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Author Topic: Vent on left side of fuselage, myth or magic?  (Read 843 times)

Offline frank mccune

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Vent on left side of fuselage, myth or magic?
« on: October 29, 2021, 08:09:26 AM »
     Hello All:

     In the past I have noticed that the vent for the fuel tank has been routed to face forward on the left side of the fuselage.  When I asked why this was done, the only explanation that was given was that it works better.

     Can someone explain why this would “Work better.”

      Tia,

      Frank

Offline bob whitney

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Re: Vent on left side of fuselage, myth or magic?
« Reply #1 on: October 29, 2021, 10:57:17 AM »
i agree with Laura .  the important thing is to get the vent out of any turbulence which usually is at least 5/8 inch away from the fuse RAD

i have seen on some profiles where a standard combat chicken hopper tank would quit on takeoff ,adding a piece of fuel tubbing and bringing the vent tube over to the inside of the fuse and facing it forward took care of the problem
rad racer

Online Brett Buck

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Re: Vent on left side of fuselage, myth or magic?
« Reply #2 on: October 29, 2021, 11:11:31 AM »
     Hello All:

     In the past I have noticed that the vent for the fuel tank has been routed to face forward on the left side of the fuselage.  When I asked why this was done, the only explanation that was given was that it works better.

   Because you aren't counting on sucking air in against centrifugal force - that's why it is on the left. Facing forward - to guarantee it doesn't siphon fuel out, which it frequently does if you have it at 90 degrees. There are some potential downsides to the ram air you get that way, but you can't have raw fuel streaming out of the vent. This can happen even if it is the only vent, it seems like it can suck fuel down one side of the tube and still suck air (at an infinitesimal rate) down the other side of the tube.

    Of course, it will "work" to an extent in other positions, but *why try to make things harder on yourself when there is something simple to make correct*?  Take 10 things that are marginal or small problems and it becomes a big problem.

     Brett
« Last Edit: October 29, 2021, 11:36:01 AM by Brett Buck »

Offline Dennis Toth

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Re: Vent on left side of fuselage, myth or magic?
« Reply #3 on: October 29, 2021, 11:29:25 AM »
This is something I have experience with and it depends on the tank position, fuse shape and if you are running pressure or not. I had a TF Tutor profile and could never get a solid engine run, change engines, tanks but always had the uniflow vent out of the tank top. The only engine that came close to a steady run was an OS FS26. But the fact was it was the position of the vent that caused the problem. It was down below the top edge of the fuse and in the turbulent airflow, never gave it much thought.

Many years ago at one the the GSCB stunt forums one of the local top fliers gave a talk about tank vents, at the time they were using conventional vented tanks. He indicated that the vents should be on the inboard side of the fuse about 1/8 - 3/16" ish off the side pointed into the free air stream. The reason was to eliminate pressure fluctuations in the airflow around the fuse. Now my experience was with my OTS El Diablo where I had the uniflow vent tucked in back of the engine (I was thinking that this would keep in out of the upstream/downstream pressure variations on windy days), for several years I fought the engine run and tried changing tank designs from round plastic to pinched back hard tanks. Finally, a few years ago I remembered the GSGB forum comments and decided to try it on the El D by adding a vent off the rear mount bolt pointing into the free airstream. Instantly the engine settled in and could set it any place I wanted and it held the setting. I also had a similar problem on my S1 Ringmaster, again had the vent just out of the tank (typical profile tank). Had very inconsistent engine runs, some good many all over the place. I change the vent to off the rear mount bolt and up in the free airstream, solid engine run ever since. 

Best,   DennisT

Online Brett Buck

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Re: Vent on left side of fuselage, myth or magic?
« Reply #4 on: October 29, 2021, 11:48:37 AM »
How significant difference does it make if you suck air against centrifugal force, Brett? Is it a noticeable difference really? I've never thought about it.
My engine sucks fuel about 0,25ml/second, that much replacement air is needed every second. L

    It is easily possible that the fuel can run out one side of the tube, while air runs in the other side. You can probably get around it by putting in a smaller diameter tube (so surface tension prevents it from separating), but, then, you can't get fuel in it. And before anyone jumps on "well, that never happens to me", right, it never happened to some people, it has happened in rare cases.  So, again, why create a potential problem, that might or might not crop up, when it is trivial to do it correctly?

  If nothing else, you can stream out fuel (pressure = 2.5gs x the density of the fuel x the distance from the fuel surface) while the engine keeps running and taking out infinitesimal through the spraybar, and blurp a bit of air to keep it running, then stream out more fuel. And yes, I have seen that happen, it is not impossible. Of course while it is streaming fuel out, it's also leaning out, making your 2.5gs go to 3 gs and making it stream faster.

    There are a million little things like this, none of them all that big by themselves, but the entire point of asking people with more experience it to take advantage of is to move beyond a small bubble of "I tried it once and nothing bad happened".

   Frank asked a question, it was not a silly question, I gave my answer, I think I explained it as well as I can. As always, it is just advice, no one is compelled to follow it, and the chances that you will have a problem (with this by itself) is not high.

     Brett

Offline frank mccune

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Re: Vent on left side of fuselage, myth or magic?
« Reply #5 on: October 29, 2021, 02:34:25 PM »
     Hello again:

      Thanks for all of the replies!

      There are so many small “things” that appear to pop up in this hobby to keep on on their toes.

      A flying mate of mine advised me to take care of all the small things to prevent them from becoming big problems!  It appears that this is so true when we play with our toy airplanes. Lol

      Frank

Online Fred Quedenfeld jr

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Re: Vent on left side of fuselage, myth or magic?
« Reply #6 on: October 29, 2021, 04:59:38 PM »
So the next time your are at Potato road with a profile  we expect  to see a vent tube  like this

Tube soldered to thin plate bolted to longer engine bolt

Fred Quedenfeld

Offline Mark wood

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Re: Vent on left side of fuselage, myth or magic?
« Reply #7 on: October 29, 2021, 07:37:32 PM »
So the next time your are at Potato road with a profile  we expect  to see a vent tube  like this

Tube soldered to thin plate bolted to longer engine bolt

Fred Quedenfeld

That'll work.
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Why do we fly? We are practicing, you might say, what it means to be alive...  -Richard Bach
“Physics is like sex: sure, it may give some practical results, but that’s not why we do it.” – Richard P. Feynman

Offline frank mccune

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Re: Vent on left side of fuselage, myth or magic?
« Reply #8 on: October 30, 2021, 08:10:17 AM »
     Hi Fred:

     Thanks for the reply! I will indeed make the change on my Magician ASAP.

     I can’t thank you and Dr. Pete for all of the help that you provide when we are flying.  Am I that old? Lol

     Stay well,

     Frank

Offline Howard Rush

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Re: Vent on left side of fuselage, myth or magic?
« Reply #9 on: November 02, 2021, 09:30:46 AM »
Alan Resinger puts his on the other side.
The Jive Combat Team
Making combat and stunt great again

Offline Dennis Toth

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Re: Vent on left side of fuselage, myth or magic?
« Reply #10 on: November 02, 2021, 09:55:10 AM »
With the vent on the inboard side in the free airflow stream you can't go wrong. Some ships work with it on the other side but as with many things in stunt the old timers did things that worked repeatedly, this was one of them. Likely started cause the conventional vents needed to be on the inboard tank wall and it was easy just to poke them through the side of the ship. If you are having inconsistent runs on uniflow non pressure setups and the vents are in back of the engine or outboard side of the fuse move them to the inboard side and you will likely be surprised how good that old engine runs. I had this on both my S1 Ringmaster and El Diablo and the change was dramatic.

Best,   DennisT


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