Well after spending a day at the Arizona jet rally i seem to have caught a bug . I recently ordered 2 cheap 70mm fan/motor units from HK and want to build a Avro CF-105 arrow to put them in . Our government wont built it so i might as well . I'm wondering it someone could recommend at what scale i should be building at to fit the 2 fans i have coming . Working on plans in Sketchup and am getting to the point where i need to resize it before i continue .

Chris

With Sketchup, it should be easy to figure out. You want to scale your drawing so the area of each intake is equal to the Fan Swept Area (FSA) for the fan. The Wemotech 70mm fan FSA is 26.2 sq cm, you can use that number for your fans too.

Sketchup will tell you the area of any face in the model. Just scale it until the area of the face over each intake is 26.2 sq cm. That will probably result in a pretty good sized jet, maybe too large for your first one.

Thank you for the info .

Is it more important to size the intake or exhaust ? While drawing i feel i have some room to play on the intake side but on the exhaust i need it a certain size to have it look right .I was guessing i was going to be around 40" span before i started

Another question while im at it . Ive been reading and will continue but i want to confirm that intake length and outlet length arent that critical ? I would like my Fans pretty far forward so that will leave me with a long outlet , is this bad ?

I am in about the same situation as you are, maybe a little further down the track research-wise, but I have basically zero EDF experience at this point. I'm working slowly on designing a 90mm F-100 for my second build. For my first, I'm using plans for a 70mm F-4 from the forum here.

Having said that, IMHO you have to get both the intake and exhaust right for the chosen fan to get the kind of performance you want. You can design for high static thrust to give good acceleration, high efflux (exhaust speed) to give high top speed, or something in the middle.

It's all based on FSA. Some general guidelines, people do go both higher and lower than these figures. 100% FSA intake and 100% FSA exhaust will give high static thrust but low top speed. 100% FSA intake and 85% FSA exhaust is in the middle. 85% FSA intake and 80% FSA exhaust would be for high top speed.

Notice that these sizes are fairly close together. This is one difference between turbines and EDFs. Turbines sometimes have much smaller exhausts than intakes.

My suggestion is to size the intake based on your fan and design goal. This sets the size of the plane, then adjust the size of the exhaust based on your design goal. This may result in a non-scale exhaust, but unless you are willing to compromise performance possibly drastically, that's the way it is.

Based on my reading, intake and exhaust length aren't that critical. But everything else being equal, it is better to have a longer intake and a shorter exhaust. This will result is less power loss, since the "fast air" of the exhaust is traveling a shorter distance.

Fan position heavily influences CG however. The fan and the batteries are the two heaviest components, so you put the batteries ahead of the CG and the fan behind the CG. You can move or change the batteries to make CG, but if the batteries run into the fan before you balance, you will have to add weight to the tail. In this case, you might as well have positioned the fan a little further back to start with.

Thank you bill It all makes sense and confirms what ive read . I went 10 pages deep in the forum last night reading whatever i could find about inlets and outlets . Kinda surprised there was no sticky . Sizing the intake gives me alot of room to play with the exhaust side . We'll see where i decide to go with the EDF's , i'll put them toward the rear if needed but i think i should have alot of room for battery placement and would like to be able to mount servos closer to the rear as well .

Christmas decorations got in the way of any progress today , maybe i'll have better luck tomorrow

Dont get too overwhelmed with it. The thrust tube is mist crucial piece. Slap an edf and thrust tube on top of a plane and it will fly

Originally Posted by stingeragents

Dont get too overwhelmed with it. The thrust tube is most crucial piece. Slap an edf and thrust tube on top of a plane and it will fly

I don't know that I would say the trust tube is the most critcal. If you have poorly designed intakes theres much more loss there. Oversized intakes are just as bad as too small of ones. As BillO mentioned, there is a fine line for peak performance. All fans are not the same. For instance, a JetFan 90 for best performance needs exhaust diam of 75-76mm , that's only 90% FSA.


Just pay special attention to the intakes. What comes out the back, depends on how much and how well it goes in the front.