Fundamentals of Aerodynamics Questions and Answers

Answer: C

The flow is brought to rest, so all the dynamic pressure is recovered as static pressure and the reading is the total or stagnation pressure. This is exactly what the forward-facing tube of a pitot-static system measures.

Answer: D

That difference is the dynamic pressure, from which speed follows once the density is known or assumed. Because the instrument is calibrated for a fixed density, indicated airspeed departs from true airspeed as altitude increases.

Answer: A

With no heat added and no work done, the sum of enthalpy and kinetic energy per unit mass is conserved, so total enthalpy is constant even across a shock wave. Static properties change freely as the flow accelerates or decelerates.

Answer: B

Newton's second law in control-volume form relates the forces acting to the net rate at which momentum leaves, which is how thrust and aerodynamic forces are derived without knowing details of the internal flow. It is the basis of propulsion analysis as well.

Answer: C

It equals the square root of gamma times R times T, so for a given gas it is fixed entirely by temperature. Since temperature falls with altitude through the troposphere, the speed of sound falls too and a given true airspeed corresponds to a higher Mach number.

Answer: D

Below roughly Mach 0.3 the density change through the flow is only a few per cent, so the incompressible equations are accurate enough for most purposes. Above that the error grows quickly and compressible relations must be used.

Answer: A

Near a free-stream Mach number of one, local acceleration over a wing can carry part of the flow past the speed of sound while the rest remains subsonic, and shock waves appear where it returns. This mixed condition is the hardest regime to analyse or to design for.

Answer: B

Mach five is the usual dividing line, chosen because above it effects such as strong shock layers, high-temperature gas chemistry and viscous interaction become important. The boundary is a convention rather than a sharp physical change.

Answer: C

Viscosity retards the fluid nearest the surface, so the velocity rises from zero at the wall to the free-stream value across a thin layer. Outside that layer the flow behaves as though inviscid, which is what makes potential-flow methods useful at all.

Answer: D

Correct answer: Zero relative to the surface

Viscosity makes the fluid immediately adjacent to a wall move with the wall, so the relative velocity there is zero. Everything about the boundary layer follows from this one condition, including skin friction, separation and the whole distinction between real and inviscid flow.