13.2 Aeronautical Physics Applications

13.2 Aeronautical Physics Applications

Quick Answer: For the initial test, keep aero applications as FSc physics with aero stories: Bernoulli and continuity for lift intuition, force balance (lift–weight, thrust–drag), a simple stability idea, and stress–strain for materials. You are not tested on full CFD or aircraft design codes—just clear mechanics and fluids applied to flight.

Candidates aiming at the College of Aeronautical Engineering (CAE) pathway benefit from seeing why physics topics matter on an airframe. The selection paper still grades physics, not specialised aero modules—so always map applications back to equations you already know.

Four Forces of Flight (Steady Level Cruise)

In steady, straight-and-level flight at constant speed:

So L = W and T = D in that ideal steady case. If T > D, the aircraft accelerates (or climbs depending on attitude). If L < W in level attitude without other support, it descends. These equalities are Newton’s first/second laws applied to the aircraft as a free body—exactly FSc mechanics language.

Worked example — force balance. An aircraft of mass 8000 kg in steady level flight needs lift L = mg ≈ 8000 × 9.8 = 78 400 N. If total drag is 12 000 N, engines must supply thrust ≈ 12 000 N to hold constant speed.

Lift, Drag, and the Bernoulli Link (FSc Framing)

Air is a fluid. Continuity for incompressible flow through a streamtube:

$A_1 v_1 = A_2 v_2$

Bernoulli’s equation along a streamline (steady, inviscid, incompressible idealisation):

$p + \frac{1}{2}\rho v^2 + \rho g h = \text{constant}$

A cambered wing speeds air over the upper surface relative to the lower surface in the classic textbook picture. Higher speed → lower static pressure on top → net upward lift. Real wings also deflect airflow downward (momentum change / Newton’s third law); both stories are compatible at intro level. Exam MCQs usually want the Bernoulli pressure–speed link or “faster flow, lower pressure.”

Drag resists motion: skin friction, form (pressure) drag, and induced drag associated with lift. Drag rises with speed roughly as dynamic pressure ½ρv² times area and a drag coefficient—useful qualitative scaling even when C_D is not tabulated on the paper.

Pitot idea (bonus link): a pitot-static system uses stagnating flow (v → 0) to raise pressure; the difference from static pressure estimates airspeed—again Bernoulli, not magic.

Worked qualitative check. If true airspeed doubles in the same air density, dynamic pressure quadruples (v²). Available lift at the same angle of attack concept scales strongly with speed—hence rotation and climb performance depend on speed, not only on throttle.

Thrust, Weight, and Performance Intuition

• Thrust from engines/propellers is a force (newtons), not “horsepower” alone—power P ≈ Tv relates force and speed.
• Weight W = mg changes with fuel burn; centre of gravity shifts as fuel is consumed—important later in training, and a reminder that “weight” is a force, not mass.
• Climb: excess thrust or excess power enables climb rate; if the paper asks conceptually, “more thrust than drag allows acceleration or climb capability.”

Stay numerical only when masses, g, and forces are given. Do not invent published PAF aircraft performance numbers on the exam.

Stability — The Restoring Idea

Static stability: after a small disturbance, does the aircraft tend to return toward the original attitude?

• Longitudinal (pitch): related to relative positions of centre of gravity (CG) and the aerodynamic centre / neutral point. CG too far aft reduces or destroys static stability—an engineering safety theme you will meet at CAE.
• Lateral/directional: dihedral effect, keel surface, and vertical fin contribute to roll/yaw restoring tendencies at a qualitative level.

For the initial test, expect at most a definition-style item: stable ≡ restoring moment/force; unstable ≡ diverging; neutrally stable ≡ stays in new attitude. Tie it to torque τ = rF and moment balance you already use in rigid-body statics.

Materials and Strength Basics (Still FSc Physics)

Airframes are structures. FSc “elasticity” maps directly:

$\text{Stress } \sigma = \frac{F}{A}, \quad \text{Strain } \varepsilon = \frac{\Delta L}{L}, \quad E = \frac{\sigma}{\varepsilon}$

Worked example — stress. A wing attach bolt carries 15 kN tension on a cross-section of 50 mm² = 5.0 × 10⁻⁵ m². Stress σ = F/A = 15 000 / 5.0 × 10⁻⁵ = 3.0 × 10⁸ Pa = 300 MPa. Compare with material allowable stress in design—but the MCQ usually stops at computing σ or E.

Hooke’s law F = kx for springs is the lumped version of the same elastic idea. Plastic deformation begins beyond the elastic limit—relevant to “why structures are sized with safety factors,” without inventing regulatory allowables.

How to Study This for PAF AE

1. Re-derive lift/drag stories from Bernoulli + Newton; do not memorise unsupported slogans.
2. Practice free-body diagrams of the aircraft (four forces) like any mechanics problem.
3. Keep materials questions in stress–strain–modulus arithmetic.
4. Use this section as motivation for fluids and mechanics revision—not as a substitute for core chapters on those topics.
5. Confirm all induction eligibility, dates, and centres on joinpaf.gov.pk; training context includes NUST CAE, Risalpur, but the written paper remains FSc-based.

Key Takeaways

• Steady level flight: L = W and T = D.
• Bernoulli + faster upper flow → lower pressure → lift (FSc model).
• Forces scale with dynamic pressure ½ρv².
• Static stability means a restoring response to disturbance.
• Stress, strain, and Young’s modulus are the materials bridge to structures.