Help Wanted (with metal fuel line plumbing)

[skippydiesel]

I don't understand why small aircraft fuel tanks are vented onto a low point (belly/underwing) on the aircraft - any slight benefit from positive pressure, is outweighed by the tendency to vent fuel overboard (especially if parked on a slope). 

 

My last aircraft had a single vent , for two tanks, that went to the highest point on the fuselage, did a pig tail coil,  before exiting on the belly, just aft of the cockpit.

 

I may do something similar with this aircraft BUT will take the vent to the top of the vertical stabiliser, before exiting at the highest point

[facthunter]

I would think the less length of pipes carrying fuel in the fuselage the better. IF you are getting fuel transfer it will vent when full, regardless.. Most planes will have a max fuel imbalance but if they don't the plane will still respond to the uneven wing mass  due to fuel levels and it should be taken into consideration..  Nev

Edited May 16, 2023 by facthunter

[Area-51]

3 hours ago, skippydiesel said:

I don't understand why small aircraft fuel tanks are vented onto a low point (belly/underwing) on the aircraft - any slight benefit from positive pressure, is outweighed by the tendency to vent fuel overboard (especially if parked on a slope). 

 

My last aircraft had a single vent , for two tanks, that went to the highest point on the fuselage, did a pig tail coil,  before exiting on the belly, just aft of the cockpit.

 

I may do something similar with this aircraft BUT will take the vent to the top of the vertical stabiliser, before exiting at the highest point

The pigtail vent you describe is employed to mitigate the risk of fuel escaping and pooling under the fuselage in the event of a rollover during ground ops or crash landing and is the correct method for aviation related purposes.

[skippydiesel]

4 hours ago, facthunter said:

I would think the less length of pipes carrying fuel in the fuselage the better. IF you are getting fuel transfer it will vent when full, regardless.. Most planes will have a max fuel imbalance but if they don't the plane will still respond to the uneven wing mass  due to fuel levels and it should be taken into consideration..  Nev

It would seem to me that the aim of the pilot is to prevent liquid fuel from being in/escaping overboard via the vent pipe .

The length of vent pipe will have negligible impact on the aircrafts performance/level of complexity

 

The vent pipe has three main functions;

• Allow the entry of atmospheric pressure/air to replace the volume of fuel used by the engine
• Allow allow air/ fuel fumes to escape from the tank if transferring fuel(volume) in.
• As aircraft (tank) ascends/defends to "normalise/balance the tank pressure with the atmospheric level the aircraft is at.

 

Its secondary and less desirable function, is to allow raw fuel to escape, when tanks are overfilled (pilot error).

[facthunter]

How does this overfilling work? To most people FULL tanks means to the top. It's one of only two situations when you know exactly how much fuel you have on board. Nev

[skippydiesel]

1 hour ago, facthunter said:

How does this overfilling work? To most people FULL tanks means to the top. It's one of only two situations when you know exactly how much fuel you have on board. Nev

Simple 

 

Technique 1. Fill tank to top, on hot day. Leave aircraft or long taxi/holding in sun. Liquid fuel venting. Failure of pilot to leave room for fuel expansion. 

Technique 2. Transferring fuel in flight. Leave transfer pump on too long. Liquid fuel venting. Failure of pilot to A correctly assess fuel volume require B monitor transfer time/quantity.

 

Edited May 17, 2023 by skippydiesel

[facthunter]

CASE 1 is no big deal and more likely to happen if you park on sloped ground. Case 2 why are you transferring fuel in flight? A well designed fuel system would prevent that happening. The Non return valves I mentioned way back prevent it and allow you to safely drain every useable drop of fuel for your system. You don't want a non symmetrical distribution of fuel either as it affects the controllability of your aeroplane  Nev

[skippydiesel]

8 minutes ago, facthunter said:

CASE 1 is no big deal and more likely to happen if you park on sloped ground. Case 2 why are you transferring fuel in flight? A well designed fuel system would prevent that happening. The Non return valves I mentioned way back prevent it and allow you to safely drain every useable drop of fuel for your system. You don't want a non symmetrical distribution of fuel either as it affects the controllability of your aeroplane  Nev

So Nev -  I have three fuel tanks (one of which has no external fueling point) how would you access and monitor the levels, in flight?

[facthunter]

By timed usage based on known rates of fuel flow predominantly from the fuel amount you loaded accurately. Most gauges are not very reliable especially with long thin section tanks. You should not have to transfer fuel in flight. In multi engines you do  cross feed when one is shut down. IF you select BOTH you have no idea of the rate fuel is coming-out of  either tank and it's often affected by where the BALL is in your turn and slip.  Nev

[Thruster88]

3 hours ago, skippydiesel said:

So Nev -  I have three fuel tanks (one of which has no external fueling point) how would you access and monitor the levels, in flight?

Beavers have three tanks in the floor. The selector has only four positions, front, centre, rear and off. Such a fuel selector would be perfect for your three tank aircraft. 

 

No external filler on one tank? Why?

[skippydiesel]

I agree with the fuel quantity /  time method - its the only safe way to do it .

 

Unfortunately this dos not answer the question of how deal with a human pilot.

 

With thee tanks , 2 x30L wing & 1 x 38L fuselage,  I fly on the fuselage tank, which must be replenished, from the wing tanks, in flight.

 

Going back to "How does this overfilling work? To most people FULL tanks means to the top. It's one of only two situations when you know exactly how much fuel you have on board. Nev"

 

If I do everything just right, the transfer of  fuel will occur as planned, and there will be no venting of fuel. This is not a complex aircraft with computer systems to aid in fuel transfer.  Even with the best laid plans, there is always the possibility of pilot error, overfilling the fuselage tank and venting fuel overboard.

 

"No external filler on one tank? Why?"

 

I purchased a, not quite complete, plans built Sonex - its the way it came. 

Edited May 17, 2023 by skippydiesel

[facthunter]

Transferring to a centrally located tank is OK but still prone to error. Most good cockpits have stop watches with total elapse and stop functions together. If you are doing something critical have a card reminder that's where you can't miss seeing it somewhere Prominent OR  set an alarm.  Nev

[spacesailor]

The standard Hummel bird tank Can hold 25 litres,  but is only supposed to take US 5 gallons. 

( 20 litres ) . The " overfill " is for long taxi & wait times , before take-off. 

But !, " all take-offs are overweight ".

So it is said.

spacesailor

 

 

[onetrack]

Spacey, 5 US gallons isn't 20 litres, it's 18.972 litres. 1 US gallon - 3.785 litres.

[skippydiesel]

2 hours ago, facthunter said:

Transferring to a centrally located tank is OK but still prone to error. Most good cockpits have stop watches with total elapse and stop functions together. If you are doing something critical have a card reminder that's where you can't miss seeing it somewhere Prominent OR  set an alarm.  Nev

Yes that is what is planned. The topic moved to fuel tank vents and their purpose, one of which is the last line of defence if a tank is overfilled for whatever reason/error.

[ClintonB]

On 2/5/2023 at 4:37 PM, Marty_d said:

I put metal fuel line in mine, bought a little beading tool from Aeroflow - AF98-2020 - PIPE BEADING TOOL SUIT 3/8" 

That allows you to make your own barbs at the end of the line,  then hose clamp regular fuel line to it. 

A very good tool to own, I use them in automotive stuff as well.