Ducted Fan

The anatomy of a ducted fan

A ducted fan has three structural parts. The rotor — usually a single axial fan stage with anywhere from two to fifty blades — sits inside a precision-machined housing. The shroud (also called the duct, fan cowl, or nacelle) is a short cylindrical airfoil whose inner surface forms the boundary of the flow tube; its leading edge is rolled into a small inward-curling lip. A ring of stator vanes immediately downstream of the rotor straightens the swirl out of the wake and recovers a portion of the rotational kinetic energy as additional static pressure. Some designs also place inlet guide vanes upstream to pre-swirl the flow into the rotor; turbofans and many lift fans do.

The aerodynamic story is unfussy at the inlet and unfussy at the exit, but interesting at the rotor and at the lip. At the rotor, an axial pressure rise accelerates the column of air through the duct. At the lip, the inward radial pressure gradient that any thrusting fan creates is captured by the shroud as forward suction — the shroud itself develops thrust. The combination of rotor work and lip thrust is what gives the ducted fan its static-thrust advantage.

Why the shroud helps: tip vortex and lip thrust

An open propeller has higher pressure on the downstream face of each blade than on the upstream face. Air at the blade tip leaks around the edge from high to low pressure, rolling up into a tight helical tip vortex that streams downstream. The vortex represents real kinetic energy spent on rotation rather than on axial momentum — a parasitic loss that reduces the propeller's effective disc area. The tip vortex is also a major source of broadband noise and is responsible for the characteristic "buzz" of small drones and the slap-slap-slap of low-speed propeller aircraft.

A ducted fan blocks the leakage. With the shroud sitting only a few millimeters from the blade tip, there is essentially no path around the blade. The flow on the suction face stays attached, the effective disc area is the full geometric disc area, and the vortex never forms. That alone recovers several percent of the open-rotor's loss budget.

The shroud then does something extra. The accelerated air leaving the rotor produces a local low-pressure region at the duct's inner lip. The lip is itself a small cambered airfoil presented to that pressure field at a slight angle of attack, and it develops aerodynamic suction pulling forward on the airframe. In a well-designed static-thrust duct, lip suction accounts for roughly one-third of total thrust; the rotor produces the other two-thirds. This is why a ducted fan can out-thrust an open propeller of equal disc area on the same shaft power: the rotor doesn't have to do all the work.

The static-thrust gain in numbers

Actuator-disc theory makes the comparison precise. For a propeller of disc area A moving air with axial velocity v, the ideal static thrust is

T_open = 2 ρ A v²        (Froude, open rotor)
T_duct = 2 ρ A v² × (1 + σ_lip)   (with shroud lip thrust)

where σ_lip is the lip-thrust coefficient, typically 0.3 to 0.5 in well-designed static ducts. The ideal hover power for thrust T on disc A is

P_ideal = T^(3/2) / √(2 ρ A)

so a ducted fan with σ_lip = 0.5 delivers the same thrust on roughly 2/3 the shaft power, or equivalently produces about 22 percent more thrust on the same power. Real ducts achieve 15 to 25 percent gain after accounting for tip-clearance losses, stator drag, and internal duct skin friction. The figure of merit — the ratio of ideal to actual hover power — runs 0.80 to 0.90 for good ducted fans versus 0.70 to 0.80 for open rotors of similar disc loading.

Configuration	Disc loading	Figure of merit	Best for
Helicopter main rotor (open)	30 – 80 N/m²	0.70 – 0.80	Long-duration hover
Tilt-rotor proprotor (open)	200 – 400 N/m²	0.75 – 0.82	Hover + cruise
Ducted lift fan (eVTOL)	500 – 1500 N/m²	0.78 – 0.88	VTOL with small footprint
High-bypass turbofan	30 000 – 50 000 N/m²	n/a (cruise)	Subsonic transport
Hovercraft propulsion fan	200 – 600 N/m²	0.65 – 0.75	Forward thrust at low speed

The pattern is consistent. As disc loading rises — as you ask a smaller disc to produce more thrust — the duct's value grows. Helicopters with their enormous, lightly loaded rotors don't need ducts. Lift fans and turbofans, which load the disc heavily for a compact installation, almost always do.

Why ducts lose at cruise

The advantage that holds in hover collapses in forward flight. Two effects make the duct an active liability above 100 knots or so. First, the shroud has surface area and develops skin-friction drag that scales as ρv²S. Second, in forward flight the rotor is no longer ingesting only static air — it sees a freestream velocity component that reduces the relative axial acceleration the rotor produces, which in turn reduces the lip suction. By cruise speed the lip thrust has shrunk and the parasite drag of the shroud has grown, and the ducted fan's static-thrust premium has evaporated.

This is why conventional fixed-wing aircraft use unducted propellers. A Cessna 172 at 120 knots simply has nothing to gain from a shroud — it would weigh kilograms, cost drag, and offer no thrust benefit. The crossover point is roughly 70 to 90 knots for general-aviation-class disc loadings, far higher for heavily loaded turbofan-class disks. That crossover is why the turbofan, which never operates at airspeeds where it would lose, is universally ducted, while the open turboprop is preferred in the 250-to-450-knot turboprop regime where its lighter shroud-free installation wins.

The turbofan: a giant ducted fan with a hot core

Every commercial jet engine flying today is, structurally, a ducted fan. The fan is the large-diameter front rotor visible at the engine inlet. Behind it sits a much smaller gas-generator core — a compressor, combustor, and turbine — that does the chemical-to-mechanical work of burning fuel. The turbine drives a shaft that spins the fan; the fan ingests air, accelerates it through the bypass duct, and ejects it from the rear. Only a small fraction of the inlet air passes through the core; the rest bypasses around it.

The bypass ratio is the mass-flow ratio of cold bypass air to hot core air. Early jet engines (the JT8D on a 737 Classic) had a bypass ratio of 1:1. Modern high-bypass turbofans (Rolls-Royce Trent XWB, GE9X, Pratt PW1100G) run 9 to 12. On a Trent 1000, about 80 percent of the engine's net thrust comes from the ducted fan accelerating bypass air; only the remaining 20 percent comes from the hot jet. The core has become almost a shaft engine whose only job is to drive a fan.

Engine	Aircraft	Fan diameter	Bypass ratio	Fan share of thrust
JT8D-200	MD-80	1.25 m	1.7	~55 %
CFM56-7B	737 NG	1.55 m	5.1	~78 %
GE90-115B	777-300ER	3.25 m	9.0	~82 %
Trent XWB-97	A350-1000	3.00 m	9.6	~82 %
PW1100G geared	A320neo	2.06 m	12.5	~85 %
GE9X	777X	3.40 m	9.9	~83 %

The drive toward higher and higher bypass ratios is propulsive efficiency. Moving a large mass of air slowly is more efficient than moving a small mass quickly when the airframe is subsonic. The geared turbofan adds a reduction gearbox between the LP turbine and the fan so the fan can spin slowly (~3000 rpm) while the turbine spins at its own preferred 8000 rpm — a trick that lets the bypass ratio climb past 12 without choking either machine.

Lift fans and the F-35B

The F-35B Lightning II uses a Rolls-Royce LiftFan that is the largest ducted lift fan in service. It is a two-stage counter-rotating axial fan mounted vertically behind the cockpit, 50 inches in diameter, driven mechanically by a clutch and 90-degree gearbox from the F135 main engine's low-pressure turbine. In a vertical landing, the clutch engages, the engine spins the LiftFan, and ducted cold-air thrust of about 20,000 lb exits a swiveling lower nozzle. Behind the engine, the three-bearing swivel module redirects another 18,000 lb of hot exhaust downward. Roll-control posts at the wingtips feed bleed air. The aircraft balances on three points of thrust, two cold and one hot.

The LiftFan exists for one reason: every kilogram of thrust the F-35B produces in hover must come from the engine, and the engine alone — without the fan — is roughly 22% short of the thrust required to hover the airframe vertically. The LiftFan provides the extra cold thrust while shifting the thrust center forward so the aircraft balances. It is a clutch-engaged, mechanically driven ducted fan that operates for a few minutes per flight at most — a marvel of single-purpose engineering.

eVTOL and urban air mobility

Battery-electric vertical-takeoff aircraft for urban air mobility have become the highest-profile recent application of ducted fans. The argument is partly safety — exposed open rotors at passenger height are a hard regulatory and public-perception sell — and partly noise. Urban operations face strict day-night noise caps that are not negotiable; a 5- to 10-dB advantage at takeoff and hover can be the difference between certifiable and not.

The Lilium Jet is the most aggressive example. Its fixed wing carries thirty-six small ducted electric fans distributed along the trailing edges of the canard and main wing. Each fan is small enough to be safely enclosed and individually replaceable. The fan banks pivot upward for vertical takeoff, then tilt rearward for cruise — distributed electric propulsion with ducting integrated into the wing structure. The cost is mass and complexity; the reward is multi-failure tolerance, low tip speeds, and acceptable noise at low altitude.

Other eVTOL designers take different approaches. Joby and Archer use open tilt-rotors: fewer, larger, unducted propellers that tilt as a whole between hover and cruise. Their argument is cruise efficiency — they shed the shroud weight and drag once in forward flight. The ducted/open choice is currently the central architectural fork in the eVTOL design space, and it is not yet clear which side will dominate the certified-passenger market.

Hovercraft, drones, and a hundred million HVAC fans

Aerospace is the visible tip of a much larger ducted-fan industry. Hovercraft typically use one or two large ducted fans for forward propulsion and a separate lift fan to pressurize the air cushion; the rear thrust fan is fully ducted with steerable rudders inside the duct. Hovermower lift fans, leaf blowers, computer cooling fans, HVAC blowers, and the small fans inside every gaming console and laptop — all are ducted axial fans optimized for static pressure rise. Annual unit production across all categories runs to the hundreds of millions; the global market is dominated by HVAC and electronics cooling, with aerospace contributing a small but expensive subset.

Wind tunnels are driven by enormous ducted fans whose diameter can exceed 10 m and whose only job is to push tens of cubic meters per second through a closed-circuit test section. The NASA 80×120 Foot Wind Tunnel at Ames uses a 24 m diameter fan; the German-Dutch Wind Tunnel uses a 12.4 m fan turning at 270 rpm. Wind-tunnel fans operate at the low end of disc loading — they are optimized for efficiency rather than thrust per area — but they are recognizable ducted fans nonetheless.

Military VTOL UAVs have used ducted fans extensively. The Bell Eagle Eye, the Boeing X-50A Dragonfly, the Honeywell T-Hawk man-portable hover-drone, and several NASA Convergent Aeronautics concepts all rely on enclosed ducted-fan propulsion for hover, with a duct that doubles as the obstacle that lets you fly close to walls without slashing the rotor.

Common pitfalls

• Confusing static-thrust gain with cruise efficiency. The 25 percent advantage is a hover number. Above ~100 knots the shroud's drag dominates and an open propeller wins. Designers pick ducts for the operating point they actually live at — hover, VTOL, takeoff — not for "more thrust" everywhere.
• Treating the duct as a free wind tunnel. The duct only develops lip thrust when there is a strong radial pressure gradient at its inlet, which only happens when the rotor is highly loaded. A lightly loaded fan in a duct gives back most of the loss in shroud drag — you can lose thrust by ducting an underloaded rotor.
• Ignoring tip clearance. Tip clearance between blade and shroud must be a small fraction of blade chord. Excess clearance leaks flow around the blade and brings back the tip-vortex loss the duct was supposed to eliminate. Production ducted fans hold tip clearance to under 0.5 percent of blade radius using precision machining and active-clearance control.
• Assuming higher bypass ratio is always better. Higher bypass means a slower jet (good for propulsive efficiency at subsonic Mach numbers) but also a much larger fan (drag, mass) and a turbine that has to do more work to spin it. The optimum bypass ratio for a given mission was about 5 in the 1980s and is now about 12; the curve flattens above that.
• Forgetting that "fan" and "compressor" are the same machine. A turbofan's fan stage is just a first-stage axial compressor that vents most of its flow into the bypass duct instead of into the core. The aerodynamic and structural design problems are the same: blade loading, surge margin, vibration modes, tip clearance. The ducted fan is the front-end of a much longer aerodynamic story.