Vented fuel caps with forward facing inlet tubes

[NT5224]

Ladies and  gents of the brains trust. Seeking some insight

 

Many aircraft that don't  actively pump fuel  to the motor have vented fuel tanks to ensure air pressure is on the side of gravity in ensuring a smooth fuel flow.

 

My fuel caps have a little forward facing tube bent through 90 degrees. I'm guessing that by facing them into the direction of airflow the intent is to prevent rain dropping in but more importantly to force air in and actively pressurise the fuel tanks to improve fuel flow. 

 

I run my electric fuel pump on take-off and landings and at full power but tend to switch it off in cruise.

 

What would happen if I replaced my forward facing vent fuel caps with regular vented caps  (just a hole in the top)?  Is there much difference? Has anybody else tried this?

 

Just curious in case I need to change caps or carry spares...

 

Cheers

 

Alan

 

 

[Old Koreelah]

Alan I’ve done a lot of mucking about with fuel vent locations and have learned that the tank must have positive pressure.

Rearward-facing vents may result in air being sucked out, leading to fuel starvation. 

Besides the direction it faces, the location of the vent is also important. I once took off without properly securing the cap on one wing tank. The low pressure around the cap sucked out some fuel and when I switched to that tank the engine spluttered. Even my wingtip vents have been affected by vortex currents; I had to find the best spot and align the vent into the airflow.

 

Another aspect to allow for is fuel flow if you end upside down, which is why my vent are as far from me as possible.

 

Despite all that experience, some designers seem to get it right. I was amazed to see one European aircraft has a pinhole vent on the trailing edge of each wing

[Kyle Communications]

Half the savannas around have modified the breather for the tank caps..the original comes out a tube that then goes through the wing to the bottom and you cut that tube with a angle to make sure the large open area faces the airflow. Most change this to just the tubes coming out the top and folded forward facing slightly down. You put some rolled up mosquito wire netting inside and that seems to stop the insects and water getting in. The originals are too small and insects get in and block off the flow out of that tank. This is why I changed mine to the alu tube ones as that very thing happened to me. It locked out a full tank from being used

 

[derekliston]

On my CH701 I have small tubes with about 3-4mm bore bent forward just above the cap. Every daily inspection I run a pipe cleaner through them to make sure they are clear. This has worked fine for 4+yrs apart from one very hot day with full tanks in the hangar and expansion caused fuel to leak out of the vents.

[skippydiesel]

4 hours ago, Old Koreelah said:

 

 

Despite all that experience, some designers seem to get it right. I was amazed to see one European aircraft has a pinhole vent on the trailing edge of each wing

Yeah! The ones I know, of have wing tank vents on upper tip ,trailing edge of upturned (not quite winglets) wing tip. Puts the vent above the tank max fuel  level but would seem (to me) to be in low pressure zone. They work.

[skippydiesel]

On my Zephyr I have combined both my main & auxiliary tank (both in fuselage) vent line into one. It goes up to the highest point, of the fuselage, does a loop and then straight down (a bulkhead), exiting into the hopefully high pressure air zone below the fuselage. Seem to work - never had a problem.

 

Sorry didnt address the Q - My point in the above description, is to demonstrate you dont need any fancy forward facing vents, you just need to vent into a high air pressure zone, to get positive pressure in your tank(s)

Edited July 28, 2021 by skippydiesel

[Markdun]

Alan, I have two aircraft, both powered by a Jab2200 with a mechanical fuel pump and with a facet electric ‘boost’ pump in series for T/O and landing.  One has a mid wing and tank (shoulder level) with a simple vented cap, probably with some negative pressure as it slopes to the back.  This works fine, and will go ok with just gravity feed.  The other aircraft has a low wing and the tank is mounted above ones’ knees.  It has a forward facing vent.  I tried gravity feed with a boost pump in parallel, but that set up failed the gravity feed ground test, hence the mechanical pump plus facet.  Over 200 hrs in each.  I can’t tell the difference.  Both tanks have finger strainers and fuel filters to eliminate problems of bugs and blown grass in the tank. Cheers, Mark

[Methusala]

Only time I recall having problems was when I ran out of fuel! (Only happened once)

[Markdun]

 If I recall correctly didn’t you run out of fuel after landing and while taxiing to the fuel bowser after a flight from Porepunka?

[Marty_d]

I've heard an urban legend about a penny-pinching pilot at Cambridge airport  (Hobart) who would turn the engine off after touch down if it looked like the Hobbs meter was about to click over.  

Guess he would have got some exercise pulling the 172 back to the apron. 

[Markdun]

Yep, and some yank could do that in a twin at a thousand foot or so, dead stick, do a wing over or two, land and roll to his gate.  Some guys have energy management down to a fine art.

[NT5224]

Thanks for the discussion folks but no clear response to my question.

 

Switching from forward facing air vents  to just open top ones, will that effect my fuel flow?

 

Cheers 

 

Alan

[onetrack]

Alan, having open top tank vents at right angles to the airflow, would just create a venturi effect at flying air speeds, and suck fuel out of the tanks - and it would also create a vacuum in the tanks.

[Markdun]

Alan, no it won’t, unless you have gravity feed, ie. no mech pump.

[skippydiesel]

2 hours ago, NT5224 said:

Thanks for the discussion folks but no clear response to my question.

 

Switching from forward facing air vents  to just open top ones, will that effect my fuel flow?

 

Cheers 

 

Alan

Probably not BUT as I said erlier you can have a positive pressure in your tank without exposing your system to the possibility of water egress. Just plumb your breather (s) into a high pressure area (underside of your fuselage/wing/wherever.

[NT5224]

OK, so reading comments above that's a 'yes', a 'no' and a 'maybe'. Plenty to work with. 🤣

 

But One Tracks point about  the Venturi effect makes sense. Does that mean that all aircraft with upward vented fuel caps have mechanical fuel pumps?

 

I'll take from this that with my gravity fuel system I need to stick with forward facing vents which pressurise my tanks and boost fuel flow...

 

Thanks fellas! 

 

Alan

[derekliston]

23 hours ago, skippydiesel said:

On my Zephyr I have combined both my main & auxiliary tank (both in fuselage) vent line into one. It goes up to the highest point, of the fuselage, does a loop and then straight down (a bulkhead), exiting into the hopefully high pressure air zone below the fuselage. Seem to work - never had a problem.

 

Sorry didnt address the Q - My point in the above description, is to demonstrate you dont need any fancy forward facing vents, you just need to vent into a high air pressure zone, to get positive pressure in your tank(s)

Nothing fancy about my forward facing vents and they work quite satisfactorily!

[skippydiesel]

5 minutes ago, derekliston said:

Nothing fancy about my forward facing vents and they work quite satisfactorily!

Is it possiblew to put the cap on facing backwards - if so the possibility of venturi effect greatly heightened

[derekliston]

1 minute ago, skippydiesel said:

Is it possiblew to put the cap on facing backwards - if so the possibility of venturi effect greatly heightened

Probably is possible, but why would you?

[skippydiesel]

2 hours ago, derekliston said:

Probably is possible, but why would you?

Human frailty!

 

If it can be done wrong, someone sometime, will probably do so.

Edited July 29, 2021 by skippydiesel

[skippydiesel]

I am interested to know why know one has taken me up/to task, on venting into a high pressure area?

 

Is it lack of understanding or disagreement.

[derekliston]

3 minutes ago, skippydiesel said:

Human frailty!

 

If it can be done wrong, someone sometime, will probably do so.

I am the only one who flies it and I built it so not likely to make that mistake. I also think that if you fit the caps 180° out the tubes wouldn’t be facing rear wards. Not sure how to explain that but the notches in the fuel tank flanges don’t align!

[derekliston]

7 minutes ago, skippydiesel said:

I am interested to know why know one has taken me up/to task, on venting into a high pressure area?

 

Is it lack of understanding or disagreement.

Neither on my part, just that I have no idea!

[skippydiesel]

13 hours ago, derekliston said:

Neither on my part, just that I have no idea!

Are you seeking an explanation or just dismissing my suggestion ?.

 

OR

 

neither for or against apathy?

Edited July 30, 2021 by skippydiesel

[skippydiesel]