GENERAL RULES OF DUCTED FANS

Though GDF's and EDF's are intrinsically different in terms of how the fan is driven, the aerodynamics and rules of thumb of the ducted fan remain the same regardless of the power source. We've compiled a list of items and frequently asked questions that should be helpful to  you in your DF jet flying. If there is something we missed , please let us know!

How a ducted fan works - Ducted fans operate as they sound:  they are an internally ducted high performance fan which takes 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 generally have inlets sized to approximately 90%-100% of the Fan Swept Area (FSA). Bigger is not always better as inlet sizes can be less, which can actually increase your top end speed (less ram drag), but at the expense of takeoff performance. Though, restricting the inlet too much can result in the fan starving for air no matter how fast the aircraft is travelling. Exhaust outlets are generally sized to approximately 75%-85% FSA.  Choking down airflow at the exhaust, increases exhaust velocity. This in turn will increase top end speed of the aircraft. There is, however, a point of diminishing returns: choking 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.

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EDF Frequently Asked Questions (FAQ)

What is an EDF? - An EDF is a ducted fan that is powered by an electric motor and battery. Required to operate an EDF is an electric motor sized for the fan, an electronic speed controller (ESC) and a source battery. These items can vary depending on the combination and the desired performance.

What do Watts and Amps tell me? - 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 can also convert into heat. Watts will vary accordingly depending on the size of fan/motor combination.

What is the best EDF setup? - The best EDF setup is the one that you are happy with.  Numerous combinations are possible, but we try to match our airplanes to systems that are best suited to them and spend many flight hours testing the setup before it is offered. Obviously, the most power for the lightest weight is the desired end.

How high a rating do I need on my ESC? - It is a good rule of thumb to have a good margin of safety to avoid thermal shutdown and over current problems. That being said, all ESCs are rated to a certain "burst" current, so as long as you are operating within the manufacturers specifications and not running continuously at this "burst" current rating, you should be ok.

 

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GDF Frequently Asked Questions (FAQ)

What's the difference between a tractor and pusher ducted fan setup? - A tractor setup has the engine behind the fan and the tuned pipe straight back from the exhaust; a pusher has the engine in front of the fan and the tuned pipe interrupting the inlet air flow. Which is better? - The tractor setup is more efficient than the pusher; it also helps keep the engine cooler.

What kind of fuel should I use in my ducted fan system? - We recommend synthetic oil based 15% sport fuel (no ducted fan blends) to each gallon of which should be added one generous ounce of turbine engine oil. Turbine engine oil acts as a true lubricant, thereby reducing friction and increasing rpm. Ducted fan “blends” contain castor oil in addition to synthetic oil. Contrary to popular opinion, castor does not lubricate at higher temperatures, and is primarily used as a rust inhibitor. Synthetic oil can also burn at high exhaust temperatures; however, turbine engine oil does not burn until it reaches about twice the operating temperature of a ducted fan engine. Most “blends” contain too much oil, which dilutes the percentage of nitromethane—the component required for combustion.

Is the tuned pipe length important? - Tuned pipe length is crucial to performance! Having the correct tuned pipe volume is critical to your engine's ability to idle, transition, and produce maximum rpm on the top end. These 3 elements are the key to successful ducted fan flight! We cannot stress the importance of the pipe length enough; it is absolutely crucial directly affects your aircraft performance.

What is the apex of the pipe? - The apex is the point at which the pipe has its greatest volume and pressure. It is generally where pipe is seamed together so can be identified by a slightly darker gray band that appears on the exterior surface of the pipe. The apex is where the pressure fitting should be located.

Why are most glow ducted fan units 5” in diameter? - Engine rpm is inversely proportional to fan diameter; this means the smaller the fan diameter, the more rpm are required to turn it. Minimum rpm required for ducted fan flight is 20,000 rpm. The 5” fan unit is best suited to engines in the displacement range of .46-.91ci.