How can dynamic and static pressure be explained?

How exactly do you explain dynamic pressure? If someone asked me what the ASI reads, I would say it reads the dynamic pressure, which is the

ram air pressure (total pressure) - static pressure = dynamic pressure

But would explaining dynamic pressure as "the pressure the pitot tube experiences through the air?

As for static pressure, how would you explain that? Would you say static pressure is the pressure the aircraft feels whether it is in motion or not?

• Actually, the ASI reads square root of dynamic pressure (but that's just calibration of the scale, of course). Commented Mar 29, 2017 at 17:46

You are very close. Your explanation of static pressure is correct, but not your dynamic pressure. Ram air pressure is what the pitot tube measures, in other words the total pressure experienced. It is your airspeed gauge which measures dynamic pressure by mechanically (in the case of a traditional pitot-static system) subtracting static pressure from ram air pressure.

If I was explaining this to a layman I would say that ram air pressure is equivalent to sticking your hand out the window of a moving car, while static pressure is the pressure inside the car. That's a bit of an over-simplification as the pressure inside the car will be lower than outside due to the movement through the air, but it gets the point across.

• Oops! Thanks @JanHudec.
– GdD
Commented Mar 29, 2017 at 21:41
• I get that total pressure minus static pressure gets you dynamic pressure, but how exactly would you explain dynamic pressure. Could you say that dynamic pressure is the pressure the aircraft feels through the air without experiencing the pressure acting on the aircraft? Commented Mar 30, 2017 at 2:01
• Actually, in one sense, the Dynamic pressure is not felt by the aircraft at all, as, at each point on the surface of the airframe, the Dynamic pressure is the component which lies parallel to the surface, (it is not pushing on the surface at all). The static pressure is the component that is normal or perpendicular to the surface at each point, and is felt by the aircraft. Of course, the magnitude of the dynamic pressure at each point, (a measure of the indicated airspeed), indirectly tells us how much the static pressure is pushing on the airframe at that point, so .... Commented Jul 13, 2023 at 16:01

Use an energy analogy:

• Dynamic pressure equals kinetic energy,
• Static pressure equals potential energy.

Total energy = total pressure.

For the more math inclined:

• In the gravity field of earth, potential energy is mass times gravity acceleration times height: $$E_{pot} = m\cdot g\cdot h$$.
• Kinetic energy is mass times speed squared, divided by 2: $$E_{kin} = m\cdot\frac{v^2}{2}$$.
• Dynamic pressure $$q$$ is similarly density times speed squared, divided by 2: $$q = \rho\cdot\frac{v^2}{2}$$, which makes it a volume-specific kinetic energy.

Static pressure is the weight force of the mass of a column of the atmosphere all the way to space resting on the base area of this column. In other words, the weight of this column of the atmosphere compresses the air lower down so this pressure can support all the air resting on top of it.

If you remember that the air is really a swarm of molecules moving in all directions at very high speed, and that pressure -static or dynamic- is caused by the impact of those molecules, then everything is very clear...

It's simple. Pressure is a force, due to the change in Momentum when a particle bounces off a surface. Because it is a force, it is a vector, (it has a magnitude and a direction), like velocity or acceleration, not a scalar, like temperature or volume, or electric charge.

Secondly, the total pressure must remain constant (principle of conservation of momentum - Newton's third law). So when velocity increases, since the apparent direction of this pressure vector shifts in the direction of the motion, (it appears to align more with the flow motion), the component that is parallel with the motion (dynamic pressure) increases, and the component that is normal or perpendicular to the flow decreases. The total pressure (magnitude) hasn't changed. Only the components have changed due to a perceptual change in the vector's direction relative to the motion of the observation.

Once you understand this, many things make more sense. A common instrument for measuring Angle of attack these days is a pitot tube with two orifices. One points straight ahead and measures the pressure of the flow vector from that direction. The other is oriented 45 degrees downwards from the forward-oriented orifice and measures the pressure of the airflow vector at that angle. Comparing the two (with appropriate software), allows a computer to calculate the Angle of Attack.