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Feb 17, 2015, 11:28 AM
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Edf


1 hp = 746 Watts.
EDF - 85% exhaust reduction Fan swept area( FSA)
you want it the exact same size as the rear of the edf tube, but in a normal configuration, you could vary the size of the opening depending on what speed you want more thrust at... a smaller opening will give less initial thrust, but will give more thrust when the plane is up to speed, allowing a higher top speed, where a larger opening will give more initial thrust, but will give less thrust once the plane is up to speed, meaning a lower top speed....
FSA caluclater -
https://www.rcgroups.com/forums/show....php?t=1160865
http://www.radiocontrolinfo.com/info...alculator/#FSA

Micro EDF Jets and Power Systems
https://www.rcgroups.com/micro-edf-j...r-systems-650/

the longer the inlet duct, the larger the inlet area should be. a good compromise would be an 85% FSA inlet, with an 85% FSA exhaust area. as a rule of thumb, you should never go smaller on the inlet, than the exhaust are

Intake area depends on the complexity of the internal ducting. corsair Nut had and F86 with an intake area roughly 70% of FSA and it performed very well. With a straight duct, smaller intake.area will work well. In complex shaped duct, I doubt that intake area would have worked as well.

Having said that, try to aim.for an intake area between 85% and 100% of your fans FSA. This will give you the beas performance overall. The smaller the intake area, the faster your top end speed will be, but static performance will be lower.

Always remember to use a round, or elliptical inlet lip when making inlets. Also, you need take into account , half the lip diameter as part of your intake area. Not just the throat area. If you don't and your throat area is 100% FSA, then add in your half lip area, it could end up as much as 110% FSA. Just thought I'd point that put to you.

The reason it performs better on the bench with a lip, is because with the lip, it not only pulls air in more.smoothly, but it also allows the fan to pull in air.from behind the lip too. Without the lip, the air has to make a sharper turn, which creates a zone of seperation, which robs power. Simple fluid dynamics.

Ducts for EDF
http://www.velocity-rc.com/cgi-bin/v...d=2&cid=&idx=1

catchment area vs intake area
https://static.rcgroups.net/forums/a...nletflow02.jpg

Try and keep the inlet ducting close as possible to 100%-120% fan swept area. Reducing it to around 95% is OK with short ducting but not desired. Thust tube can be tapered down to as much as 85%. The FSA of the thrust tube should never be greater than the FSA of the intake. Thrust tubes should be kept no longer that 3 fan diameters in length. If you are forced to use a thrust tube longer than 3 fan diameters then keep the thrust tube at 100% FSA unless your inlet is less. If the inlet is less than 100% FSA then try and match the FSA with the thrust tube.

The EDF 50 likes weights in the 6oz region where the EDF 55 likes weights in the 10-14oz region or wing loadings in the 9-12oz/ft2. This has been my observation and experieinces with these two fans.

The rule of thumb for inlet and outlet ducting is that the inlet should be about equal to the 'FSA' or slightly greater, say 100-120% of FSA. Inlets slightly smaller than FSA can work but thrust will suffer.

The outlet can be slightly less than FSA, 85% of FSA as an optimal figure is often quoted.

'FSA' = fan swept area, this is the cross section area of the fan shroud less the cross section area of the rotor hub.

Generally the smaller the outlet the lower the static thrust but slight reduction in outlet (to about 85%) can enhance thrust at high speed with only a small loss in static.

Ducting should be as short as possible but on scale models there is little that can be done to shorten ducting. It's very important to have a nice radius on the lip of the inlet.

the optimum inlet area for a DF is arround 120% of FSA, the outlet optimium area is between 80-90% of FSA. 100% of FSA can be used but to use a larger area will degrade performance as velocity will slow and you will lose the energy put into the airflow to start with.

general thrust equation
http://www.grc.nasa.gov/WWW/k-12/airplane/thrsteq.html

1. What should the ducting inlet opening area be compared to the FSA (fan swept area).

90-100% of FSA

2. Is there a length of inlet ducting that is desirable, ie the distance from the inlet to the actual EDF unit.

shorter is better in all cases - more ducting = more drag

3. Same question for exhaust....like is it better to just have it dump out right after the EDF fan or should there be more ducting.

shorter is better in all cases - more ducting = more drag

4. If 100% of FSA is optimal for thrust then is the reason many scale modles seem to have a less then 100% FSA only to make it look scale? I was under the impression that if the exhause air has a smaller exit hole it will increase it's velosity therefore increasing the thrust. Maybe this is true in theory but our EDF's just don't have the power to push the air out a smaller hole to gain this benefit where a real jet engine does?

Thrust verses speed is a trade off. 100% FSA on the outlet = max thrust. Smaller outlet duct exits, down to about 80% FSA, give more speed but reduce the thrust. Smaller than 80% just makes you use more power and gives less speed and less thrust.

5. Is ducting that's optimal for low speed thrust the same as ideal ducting for best high speed thrust/flight? I guess this question can be analogus to car engines where an engine can either be designed for low end torque or high end horse power but not really both.

see answer #4

some long reading for EDF
http://southamptonmac.org.uk/doc/RS_...ractice_v5.pdf

FSA
http://www.parkjets.com/home/2010/2/...df-lovers.html

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.
Last edited by bigtruck169; Feb 17, 2015 at 03:59 PM.
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