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2 Answers

Pitot Static Pressure

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Aerodynamics, Aircraft Systems, Flight Instructor

Why is ram air pressure higher than static if the ram air pressure is moving faster? Based on Bernoulli's principle one would think that the pitot tube would act somewhat like a venturi and that fast moving air would have a lower pressure compared to static presure.

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2 Answers

  1. Best Answer


    Brian on Oct 03, 2010

    Matthew,

    You’re confusion appears to be regarding the types of pressure. For a quick background on Bernoulli, just to ensure we are all on the same page:

    Bernoulli did not say that all pressure goes down when airspeed goes up. In fact, few pilot books (I haven’t found one yet) will ever explain to you what Bernoulli’s discovery was. Bernoulli taught us that pressure follows the laws of conservation of energy. What does that mean?!

    Most of us have heard that energy cannot be created nor destroyed, but only transferred. We are taught that kinetic energy (KE) is transferred to potential energy (PE) and vice versa to keep total energy (TE) constant. Often this is done using the example of a roller coaster going up and down hills transferring its energy from KE to PE and back without changing TE.

    Similarly, dynamic pressure and static pressure will be transferred to conserve total pressure. Dynamic pressure is equivalent to our kinetic energy. In other words, it is the measure of pressure in motion and sometimes called velocity pressure. Static pressure is like our potential energy, it measures the pressure at a given point.

    If you haven’t guessed it yet, dynamic pressure is the pressure measured by the Pitot tube, not static pressure. Dynamic pressure is a function of one half the air density times velocity squared. Therefore, more airspeed equals more dynamic pressure. More dynamic pressure, per Bernoulli’s discoveries, equates to less static pressure so as to satisfy the law of conservation of energy.

    In conclusion, the static pressure did go down when airspeed went up. However, the pitot tube doesn’t measure that, it measures dynamic pressure. Since dynamic pressure is directly proportional fluid velocity then increased fluid velocity results in increased dynamic pressure.

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  2. Steve Pomroy on Jan 04, 2011

    Hi Matt.
     
    Brian is right about Bernoulli’s equation being a statement of the conservation of energy in a flowing (frictionless, incompressible) fluid.  And indeed, dynamic and static pressure can be converted back and forth just as kinetic and potential energy can be.  Increase the speed of a flow (without adding or subtracting energy), and dynamic pressure increases while static pressure decreases (and vice versa).
     
    But from the two posts above, I’m still getting a sense of misunderstanding of some relevant points.
     
    QT:  “Why is ram air pressure higher than static if the ram air pressure is moving faster?”
     
    First things first.  Terminology.  Ram pressure is not the same thing as dynamic pressure.  Ram pressure is a somewhat informal term, the more technically correct term is “total pressure”.  The ram (e.i. – total) pressure of the airstream is the dynamic and static pressures combined.  This is what the pitot tube samples.  It does so by acting as an obstruction in the flow.  At the front of any obstruction, there is a point called the “stagnation point”.  At this point, the airflow is brought to a stop, and all dynamic pressure of the freestream is converted to static pressure locally.  So the ram air pressure will always be higher than static since it includes static as a component.
     
    As for the higher or lower values, dynamic pressure is normally much lower than static pressure.  For example, at 200 knots at sea level, the dynamic pressure is approximately 5%-6% of the static pressure.  At dynamic pressures this high, compressibility is starting to become important, and Bernoulli is becoming less accurate.
     
    QT:  “Based on Bernoulli’s principle one would think that the pitot tube would act somewhat like a venturi and that fast moving air would have a lower pressure compared to static presure.”
     
    A venturi tube has air flowing through it, which is then influenced by the variable cross-section area — leading to velocity (i.e. – dynamic pressure) increases and the resulting (static) pressure decreases a-la Bernoulli.  A pitot-tube has air flowing around it, and samples from the stagnation point at the front —  where a reduced velocity (i.e. – dynamic pressure) results in increased (static) pressure a-la Bernoulli.
     
    In other words, a venturi is used to increase the local flow velocity, but a pitot is used to decrease it (stop it completely, in fact).  There is no air flowing through the pitot tube.  If you follow the plumbing of a pitot tube, you will discover that it dead-ends at the airspeed indicator.
     
    QT:  “If you haven’t guessed it yet, dynamic pressure is the pressure measured by the Pitot tube, not static pressure.”
     
    Close, but no cigar.  The pitot tube samples total pressure which includes both dynamic and static.  The airspeed indicator measures dynamic pressure.  It does this by comparing total pressure (from the pitot) to static pressure (from the static port).  The difference is dynamic pressure, and this results in the deformation of the aneroid capsule, which ultimately shows up on our dial as an airspeed reading.
     
    QT:  “In conclusion, the static pressure did go down when airspeed went up.”
     
    Not really.  The static pressure in the freestream will be whatever the atmospheric pressure is at our altitude.  It doesn’t change.  However, as we change airspeed, the dynamic pressure (and therefore the total pressure) will change.
     
    Cheers,
    Steve Pomroy
    http://www.flightwriter.com

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