Vented fuel caps with forward facing inlet tubes

[IBob]

Skippy,

Controlling the pressure cannot be done by reducing the aperture (unless it is made very tiny).

 

 

 

[Bruce Tuncks]

 Years ago, when my Jabiru was new and I was young and silly, I flew for months with no tank vent at all! On a one hour flight, not enough fuel was used to notice.

Then on a nav flight with the CFI, we flew for nearly 3 hours and on coming home, there was a loud "bonk" as the top of the fuel tank was sucked in.

The cause was found to be a bit of packing polystyrene in the fuel vent pipe. Under the fuel fumes, this had become like chewing gum and it had blocked the vent. The engine never missed a beat though.

[Bruce Tuncks]

I hasten to add that the setup had passed the fuel-flow test before first flight... this must have been before the vent blockage.

[IBob]

When it comes to multiple high-wing tanks with individual tank vents under the wing:

 

At least one highly experienced member here has written of looking out in flight to see fuel streaming from one underwing vent. And once that starts to happen, you have a siphon which will just keep on going if the tanks are interconnected with no stop valves. I believe that event was caused by the underwing vent ends pointing in different directions, so introducing differential pressure. The (ferry) pilot landed and swapped in his own snorkel style fuel caps.

 

I was told of a similar event where the aircraft was parked sideways on a slope with fairly full tanks: the downslope tank began to empty through the vent and levelling the aircraft did not stop it once the siphon was established. Popping the fuel cap, so breaking the siphon, fixed it on that occasion.

 

And I had an event where, 20mins into a local flight, I looked at the levels on the inboard tanks I was using and was surprised to see one going down and the other going up. The reason turned out to be a fuel cap not sealing properly due to a light deformity in the tank lip (and my failure to pull it down tight).

 

I'm an avid believer in the KISS principal, but I also think we need to be aware of these possibilities.

 

FWIW, with twin unvalved tanks 180mm deep (for example), a differential pressure of just 0.065PSI between tanks will crossfeed all the fuel from one tank into the other. Which I guess is why Cessna tanks share a common vent, and why I'd like to be doing the same...........)

 

 

[spacesailor]

One stupid question !.

If  your trim goes crazy and you start heading virticly down, 

Could doing an inverted flight make that down trim, an UPTRIM, !

Often thought those big jets could have rolled over to get a moments grace.

spacesailor

[Marty_d]

It worked in that movie with Denzel Washington.  But that was a movie.

[skippydiesel]

4 hours ago, IBob said:

Skippy,

Controlling the pressure cannot be done by reducing the aperture (unless it is made very tiny).

 

 

 

Of course you are technically correct, however in flight the size of the aperture, in relation to the rate of out flow /consumption/transfer of fuel, will influence the internal tank pressure. .

Edited August 3, 2021 by skippydiesel

[IBob]

12 minutes ago, skippydiesel said:

Of course you are technically correct, however in flight the size of the aperture, in relation to the rate of out flow /consumption/transfer of fuel, will influence the internal tank pressure. .

Only if it's a very tiny aperture: do the maths.

[IBob]

Cross sectional area of 2mm vent pipe = 3.14 x .1 x .1 = .0314 sq cm

Aircraft at 80kts = 41M/sec = 4100cm/sec

So air arriving at face of pipe = .0314 x 4100 = 129cc per sec.

 

Fuel leaving tank = 15L/hr = 15000/3600 = 4.2cc per sec

 

And that's for a very small pipe, with fuel taken from only one tank.

 

So there is a huge imbalance between air arriving at pipe and fuel leaving the tank, and the result will be pressurisation of the tank, even with a very small pipe, and the relatively small amount of fuel taken will not offset that.

[perrynz]

2 hours ago, IBob said:

And I had an event where, 20mins into a local flight, I looked at the levels on the inboard tanks I was using and was surprised to see one going down and the other going up.

IBob. Does the return fuel to your RH inboard tank feature in the above?

[IBob]

1 minute ago, perrynz said:

IBob. Does the return fuel to your RH inboard tank feature in the above?

Hi Perry, in short, no.

I don't know what the return flow is, but it's through a very small orifice and certainly would not cause the RH tank level to go up when running on 2 tanks.

As I recall, I set off with approx 20L in each tank, by the time I got close to your place (20min?) I had less than 10 in the L and about 25 in the R.

So I went home.

When I got back, there was fuel staining on the upper RH wing. So the cap was not properly sealed, I had low pressure there and hi pressure in the L tank resulting in fuel crossfeeding.

 

The question then becomes what happens if you fly the L tank to empty.

Some think the fuel from the R tank would somehow then make it's way down.

But I'm not so sure.

 

 

 

[skippydiesel]

2 hours ago, IBob said:

Only if it's a very tiny aperture: do the maths.

That's why most tank "breathers" are very small. They dont need to be large to archive the balance of fuel out / air in (as the maths points out) and by being small they reduce the potential of foreign objects entering, conversely are prone to blockage.

 

Most of the RAA/ LS aircraft do not need/benefit from a pressurised fuel tanks hence the small  hole (2mm?) in the fuel cap approach.

 

Oh! FYI A few additional points to show your maths is unique to your aircraft type:

 

When I do my inflight fuel transfer (Aux to Main) its more like 2L/min (120L/hr)

My aircraft econamy cruise is 100 knots for sub 13L/hr consumption (I flight plan at a conservative 14l/hr)

[IBob]

That's wonderful, Skippy!

I, on the other hand, have a fat wing that develops so much lift, I must take off before lunch if I am to have any hope of being on the ground by tea-time...........)

[Thruster88]

The real reason Cessna placed the vent tube behind the strut is to prevent the strut fairing from sliding down far enough to allow the two inspection panels to be removed with a normal screw driver. Another win for designers over mechanic's