape, and it would be different for any AoA. But a spherical one comes close, as your Figure 20 shows - the first bit of a parabola is close to a sphere anyway. The larger the sphere radius, the closer it is to the optimum. And a sphere is a sphere at any angle.

Edit How can the shape suck itself into the air in front of it?

The pressure disturbances warn the air to get out of the way. Bernouilli is valid for low subsonic flow:

$$p_t = p_s + \frac{1}{2} \cdot \rho \cdot V^2$$

or: total pressure = static pressure plus dynamic pressure. Far in front of the moving body, $p_t$ is the total pressure is environment static pressure. As soon as the air starts to move, local dynamic pressure increases and local static pressure decreases.

Edit2

Yeah, elliptical for finite body dimensions.

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  • $\begingroup$ Oh, that’s an interesting derivation of the shape! But I don’t really understand it, is there a source with more details because where in the first image do you get a parabola? And why does the parabola suck itself into the airflow (google search only gave computer-related answers)? Once I understand that, the AOA/pressure drag part of my question is probably answered to my satisfaction. $\endgroup$ – Aerocurios May 28 at 14:45
  • $\begingroup$ The parabola is at the connection of the circle outlines. The 3D shape with a nose shape that matches that of what happens with a point disturbance moves the air out of the way isentropically (without losses). $\endgroup$ – Koyovis May 29 at 2:03
  • $\begingroup$ Sorry, my reference books are old uni books in a foreign language. $\endgroup$ – Koyovis May 29 at 2:36
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From information provided by the learned writers we can gather bluntness of an aircraft nose design may fall into 2 considerations. First, avoidance of wave drag, and air speeds above a number where it increases significantly, favor a blunter nose subsonicly. At lower speeds, the sharper nose, as seen in gliders, appears more favorable.

These optimal nose shapes can be described as elliptical and parabolic.

Elliptical Volume = pi x diameter squared x height/6, Parabolic Volume = pi x diameter squared x height/8

At very low Mach numbers the air is simply parted with insignificant compression ahead of the aircraft. Here the sharply pointed elliptical cone works best.

At higher subsonic Mach numbers (less than 1) air begins to compress in front of the aircraft. Here the parabolic cone is better. Removing the sharp point keeps the surface of the nose out of the more draggy compressed air "wave" and in a lower pressure "suction zone" just behind it. The aircraft still has to push the air out of the way, but avoids some drag with a blunter nose. This is the high subsonic design of air liners, and also of "bulbous bow" ships.

However, in these designs, as in bullets, a constant curve with no "sharp" edges, as seen in figure 20 above, provides significant drag reduction benefits.

"Bluntness" is simply management of higher speed effects and/or larger changes in angle of attack.

Now that we have split the "fluid" aside with our "bow", how do we manage its return (drag recovery)? Again, the answer may also lie in the constant curve. This will be left to another discussion 😊.

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