Vented fuel caps with forward facing inlet tubes

[Thruster88]

14 hours 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.

Skippy you will be pleased to know that the worlds best aircraft, the Cessna 172 has no fuel pumps on the carburetor versions and Vents in a single high pressure area under the wing. The caps are also vented but with one way valves, air in but no air or fuel out. This is good if you land upside down. The multiple vents would also be good in icing conditions. 

[IBob]

52 minutes ago, Thruster88 said:

Skippy you will be pleased to know that the worlds best aircraft, the Cessna 172 has no fuel pumps on the carburetor versions and Vents in a single high pressure area under the wing. The caps are also vented but with one way valves, air in but no air or fuel out. This is good if you land upside down. The multiple vents would also be good in icing conditions. 

To add detail to that:

The vent is forward facing, but tucked behind the strut to avoid overpressurising the system, which would risk structural damage. And the tanks are then cross-vented to give equal pressure in each side, with the aim of delivering equally from each tank and avoiding transfer of fuel between tanks.

[derekliston]

2 hours ago, skippydiesel said:

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

 

OR

 

neither for or against apathy?

Neither of those, simply saying I am of no help because I have no idea of the efficacy or otherwise of your suggestion and no need to find out because what I have is, for my requirements, perfectly adequate. If I had any idea or suggestions I would tell you!

[skippydiesel]

4 hours ago, IBob said:

To add detail to that:

The vent is forward facing, but tucked behind the strut to avoid overpressurising the system, which would risk structural damage. And the tanks are then cross-vented to give equal pressure in each side, with the aim of delivering equally from each tank and avoiding transfer of fuel between tanks.

I would speculate that:

• the forward facing bit is just aesthetics - there would be no benefit in terms of pressure capture.
• behind the strut is probably more to do with A reduced chance of damage from passing skulls B reduced chance of damage to passing sculls.

 

The positive pressure will vary according to location and angle of attack - the shape of the breather will have little impact on the pressure entering the system.

[skippydiesel]

3 hours ago, derekliston said:

Neither of those, simply saying I am of no help because I have no idea of the efficacy or otherwise of your suggestion and no need to find out because what I have is, for my requirements, perfectly adequate. If I had any idea or suggestions I would tell you!

Its a perfectly viable option, used in many aircraft, with the added benefit of less chance of water egress. It also reduces/removes concerns about boundary lair / turbulence / chance of low pressure zone, impacting negatively, on your fuel delivery system.

[kgwilson]

Venting via the cap a simple & cheap way to do it but won't create as much positive pressure as how I have vented mine. 

 

I installed 3 tanks in my aircraft and the vent on each (2 in wings & 1 in fuselage) exit the tank at the highest point and then go straight down through the bottom of the wing/fuselage about 25mm and are cut at a 45 deg angle into the airflow. This maintains a positive pressure in each tank. I have not bothered with a finger strainer & have not had issues with bugs. They are easily tested by creating a suction with the fuel cap off. The tank end and the exit end are 1/4" aluminium tube with clear plastic tube between each bit.

[skippydiesel]

16 hours ago, kgwilson said:

Venting via the cap a simple & cheap way to do it but won't create as much positive pressure as how I have vented mine. 

 

I installed 3 tanks in my aircraft and the vent on each (2 in wings & 1 in fuselage) exit the tank at the highest point and then go straight down through the bottom of the wing/fuselage about 25mm and are cut at a 45 deg angle into the airflow. This maintains a positive pressure in each tank. I have not bothered with a finger strainer & have not had issues with bugs. They are easily tested by creating a suction with the fuel cap off. The tank end and the exit end are 1/4" aluminium tube with clear plastic tube between each bit.

Excellent work - just being picky (cause I can);

• Why project more than a few mm below the aircraft "skin" ?

and

• Why bother with the aesthetically pleasing but probably ineffective, 45 degree angle into apparent wind?

[Markdun]

Mmmm....reminds me of a ‘fuel leak’ from the port wing root in a Tecnam noted in a condition report.  A friend had just purchased the plane and had a LAME at Northam airfield fixing the list of faults (there were a few).  I was there to do the delivery from WA to the east.  Check out the plane, including insisting a W&B after the two blade wood prop was replaced with. a heavier 3 blade carbon & alloy job...LAME said ‘they’re about the same’.  Anyway took it for a test fly...very nice, except the electric trim was not working...or only intermittently..another fault noted on the condition report and signed off as fixed by the LAME.  Took the plane to fill her right up for the flight to Kalgoorlie, hopped in the left seat to be given an Avgas shower from the port wing root...but that was fixed by the LAME, right?  ‘ We couldn’t find the leak so we just cleaned up the stains down the fuselage caused by the leak’....back to the shed.

 

OK the cause.  A plastic vent line of the  port wing had cracked.  The tube runs from the outboard top front of the tank to the wing tip. It had cracked at a low point where the vent line was resting on the bottom skin of the wing. If the tanks are full and the plane was inclined to port by a small amount, the vent would siphon fuel out into the wing, and this would then flow inside down to the wing root, until the fuel level dropped below the vent height.  I reckon you could lose 10l; more if you left the two fuel cocks on.

So just be careful with those vent lines.

oh, I won’t mention the wrong diameter oil lines, the loose jubilee clips,  the coolant hose touching the exhaust, or the radiator rubbing on the cowl from the rubber replacement done at the same time.

[IBob]

2 hours ago, skippydiesel said:

Excellent work - just being picky (cause I can);

• Why project more than a few mm below the aircraft "skin" ?

and

• Why bother with the aesthetically pleasing but probably ineffective, 45 degree angle into apparent wind?

I can promise you it's not 'probably ineffective', Skippydiesel:
My tank breathers emerge under the wing, and I can vary the take from L vs R tank by cutting slightly different (forward facing) angles on them.

[kgwilson]

The reason they protrude a bit is so I can put the cover with the "Remove before flight" tag on easily. It is just a bit of pvc clear tube that fits over the aluminium breather to stop the bugs getting in. Also I reckon the airflow is a bit higher there. The 45 deg angle must create more positive pressure as the opening is double the area of the tube cut flat.

Edited July 31, 2021 by kgwilson

[skippydiesel]

6 hours ago, IBob said:

I can promise you it's not 'probably ineffective', Skippydiesel:
My tank breathers emerge under the wing, and I can vary the take from L vs R tank by cutting slightly different (forward facing) angles on them.

Interesting - how have you determined this?

 

5 hours ago, kgwilson said:

The reason they protrude a bit is so I can put the cover with the "Remove before flight" tag on easily. It is just a bit of pvc clear tube that fits over the aluminium breather to stop the bugs getting in. Also I reckon the airflow is a bit higher there. The 45 deg angle must create more positive pressure as the opening is double the area of the tube cut flat.

The point with exiting into a positive air pressure area, is that you dont have to try and "capture" the apparent wind by having potentially damageable/damaging pipes.

[IBob]

1 hour ago, skippydiesel said:

Interesting - how have you determined this?

 

The point with exiting into a positive air pressure area, is that you dont have to try and "capture" the apparent wind by having potentially damageable/damaging pipes.

Sure, Skippydiesel:

My fuel valves for L and R tank are both on the fuselage wall behind the passenger, where I can see them. They then feed down into a receiver behind the passenger. So all lines go to the RH side of the fuselage then down. And initially more fuel was generally taken from the RH.

I tried a number of things like adding ballast to ensure I was flying level, without results.

 

I then straightened the LH fuel hoses where they come across the fuselage, by cable-tieing them to a length of al. angle, so removing all undulations. This resulted in a definite improvement.

 

I then fooled with the angle cut on the underwing breathers. They are nylon air line, and do not come out of the wing perpendicular, but tend to lean this way and that. So angling them all the same does not present them all at the same angle to the air flow. One or two cuts on successive flights had the tanks feeding reasonably evenly.

The next step would be to run solid tubes down through the wing, instead of the nylon air line. that way i could be confident the tube ends were presenting in a similar fashion to the air stream. I just need to come up with a simple way to ensure the tubes don't rotate on their axis....

 

It's an interesting problem, and I do think the Cessna setup (cross-porting the upper tanks) is the way to go, if at all possible........
 

[skippydiesel]

Well - my system may not work exactly the same but I have two tanks (main & aux). Both tank breathers are connected into one tube. The one tube is taken to the highest point on the fuselage body, does a one turn curl & then goes straight down a bulkhead, through the lower fuselage skin, exiting a few mm proud of the surface, into what I hope/assume, is a relativly high air pressure area. Works like a charm - that is to say I have no idea what the inflight tank pressure actually is, however I never have any problems with delivery - aux to main - main to engine via boost pump (only for TO  power & landing) and mechanical pump.

Edited July 31, 2021 by skippydiesel

[Steve L]

Markdum did you find out what the intermittent trim was, my Tecnam was doing the same.

[IBob]

12 hours ago, skippydiesel said:

Well - my system may not work exactly the same but I have two tanks (main & aux). Both tank breathers are connected into one tube. The one tube is taken to the highest point on the fuselage body, does a one turn curl & then goes straight down a bulkhead, through the lower fuselage skin, exiting a few mm proud of the surface, into what I hope/assume, is a relativly high air pressure area. Works like a charm - that is to say I have no idea what the inflight tank pressure actually is, however I never have any problems with delivery - aux to main - main to engine via boost pump (only for TO  power & landing) and mechanical pump.

Skippydiesel, I certainly don't claim to be any expert, I just know what I have observed with my own aircraft so far, and what I have learnt speaking with other pilots. I have L and R wing tanks which each have their own vent, out of the fuel cap and down through the wing (I've also got L and R outer tanks too, but let's not go there!) I got interested when I noticed the uneven feed from them.

I've since learnt that many aircraft with such a  configuration feed unevenly, and this is managed in some of them by flying for set periods on alternate tanks.

I'm happy that I've been able to at last reduce the unevenness. I think if I was building again I would look at some system like Cessna, with a single vent then cross ported to the upper tanks.

The other popular alternative seems to be the solid pipe set into the fuel cap, upwards then  bent forward (and down a bit).

 

I doubt there is any one system works well across all aircraft or all tank configurations. And it seems to me that what is vital is to have a working understanding of whatever fuel system you are flying.

[bushcaddy105]

Don’t overthink the issue. My kit-supplied caps for the wing tanks are of CNC machined alloy and have two tiny holes cross-drilled at 90 degrees to each other. Their orientation to the airflow over the wing doesn’t seem to matter and depends on how tight the caps are screwed down. Fuel feed is gravity only in cruise, with a series electric pump used only in climb but shut down on leveling out. Never a problem with fuel feed or loss of fuel from the vents.

[skippydiesel]

21 hours ago, IBob said:

.......................................................

 

I doubt there is any one system works well across all aircraft or all tank configurations. And it seems to me that what is vital is to have a working understanding of whatever fuel system you are flying.

I agree. However, we seem to be discussing the options for the vent, as it is presented to the atmosphere (not so much the system/lack of behind the vent) 

 

There are principals which should work across all aircraft. The very definition of an aircraft - is that it must/should be capable of flight. Flight suggests a high air pressure zone below airfoils and a low pressure above (most often this is also the case for the fuselage).

 

I appreciate/support the idea that the KISS principal should a be applied and extra possibly unnecessary plumbing may not be the way to go. It concerns me that so many of the respondents seem to be unaware of the opportunities afforded by basic physics and instead want to go with what is, in most cases, an ornate decoration (the forward facing tube scoop) rather than a simple practical solution.

[IBob]

Skippydiesel, if you blow a jet of air across the end of a tube in a certain manner, it creates a low pressure there, drawing air out of the tube.

So, while you are correct in thinking the underside of the wing is a (relatively) high pressure area, it is also possible to create a low pressure inlet under there, because the relative air is flowing, not static. For this reason, the likes of Cessna (for instance), who have spent a great deal more time and effort on these issues than you and I, have opted for a forward facing vent, albeit placed behind the strut to limit the pressure.

If you have a system that works well in your aircraft, then that is what you have. But it certainly does not convince me that what you describe is all that is required, and everyone else has got it wrong.

[spacesailor]

Vent behind a strut.

Sounds like it,s in the low pressure area there as well.

spacesailor

[IBob]

Spacesailor, it's quite a large pipe , bent forward to face into the airflow, but positioned behind where the strut meets the wing.

My guess would be that if it was not sheltered behind the strut, it would result in excess pressure at speed, which would cause the fuel tanks to bug out and various other possible complications.

[Old Koreelah]

The KISS principle is important, but simplicity can catch you out. 

I have a one litre collector tank mounted a poofteenth below the two wing tanks that supply it. Years ago I decided to test the changeover time if one tank was to run dry. I circled our quiet strip until the first sign of fuel starvation (the fuel pressure gauge starts to flicker wildly a few seconds before the engine sound changes).

 

After changing tanks, it took 11 seconds for the engine to recover. I discovered this delay was due to my small collector tank not having a breather line of its own. 
 

After installing a separate breather line, all went well for years, until I stupidly started up with the main tank valves closed. The one litre collector, plus a couple of meters of fuel line and the carby bowl, contained enough juice for me to warm up, taxi out and take off. When about 200’ off the ground the engine noise changed and I realized my mistake.
This time, the recovery was almost instantaneous.

[IBob]

I reckon that collector tank is a great innovation, Old Koreelah.

On the Savannah it's 5 to 6 litres, with a level switch in the top that lights an indicator 2.5minutes after valving off all tanks when in cruise (I recently measured it).

Older Savs did not have a vent line on the collector, so occasionally gave false activations of the indicator due to air trapped in the tank.

They now fit a breather back to the top of the LH tank.

 

There is a button on the dash to test the indicator. The gotcha there is that's what it does: test the indicator, but not the switch in collector tank.

So if intending to fly tanks to empty (which I rarely do here in NZ), I valve off all tanks prior to leaving and test both switch and indicator.

It works for me, as we have a reasonably long taxi from hangar to strip, and both my prestart and preflight checks include identical runs through the fuel system, tanks to engine.

[Markdun]

Steve, in answer to your question regarding the Tecnam trim problems...yes. We had thought originally the issue was a dry joint in the cheap audio connector to the trim motor in the tail. I took this apart and cleaned all the red bull dust out at Kal.  And then on the flight back, we noted the trim worked better on the right stick compared to the left.  That meant the issue was either the ‘left-right’ toggle, or the left stick microswitches.  The owner (an electronics guy) first replaced the toggle, then the stick microswitches.  However, he eventually found tge problem as being a combination of the stick microswitches and the savage bending and compression of the wires to those microswitches....it was in the top of the control stick.  It took a fare bit of effort to resolve, because once you released the compression and bending when you disassembled the head of the stick the trim worked.   Put it back together, and then the fault again.

[skippydiesel]

12 hours ago, IBob said:

Skippydiesel, if you blow a jet of air across the end of a tube in a certain manner, it creates a low pressure there, drawing air out of the tube.

So, while you are correct in thinking the underside of the wing is a (relatively) high pressure area, it is also possible to create a low pressure inlet under there, because the relative air is flowing, not static. For this reason, the likes of Cessna (for instance), who have spent a great deal more time and effort on these issues than you and I, have opted for a forward facing vent, albeit placed behind the strut to limit the pressure.

If you have a system that works well in your aircraft, then that is what you have. But it certainly does not convince me that what you describe is all that is required, and everyone else has got it wrong.

IBob - its called the venturi effect and is likely to be the opposite of what people are trying to achieve when they vent there fuel tank.

 

I suspect the underwing curved vent on Cessna's has more to do with safety (& aesthetics) than controlling pressure (easily done by reducing aperture). You are less likely to open your scalp/ poke an eye on a nicely curved tube placed behind a strut (where it is not obvious). 

 

My musings on the matter:

 

What is the primary purpose of a vent in a fuel tank - answear; To allow atmospheric pressure to enter the tank, replacing the fuel, as the level drops due to consumption/or transfer, facilitating flow to carburettor/other tank. Every other factor/consideration is, I would suggest, secondary.

 

Factors we might want to consider in assenting the efficiency of the vent:

• Is it meeting the prim function, as articulated  above. If not why? 
• Where there is more than one tank - does the vent(s) facilitate equal fuel extraction/flow
• Is the vent allowing undesirable "stuff" to enter eg water, bugs, dust - if so this may need to be corrected.
• Is the vent likely to be damaged/blocked by some non aviation activity eg passing person making contact with it, washing/polishing the aircraft .
• Is there potential for the vent to cause damage to the unwary i.e. does it project into a location where persons, animals may come into contact & be injured.

After the above you may want to consider:

• Improving tank fuel extraction/delivery performance (especially for high wing gravity supply system) by creating a higher than atmospheric pressure in the tank. To achieve this, using the vent, will require accessing the high speed/pressure air flow above wing/fuselage or the high air pressure zone, usually found under the wing and with less certainty the fuselage. Some sort of barometer that you can plug into temporary tubes, taped to the most likely locations and termination inside the cockpit,  may assist in selecting the best site.

To access the high speed air flow will require placing the venturi at sufficient hight above the "skin"  so that it is in relativly smooth air and of course facing it into "wind". Considerations might be;

 

Added turbulence and drag - may increase noise and reduce efficiency of control surfaces/ aircraft as a whole (probably minimal)

Forced entry of "stuff" water/bugs/dust (likely)

Over pressurising the fuel tank fuel reticulation system (unlikely)

Potentially vulnerable to mechanical damage (likely)

Vent presentation to high speed/pressure air flow may vary with aircraft attitude - is there a potential for low pressure to be created in tank (possibly by curved tube acting as a venturi)

Can be blocked by insect body/nest

Aesthetically  (eye of the beholder) - interrupts smooth lines of aircraft

 

To utilise the, already relativly (to atmosphere) high air pressure, under the wing/fuselage

 

Can be flush or only slightly proud of skin - nil impact on drag/noise/flight surfaces/aesthetics. 

Almost no chance of mechanical damage or water entry

Can be blocked by careless polishing/insect body/nest

Location still requires some analysis/reserch/testing of likely high air pressure areas

High pressure air, on underside of aircraft, less likely to be impacted by turbulent air flow during normal changes in attitude

 

Cant think of anything else - discuss!

[Steve L]

Markdun, thanks for the reply and the reason I ask:  I had a similar problem with my Tecnam trim recently. After purchase I flew it from Qld, home to SA. At 065 over Dalby I handed control to my mate (also a pilot) and I switched trim control to the right yolk. After a few minutes the trim run away to a full nose down attitude. Not fun with a full flying tail plane. Done the usual, undone everything I performed earlier and got it flying again. Thought it was switches in the control column so I contacted a licensed aircraft electrician and he found the same audio plug connector in the tail corroded so he removed it. Don’t know why but the next day in my hangar I got my wife to operate the right trim while I rattled connectors and wiring that ran through the fuselage, suddenly it went into a full nose down attitude then popped the circuit. Found the problem to be chaffed loom wires deep in the fuselage, one chaffed near through, i had to use a wand camera to see the problem. (I have pics) Maybe the Tecnam you ferried has the same problem. 
 

steve