Inlet / outlet ducts for EDF50 - RC Groups
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May 31, 2002, 10:35 AM
Michele Somaschini, Spain
RookieOne's Avatar

Inlet / outlet ducts for EDF50

I'm planning my first EDF project.

I know ducts are very important but I've not idea about how they bust be: length, area, shape ...........!!

EDFs location? Close to the wing, to the tail, to the CG.......?
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Jun 01, 2002, 07:41 AM
Registered User
Hi RookieOne

It is important to remember that you want as little length of duct as possible after the ducted fan, this is because as the duct gets longer energy losses appear. You also have the problem if you put the EDF a long way back, a lot of weight (battery pack?) is needed in the nose to counteract this weight. The probelm then is you have a lot of mass at opposite ends of the model and so inertia then steps in to produce what could be a very sluggish to respond model.

You want the area of the duct to be the same as the swept area of the fan or maybe slightly bigger and perhaps down to 10% smaller if you are trying to keep to scale dimensions though you must expect a drop in thrust if you reduce areas.

So to sum up,
1. Put fan around 2/3 - 4/5 of the way down the model presuming regular shaped fighter etc, try to keep weight close to the c of g.

2. Duct length is determined by the model size but area should remain close to the swept area of the fan, any changes in area in a duct should not exceed 7 degrees particularly for diverging ducts.

3. Try to keep all joins smooth particularly the one attaching the front of the EDF to the duct, you don't want a turbulent 'donut' tumbling into the front of the fan face.

4. The inlet should be nicely rounded a 2:1 eclipse is the idealised shape for the subsonic regime.

Hope this helps, I think I've answered your questions. If you have anymore feel free to contact me

Dan Armstrong
Jun 01, 2002, 04:23 PM
Registered User
stegla's Avatar
Originally posted by DevelopedBug
any changes in area in a duct should not exceed 7 degrees particularly for diverging ducts.
Dan Armstrong
What exactly does this mean?


Jun 01, 2002, 04:52 PM
Registered User

I guess I should have said the slope of the duct shouldn't exceed 7 degrees when changing the duct area

A diverging duct is one that goes from a smaller diameter to a larger one. The simplified reason you don't want to exceed 7 degrees is that the air can't expand quick enough to fill the tube and can trip and tumble down the duct, causing excessive losses inside the duct. The opposite of the diverging duct is the converging duct where large becomes small - there are laws involving this as well as only so much air can get down the pipe without having to change pressure or velocity but generally problems don't arise in what we are involved in, EDF conditions.

I think that makes sense......

Jun 03, 2002, 12:06 PM
Michele Somaschini, Spain
RookieOne's Avatar
Assuming that GWF EDF50 are not really super-mega-top-performance EDFs, a normal tube would be good enough?

As you maybe have seen in other threads, I'm planning some kind of EDF trainer which will be a flying tube on wings and T tail.

It will be simple as much as possible and quick to build.

Last question, in and out ducts must be at right angle with the EDF axle?

I mean: take the example of the F-86. The "mouth" area is bent some degrees down so it seems to have a nose. The tail exaust too.

This will influence the performances of the EDF???

(Sorry for my bad english)
Jun 04, 2002, 07:59 AM
Registered User
Hi RookieOne.

Yes, a tube made from a rolled up piece of stiff card would be fine for an EDF50 size fan.

I haven't seen your posts on your EDF model - one thing I must say is that these little fan units don't give very much thrust so model weight must be kept to a minimum if the model is to fly at all, have you considered using 2 EDF's instead? Perhaps using 2 ducts side-by-side? You could also keep your batteries and radio gear in between them.

Yes, a change in direction of the airflow will produce slight losses in performance - how much depends on the change. Try to keep the ducts as straight as possible.

Don't worry about your English it seems fine to me