AFCAT (Air Force Common Admission Test) EKT - Mechanical and Aeronautical Stream Syllabus

State Pascal's law of fluid statics.

Pressure applied to a confined fluid is transmitted equally and undiminished in all directions throughout the fluid. It is the basis of hydraulic systems.

Give the equation for hydrostatic pressure at depth h in a fluid.

P = rho * g * h (gauge pressure), where rho is fluid density, g is gravitational acceleration, and h is the depth below the free surface.

State the continuity equation for incompressible flow.

A1*V1 = A2*V2 (mass conservation: the volume flow rate is constant). For compressible flow: rho1*A1*V1 = rho2*A2*V2.

State Bernoulli's equation and its assumptions.

P + (1/2)*rho*V^2 + rho*g*z = constant along a streamline. Assumptions: steady, incompressible, inviscid (frictionless) flow along a streamline.

Define a Newtonian fluid and state Newton's law of viscosity.

A Newtonian fluid has shear stress directly proportional to the velocity gradient (rate of shear strain): tau = mu * (du/dy), where mu is the dynamic viscosity. Water and air are examples.

Distinguish laminar from turbulent flow using the Reynolds number for pipe flow.

Laminar flow (smooth, ordered layers) occurs for Re < 2000; turbulent flow (chaotic mixing) for Re > 4000; the range in between is transitional. Re is based on pipe diameter.

What is the four-force balance acting on an aircraft in steady level flight?

Lift (upward) balances Weight (downward), and Thrust (forward) balances Drag (rearward). In steady, unaccelerated level flight all four forces are in equilibrium.

Give the lift equation for an airfoil.

L = (1/2) * rho * V^2 * S * C_L, where rho is air density, V is airspeed, S is wing planform area, and C_L is the lift coefficient.

Define the two main types of drag on an aircraft.

Parasite drag (skin friction + form/pressure drag) increases with the square of speed; induced drag (due to lift-generating wingtip vortices) decreases with speed. Total drag is their sum, minimum at the best L/D speed.

Define angle of attack and what happens at the stall angle.

Angle of attack is the angle between the chord line and the relative airflow. As it increases, lift rises until the critical (stall) angle, beyond which airflow separates, lift drops sharply, and the aircraft stalls.

Name the key geometric terms of an airfoil: chord, camber, and mean camber line.

Chord: straight line from leading edge to trailing edge. Camber: curvature of the airfoil (asymmetry between upper and lower surfaces). Mean camber line: line midway between the upper and lower surfaces.

Define the Mach number and the flow regimes it defines.

Mach number M = V/a (flow speed over local speed of sound). Subsonic M<1, Transonic ~0.8-1.2, Sonic M=1, Supersonic M>1, Hypersonic M>5.

Give the formula for the speed of sound in an ideal gas.

a = sqrt(gamma * R * T), where gamma is the specific heat ratio, R is the specific gas constant, and T is absolute temperature.

What is a normal shock wave and how do properties change across it?

A normal shock is an abrupt, thin discontinuity perpendicular to supersonic flow. Across it, velocity and Mach number drop (to subsonic), while pressure, temperature, density and entropy increase. It is an irreversible (non-isentropic) process.

Compare normal and oblique shock waves.

A normal shock is perpendicular to the flow and always leaves subsonic flow. An oblique shock is at an angle to the flow (caused by a wedge/turn); it is weaker, and the downstream flow can remain supersonic. Both increase pressure, temperature and entropy.

What happens to flow in a converging-diverging (de Laval) nozzle?

In subsonic flow it accelerates in the converging section, reaches M=1 at the throat, and then accelerates to supersonic in the diverging section. It is used to produce supersonic exhaust in rockets and jet engines.

Define stress and strain, with their units.

Stress = force per unit area (sigma = F/A), unit Pa or N/m^2. Strain = deformation per unit length (epsilon = change-in-length / original-length), dimensionless.

State Hooke's law and define Young's modulus.

Within the elastic limit, stress is proportional to strain: sigma = E * epsilon. Young's modulus E is the constant of proportionality (stress/strain) in uniaxial loading, with units of Pa.

Define Poisson's ratio.

Poisson's ratio (nu) is the ratio of lateral strain to longitudinal strain under axial loading: nu = -lateral strain / axial strain. For most metals it is about 0.25-0.35.

Give the relations among the elastic constants E, G, K and nu.

E = 2G(1+nu) and E = 3K(1-2nu), where E is Young's modulus, G is shear (rigidity) modulus, K is bulk modulus, and nu is Poisson's ratio. Combined: E = 9KG/(3K+G).

State the simple bending (flexure) equation.

M/I = sigma/y = E/R. Here M is bending moment, I is second moment of area, sigma is bending stress at distance y from the neutral axis, E is Young's modulus, and R is the radius of curvature.

State the torsion equation for a circular shaft.

T/J = tau/r = G*theta/L. T is torque, J is polar moment of inertia, tau is shear stress at radius r, G is shear modulus, theta is angle of twist, and L is shaft length.

Define shear force and bending moment in a beam.

Shear force at a section is the algebraic sum of all transverse forces on one side of it. Bending moment at a section is the algebraic sum of the moments of those forces about that section. Bending moment is maximum where shear force is zero (or changes sign).

What is the polar moment of inertia of a solid circular shaft of diameter d?

J = pi * d^4 / 32. It represents the shaft's resistance to torsional deformation; for a hollow shaft J = pi*(D^4 - d^4)/32.

Define a kinematic pair and a kinematic chain.

A kinematic pair is two links joined so they have constrained relative motion (e.g., revolute, prismatic). A kinematic chain is a series of links connected by pairs such that the relative motion is definite; a mechanism is a chain with one link fixed.

State Grubler's/Kutzbach's degrees-of-freedom criterion for planar mechanisms.

F = 3(n-1) - 2*j1 - j2, where n is the number of links, j1 is the number of lower (one-DOF) pairs, and j2 is the number of higher (two-DOF) pairs. F is the mobility (degrees of freedom).

What is Grashof's law for a four-bar mechanism?

For at least one link to make a full revolution, the sum of the shortest and longest link lengths must be less than or equal to the sum of the other two links (s + l <= p + q).

Define the natural frequency of an undamped single-degree-of-freedom spring-mass system.

omega_n = sqrt(k/m), where k is the spring stiffness and m is the mass. The natural frequency in Hz is f_n = (1/2pi)*sqrt(k/m).

What is resonance and why is it dangerous?

Resonance occurs when the forcing frequency equals the system's natural frequency, causing vibration amplitude to grow very large (limited only by damping). It can cause structural failure and excessive stresses.

Distinguish static balancing from dynamic balancing.

Static balancing eliminates the net unbalanced force (centre of mass on the axis of rotation). Dynamic balancing eliminates both the net unbalanced force AND the net unbalanced couple, so the system is balanced while rotating.

Name the three axes of an aircraft and the rotation about each.

Longitudinal axis: roll (controlled by ailerons). Lateral axis: pitch (controlled by elevators). Vertical (normal) axis: yaw (controlled by the rudder).

Distinguish static stability from dynamic stability of an aircraft.

Static stability is the initial tendency to return to equilibrium after a disturbance. Dynamic stability describes the motion over time, whether the resulting oscillations damp out (positive), persist, or grow (negative).

Define the load factor (n) and stall speed relationship in a turn.

Load factor n = Lift/Weight (expressed in g). In a level turn n = 1/cos(bank angle). Stall speed increases with load factor as V_stall_turn = V_stall * sqrt(n).

What is the function of an aircraft wing spar and rib?

The spar is the main spanwise structural member carrying bending and shear loads (the wing's backbone). Ribs give the wing its airfoil shape and transfer aerodynamic loads from the skin to the spars.

What is a semi-monocoque fuselage structure?

A construction where the skin carries much of the load but is reinforced by internal frames/formers (rings) and longitudinal stringers/longerons. It combines strength with light weight and is the most common aircraft fuselage type.

Name the primary and secondary flight control surfaces of an aircraft.

Primary: ailerons (roll), elevator (pitch), rudder (yaw). Secondary: flaps and slats (increase lift), spoilers (reduce lift/increase drag), trim tabs, and slats/leading-edge devices.

Compare casting, forging, and machining as manufacturing processes.

Casting: molten metal poured into a mould to form a shape. Forging: metal shaped by compressive force (hammering/pressing), giving good grain flow and strength. Machining: material removed by cutting tools to achieve precise dimensions and finish.

What is the difference between hot working and cold working of metals?

Hot working is done above the recrystallization temperature (lower forces, no strain hardening, coarser finish). Cold working is below it (higher strength via strain hardening, better surface finish and tolerance, but higher forces and residual stresses).

What is a factor of safety in machine design?

Factor of safety = ultimate (or yield) strength / allowable (working) stress. It accounts for uncertainties in loading, material, and manufacturing, ensuring the component does not fail under expected service conditions.

Distinguish a clearance fit, interference fit, and transition fit.

Clearance fit: hole is always larger than the shaft (free movement). Interference fit: shaft is always larger than the hole (force/press fit, no movement). Transition fit: tolerances overlap so the result may be slight clearance or slight interference.