Aerodynamics is chiefly concerned with
Answer: A
Aerodynamics studies the motion of air relative to a body and the resulting pressure and shear distributions, from which lift, drag and moments follow. Composition, structure and systems belong to atmospheric science, structures and avionics respectively.
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The continuum assumption in aerodynamics treats air as
Answer: B
Because the mean free path is tiny compared with any aircraft dimension at ordinary altitudes, properties such as density and pressure can be defined at a point and the flow described by differential equations. The assumption fails only in very rarefied flow at extreme altitude.
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The two sources of the aerodynamic force on a body are
Answer: C
Every aerodynamic force and moment, without exception, arises from the pressure distribution acting perpendicular to the surface and the shear stress distribution acting along it. Integrating those two over the whole surface gives the resultant force and moment.
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Dynamic pressure is defined as
Answer: D
Dynamic pressure, written q, equals one half rho V squared, and it appears in the definition of every aerodynamic coefficient. It is not itself a pressure that a static tapping would read, but a convenient grouping with the units of pressure.
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Mach number is the ratio of
Answer: A
Mach number measures how fast the flow moves compared with the speed at which pressure disturbances travel, so it governs whether compressibility matters. The ratio of inertial to viscous forces is Reynolds number, a quite different group.
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Reynolds number is the ratio of
Answer: B
Reynolds number determines whether a flow is dominated by inertia or by viscosity, and therefore whether the boundary layer is laminar or turbulent and how thick it is. Two flows with the same Reynolds number over geometrically similar bodies behave alike.
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The continuity equation expresses conservation of
Answer: C
Continuity states that mass is neither created nor destroyed, so for steady flow through a stream tube the mass flow rate is the same at every station. Momentum and energy conservation give the other two governing equations of fluid motion.
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For steady one-dimensional flow, the mass flow rate through a duct is given by
Answer: D
The product rho times A times V has the units of mass per unit time and is constant along the duct for steady flow. For incompressible flow the density cancels and the relation reduces to A times V being constant, so a narrowing duct speeds the flow up.
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Bernoulli's equation may be applied only to flow that is
Answer: A
The derivation assumes no viscosity, no change of density and no time dependence, and the result holds along a streamline. Applying it across a boundary layer, through a shock or at high Mach number gives answers that are simply wrong.
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According to Bernoulli's equation, where the velocity of a flow increases, the static pressure
Answer: B
The sum of static and dynamic pressure is constant along a streamline, so any gain in speed must be paid for by a fall in static pressure. This inverse relationship is the basis of the simplest explanation of how an airfoil generates lift.
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