# Why is the pressure lowest towards the front of an airfoil? [duplicate]

According to this answer, the low pressure on top of an airfoil is caused by the curving away of the wing (what I interpreted it as). If you look at the lift distribution over an airfoil, you'll see it looks something like the picture below.

My question is, why is the pressure lowest towards the front of the airfoil, when the wing only starts to curve away from the air at the back section? If you look at the waterfall analogy in the link I provided, you'll see what I mean about the "curving away" part.

Thanks! (Also, if there's any answer that answers this question that I didn't see, that's my bad, I seem to have a habit of doing that haha)

According to Bernoulli, the pressure drops where the flow speeds up. The flow hits its highest velocity as it rounds the tight corner around the leading edge -- don't forget to include angle of attack when considering turning.

• oh okay, thanks. Do you know if there is an answer explaining why the pressure dops when it speeds up? Commented Jan 5 at 19:55
• Answer: conservation of energy. en.wikipedia.org/wiki/Bernoulli's_principle, and especially en.wikipedia.org/wiki/Bernoulli's_principle#Misconceptions Commented Jan 5 at 20:05
• @CamilleGoudeseune oh okay, got it. So basically the air has to have a low pressure zone ahead of it to accelerate, and that will kind of 'stretch' the air out, making it low pressure? Commented Jan 5 at 20:27
• Remember, in every diagram, or explanation, or discussion of this, they are generally not talking about the absolute pressure, or aerodynamic force, they are talking about the pressure relative to the free-stream pressure. I. E., there is really not any suction going on on the top of the wing. The air molecules there are still pushing down on the upper surface. They are just pushing down less than the ones on the bottom are pushing up. Commented Jan 6 at 14:36
• The diagram would be less mis-leading and more accurate if all the arrows pointed at the wing, but were longer, (representing a stronger force), on the bottom. That the artist already uses arrows of different lengths, implying that the length implies the magnitude of the pressure or force, only adds to the mis-representation. Commented Jan 6 at 14:38

The pressure differential between upper and lower wing surfaces provides lift, and at both the leading and trailing edges of the wing it is zero. Your question is really why does the change in this differential occur more steeply at the front of a wing than at the rear.

The answer is that the structure of front of the wing can abruptly push air out of its path without flow separation while the rear has to use a more gradual curve to keep flow attached to the trailing edge, at least for the upper surface. The thickest section of the wing is usually around the quarter chord and ahead of this a proverse pressure gradient keeps flow attached. Beyond this point an adverse pressure gradient and friction demand more gentle treatment of the flow.

From MIT:

Fluid particles flowing along the top of the wing surface experience a change in pressure, moving from the ambient pressure in front of the wing, to a lower pressure over the surface of the wing, then back up to the ambient pressure behind the wing. The region where fluid must flow from low to high pressure (adverse pressure gradient) is responsible for flow separation. If the pressure gradient is too high, the pressure forces overcome the fluid's inertial forces, and the flow departs from the wing contour. Since the pressure gradient increases with an increasing angle of attack, the angle of attack should not exceed the maximum value to keep the flow following the contour.

The lower surface generally has a proverse pressure gradient on the rear of the wing all the way to the trailing edge and is not critical.