Does the lift force comes from the bottom or from the top of the wing?

Question

My team and I discussed yesterday how lift is generated based on the pressure analysis. The most popular explanation from books and other sources is that pressure is lower on the top and slightly greater on the bottom and "pressure difference" makes the force. My argument is that because the pressure on the top is lower than the atmospheric pressure the pressure on the bottom makes a pushing force upwards. Others from my team say that because the pressure on the top is lower there is a great pulling on the top and a small pushing force from the bottom. The thing is that our simulation program can generate this kind of plot:

And its not only our simulation software, this type of image can easily be found all around the internet representing lift. I say this lines are just a measure of pressure, the orientation shows if its lower or greater than the atmospheric pressure and the direction is normal to the surface just because it is convenient. Some of my teammates say that these vectors are actually force vectors that simulate the lift force direction if we sum all of them. What do you think?

Answer 1

Pressure always acts normal to a surface. It isn't just convienent.

Away from a surface, we can talk about the pressure of a fluid element (say a tiny cube) -- and pressure would act normal to the six sides of the cube.

The other kind of aerodynamic force is shear. Shear acts tangent to a surface. For the fluid element, it acts parallel (tangent) to the six faces of the cube.

The total aerodynamic force is the integral of the pressure and shear forces on a body.

Usually, the shear forces are insignificant in lift -- so the integral of pressures would be the dominant component of the lift force.

In particular, if you are doing an inviscid analysis, the shear forces will be exactly zero and the integral of the pressure is the forces.

Answer 2

In short: Both the pressure on the top side and on the bottom side push against the airfoil. The airfoil is overall pushed up, because the force on the bottom is larger than the force on the top.

Lift results from the pressure difference. Still we could say the top side (usually) participates more to the creation of this difference than the bottom side, in the sense the top side decreases the pressure below the ambient pressure more than the bottom side increases the pressure over the ambient pressure.

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There is no "physical" lift force, lift and drag are engineering concepts. Lift is the vertical portion of the sum of all tiny pressures acting on the airfoil surface.

Conventional representation of the pressure variation created by airflow on an airfoil with positive lift:

Source.

Be aware of two confusing aspects in the picture:

• There is no volumetric notion here , the lines indicate the pressure magnitude and direction directly at the airfoil surface. Pressure at any distance from the surface is unknown.

• Arrows seem to indicate a suction effect on the top side, and a pushing effect on the bottom side. Remember pressure only pushes against the surface, it doesn't pull. Arrow indicate the sign of the pressure differential with atmospheric pressure, not the direction of the force. Arrows are exactly redundant with color here.

Red lines depict how much the pressure is in excess of the atmospheric pressure, blue lines depict how much the pressure is below atmospheric pressure. Pressure acts perpendicularly to the airfoil surface, hence the direction of the lines.

• The bottom side sees a local pressure gradient which values are greater than atmospheric pressure, hence overall this side tends to push the wing up, more than atmospheric pressure alone.

• The up side (unfortunately called the suction side in English) sees a local pressure gradient which values are lower than atmospheric pressure, hence overall this side tends to push the wing down, but less than atmospheric pressure alone.

Because the top pressure gradient is overall smaller than the bottom pressure gradient, the latter wins, the wing moves up. Whatever, all these pressures with disparate magnitudes and orientation don't combine in a perfect vertical force, the force is actually oblique. (And because the pressure differential at the leading edge is not the same than at the trailing edge, some positive or negative pitching moment is created too.)

Pressure force and aerodynamic force

To simplify, engineers mathematically integrate tiny pressures into a single symbolic larger force, the total aerodynamic force. For the airfoil above, the total force has an oblique orientation, up and back, hence the airfoil is pushed up and back.

Source.

Breakdown of the aerodynamic force into lift and drag

Engineers like to split this total force into a sum of two other symbolic forces of particular directions, drag and lift, to ease common computations. Note this decomposition is purely mathematical and has no actual physical interpretation.

The handy directions are the vertical and the direction of the flow. Lift, the vertical component, can be compared to weight to determine whether the aircraft will gain or lose altitude. Drag, the flow direction, can be compared with the propulsive force, to determine the velocity of the aircraft.

Varying the total aerodynamic force

The shape of the airfoil is the mean to change the magnitude and the orientation of these tiny forces, so every airfoil has a different distribution of the tiny pressures, and the distribution can change with the orientation of the airfoil:

Source.

Ultimately lift and drag conventional forces can be adjusted. With a negative angle of attack (top case), the aerodynamic force can be oriented down, lift has a negative magnitude.

From pressure to speed

Note how the pressure on the top side is mostly smaller than atmospheric pressure (- sign) and larger on the front part of the bottom side. Intuitively if you oppose a fluid (air or water) flow with your hand, or if your hand moves in a stationary fluid, you sense a force, fluid pressure has increased. The reason is because the flow has been slowed down. Conversely, if the flow is accelerated by some mean, pressure decreases (hand is not a good sensor for this negative pressure change).

Thus the sign of the pressure on the top side (also) indicates where the flow accelerates and where it slows down. Namely it always accelerates after some distance from the leading edge, even when the angle of attack is negative. This acceleration, combined with the deceleration on the bottom side results in a up force. Airfoil designers look for the "best" combination of changes to the flow speed at different angles of attack, e.g. one resulting in a near vertical sum.

Which side creates more pressure?

The total lift is the integral of all pressures, hence it corresponds to the difference between the sum of vertical components of all red and blue arrows of the first picture. Without performing the actual integration, it appears on the examples provided, the largest part of the lift comes from the top side, specially at high angle of attack. The stall angle is the limit, past the stall angle air acceleration starts to be problematic due to the apparition of vortexes. This is the direct answer to your question.

Answer 3

The image doesn't convey the entire picture.

The main force is Newtonian action/reaction. A wing drives air down, more or less "planing" on it (hence, airplane), like a boat on the step. A secondary force is the wing being more or less sucked upward by the low pressure zone on top.

But mainly, the wing is inducing a large package of air, mostly above and some below, to move downward as it passes. The result is downwash, the final result of the Newtonian action/reaction. You can make a wing from a flat plate, but the lift it can generate will be limited mostly to what air can be directly deflected mechanically by the plate's angle to the flow (the boat planing on water effect). Lots of light planes with steel tube structures have tails that are basically flat plates, and get by on air deflection alone for stabilizing and maneuvering forces, because the lift coefficients don't need to be very high.

On a normal airfoil, the curved surfaces and resulting pressure distribution that ends up with suction on top, in addition to the wing to wanting to move into the low pressure, allows a much larger package of air, well above the wing, to be induced to move downward than by direct mechanical deflection, and does this across a range of angles to the flow. The affected area above the wing, that is, the zone containing molecules that ended up lower than they were before the wing passed and ended up as part of the wing's downwash, may extend several chord lengths above.

Answer 4

We have a simple fact that can resolve the question in one go -

A gas (e. g. air) is physically unable to "pull" something.

Solids are capable of pulling (i.e. experience negative pressures). Liquids can do some pulling based on their surface tension. But a gas always exerts a positive pressure, it "pushes".

Whith this in mind, we can only conclude that the wing is pushed from below.