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Author Topic: Uniflow vent direction....  (Read 1093 times)

Offline Steve Helmick

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Uniflow vent direction....
« on: August 13, 2006, 07:49:07 PM »
I noticed this past week that Bruce Perry's uniflow vent was pointed straight down, instead of straight forward. He said that Randy Smith recommended it, but I can't remember why, or what ills it cured. I'd expect a lot of fuel to siphon out. I've seen plenty of fuel siphon out of several uniflows that faced forward, anyway. Yet it didn't seem to. Please, oh, PLEASE, Randy Smith, explain what's up with that?
 #^ Steve

Hello Steve

Many people dislike the speedup slowdown  or  rich into the wind  lean downwind  when running a open uniflow tank when the wind blows. What happens is the added wind speed into the wind and  less  speed  downwind changes the fuel head pressure and makes the engine go lean and rich. I have been for years  using a 90 degree  end on the uniflow vent of the fuel tank. It doesn't really matter if you turn it down out  or  up. It stops   the fuel head pressure changes  when flying in windy weather.  It does not affect  the run of the engine or  siphon fuel from the tank. I have recommend this to Most of the guy I fly with, and  all use it. I stock the super light 90 degree plastic turn downs  or  you can make one out of Alum tubing. Just make sure you don't crimp it.
I will try to post a photo of it soon

Regards
Randy
« Last Edit: August 14, 2006, 09:22:51 PM by RandySmith »
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Offline peabody

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Re: Uniflow vent direction....
« Reply #1 on: August 13, 2006, 08:59:17 PM »
A uniflow in a "non-standard" orientation should allow a more uniform engine run...
Especially in high winds, when the aeroplane heads into the wind, and wants to be a little leaner (for more power), traditional tube-forward uniflow pressurized the fuel, causing a rich condition, slowing the plane and costing power.
Conversely, a "downwind) run will sometimes lean the engine...a uniflow tube out of the airstream, like inside the cowl, should work more uniformly.
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Offline Chris Gilbert IRL-1638

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Re: Uniflow vent direction....
« Reply #2 on: August 14, 2006, 07:01:16 AM »
I'd have concerns about a uniflo vent inside the cowl. My current stunter had problems caused by a forward facing vent  placed too close to the engine/fuselage, resulting in wildly inconsistent engine runs. I guess the important thing is that the vent is in clean air. Until I re-vented the tank I was convinced that I had a serious engine problem. Once my vent was out in the breeze everything was fine.

Another thing I've noticed on this particular model is that if the engine setting is wrong, too rich/lean the speed up/slow down as the model travels up and down wind is quite pronounced, but once the engine is set correctly this is not noticable (certainly at my level of flying).

Based on this recent experience I'm gravitating towards pressurised fuel systems, using muffler pressure, but I can't do that with all of my engines.
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Offline Jim Thomerson

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Re: Uniflow vent direction....
« Reply #3 on: August 14, 2006, 07:25:35 AM »
I put a one way valve on my uniflows and point them where convenient.  I use  KK valves and keep track of them because they are no longer available.  I have heard that ROJETT has some similar ones. 

I do have one exception.  The uniflow sticks out behind an upright engine.  When the engine is running there is a thin stream of fuel shooting out the one way valve.  Yes the one way valve is in the right way, there are no tank leaks, and air still makes it in past the stream.  It works OK but that airplane has other issues and I have never done anything about it, like moving the tube around a little.
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Offline RandySmith

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Re: Uniflow vent direction....
« Reply #4 on: August 14, 2006, 09:23:31 PM »
Quote from: Steve Helmick on August 13, 2006, 07:49:07 PM
I noticed this past week that Bruce Perry's uniflow vent was pointed straight down, instead of straight forward. He said that Randy Smith recommended it, but I can't remember why, or what ills it cured. I'd expect a lot of fuel to siphon out. I've seen plenty of fuel siphon out of several uniflows that faced forward, anyway. Yet it didn't seem to. Please, oh, PLEASE, Randy Smith, explain what's up with that?
 #^ Steve

Hello Steve

Many people dislike the speedup slowdown  or  rich into the wind  lean downwind  when running a open uniflow tank when the wind blows. What happens is the added wind speed into the wind and  less  speed  downwind changes the fuel head pressure and makes the engine go lean and rich. I have been for years  using a 90 degree  end on the uniflow vent of the fuel tank. It doesn't really matter if you turn it down out  or  up. It stops   the fuel head pressure changes  when flying in windy weather.  It does not affect  the run of the engine or  siphon fuel from the tank. I have recommend this to Most of the guy I fly with, and  all use it. I stock the super light 90 degree plastic turn downs  or  you can make one out of Alum tubing. Just make sure you don't crimp it.
I will try to post a photo of it soon

Regards
Randy
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Offline Busby

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Re: Uniflow vent direction....
« Reply #5 on: August 25, 2006, 11:46:03 AM »
In addition to the above comments concerning the location of the vent tube for the uniflow tank on full fuselage  planes ,I agree with Randy it stabilizes the run .

On profile planes I don't go to the trouble of fabricating an offset gooseneck to mount on the fuselage but simply make a longer piece with a restrictor with a dia of about .040 at the end, slide this on to the uniflow vent of the tank and tuck it into the hollow of the backplate.

It appears to accomplish the job of stabilizing the engine run,
no or very minor changes in the wind.
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Marshall Busby

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