Ducted Fan Power Calculator Based on Thrust

Enter ducted fan design inputs

Use total system thrust or per-fan thrust. Efficiency values use decimal form.

Required thrust

Enter the thrust target before thrust margin.

Thrust basis Select how the required thrust value should be interpreted.

Number of ducted fans Equal thrust sharing is assumed across all fans.

Internal duct diameter

Use the open aerodynamic diameter, not outer shroud diameter.

Air density, ρ (kg/m³) Sea-level standard air is approximately 1.225 kg/m³.

Freestream flight speed (m/s) Use zero for hover, bench testing, or static thrust.

Aerodynamic efficiency / figure of merit Use fan test data when available. Typical screening values are 0.60–0.80.

Motor efficiency Use the expected loaded efficiency, not the peak catalog value.

Controller and wiring efficiency Includes controller and practical wiring losses.

Nominal supply voltage (V) Used to estimate DC current. Check voltage sag separately.

Thrust margin (%) Adds thrust capacity above the entered target.

Electrical power reserve (%) Adds selection headroom after modeled electrical losses.

This tool is for preliminary engineering. Validate final designs with fan maps, manufacturer limits, and controlled measurements.

Formula used

The calculation uses actuator-disk momentum theory. It treats all fan disks as one combined airflow area. The model estimates induced velocity before applying practical losses.

A_total = fan_count × π × D² / 4 T_design = T_entered × thrust_margin_factor v_i = [-V∞ + √(V∞² + 2T_design / (ρA_total))] / 2 P_ideal = T_design × (V∞ + v_i) P_shaft = P_ideal / η_aero P_electrical = P_shaft / (η_motor × η_controller) P_recommended = P_electrical × (1 + power_reserve)

Here, T is thrust, D is duct diameter, ρ is air density, V∞ is freestream speed, and v_i is induced velocity.

How to use this calculator

Enter the required thrust and choose whether it applies per fan or to the full system.
Enter fan count and the usable internal duct diameter.
Use local air density when altitude or temperature differs from standard conditions.
Enter zero flight speed for static operation, hover, or a thrust stand test.
Set conservative aerodynamic, motor, and controller efficiency values.
Add thrust margin and electrical reserve for real operating conditions.
Review power, current, pressure rise, disk loading, and velocity together.
Export the result, then compare it with supplier fan curves and thermal limits.

Example input data

These examples show useful starting points. They are not manufacturer performance claims.

Case	Required thrust	Fans	Duct diameter	Flight speed	Aero efficiency	Motor efficiency	Voltage
Static lift module	120 N total	2	90 mm	0 m/s	0.72	0.90	22.2 V
Compact test rig	45 N per fan	4	70 mm	0 m/s	0.65	0.88	14.8 V
Cruise propulsion module	300 N total	2	120 mm	25 m/s	0.76	0.92	44.4 V

Power, thrust, and practical fan selection

Why thrust alone is not enough

A ducted fan does not convert input into thrust perfectly. Air must accelerate through the inlet, rotor, duct, and exit. Each stage creates losses. A thrust target is therefore only the starting point. The calculator estimates ideal air power first. It then applies aerodynamic, motor, controller, and reserve factors.

Disk area strongly affects the answer. A larger duct moves more air at lower induced velocity. This usually reduces static induced power. A smaller duct can fit compact equipment. It also raises disk loading. Higher disk loading needs more pressure rise. It can increase noise and heat. Compare several diameters before selecting hardware.

Static and moving conditions

Static operation is the hardest condition for many lift fans. The surrounding air begins at rest. The fan must create all induced velocity itself. Forward motion changes the inlet flow. Existing airspeed contributes to flow. The required induced velocity can decrease for the same thrust.

Enter zero flight speed for hover, bench, or static testing. Enter expected freestream speed for cruise analysis. This estimate uses actuator disk momentum theory. It provides an engineering screening result. It does not replace measured fan maps. Inlet shape, blade pitch, and exit nozzle details can shift real performance.

Efficiency and electrical demand

Figure of merit represents aerodynamic quality. It captures how closely the rotor and duct approach ideal momentum performance. Use a conservative value when no test data exists. Motor efficiency converts shaft demand into electrical demand. Controller and wiring efficiency account for electrical losses before the motor.

The power reserve is useful. It covers warm weather, manufacturing variation, battery voltage sag, and operating uncertainty. It does not create extra thrust in the model. It simply increases the recommended electrical rating. Select a battery, controller, and wiring system that can supply this current continuously.

Using results for practical sizing

Review induced velocity, disk loading, pressure rise, and current together. Low power alone is not enough. Excessive current can overheat cables. High disk loading can increase noise. High exit velocity may create uncomfortable jet wash. A design with slightly larger ducts can sometimes lower electrical demand.

Results are estimates. Use them to narrow design choices. Then confirm the selected ducted fan with manufacturer curves or testing. Measure thrust, voltage, current, temperature, and rotational speed. Recheck calculations when duct diameter, fan count, or mission speed changes. Measurements make a dependable propulsion system today.

Frequently asked questions

1. What does the calculator estimate?

It estimates ideal air power, shaft power, electrical input, recommended power rating, current, disk loading, pressure rise, and induced velocity for a ducted-fan system.

2. Should I enter total thrust or thrust per fan?

Select the matching thrust basis. Total system thrust is divided across all fans. Per-fan thrust is multiplied by the entered fan count.

3. Why does duct diameter change the power result?

Diameter changes disk area. More area moves air at lower induced velocity. That usually reduces ideal static power for the same thrust.

4. What air density should I use?

Use 1.225 kg/m³ for standard sea-level air. Use a lower value for hot weather or higher altitude conditions.

5. Is zero flight speed correct for hover?

Yes. Set freestream flight speed to zero for hover, a fixed test stand, or static thrust operation.

6. What aerodynamic efficiency is reasonable?

Use measured fan data whenever possible. For early estimates, values from 0.60 to 0.80 can be useful, depending on the duct, rotor, and operating point.

7. Does this replace a manufacturer fan curve?

No. The calculator provides a first-pass momentum-theory estimate. Manufacturer curves and controlled testing remain necessary for final component selection.

8. Why is recommended power larger than electrical power?

Recommended power includes the selected electrical reserve. This supports practical component sizing for uncertainty, temperature, voltage sag, and operating variation.

9. What does disk loading mean?

Disk loading is thrust divided by combined fan disk area. Higher values normally require greater pressure rise and can increase noise.

10. Can I calculate battery current?

Yes. The current estimate divides recommended electrical power by nominal supply voltage. Check actual voltage sag and battery discharge limits separately.

11. Can this model be used for forward flight?

Yes. Enter an expected freestream speed. The model adjusts induced velocity using that speed, but it still remains an approximate preliminary design method.