I am looking at maybe doing a Convair Sea Dart should be a twin. I would think doing a single edf with bifurcated inlet and outlet would help to save weight.
My question here is I intend to use a 70mm unit and since the inlet ducts are rather small I would scale so that each is equal to half the size of the fan. With the outlet however I am slightly stumped. I know I need to work with the 5-10% FSA rule. The airframe the EDF came from had a 65mm outlet. This by the way came from one of the china F-16's. My question then is do I half the 65mm for the two outlets or maybe go a little larger? If I do that it would be some rather small outlets. Am I correct in my thinking about the inlets as well?
Any help and suggestions are greatly appreciated

The inlet area has to be equal to or greater than the FSA of the fan. The area of the exhaust tube opening should be no less than 80% FSA for a single opening. On the bifurcated exhaust you can try dividing it in half but there will be a lot of losses so design the pipe smoothly to keep the flow efficiently moving through the pipe.
Start there and adjust the tail pipe diameter for effect.
Make sure you calculate FSA on that particular EDF unit well so you have a good starting point.
You may benefit from a higher blade count on one of those budget HK EDF units.

The rotor on the unit is either a 10 or 12 blade already with an out runner motor. It has an 80a esc I believe it ran on a 5s battery

Originally Posted by Chris Nicastro

The inlet area has to be equal to or greater than the FSA of the fan.

False. Inlet area can be as low as 65% FSA if the shape is just right. 80% is a reasonable number to shoot for, especially with new fans having more than 4 or 5 blades.

For the exhaust, make sure you're looking at half the area, not diameter. As long as the total area is equal or less than 100% fsa, I think it will work ok.

a 2.250 exit dia would work ...if you split it do 1.250" on eaxh tail pipe,that will help make up for splitting it ,there will be some drag / turbulence .with a bi exhust

Originally Posted by Steve C

False. Inlet area can be as low as 65% FSA if the shape is just right. 80% is a reasonable number to shoot for, especially with new fans having more than 4 or 5 blades.

For the exhaust, make sure you're looking at half the area, not diameter. As long as the total area is equal or less than 100% fsa, I think it will work ok.

Steve if this is the case then it might be do-able in scale. The 2 inlets are rather small. I will have to run the area numbers again tomorrow to see if it is anywhere near the 65-80%. If scaled to meet the 100% inlet diameter the exhaust outlets would look way too small at the 1.25" suggested.
Should the inlets have a twist to them as well as the outlets? I think this could help to smooth the airflow coming into the rotor and may also help with the vortice of the outlet as the air takes on a swirl when forced by the rotor. I think this can also help to get past any high or low pressure areas in front of or behind the rotor.

The stator straightens the air, so there is no twist after the fan.

have everything as smooth and straight at you can. Stu did a thread once for his F-5. Certainly worth looking into.

CapMike
I think you will learn a little by looking at my latest model the T -2 C buckeye in the scratch-build thread.
Fans are individuals. Some can handle long intakes, some not.
the exit for a WM 400 mk 2 can be as little as 48 mm diameter for a single duct.
that is 37mm dia for a bifurcated exit

Im suggesting a larger intake simply because on the Sea Dart they are very small and located on top of the fuse.
The exhaust is more simple than an F-4 exhaust for example. On the Sea Dart the openings are tangent so the divergance of the two tubes is minimal unlike an F-4. When you make the exhaust tube you can make a minimal splitter between the pipes.

Originally Posted by clausxpf

CapMike
I think you will learn a little by looking at my latest model the T -2 C buckeye in the scratch-build thread.
Fans are individuals. Some can handle long intakes, some not.
the exit for a WM 400 mk 2 can be as little as 48 mm diameter for a single duct.
that is 37mm dia for a bifurcated exit

Looking at your plane it looks as though the intake inlets are almost as small as that on the Sea Dart.

Originally Posted by Chris Nicastro

Im suggesting a larger intake simply because on the Sea Dart they are very small and located on top of the fuse.
The exhaust is more simple than an F-4 exhaust for example. On the Sea Dart the openings are tangent so the divergance of the two tubes is minimal unlike an F-4. When you make the exhaust tube you can make a minimal splitter between the pipes.

I was thinking that as well, but if you look at the plane it aslo has cheater screens over the top. One concern is that it does not get spray up into the intakes

Having Autocad is great. Using the calculator for area I have run some numbers to have a look at doing this scale. I still need to measure the rotor to get FSA but here is what I have come up with. If doing the Dart at say 1:20th scale the inlets combined would be 2.3994. The outlets if both are a 1.25 diam would give me a 2.4544 area. This does not take into account the inlet cheaters that could potentially give me an area of 5.3994. Off to look for my fan to measure the FSA for better information for those who have helped so far

Small intakes can work, but you may end up tinkering a lot if it's less than about 80%. I'm not trying to talk you in to using exact scale inlets, but I wanted to set the record straight about needing 100% or greater inlets. It simply is not true and some of the highest performance EDF's I've seen had smallish inlets.

I don't think we see much in the way of experimenting with bifurcated thrust tubes. These are always on scale airplanes and once it flies, that's tends to be good enough. I have not played with these, but It makes sense to do some things. Avoid sharp angle turns. Don't allow the total area to increase where it goes to two ducts. Where they split, the center should be rounded like the leading edge of a wing. BTW, the inlet is sharp there, like the trailing edge of a wing.

I have a small F-86 that was such a pain that I did in fact put a cheater hole in it. I know, I have never been a fan of the things, but the darn thing wouldn't climb and it was a long time ago. The cheater hole helped a bunch. It would fly, but it was slow and I wanted more. I kept at it (over a few years) and finally hit on a combo with the HK 7 blade 64mm rotor that far exceeded previous performance.
If you just want it to fly, go with either something larger or with cheater hole, but keep in mind that the cheater hole will never match the performance of a clean working inlet duct.

Ok here is the spec on the EDF. It's an 8 blade unit motor kv unknown using a 40a Raiden ESC. The inlet diameter is 82.18mm or 3.2355 inch. The stator is 80mm or 3.1520 inch. This gives a FSA of 7.803. I think I need to scale the airframe up for larger intakes maybe.
Steve what do you think here? Would I have enough with the cheaters? Otherwise my outlets would really need to be pretty small, or I might need to go for a twin but the added weight might be a detriment to getting the plane to rise off the water

I was going to stay out of this thread but decided to add a bit of (confusion? ) information.

Look at this post for a general idea of typical Fan Swept Area:
http://www.rcgroups.com/forums/showp...00&postcount=1

Typical 100% FSA for a 70mm fan would be approximately 29-30sq centimeters (4.5sq")

As well; a decent starting point for outlet diameter of a single exhaust 70mm fan is 55mm (or 2.16")
The AREA of this exhaust is 23.64^centimeters (3.66^")
Now for split/bifucated exhaust you can't just divide the single pipe DIAMETER in half.......you have to divide the AREA in half.
So: total area of 23.64^centimeters (3.66^") divided by 2 equals 11.82^ cm (1.52^")
Take that AREA and work the calculations backwards and you get a DIAMETER of 3.87centimeters (38.7mm) or 1.52" for each exhaust outlet.

And to reinforce what Steve C and Claus mentioned; you do not need 100% FSA with typical modern EDF power. Anywhere between 80% and 100% that suits you will be fine. Just make sure that you don't have a sharp inlet lip....