CTSW Fuel Tank Vent Design

[Skunkworks85]

Disclaimer: the following is a thought experiment only, 

 

Today I was messing around with some computation fluid dynamics, running simulations on the CTSW fuel Vent design. 

I inferred my best about how the vent is made, but looks like its pressed into the cap with 2 flats 90° from air flow, with a gap at the top. 

If anyone has any better info about how these are made, internal flow please let me know. 

Here is section of my perceived construction:

 

 

Here is simulation of the vent air pressure at 100 knots at standard temp and pressure, As you can see, the pressure in the tube that goes into the tank is slightly higher that outside pressure. ( Side view, cross sectioned in the center,  airflow from left to right.) 

 

Here is some close up detail ( blue = low end of scale and red = High end)

 

Now that baseline is established, Here is what I came for. 

This is with a 1.5mm hole in the back side of the cap.

 

So the question is why? Well, I was thinking of ways to mitigate icing of the vents, and was curious if a hole would effect the positive pressure of the vent.  This hole would be a "back up" in the event that vents gets iced over. The hole has no effect on the performance of the vent. Now i will step up the hole size to see how big is to big. 

 

Any Thoughts? How would you do it?

 

 

 

 

 

 

 

 

[Roger Lee]

I did testing and research back in 2007. I tried several things with the original vent cap. No direction change or anything else worked. It's too short and fuel can slosh or come out in turning banks. I did find a solution and tested it for 1300 hrs and no more fuel over flows or sloshing.  

 

[cdarza]

Will this make my CTSW faster ?   lol   Joking aside - i wish i had the brain power to process what you have simulated here.   Interesting, but i need to study to understand - In the meantime, the colors look great and i think i see a cows head somewhere in the image.

[FlyingMonkey]

9 hours ago, Roger Lee said:

I did testing and research back in 2007. I tried several things with the original vent cap. No direction change or anything else worked. It's too short and fuel can slosh or come out in turning banks. I did find a solution and tested it for 1300 hrs and no more fuel over flows or sloshing.  

 

So you are just going to leave us hanging and not tell us the solution?  Is this the one you mentioned before using just a curved 90° tube as a vent?

[Skunkworks85]

10 hours ago, Roger Lee said:

I did testing and research back in 2007. I tried several things with the original vent cap. No direction change or anything else worked. It's too short and fuel can slosh or come out in turning banks. I did find a solution and tested it for 1300 hrs and no more fuel over flows or sloshing.  

 

Roger,

 

I am very curious what you did, Again though this experiment is to see effects of potential icing mitigation techniques. 

I have 2 other options in mind too

1. Add a vent, with check valve to tank inside wing ( similar to high wing Cessnas)

2. Add check valve facing back to fuel tank vent, 

[Madhatter]

This is the new cap for super cub. I have it on my modified J3. No fuel vent leakage.

[FlyingMonkey]

I did find that most of my leakage over the wings was not through the vent tube itself, but around the nut at the base of the tube.  I put some fuel tank sealant around the nut and so far have been leak free for several months.  I still sometimes get vaporized fuel behind the vent leaving light streaks, but that's from maneuvering with full or nearly-full tanks.  I used to get huge amounts of full sloshing out over the wings when parked on even surfaces, and that problem appears to be gone.  I still try to park as level as possible.

[Anticept]

 Can't comment on the SW, but for CTLS, the leakage in my experience has been from spillage, or o-ring leakage. Happens often.

[FlyingMonkey]

12 minutes ago, Anticept said:

 Can't comment on the SW, but for CTLS, the leakage in my experience has been from spillage, or o-ring leakage. Happens often.

Doesn't the LS have wingtip vents?

[Anticept]

It does. My point is that the LS also loves to leak, sometimes its from spillage while adding fuel, but I've also seen it leak around the o-ring.

[FlyingMonkey]

23 minutes ago, Anticept said:

It does. My point is that the LS also loves to leak, sometimes its from spillage while adding fuel, but I've also seen it leak around the o-ring.

When the LS sits one wing low for a while with a lot of fuel, will it pour out the tip vent onto the ground?  That's how my SW has issues, and it's made worse in that my right wing is 1.5" lower than the left.  The fuel drains into the low wing tank until it gets to the cap and pushes past the 10mm nut on the vent tube.  I have had fuel literlly pouring off the bottom of my wing more than once.

[Anticept]

The tube doesn't run straight to the outboard side, it does a U in the wing to help prevent that. It has to be tipped considerably with near full tanks to get it to drain.

[Jim Meade]

What a plumber would call a P trap?  All house plumbing has it so the water in the trap stops the odors from coming back up from the sewer system.

[Anticept]

No, it attaches somewhere outboard in the tank, runs inboard then back outboard to the wingtips. Keeps you from losing fuel if you fly uncoordinated.

I had tanks full and weighed it at A&P school years back, and the scales used were meant for very light jets and smaller. They sat about 4 inches high. I picked up the wing and set it down on the scale, and the someone said the other side started leaking fluid. It was the fuel. I had tanks TO THE FILL NECK, which is probably actually overfilled a little.

I've picked up the wing and walked inboard to where the other wing was almost touching the ground, and nothing leaks when the tanks are down a bit.

[Roger Lee]

9 hours ago, FlyingMonkey said:

So you are just going to leave us hanging and not tell us the solution?  Is this the one you mentioned before using just a curved 90° tube as a vent?

Hi Andy,

I thought you'd never ask. LOL

Yes Andy it is the 90 degree facing forward vent tube. It worked great for years. Never will change the fuel flow differential between wings, but you will not have any more sloshed fuel from turning or over filling or fuel on the wing. I bought the tubing at Ace Aviation (Ace Hardware) and just used a bender and then threaded the one end to screw into our existing cap. Even used the original nut. I did this when I was testing to see if the vent tube affected the fuel differential between tanks. I liked it so much it stayed there for years.

[Omerrett]

1 hour ago, Roger Lee said:

Hi Andy,

I thought you'd never ask. LOL

Yes Andy it is the 90 degree facing forward vent tube. It worked great for years. Never will change the fuel flow differential between wings, but you will not have any more sloshed fuel from turning or over filling or fuel on the wing. I bought the tubing at Ace Aviation (Ace Hardware) and just used a bender and then threaded the one end to screw into our existing cap. Even used the original nut. I did this when I was testing to see if the vent tube affected the fuel differential between tanks. I liked it so much it stayed there for years.

And your vents are still going strong… they just got a fresh bit of paint :)

[FlyingMonkey]

Thanks for the picture...I might have to replicate that.  It's such a simple design and mimics a lot of vents on many different kinds of airplanes.  By Buddy's Avid Flyer has the same setup.  I'd still say the critical piece here is a good seal between the tube and the cap to not allow fuel past the 10mm stop nut on the tube. 

There has to be a sweet spot for the length of that tube...too short will allow fuel to puke out when parked on uneven ground or when maneuvering in flight, too long is just unnecessary drag.  It looks like the tube in the picture is about 6-7" tall from base to top of the forward facing inlet, is that true?  I also notice and like that the inlet is still facing slightly upward, which buts a little more fuel overflow protection and still provides plenty of ram air pressure in the tank.  Plus the higher pressure ram air going into the tank will help keep fuel from coming out in flight.  

Only downside I can think of is this is possibly more prone to icing or water ingestion, but if you are flying in icing or heavy rain you are doing CT wrong, and you should be sumping tanks before each flight and you'll see any water that gets in.

Roger, do you recall the diameter tube you used? 

[Roger Lee]

I believe it was 1/4". It was only for a test, but liked it so much I left them in place. Used a tubing bender to shape it and then tapped one end for the threads in the cap. It was very easy to do.

[Mike Koerner]

Comments on vents with bend forward tubes and pressure equalization between the wings have nothing to do with the original post. In fact, they represent the exact opposite - rain and icing collectors. And no matter how hard you try to avoid rain and icing, someday they may find you (except in the Philippines). Maybe it will be after you leave the plane parked outside on a trip and try take off the next morning!

Good work Skunk Works! My guess is that the back hole would have little effect on pressure recovery until its area is a substantial portion of the area of the back skirt of the helmet - maybe about the OD of the vertical tube? I'm a bit surprised you don't see better pressure recovery in the vertical. I would have guessed it would be closer to stagnation pressure. Do you have the vent flow rate set to around 10 cu-in/min (1/2 x 5 gal/hr x 231 cu-in/gal x 1 hr/60 min). Since that's a small fraction of the air flow impinging on the inside of the back skirt of the helmet at 100 kts, you can probably just leave it dead-headed. I’m not sure to what degree the hole will relieve icing. The fact that air can still travel up into the helmet means ice particles may be able to too, though with little vent velocity their weight will inhibit this to some degree. Ultimately though, you may need to conduct icing tests of your final design!

[Roger Lee]

Aircraft have been using this curved vent on caps and fuel systems for many decades. If there was a serious issue from them we would have heard a long time ago. Plus I myself and two others I know have used them for 1500+ hours each in all climates and flying and parking outside is not an issue. Rain or icing is not an issue. I was running mine up at Page, Az all those years and people ask about them. Think about rain trying to hit that tiny hole? Plus it would have to be an awful hard rain and I doubt any CT is in that kind of mess.

[Tom Baker]

Roger, I am pretty sure the whole idea of this post is to eliminate the possibility of ice blocking the fuel vent, while still maintaining adequate pressure for the fuel system. Living in the Midwest and working at airports for over 40 years, I can tell you that little protruding mast like the fuel vents on the CTSW, and OAT's are the first place ice will start to form when icing conditions occur. The whole prohibition for light sport airplanes flying in IMC came about from two incidents of a CTSW having an emergency landing, because the fuel vents iced over and the engine quit. Granted the airplane was flying in IMC conditions. Neither of the incidents caused any severe damage or injuries, and both may have been the same pilot.

I have witnessed ice forming on the fuel vent on a Cessna first hand, and that is part of the reason there was an AD issued for adding vented fuel caps to many of the early Cessna airplanes.  

[Anticept]

We were in MVFR, with the TKS running. SR22T.

When ice starts to build, it can build very fast.

[Roger Lee]

Since you aren't sup[posed to be flying in IMC conditions or temps so low with rain that it may freeze then it shouldn't be an issue. They've been using these style vents before we were born on tons of aircraft that fly in worse conditions than us.Bottom line an icing condition that would cause this to block up would be pilot error and not system failure. In anything we do good pilot judgment is always a key factor. 

[Tom Baker]

There are some people who are ELSA, and want to do limited IFR for traveling purposes. I don't think anyone would intentionally fly in icing conditions, but sometimes inadvertent icing conditions happen. I have had ice before in VFR conditions, and can confirm with 40 plus years of flying that actual conditions can be different than what was forecast. 

[Madhatter]

IFR in a CT is good for easy access to simple flight routing in the system when VFR, it's something I had thought about. But for long cross-country flights not so sure the CT is the way to go. Just remember the CT pitot tube design is less than desirable in precipitation also. Not just ice but will not shed internal water like certified pitot tubes. If you don't think water gets in you will be wrong when in moderate rain. I have seen it a lot . The CT pitot lines flow continuously downhill with no rise right after the pitot tube so no water can drain out when on the ground. It will go right to the airspeed indicator over time, even faster if you have a small leak in the airspeed. I had to design a water trap due to $3000 worth of air data computers that can not tolerate any water in my CT.