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Old Feb 08, 2008, 01:17 PM
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Yah, I am thinking about possibly plugging it. That's when that guy with the CNC I know may come in handy. All I would have to do is model it up in CAD, and cut out the solid plug. Little bit of hand work and I could start molding. Its just hard for me to picture a fiberglass fuse lighter than a balsa one. But with the complex curves of the F22 it may be a better option.
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Old Feb 08, 2008, 01:33 PM
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yeah you never know weight wise till you see where balsa parts get swapped for ply and such for strength. just from experience though from F-15'a and Mig 25's and by the material into my plug, I would say it'd be close enough to not make a huge diff. so maybe end of day 3 to 4 oz diff(glass being tad heavier?), for that I'll take a easy repeatable fuse thats open inside and easy to work on and adapt.

Barry
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Old Feb 08, 2008, 01:58 PM
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Well if that's the case then Barry. In your plugs are you using bulkheads on the inside for strength. Or are you just relying on the strength of the glass to keep everything straight and strong?
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Old Feb 08, 2008, 02:48 PM
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Quote:
Originally Posted by nem3
Well its kind of a catch 22. You either do it the way I do, or the way you suggest. Either way you end up with a problem. If you do it your way, the entire intake would have to be roughly 130% of the FSA. And once the air got to the spinner section, there would be an abrupt change in area and velocity. Or you could start off with a 100% FSA intake and then go to the fan dia. But then your going from small to big to small again. The would be two changes in area and velocity. I know that many won't agree with me and that O.K. I would rather try it my way and see what happens, than just say it'll never work.

Any ways here are a few more pics to help you guys sleep at night. It's 2:10a.m. and I think I'll be heading off to bed now.

Just my opinion,I'm not pushing you.
Air has no problem to increase speed ,but get a lot going against a pressure increase that can be found whenever speed decreases.
It's not the same case,but the most drag in a wing profile is caused by boundary layer separation.
I think that the best designed air duct in the modelling field are those found in Violett kits,take a look at them.
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Old Feb 08, 2008, 03:23 PM
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I am not sure I totally understand you. The way I understand it is that as you neck down the intake. Air velocity will increase. Also, in a perfect world, pressure would not necessarily increase due to the increase in air velocity. Yes this does happen to some extent due to inefficiencies in the duct design. Air can't move through it quickly enough so pressure builds. The same thing would happen if you went from a 4" hole down to a 1/32" hole.

I keep going back and rereading your post. You have to be very careful with the wording. Because if said backwards, it's wrong. If you have air traveling through a duct with an area of 30 sq. cm. And at some point to duct begins to transition down to 15 sq. cm. Air velocity will increase and pressure will also increase. But this has some limitations, if there is enough drag in the surface, or the duct is very poorly designed, the air velocity would decrease and air pressure would severly increase.

But lets say you have another duct that has an area of 30 sq. cm. And at some point the duct transitions up to an area of 120 sq. cm. Air velocity will decrease and air pressure will decrease. And there isn't much in the way of design that will change this outcome. Unless after some period the duct transitioned down to 30 sq. cm. At that point the air pressure will increase again. And if the whole duct goes from 30 to 120 to 30, there will most likely be a pressure increase in the section with an area of 120 sq. cm. Right

Just because air velocity decreases doesn't mean the pressure will increase.

I am also not sure where your going with the whole boundary layer thing. But boundary layer drag is the reason I decided to stay away from sticking more things into the intake duct.

I should also point out that I do not claim to be any kind of expert in this area. I am simply applying what seems logical to me. If it doesn't work, and it may not, then fine. But at least I eleminated the option.

I just looked at the duct system for BVM's F-4 phantom, and I fail to see how his design is any different that what I am trying to do. Could you please explain.

Thanks, Nick
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Old Feb 08, 2008, 05:19 PM
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Quote:
Originally Posted by nem3
Just because air velocity decreases doesn't mean the pressure will increase.
Nick,
Bernoulli's says this is exactly what happens. Pressure and velocity are the two (major) types of energy airflow has. Unless you do work on it, i.e. the fan itself, all you can do is change one to the other. Therefore, in the duct, if the velocity reduces, the pressure must increase.

That said - ignore the theory! Looking at your designs, I'd say you're spot on. Even without understanding pipe flow, you've come up with a very good duct - good work! Short of cranking out CFD, you've done as much as possible.

(BTW, I hope you don't mind, but I'm borrowing your method for my S.32 )

Odysis
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Old Feb 08, 2008, 05:47 PM
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Thanks Odysis. Your more than welcome to use what ever you'd like.

I think I just figured out why I am trying to confuse myself. I am thinking in terms of pushing air into the intake duct. The way I am thinking is that if you push air through a small opening, not only will its speed increase, but the pressure would have to increase since the air will be resisting going through the small opening. Now take the intake and stick it on the front of a fan. Go to full power and what happens. An area of low pressure (vacuum) and high velocity is created at the small opening and lower velocity and higher pressure is created at the large opening.

Am I right this time
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Old Feb 08, 2008, 07:01 PM
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Man, don't get wrapped around the axles with the whole aerodynamic stuff! There are some rather heated discussions every time aerody comes up, the main reason being there is no simple explanation for any flow!
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Old Feb 08, 2008, 07:38 PM
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WOW nem that is very nice. I've have to have one
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Old Feb 08, 2008, 11:07 PM
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in the plug itself the formers are there for shape, the glass on it is what makes up the strength.

and BVM are the worste ducts to be looking at since there for gas fans or slow running electrics fans. granted for gas stuff they were and are top notch! And since EVF mimicks a really fast gasser fan its also ideal. But for real velocity fast spinning fans they are way to short and bent, and the Y's end too short and should be right up as close to spinner as possible. And boundry layer at these speeds is ofcourse existant, but not nearly as thick as you'd expect, and frankly they do actually offer some relief from wet area drag.

Barry
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Old Feb 09, 2008, 01:44 AM
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Quote:
Originally Posted by nem3
I am not sure I totally understand you. The way I understand it is that as you neck down the intake. Air velocity will increase. Also, in a perfect world, pressure would not necessarily increase due to the increase in air velocity. Yes this does happen to some extent due to inefficiencies in the duct design. Air can't move through it quickly enough so pressure builds. The same thing would happen if you went from a 4" hole down to a 1/32" hole.

I keep going back and rereading your post. You have to be very careful with the wording. Because if said backwards, it's wrong. If you have air traveling through a duct with an area of 30 sq. cm. And at some point to duct begins to transition down to 15 sq. cm. Air velocity will increase and pressure will also increase. But this has some limitations, if there is enough drag in the surface, or the duct is very poorly designed, the air velocity would decrease and air pressure would severly increase.

But lets say you have another duct that has an area of 30 sq. cm. And at some point the duct transitions up to an area of 120 sq. cm. Air velocity will decrease and air pressure will decrease. And there isn't much in the way of design that will change this outcome. Unless after some period the duct transitioned down to 30 sq. cm. At that point the air pressure will increase again. And if the whole duct goes from 30 to 120 to 30, there will most likely be a pressure increase in the section with an area of 120 sq. cm. Right

Just because air velocity decreases doesn't mean the pressure will increase.

I am also not sure where your going with the whole boundary layer thing. But boundary layer drag is the reason I decided to stay away from sticking more things into the intake duct.

I should also point out that I do not claim to be any kind of expert in this area. I am simply applying what seems logical to me. If it doesn't work, and it may not, then fine. But at least I eleminated the option.

I just looked at the duct system for BVM's F-4 phantom, and I fail to see how his design is any different that what I am trying to do. Could you please explain.

Thanks, Nick

Odysis has already explained the reason for pressure changing with speed:energy remains constant ( except for losses due to drag ).
In very simple words:I mean that a 5 degrees deviation ( enlargment) in duct walls can cause flow separation and produce bubbles .
Where did you see Phantom ducts ?
I would like to take a look at them too.
There is another discussion about how ducted fans work here in RC Groups ,the writer put a lot of pdf files,I'm sorry I can't remember his name.
Try a search.
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Old Feb 09, 2008, 11:39 AM
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5 degrees instant yeah, but gradual won't, specially at our speeds. go super sonic then yeah all kinds a things can happen. Just look at a nose cone itself, its got angles on the tip of more than 5 degree's, now look at the base of the blades, how much compensation there shows you how much shrouding happens with flow, you'll notice it isnt much, if it were then blades would not start till much further outside the hub. And the air is so tight even around a spinner, that look at a ds fan. the ds uses by pass air around the spinner to cool the motor, look right down the fan, and you will see just how small the slot is. And you could be talking about Klaus? he posted a lot of data on here along with papers and pdf's and illustrations, great reading for sure.

Barry
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Old Feb 09, 2008, 01:16 PM
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Quote:
Originally Posted by monkamarm2000
5 degrees instant yeah, but gradual won't, specially at our speeds. go super sonic then yeah all kinds a things can happen. Just look at a nose cone itself, its got angles on the tip of more than 5 degree's, now look at the base of the blades, how much compensation there shows you how much shrouding happens with flow, you'll notice it isnt much, if it were then blades would not start till much further outside the hub. And the air is so tight even around a spinner, that look at a ds fan. the ds uses by pass air around the spinner to cool the motor, look right down the fan, and you will see just how small the slot is. And you could be talking about Klaus? he posted a lot of data on here along with papers and pdf's and illustrations, great reading for sure.

Barry

the lower is the speed the lower is Reynolds number.
With low Re air has an heavy work to do to stay put an enlarging duct where speed slows down and pressure increases.
I think that duct should have minor direction changes and the least possible section changes,most important whenever area increases.
No problem with a spinner pointing into the air,serious problems may arise from a spinner pointing back.
I can't understand what you are meaning speaking of ds.
Sorry.
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Old Feb 09, 2008, 02:19 PM
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yeah guess ya kinda have to see ds, kinda hard to describe. basically the spinner is deliberatley smaller than motor tube so that air around it can pass into tube rather than shrouding over the motor tube stream lining it. and i see what your saying about duct diverging, but that usually only a problem when it does it on its own. In a properly made duct where it diverges in unison with spinner, the air doesnt know its diverging cause as it goes out spinner is also pushing air out in same ratio. so air see's same size path, and long as its not real sharp it wont seperate.

Barry
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Old Feb 10, 2008, 02:11 AM
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Originally Posted by monkamarm2000
yeah guess ya kinda have to see ds, kinda hard to describe. basically the spinner is deliberatley smaller than motor tube so that air around it can pass into tube rather than shrouding over the motor tube stream lining it. and i see what your saying about duct diverging, but that usually only a problem when it does it on its own. In a properly made duct where it diverges in unison with spinner, the air doesnt know its diverging cause as it goes out spinner is also pushing air out in same ratio. so air see's same size path, and long as its not real sharp it wont seperate.

Barry
anyway ,if you have the air inlet area larger than the fan area,I mean the area determined by the fan outside diameter less the area of the spinner cone,the only need is to reduce the area gradually as you do with the form of the sections of the duct.
No need to reach the fan area in a point coincident with the spinner nose.
I'm telling,one more time, that less curves and deviations means less drag.
look at a jet inlet.
They too have a spinner cone,but the walls of the inlet are curved at leading edge and then remains straight.
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