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Subsonic Flow Characteristics
Topics Covered in this session: !
1
Overview
!
2 Moving Airplane In Stationary Air or Moving Air Over Stationary Airplane?
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3
Streamlines
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4
Airflow Characteristics versus Mach Number
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5
Constant Density or Incompressible Flow
What needs to be done prior to class (WNTBPC) !
Read the ANA text pages 4 through 9, Bernoulli’s principle and subsonic airflow, and Pilot’s Handbook of Aeronautical Knowledge (PHAK) p. 2-4 Airfoil Design through p.2-6 High Pressure Below.
!
Watch this lecture
!
Create your own notes in word, save it as a .pdf and post it in Dropbox as homework. !
This should be no more than a page of YOUR OWN version of notes
1. Overview !
Characteristics of Subsonic airflow
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Drawings of stationary vs moving airflow
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Use of streamlines as graphic and visual depiction of invisible airflow
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Subsonic airflow! !
Density changes are negligible Considered incompressible
2. Airplane moving through stationary air, or air moving over stationary airplane? !
Airplane in flight is moving through non-moving (stationary) air.
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Studying aerodynamics on the ground – we must ‘blow’ moving air over a stationary airplane.
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Leonardo Da Vinci: !
“The action of a medium upon a body is the same whether the body moves in the quiescent medium, or whether the particles of the medium impinge with the same velocity upon the quiescent body.”
3. Streamlines !
Streamlines are a visible representation of an invisible airflow
Streamline Relationships
4. Airflow Characteristic versus Mach number !
Mach number is defined as airflow (or aircraft) velocity divided by the local speed of sound:
Airplane velocity vs Airflow velocity
As Aircraft moves through the air – shape causes some airflow to be faster than aircraft, some airflow is slower compared to AIRCRAFT VELOCITY
Mach numbers !
Airflow traveling slower than M = 1 is called subsonic flow. Airflow traveling faster than M = 1 is called supersonic flow. Subsonic and supersonic flows have very different pressure, temperature, and density characteristics.
5. Constant Density or Incompressible Flow !
Subsonic flow: !
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Symbol for air density is rho ! !
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Constant density or incompressible flow ρ.
Air density decreases as altitude increases and vice versa. At a constant altitude, the air density of subsonic flow is a constant for all practical purposes. ! Therefore, rho is constant in subsonic flow at a given altitude.
In aerodynamics, incompressible flow means the density does not change. It does NOT mean that the pressure does not change! Pressure in fact does change depending on the circumstances.
Review !
1
Overview
!
2 Moving Airplane In Stationary Air or Moving Air Over Stationary Airplane?
