Aerospace Engineering

115 flashcards by MentisLibrary

The standard atmosphere, aerodynamics and airfoils, aircraft performance and stability, air breathing and rocket propulsion, orbital mechanics, and airframe structures, for undergraduate aero students and pilots studying the theory.

What does the International Standard Atmosphere specify at sea level?
Temperature 288.15 K (15 C), pressure 101325 Pa, density 1.225 kg/m3, and a speed of sound of 340.3 m/s. Every published performance chart is referenced to these numbers.
What is the ISA temperature lapse rate in the troposphere, and where does it stop?
Temperature falls 6.5 K per kilometre from sea level to the tropopause at 11 km, where it reaches 216.65 K, or -56.5 C.
What happens to standard temperature between 11 km and 20 km?
It is constant at 216.65 K. Most jet transports cruise inside this isothermal layer, so cruise temperature barely changes with a step climb.
How does pressure vary with altitude inside an isothermal layer?
Exponentially. The hydrostatic equation with constant temperature integrates to a decay with scale height RT/g.
State the hydrostatic equation used in atmospheric modelling.
The pressure change across a layer of air equals minus the weight of that layer per unit area. It is the starting point for every atmosphere table.
How is the speed of sound in air computed, and what is it at 15 C?
From the absolute temperature alone for a perfect gas. At 288.15 K it is 340.3 m/s; at the tropopause it has fallen to 295 m/s.
What is dynamic pressure and why does every aerodynamic force use it?
It is the kinetic energy per unit volume of the oncoming flow. Lift, drag and moment are all written as a coefficient times dynamic pressure times a reference area.
What is the difference between pressure altitude and density altitude?
Pressure altitude is the ISA altitude matching the ambient pressure, which is what the altimeter reads at 29.92 inHg. Density altitude corrects that for temperature, and it is what the airplane actually feels.
Why does a hot, high day hurt takeoff performance so badly?
Low density cuts lift and engine mass flow at the same time, so the aircraft needs a higher true airspeed to lift off while the engine produces less thrust to reach it.
Distinguish indicated, calibrated, equivalent and true airspeed.
Indicated is the raw instrument reading. Calibrated corrects position and instrument error. Equivalent corrects compressibility. True is the actual speed through the air mass.
What does the Reynolds number compare, and what does a high value mean?
It is the ratio of inertial to viscous forces. High Reynolds number flows are dominated by inertia and go turbulent; low Reynolds number flows are viscous and stay laminar.
Name the flow regimes by Mach number.
Subsonic below about 0.8, transonic 0.8 to 1.2, supersonic 1.2 to 5, and hypersonic above 5, where real gas effects and intense heating dominate.
When is Bernoulli's equation actually valid?
Only along a streamline, in steady, incompressible, inviscid flow with no work or heat added. It is a poor model inside a boundary layer or across a shock.
State the one-dimensional continuity equation for steady flow.
Mass flow is the same at every station of a stream tube, so density times area times velocity is constant.
What is the boundary layer?
The thin layer of air next to a surface where viscosity slows the flow from zero at the wall to freestream speed. All skin friction drag is generated inside it.
Laminar or turbulent boundary layer: which do you want, and why is it a trade?
Laminar gives far less skin friction, but it separates easily under an adverse pressure gradient. Turbulent costs friction and buys attachment, which is why golf balls and many wings trip the flow deliberately.
What causes flow separation, and what does it do to an airfoil?
An adverse pressure gradient that the boundary layer cannot climb. The flow detaches, pressure recovery stops, lift falls off and pressure drag rises sharply. Full separation is stall.
How do skin friction drag and pressure drag differ?
Skin friction is the shear the boundary layer exerts along the surface. Pressure drag, also called form drag, is the fore and aft pressure imbalance a wake leaves behind.
What does the Prandtl-Glauert rule correct, and where does it break down?
It scales incompressible pressure coefficients for subsonic compressibility. It is a linearised result and blows up as Mach approaches 1, so it is untrustworthy above about M = 0.7.
Define critical Mach number.
The freestream Mach number at which the flow somewhere on the aircraft first reaches Mach 1 locally. Above it, shocks appear and transonic drag begins to build.
What changes across a normal shock wave?
Static pressure, temperature, density and entropy all rise; total pressure drops; and the flow leaves subsonic. The process is adiabatic but strongly irreversible.
How does an oblique shock differ from an expansion fan?
An oblique shock forms when flow is turned into itself: it compresses, loses total pressure, and is weaker than a normal shock at the same Mach. An expansion fan forms when flow turns away: it accelerates the flow isentropically.
What is the area rule, and what problem did it solve?
Whitcomb's finding that transonic wave drag depends on how smoothly total cross sectional area varies along the body. Waisting the fuselage where the wing joins removes the bump and cuts drag.
What is drag divergence Mach number?
The Mach number at which drag coefficient starts climbing steeply because shocks on the wing have grown strong enough to separate the flow behind them. Cruise Mach is chosen just below it.
How is the lift coefficient defined?
As lift non dimensionalised by dynamic pressure and reference wing area. It packages angle of attack, camber and configuration into one number.
What do the four digits of a NACA 4-digit airfoil mean?
First digit: maximum camber as a percentage of chord. Second: the chordwise location of that camber in tenths. Last two: maximum thickness as a percentage of chord.
What does camber do to an airfoil's lift curve?
It shifts the whole curve left, so the airfoil makes lift at zero angle of attack and stalls at a higher lift coefficient. The lift curve slope itself is essentially unchanged.
What lift curve slope does thin airfoil theory predict?
Two pi per radian for a two dimensional section, which is 0.110 per degree. Real airfoils come within a few percent of it below stall.
How does the lift curve slope of a finite wing differ from the section value?
Downwash from the trailing vortices reduces the effective angle of attack, so the wing slope is lower and falls further as aspect ratio drops.
Where is the aerodynamic center of a subsonic airfoil, and what makes it useful?
At about the quarter chord. It is the point about which the pitching moment coefficient does not change with angle of attack, so stability analysis can use a single constant moment.