3.6 Physics for Aviation: Forces, Fluids, and Flight

Matter, Force, Work, and Power

Matter exists as solid, liquid, or gas, and has mass (quantity of matter) and weight (force of gravity on that mass). Force is a push or pull measured in pounds; when a force moves an object, it does work. The relationships are tested directly:

Worked example: lifting a 200-lb component 11 ft does 200 x 11 = 2,200 ft-lb of work. If a hoist does it in 1 minute, power = 2,200 ft-lb/min, which is 2,200 / 33,000 = 0.067 hp. Note that holding a weight without moving it does no work in the physics sense, because distance is zero. Energy is the capacity to do work and is conserved - it converts between potential, kinetic, heat, and electrical forms but is not destroyed.

Heat is a form of energy and is measured in British thermal units (Btu) or calories; one Btu raises one pound of water one degree Fahrenheit. Heat moves by conduction (through solids by contact), convection (by fluid movement), and radiation (by waves, needing no medium) - all three matter in engine cooling and fire protection. Temperature is the intensity of heat, read in Fahrenheit, Celsius, or the absolute scales Rankine (R = F + 460) and Kelvin (K = C + 273); the gas laws below require absolute temperature.

Thermal expansion (most materials grow when heated) is why riveted and bolted assemblies are designed with the right clearances and why some torque and fit specs are temperature-dependent.

Simple Machines and the Pressure-Force-Area Law

Simple machines (lever, pulley, wheel-and-axle, inclined plane, wedge, screw) trade force for distance to give a mechanical advantage (MA) = output force / input force. A lever's MA equals the ratio of effort arm to resistance arm; a movable pulley roughly doubles force while halving distance. No machine creates energy - it cannot output more work than is put in, so increasing force always costs distance (and friction loses a little).

For fluids, the key relationship is Force = Pressure x Area (so Pressure = Force / Area, in psi). This underlies hydraulics: a small input piston creates a pressure that acts on a larger output piston to multiply force. Worked example: a hydraulic system at 1,500 psi acting on a 3-square-inch actuator piston produces Force = 1,500 x 3 = 4,500 lb. Because pressure is equal throughout a confined fluid (Pascal's principle), a small pump force on a small area yields a large force on a large area - the trade-off again being travel distance. Pneumatics use the same law with compressible air.

The Gas Laws

Gases respond to pressure and temperature in ways that matter for tires, struts, oxygen systems, and turbine theory. Temperatures must be absolute (Rankine or Kelvin) in these laws:

Worked Boyle's-Law example: 200 in^3 of gas at 30 psi compressed to 50 in^3 (temperature constant) reaches P2 = (P1 V1) / V2 = (30 x 200) / 50 = 120 psi. Charles's Law explains why a tire or strut pressure rises on a hot day - heating the trapped gas at fixed volume raises its pressure. The combined gas law is used whenever pressure, volume, and temperature all change, such as comparing a gas charge between ground and altitude conditions.

Bernoulli, Newton, Atmosphere, and Density Altitude

Bernoulli's Principle states that in a moving fluid, where velocity increases, static pressure decreases. Air accelerating over the curved upper surface of a wing has lower pressure than the slower air below, producing lift; the same principle calibrates the airspeed indicator and operates a carburetor venturi. Lift is also explained by Newton's third law - the wing deflects air downward and the equal-and-opposite reaction pushes the wing up; both views are taught together. Newton's laws also govern inertia (first law) and F = m x a (second law).

The standard atmosphere sets sea-level reference values of 29.92 in Hg (1013.2 hPa) and 15 degrees C (59 degrees F), with pressure and temperature falling as altitude increases. Pressure altitude is altitude referenced to 29.92 in Hg; density altitude is pressure altitude corrected for non-standard temperature. High density altitude - caused by high elevation, high temperature, and high humidity - means thinner air, which reduces engine power, propeller thrust, and wing lift, lengthening takeoff and degrading climb.

For maintenance, any repair to a flight surface must preserve contour, balance, and conformity to approved data, because changing a control surface's shape or weight alters its aerodynamics and can cause flutter. Heat transfers by conduction, convection, and radiation, and thermal expansion is why clearances and torque values account for temperature.