Can you explain how to calculate lift, drag, total force and how to read this pressure distribution diagram?
Not with the given information. All what can be calculated is the lift per unit of span. Pressure is force per area, so in order to arrive at a force, it must be multiplied by an area. We only have chord, no span, so there is no area given. All I can do, therefore, is to calculate the lift force per unit of span. Drag cannot be calculated at all.
Let's take 1 m as this unit: This will give us 1 m² as the wing area. Since the pressure over chord is either constant or linear, no fancy integrals are needed. The plot already distinguishes four sections, each with their own gradient of pressure:
- Upper forward: Constant pressure of -981 Pa which equals a suction force of 981 N per m². Per meter of span this is 490.5 N of upward force of the half square meter here.
- Upper rear: The pressure grows from -981 Pa to 0 at the trailing edge. Therefore, lift per meter of span is half of what it is in the first section, namely 245.25 N of upward force.
- Lower forward: Again a linear increase over chord, now from 0 to 490.5 Pa pressure at mid chord. Per meter of span this is 122.625 N of upward force.
- Lower rear: Same thing, only in reverse: Pressure drops from 490.5 Pa pressure at mid chord to 0 at the trailing edge. Again, we have 122.625 N of upward force per meter of span.
If we sum up all four sections, the result is 981 N of lift per meter of span. Suction on the upper side contributes ¾ of this force and pressure on the lower side adds the fourth quarter. Funny how this coincides with the usual approximation of gravitational acceleration! Each meter of span lifts 100 kg of mass in Earth's gravity field.
Why F3 is calculated from 0 to C/2 if the pressure distribution is drawn at the lower surface from X to C/2?
X is a variable which starts at 0 at the root of the X axis. So F3 runs from X=0 to X=c/2. F3 is the force produced by the forward pressure ramp, nothing more.
What is the pressure distribution at bottom surface from 0 to X?
X is variable. At X=0 the bottom pressure is 0. It grows to 490.4 N/m² at X=c/2 and drops again to zero at the trailing edge when X=c.
If I know the pressure distribution around the airfoil, are the shape of the airfoil and the airfoil AoA (inclination on diagram) not relevant when I calculate the forces?
Yes, the shape and AoA are not relevant (or is this: No, the shape and AoA are not relevant?) German and English answer negated questions in opposite ways when meaning the same. Try to avoid negated questions when you want a clear answer.
Why is the pressure not drawn perpendicularly to the airfoil surface?
This is to simplify the calculation. You can either integrate the cosine of the inclined pressure over the actual length of the airfoil contour, or you can integrate the absolute pressure over the projection of the contour on the X axis. Both will give the same result when you want to find the lift force.