Chapter 2: Propulsion & Intake / Exhaust Shaping

Created by Sarah Choi (prompt writer using ChatGPT)

Propulsion (Prop, Turbojet/Fan) & Intake/Exhaust Shaping — Air Vehicles: Fixed‑Wing

Propulsion is the heartbeat of fixed‑wing craft. The size, placement, and shaping of propellers, inlets, nacelles, and exhausts determine not only thrust and efficiency but also silhouette, stealth, noise, and handling. For vehicle concept artists—on both the concepting and production sides—encoding propulsion truth early makes fighters feel quick‑witted, transports dependable and quiet, and gliders plausible when powered. This article compares propeller and turbine systems, shows how fighters, transports, and gliders express them differently, and translates those choices into buildable deliverables for modeling, rigging, physics, VFX, audio, and UI.

Big picture: matching propulsion to mission

Fighters live near transonic/supersonic regimes with high throttle transients and maneuver demands. Intakes must feed clean, stable flow across angle‑of‑attack and yaw, sometimes at supersonic speeds; exhausts manage afterburners and thermal signature. Transports cruise efficiently at high subsonic speeds and low specific fuel consumption; nacelles emphasize low noise, high bypass ratios, and engine‑out control. Gliders and motor‑gliders prioritize low drag with either retractable props, small sustainer turbines, or tow hooks; any powerplant must vanish when not in use.

Propellers: geometry, placement, and reads

Blade variables that players feel. Diameter, blade count, planform (taper, sweep), twist and pitch schedule, and tip speed govern thrust, noise, and silhouette. Large diameters produce efficient thrust at low speeds but drive ground‑clearance needs; more blades let you absorb power at smaller diameters and lower RPM, lowering noise. Tip Mach near ~0.85–0.9 increases noise dramatically; designs often aim to keep tips subsonic.

Tractor vs pusher. Tractor props (in front) bathe wings and tails in propwash, improving control at low speed and enabling blown flaps/STOL; pushers reduce cockpit noise and improve visibility but risk tailplane ingestion and require careful ground clearance. Pylons or booms should keep the disc clear of fuselage boundary layer and debris.

Contra‑rotating and coaxial. Counter‑rotating props recover swirl, increasing efficiency and canceling torque roll. They read as dense, multi‑blade discs with tight spinners and complex hubs—great for transports (turboprops) and some fighters. Production must include hub fairings, pitch actuators, spinner join seams, and blade lock access.

Propwash & control coupling. Propwash accelerates flow over local surfaces—good for STOL transports with flaps in the slipstream, risky for tail buffet if geometry is wrong. Show intentional alignment: high‑lift flaps or flaperons directly behind discs, vortex fences where wake would strike tails.

Motor‑gliders. Use folding or retractable props to reduce drag. Visual tells: slender blades with root hinges, nose or mast fairings with doors, compact electric motors or small turboshafts. Production callouts must lock door arcs, latch logic, and pitch/fold sequence.

Turbomachinery: turbojet vs turbofan vs turboprop

Turbojets (low or zero bypass) are compact, high‑specific thrust engines for very high speed; exhausts run hot and fast, afterburners add length. Turbofans move a large mass of air more slowly; high‑bypass fans (transports) give efficiency and low noise, while low‑bypass fans (fighters) balance speed and signature. Turboprops extract shaft power to a prop; nacelles show gearbox bulges and exhaust stubs separate from the prop disc plane.

Nacelle anatomy that should appear. Inlet lip with proper radius; fan face or spinner; S‑ducts or straight ducts; fan case and stator hints; accessory gearbox blisters; pylon with fairings at wing juncture (fillets/strakes); thrust reverser sleeves (transports) or petal lines; chevron mixer edges (noise control); exhaust plug or flametube and, on fighters, afterburner spray bars and variable‑area petals.

Intakes: keep the engine fed through all attitudes

Subsonic pitot inlets (transports, props, trainers). Simple, round/oval lips ahead of boundary layer; generous radius to avoid separation; anti‑icing lips in cold operations. Place high on nacelles to avoid FOD; add splitter distance to keep wing/fuselage boundary layer out.

Supersonic/fighter inlets. Must manage shocks and deliver subsonic flow to the compressor across Mach and AoA. Families include:

  • External compression with ramps/doors: visible variable‑geometry ramps, bleed doors, and shock control plates. Reads technical and aggressive.
  • Mixed compression cones/spikes: central cones (axial spikes) that translate with Mach to set oblique/normal shocks.
  • DSI (diverterless supersonic inlets): fixed bumps and offset lips that shed boundary layer without moving parts—clean stealth silhouettes.
  • Chin or cheek inlets with boundary‑layer diverters: separate slots and fences; require bleed grills and access panels. Production must lock ramp angles, door outlines, and bleed slot locations; rigging needs axes and travel; VFX needs shock/vapor emission lines tied to lips and ramps.

S‑ducts and stealth. Fighters often hide the fan face from radar using S‑ducts and blocker vanes. Show smooth S‑curves and internal vanes through cutaways; include access panels for inspection.

Ingestion control. Transports include FOD screens or translating lips for unpaved strips; seaplanes need spray rails and deflectors. Fighters integrate boundary‑layer bleed and anti‑ice; production should call mesh aperture, bleed capacity zones, and heater mats.

Exhausts: force, heat, and signature

Transport exhausts (high‑bypass). Cool mixed flow reduces IR and noise. Visual cues: long nacelles, mixer lobes or chevrons at the nozzle, pylon fairings that keep hot flow off the wing box. Reversers: cascade doors on fan ducts, translating sleeves and blocker doors—clearly segmented with fairings called “canoes.” Production needs to specify door counts, hinge lines, and deploy angles.

Fighter exhausts. Variable‑area convergent‑divergent (CD) nozzles with petals and overlapping flaps; afterburner rings visible in cutaway; thrust vectoring paddles or axisymmetric petal gimbals on some types. IR signature reduction may add serrations, film cooling slots, or shrouds. Rigging must support area change and vectoring with believable linkages.

Turboprop exhausts. Stacks or annular ejectors that can augment thrust (Meredith effect analogs); positioning avoids prop ingestion and cabin.

Glider sustainer exhausts. Tiny, well‑shielded outlets or none for electrics; doors close flush when stowed.

Fighters: integration and extremes

Fighter silhouettes blend planform and propulsion tightly. Inlets align with chines and LEX to stay fed at high AoA; S‑ducts hide fans; bays and gear drive nacelle spacing. Exhaust petals, vectoring nozzles, and heat shields shape the stern. Concepts should propose one coherent inlet family (DSI, ramps, spikes) and show how it handles AoA/yaw; production locks ramp travel, bleed doors, and nozzle vector limits. VFX places shock diamonds and vapor cones near the inlets and nozzle lips at high power; audio stages compressor whine, ramp clacks, and afterburner roar.

Transports: efficiency, redundancy, and hush

Transports wear high‑bypass fans under wings (or rear‑mounted) with nacelles optimized for drag and noise. Pylons are structural bridges with anti‑vibration isolators and systems plumbing; inlets have large radii and anti‑ice; reversers and chevrons manage approach noise. Multi‑engine layouts demand engine‑out controllability—wing dihedral, rudder size, and nacelle placement tell that story. STOL/transports may pair blown flaps with propwash (turboprops) or upper‑surface blowing. Production must define pylon attach points to the wing box, reverser segment counts, and anti‑ice zones; rigging animates reversers, cowl doors, and fan spin with correct direction; VFX uses flap‑edge vortices and reverser dust plumes; audio emphasizes low-frequency fan beat and reverser roar.

Gliders & low‑drag thinking

Powered gliders hide power. Folding electric props with slim blades, retractable nose/stem masts, or small sustainer turbines that fold away. Inlets are tiny and flush; exhausts minimal. Doors and fairings must close flush; control schedules retract power at altitude to preserve glide. Production locks hinges, latches, and sequencing; VFX adds short electric motor whine and negligible heat shimmer.

Nacelle–wing–fuselage junctions: where drag lives

Use fillets, chines, and pylon fairings that align with local flow. On fighters, blend inlets into chines and LEX; on transports, keep pylons forward of shock/buffet zones and behind slats to preserve high‑lift device function. Avoid placing gear doors where reverser cascades will blast them. Production orthos should mark keep‑out zones for flap tracks and spoilers.

Cooling, accessories, and maintenance access

Accessory gearbox doors, oil coolers, intercoolers, and FADEC bays require panels and latches. On turboprops, show intake particle separators and inertial doors. On fighters, add APU inlets/exhausts and ECS (environmental control system) scoops with louvers. Production callouts include panel sizes, hinge axes, latch counts, and IP/thermal notes.

VFX & audio: propulsion tells

  • Props: disk blur tied to RPM; tip vortex wisps at humid low speed; blown dust stripes aligned with discs. Audio: blade smack at high AoA; constant‑speed prop pitch change without RPM change for turboprops.
  • Fans/jets: compressor/fan shimmer inside inlets; shock diamonds at high power; reverser dust/spray cones. Audio: fan beat → core whine → jet roar; afterburner crackle.
  • Glider electrics: faint inverter/motor song; quick spin‑up/down. Place emitter sockets at lips, mixers, chevrons, blade tips, and reverser cascades.

Camera‑read discipline

At far range: read engine count/placement, inlet family (pitot, DSI bump, ramp), and nozzle type (petal CD, chevron, plain). At mid: read reverser seams, pylon fillets, gear door/nacelle relations. At near: read ramp hinges, bleed doors, petal overlaps, spinner seams, and hub details. Avoid LED lines on leading/trailing edges; anchor nav/strobe lights at tips and pylons.

Concept → production deliverables

  • Propulsion brief: engine type(s), inlet family, nozzle type, mission notes (STOL, supersonic dash, stealth, short/rough fields).
  • Metrics sheet: fan/prop diameter, blade count, tip Mach target, bypass estimate, inlet area and lip radius, ramp travel/angles, S‑duct curvature constraints, nozzle area ratios, reverser door count/deflection, pylon attach coordinates.
  • Orthos (measured): plan/side/front with intake lips, splitter distances, bleed slots, ramp/door outlines, fan face location, pylon fillets, reverser seams, nozzle geometry; prop discs with ground clearance arcs.
  • Cutaway: duct routing (S‑duct, chin), blocker vanes, accessory bay, fuel/oil coolers, ECS/bleed air, reverser cascades, afterburner hardware, vector paddles.
  • Exploded views: prop hub + pitch mech + spinner; fan cowl + reverser + cascade; ramp module + actuators; nozzle petals + linkages; DSI bump structure.
  • Callouts: anti‑ice zones, particle separator modes, bleed capacity, ramp/rudder interlock logic, reverser inhibit logic, vector limits, maintenance panel sizes, FOD guard specs.
  • Rig pack: spin directions, blade pitch ranges, ramp/door axes/limits, reverser sequences, nozzle area/vec schedules; VFX/audio sockets and naming.
  • Camera‑read boards: far/mid/near day/night/fog; “must read” notes for inlet/nozzle families and reverser states.

Indie vs. AAA cadence

Indie: one evolving canvas with propulsion brief + measured ortho + compact cutaway + rig notes; validate in engine with fan/prop spin, reverser animation, and shock/vapor stub VFX.

AAA: gated: inlet/nozzle family lock → nacelle/pylon layout lock → modeling kickoff (orthos/cutaways/callouts) → rig & VFX/audio check (spin, ramps, reversers, vectoring) → camera‑read sign‑off; ship a propulsion kit (props, spinners, ramps, DSI bumps, chevron nozzles, reverser cascades) for reuse across variants.

Closing

When propulsion geometry and inlet/exhaust shaping agree with mission, the aircraft’s behavior reads before throttle moves. Fighters advertise breath and bite with disciplined inlets and animated nozzles; transports whisper efficiency through fat fans, chevrons, and reversers; gliders hide power until it’s needed. Encode those truths in silhouettes, lips, petals, and numbers, and your fixed‑wing craft will sound and move like they belong in your skies.