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Author Topic: Standard vent with pressure VS Uniflo with pressure ?  (Read 1117 times)

Offline Fredvon4

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Standard vent with pressure VS Uniflo with pressure ?
« on: February 27, 2017, 09:54:53 AM »
I think I have a good understanding how uniflo works and why

But it seems to me once CC or Muffler pressure is added the effects of uniflo are no longer true (agreeing that an open uniflo vent in the prop or wind stream causes changes in head pressure  vs hidden vent line out of any forced air)

With that (maybe wrong) understanding, I fail to see, on a uniflo vent with Muffle pressure, how adjusting the internal uniflo vent up or down relative to the spray bar has any noticeable effect

I also wonder why this seems to be also true for standard vent tank on Muffler pressure...in so much as I DO know the moving the tank up or down will change the engine run upright vs inverted flight, regardless if open vent or pressurized

I guess I am just curious why, in a pressurized system, the tank height or in the case of uniflo the vent height has any effect

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Offline Tim Wescott

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Re: Standard vent with pressure VS Uniflo with pressure ?
« Reply #1 on: February 27, 2017, 11:50:02 AM »
Oh, this is complicated:

The head pressure at the needle is equal to the muffler pressure plus any effects having to do with tank (or vent) height.  Muffler pressure adds a bit to the pressure at the needle, but in absolute terms it doesn't change how the pressure changes at the needle with respect to airplane attitude (gawd, this sounds clear as mud -- bear with me).

If the muffler pressure were Really High then you'd have the needle cranked way down and it would swamp out the effect of any head change due to tank (or vent) height.  But the muffler pressure is fairly mild.  It does reduce[\i] the effect of tank height, but not enough that you can go ignore tank height when you're trimming a plane with muffler pressure.
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Offline BillLee

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Re: Standard vent with pressure VS Uniflo with pressure ?
« Reply #2 on: February 27, 2017, 12:40:48 PM »
The pressure variation due to the centrifugal forces is the same regardless of whether the "outside" pressure is atmospheric or muffler-induced. Why you don't see so much difference when moving things around when on muffler pressure is that the PERCENTAGE CHANGE is smaller. IOW, the absolute change is the same, the relative change is smaller when muffler pressure is included.

As to moving the tank vs. moving the uniflow vent: All you are doing when you are moving the tank is moving the vent! The tank can be any old shape, it's the position of the uniflow vent in the tank vs the needle valve that's important.

If you can extract the science involved from the context of a CL Racing event, this is all written up in http://www.nclra.org/TechTopics/UniflowPhysics.pdf
« Last Edit: February 27, 2017, 07:29:08 PM by BillLee »
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Offline Steve Helmick

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Re: Standard vent with pressure VS Uniflo with pressure ?
« Reply #3 on: February 27, 2017, 02:46:58 PM »
Ummmm...the link to the .pdf doesn't work. At least, not at 12:53 Pacific Time.  :(

I've run several stunters with and without muffler pressure, and don't recall any difference that suggested changing tank height would be needed. With pressure, the NV needed to be leaned down a bit, but that was all.

On my SV-11 with piped PA .51, I needed more fuel capacity, and added a sump onto the bottom of the tank, just clearing the header. See pictures. No tank height change was needed. I didn't use pipe pressure on that one, but tried it later on my OS .46VF AAC. Again, no tank height change was needed, but I had problems with fuel siphoning and engine flooding, so I 86'd the pressure setup and plumbed the uniflow vent into the backplate cavity to eliminate ram air variations on windy days. H^^ Steve
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Offline BillLee

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Re: Standard vent with pressure VS Uniflo with pressure ?
« Reply #4 on: February 27, 2017, 07:29:52 PM »
Sorry 'bout that! Link is correct now.

Bill
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