GENERAL RULES OF DUCTED FANS
How a ducted fan works.
The following rules are for Jets with internal ducting system.
Appreciate if there is any important rule which I had missed out, please let me know.
How a ducted fan works? – Basically they are an internally ducted high performance fans which take in air from the inlet opening and blows air out an exhaust tube to generate forward thrust.
The air enters an impeller spinning at high RPM, which propels pressurized air out of its exhaust at increased pressure and velocity. A ducted fan thrives on the air fed into it so too little air will starve the fan, thus preventing it from perform at its optimum design point.
Conversely, inlets that are grossly oversized can have the same effect: a ducted fan can only process a given amount of air at any particular time, so too much air will hamper performance --not to mention the increased drag from the larger inlets.
Requirements for Ducted Fans - Ducted fans need inlets sized to approximately 95%-110% of the Fan Swept Area (FSA) with an exhaust outlet sized to approximately 75%-85% FSA. Choking down airflow at the exhaust will increase the exhaust velocity and in return will increase top end speed of the aircraft. However, a point of diminishing returns: reducing down the exhaust too much will back pressure the fan resulting in degraded performance. Additionally, a larger outlet area will increase the static thrust of the system, but lower the top end speed of the aircraft. The areas suggested above represent a good compromise between static thrust and exhaust velocity.
Fan Swept Area - The fan swept area is calculated by subtracting the area of the impeller hub from the area of the inside of the shroud. The equation is shown below:
Duct Area - Impeller Hub Area = Fan Swept Area
External Inlet Shape - The ideal inlet lip shape for a ducted fan aircraft is a 2x1 ellipse. This represents the most aerodynamic shape while maintaining good airflow into the inlet system.
EDF Frequently Asked Questions (FAQ)
What is an EDF? - An EDF is a ducted fan that is powered by an electric. Require to operate an EDF is an electric motor, an electronic speed controller (ESC) and battery. These items can vary depending on the combination and the desired performance.
What do Watts and Amps tells ? - Watts and amps tell you the power and current at which your EDF system is running.
Amps is the measurement for electrical current while Watts is the measurement of power being generated.
Amps is a measure of efficiency of the system and is related to the wind of the motor combined with the source battery and how the motor is loaded. Additionally, better quality motors will run more efficiently (same power with lower amps) than lower quality motors.
Watts is the measure of power of the system. It is directly related to horsepower and in fact, 1 hp = 746 Watts. It is a good idea to keep an eye on these things as high amps and high power also converts into heat. Watts will vary accordingly depending on the size of fan/motor combination.
What is the best EDF setup? - The best EDF setup I would say that it’s really up to individual. Numerous combinations are possible, but we try to match the power system to suit the Jet’s size and weight. Obviously, the most power for the lightest weight is the desired end.
