# 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). – Jan Hudec Mar 29 '17 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 Mar 29 '17 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? – nyorkr23 Mar 30 '17 at 2:01

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.

Short answer: It's the dynamic pressure which is at work in a sea anchor:

Let's see how we can separate effects of static and dynamic pressures.

Use the water similarity:

• Blow up a balloon and submerge it in water.
• As you go deeper, the balloon volume shrinks as the static (ambient) pressure increases.

• Move the balloon at some speed through the water, its shape changes because the dynamic pressure created by the displacement is not equally distributed over its surface.

Dynamic pressure is comparable to static pressure unevenly distributed. It's dynamic pressure that slows down a parachute. If there was no static pressure, the effect would not change.

To observe the effect of dynamic pressure alone, you can replace air by water in the parachute, this is what is at work in a sea anchor.

(You can also demonstrate this in air, but this would requires to inject air under a parachute in vacuum, not simple...)

To be complete: In this experience the balloon also receives a buoyancy force because air and water densities are not equal. A the weight is added to stabilize the balloon. When the balloon contains water, this buoyancy force disappears and the weight is not required any longer.

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...