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Why is an airplane's wing (the leading edge) not made as sharp as possible to break the air, in the same way that a ship's hull is made sharp to easily break the water?

This is unlike Concorde's nose and most jet fighters' noses, which are made very sharp.

Airbus A380's wing, obtuse angle Concorde, sharp nose F18 Hornet, sharp leading edge

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A blunter (less sharp) leading edge allows the wing to operate effectively for a wider range of angles of attack (AoA). The angle of attack is the angle between the approaching airflow and the chord line (as illustrated below)

Angle of Attack of an Airfoil

As you can see, the air has to curve around the leading edge on the upper side of the airfoil. This curvature accelerates the flow and thus creates an area of reduced pressure and thus contributes to the lift of the airfoil.

A sharper leading edge will cause a more intense curvature of the flow and therefore reduce the local pressure even further. A higher angle of attack will also cause a more intense curvature of the flow with the same effects.

Here's the problem: In order to reach pressure equality between the upper and lower side at the trailing edge of the wing, the pressure will have to increase again as you move further towards the trailing edge. The boundary layer of the flow doesn't like this at all. Strong pressure gradients will cause the boundary layer detach (generally, the boundary layer is already turbulent for commercial aircraft). A detached boundary layer (stall) causes a drastic increase in drag and decrease in lift and is therefore highly undesirable.

To increase the range of angles of attack at wich the boundary layer stays attached and to allow for a more gradual transition between normal flow and stall, the designers favour a quite blunt leading edge for most commercial and general aviation aircraft. This will increase the drag at zero AoA, but won't force the air to follow a high curvature at positive AoAs.

Supersonic aircraft such as the Concorde and fighter aircraft have to deal with a new type of drag called wave drag. Generally wave drag can be reduced by having sharp leading edges. Most boats also have to fight against some kind of wave drag (although the fluid dynamics are very different) and therefore also have sharp hulls.

This is a broad generalisation and there are a lot more things to consider in reality. If you want I can go into more detail.

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  • $\begingroup$ There is one formula binding between force (F), pressure (P), and surface area (A). $$P=\frac{F}{A}$$ or $$F=PA$$ Here, F is drag occurred at the leading edge as the airplane hit the air, the pressure P is pressure occurred at the leading edge as the product of the force over the wing surface. A is the frontal surface area. Is this formula has relation to your explanation? $\endgroup$ – AirCraft Lover Dec 27 '18 at 23:05
  • $\begingroup$ The total resultant force $F$ on the wing can be calculated by integrating the local pressure $p$ over the total wing area $S$ (upper and lower side): $$F = \iint\limits_S p \:dA $$ This is like applying your formula $F=pA$ to very small sections and then adding them all up. You need to do it this way because the pressure is different at every location. Now the component parallel to the airflow is what we call drag and the orthogonal component is lift. So essentially drag is the sum of the air "pushing" against the frontal area on the leading side and the air "pulling" on the trailing side. $\endgroup$ – Felix L. Dec 28 '18 at 10:36
  • $\begingroup$ If the plane is flying at an angle of attack, the frontal area of the wing is a lot more than just the leading edge. Look at the image in my answer: Most of the frontal area comes from the underside of the airfoil. Also, the air isn't really pulling on the trailing side of the wing, it is only pushing with less force than the leading side. $\endgroup$ – Felix L. Dec 28 '18 at 10:39
  • $\begingroup$ In reality friction is also a force acting upon the wing. The real resultant force is the sum of the pressure forces and frictional forces. $\endgroup$ – Felix L. Dec 28 '18 at 10:45
  • $\begingroup$ Thank you Felix for your nice explanation. $\endgroup$ – AirCraft Lover Dec 29 '18 at 2:07
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As long as the aircraft flies subsonically, a sharp leading edge doesn't really have much advantage. Far more important for reducing drag is to keep the flow as laminar as possible.

Simplifying the physics a lot, you can imagine that before a round leading edge, an additional “wedge” of only slightly higher-pressure air is built up, which can divert the flow around the wing almost as efficiently as a sharp edge would. This works only in the subsonic / transsonic region, because the pressure-forwarding travels as a sound wave, i.e. against a subsonic stream the wedge can be built up against the stream direction, but against a supersonic stream this is not possible. That's why supersonic planes have sharp leading edges, but this leads to its own engineering challenges. Sharp edges in general are less structurally robust and they can easily disrupt the laminar flow.

Actually, this high-pressure region is not wedge-shaped at all, rather it's a smoothly decaying “baffle”. The crucial point is that it makes a pressure gradient that's pointing up for the air approaching above the center line, and down for the air approaching below it. This accelerates the air out of the way of the wing, so not much of it actually hits the surface. You can look at the pressure in some CFD simulations. https://www.google.com/search?q=airfoil+cfd+pressure

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Rounder leading edges help the air follow the wing under high angles of attack, they are cheaper and easier to manufacture, and thicker wings are structurally stronger, reducing the weight used for structural rigidity by a big margin.

Like almost everything in aviation/engineering, it's all about trade-offs. Clearly, the advantages mentioned above are more important than the sleekness of sharp leading edges.

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The shape of the wing helps determine the lift it creates, and the reaction of the wing when the plane is pitched up enough to stop producing lift (or stalls). Everything is a tradeoff. Small enough curve to make sufficient lift and be aerodynamic, but not quick to stall when flying slower for landing. Sharp noses help with flying faster than the speed of sound. The Concorde and the jet fighter are designed for fast flight, the airliner is not. The more rounded nose also allows for weather radar to be installed. In even larger planes, the nose can be tilted up for cargo loading as well.

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  • $\begingroup$ There is one formula: $$P=\frac{F}{A}$$ or $$F=PA$$ Is any from your explanation related to this equation? F is the drag due to frontal area of the wing, P is pressure that occured at the frontal area, and A is the frontal area. $\endgroup$ – AirCraft Lover Dec 27 '18 at 14:14
  • $\begingroup$ Don't know. I'm a pilot, but not an aeronautical engineer. My particular plane model started like with a fast, thin leading edge, that was later changed to a more rounded edge for better low speed handling - the sharp edge would stall too quick, general aviation pilots were having a hard time transitioning into it after flying the more traditional rounded edge wing planes. Good article here on it avweb.com/news/features/… "The 6400 series airfoil was changed to a more conventional 2400-series" the 2400 being more rounded. $\endgroup$ – CrossRoads Dec 27 '18 at 14:36
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    $\begingroup$ The NACA 65-A015 on the '68 had a region of very low induced drag at certain low AOAs (the drag bucket). The Cardinal couldn't go fast enough to get the AOA low enough to exploit that, so the airplane's performance was no better than a 172. Plus the crappy stall behavior and the need to pay close attention to speed. They just grafted the forward part of the 172's NACA 2415 onto the '70 Cardinal's wing (which started as a scaled down Centurion wing on the '68). I put a Horton kit (LE cuff) on my '68 and it totally tamed the low speed issues with no speed penalty. $\endgroup$ – John K Dec 27 '18 at 22:26

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