Hi folks . I plan on modifying my HET mig21 intake and making a duct for it.After cutting the battery Canoe out of the way along withthe nose cone . I plan on making an intake duct (with a smaller nose spike )out of blue foam , then glassing over it and melting out the foam with gas .The battery will then sit on top of the glass intake duct .What I would like to know is , Do I need the intake ring in front of the fan , even if I run a duct ?If so would I make the end of the duct that comes up against the 6904 fan ,big enough to slip over the intake ring ? Or do I skip the ring altogether ?

Skip the ring, run the duct right up to the fan and slip it over the housing.

I'd love to see some pics before/during & after you get it done. I've got a set of MiG 21 plans and will scratch build one someday. The duct work is tricky on this one with the shock cone in the way.

Julian

Well Boogie I just ripped out the shock cone tray . So Now I'm committed .Time to build a proper 70mm duct for the old Mig and get this ball rollin...

What do you think of this idea - Make a 70mm duct and then squeeze it a little flat so that the sides meet the edge of the inside of the fuse which is about 95mm wide?The duct would then be more of an oval shape like on an f16 while still leaving enough room above for battery and esc ? The Duct would sit on the bottom of the Fuse and have to curve up to meet the fan which is centered in a bulkhead .This setup would have to flow better than the old one which was a plugged up nightmare .

That would work fine. I took a similar approach on my F-84F build, works great.

Try this:
Make a paper tube 100% FSA, 2.45" dia (62.23mm), and fit it down in the fuse and see how much room you've got. That will give you an idea how much room you'll need to make to get the batts in there.

Make the intake a bell shaped opening and have the shock cone in a teardrop shape held in place by thin ply vanes. The duct will taper down to the 2.45" dia behind the shock cone and then flare out again to slip over the fan housing.

You may have already thought of this (or something even better). I've been mulling this over for sometime and would like to see how some one else figures this one out.

Is that the FSA of the 6904 fan ?

as long as you area rule the intake, it shouldn't matter what's in the way. if you've got a center body, such as the Mig-21 or the EE Lightning, you simply have to account for the center body as part of the area ruling. as long as you've got about 85% FSA to the point where the center body tapers to a point, you'll be ok. especially since you'll have a nice rounded inlet lip, which will draw in more air than the area of the throat. if you go to Velocity RC's website, there's a drawing that might make it a bit more clear. the drawing was done by Claus.

So are you saying that the original setup is just as good as a more open setup? That flat walled canoe makes no difference ? Have you seen the intake on one of these ?

Bob Violet I think came up with the area rule thing.

It doesnt' appear in any textbook on hydraulics or fluid flow I've ever read.

It does make people think about the shape, and in general area ruled ducts have smooth, gentle changes in shape. They also only accelerate the air, not speed up/slow down/speed up etc.

Those things are more important than the area itself. If the duct looks bad, it probably is.

Did you get any numbers (power/thrust, speed) before ripping the duct out? That's how you'd know if you've done better or not

Is that the FSA of the 6904 fan ?

Yep.
2.45" dia = 4.7 sq in (rounded numbers)

Ok so If I used autocad and did the intake in sections with a hot wire cutter , and squeezed the 2.45" sections into a more oval shape , would they still have the same area ? I'm no edf pro , so I have to ask .Opinions ? Anyone ?

If your working in autocad, won't that program tell you the area of the cross sections?

But, I think you can squish a circle into an oval and maintain the same area. ...greater minds may chime in with the correct answer.

So ,a 70mm Duct would be too much air for the fan ? Boogie , you mean flare the intake a little larger to account for the intake spike right ? I guess I could also make a round lip at the front of the fuse for better flow , even if it isn't scale ..Orrrr how about a servo operated intake spike on a hollow rail with the servo lead inside of the rail ? I could mount the servo right in the spike .

Autocad will give you the area of an ellipse. I can only do 2D so my approach would be harder. The oval will work but the transition should be gentle.

What is your reason for changing the duct? For speed are better acceleration?