Fuel Flow problem.

[patsy]

I have a Kitfox II, a high wing with a fuel tank in each of the wing roots.
Until recently the fuel flowed evenly from each of the tanks. No problem.
Then, for no apparent reason fuel stopped flowing from the left hand tank.
I have disconnected the fuel line between the tank and the header tank and the header tank and the first of the fuel filters and then switched the stop-cock on in-line and the fuel flows freely. But when I put it all together again and fly, the fuel does not go through.
In fact, at the end of an hour or so flight the left tank is fuller than when I started. Fuel , strange as it may seem, must be siphoning from the right tank into the left tank.
If any one out there has had a similar problem and knows how I could sort this out I'd be very grateful. Thanks.

[Mogas]

Welcome to microlighters Patsy.
Taking for granted that there is no low pressure in the left tank above the fuel, like maybe a wrong facing breather, the only way fuel will transfer is if the one wing is higher than the other.
Are you flying long sectors with the ball out of the middle?
If not, is your ball indicator itself level?
Let us know how it goes.
Mogas
Kitplanes for Africa

[Big-D]

Welcome to Microlighters Patsy - Got a feeling we know each other - You hangered at Aeroden?

Hope you find solution to your problem


Cheers
David

[patsy]

Hi Mogas, thanks for your welcome and to you too Big D. Yup,we did meet at Aeroden. Piece of Heaven don't you think.
Mogas thanks also for your quick response to my problem. Somehow I think you are part way to solving it already.
In answer to your questions, the breather pipes are clear and facing the right way, and
the tanks are level and so is the ball indicator, to the best of my knowledge. Nothing drastic has happened to the plane in any way to make me think otherwise.
BUT, and here's the thing, when I do fly, the ball is always out to the left. This bothered me so much that I recently put a trim tab on the rudder to help correct this but quite frankly it hasn't helped at all. The ball still stays to the left and fuel does what it does.
So, where do we go from here?
I look forward to the answer.
Thanks again.

[Morph]

How far to the left? how much left boot is required to bring it straight? That is one issue

as far as the fuel goes, you say that when you unplug the fuel pipe from the header tank and open the tap the fuel flows freely? Is there not a blockage in the copper fitting on the header tank? Also in the pickup fitting inside the main tank you usualy have a fiter of sorts that has a lot of holes in it to prevent a single large object from blocking the fuel pickup. Have you checked that you are not getting a partial blockage there.

Are you flying with both taps open? Is there a t-connection between the two tanks?

I have found on my Bushbaby that if I leave both taps open my one tank will be used more than my other tank.

I have been thinking about this for a while and this is my theory.

The difference to me is the same reason as why you need to fly with a bit of right rudder (in my case) all the time (hence using a rudder trim to reduce pressure required by the foot). My prop swings clockwise, looking forward. This means the airflow is rotating clockwise, off the prop moving backwards like a cork screw. When the prop swings over the top the air is pushed backwards rotating to the right and flows over the right hand side of the roof and the top of the wing. The right tank airvent is exposed to this propwash increasing the pressure. The left tank doesn't pick up prop wash because the prop comes up from below the wing and only picks up a bit of was as it gets to the top. (the forward movement of the plane also pressurises the tanks, but I suspect the right tank to have a slightly higher pressure die to the propwash)

This to me means that since the pressure in my right tank is slightly higher than that of the left, and If I leave the taps open, the right will be used first. and if the pressure is higher than the required fuel flow the excess pressure will push the fuel from one tank to the other. Now if you close the tap to the right tank, then the engine has to use the left. But if there is a t-piece between the two tanks you will always get fuel pushing across.

Now how is that for a theory.

[Morph]

Found this on the internet

Kitfox - proper header and fuel line routing.

SERVICE LETTER #22

October 21, 1992

SUBJECT: Header Tank and Fuel Line Routing

TO: All Kitfox Builders and Owners (using a header tank)

FROM: SkyStar Aircraft Corporation

In keeping with SkyStar Aircraft policy of informing our customers of important technical information, this Service Letter is being issued as an advisory which needs your attention. While advising in nature, the subject of this letter should be considered carefully and a Service Bulletin will be issued in the near future with specific instructions regarding maintenance action you will need to perform.

SkyStar has received reports of header tanks being drained and not refilling (potentially causing fuel starvation), while there is still plenty of fuel in the wing tanks. This has occurred in aircraft with both front and rear mounted header tanks.

We have been able to recreate this fuel starvation in a special test rig. In our test the fuel lines were routed out of the wing tanks to the valve on the head rack, then across to the opposite side, down and back to the rear mounted header tank, or down under the door, forward to the firewall and up to the front mounted header tank. In each situation the header tank was vented and only one wing tank valve was turned to the “on” position. (It is feasible that this situation could occur with both wing tank valves turned on). The fuel flow to the header tank from the wing tank stopped when the wing tank was un-ported (as in a “slip” with low fuel) and did not resume because of the air introduced into the fuel line.

Further testing has shown that when the fuel line is routed directly forward out from the wing tank, above the door, then down along the diagonal brace to the front mounted header tank, or out of the tank and directly back to the rear mounted header tank, fuel flow would resume on its own to the header tank from the wing tank after un-porting.

NOTE: Those Kitfox's with the standard nose tank and wing tanks should be aware of this situation, but because of the fuel’s visibility in the standard nose tank there should be little possibility of fuel starvation.

RECOMMENDATIONS:

Until further testing, each Kitfox owner with a rear or front mounted header tank should (in the best interest of safety) take the following precautions:

(1) The highest point in the main fuel line routing should be the wing tank outlet. Tube routing should decline as rapidly as possible with no risers (high points). This will allow air to flow out of the tubing as quickly as possible. Check to make sure the fuel line coming out of each wing tank is configured in the most direct (in flight) down hill route to the header tank.

(2) If you have a wing tank installed in each wing, fly with Both fuel valves in the “open” position.

(3) Avoid “slips” when fuel in the wing tanks is less than 1/2 full.

(4) If you have not installed a clear vent line from the header tank to the wing tank you need to do so (SkyStar offers a retro-fit vent line and nipple into the right wing tank). The Tygon tubing vent line from the header tank back up to the right wing tank allows the fuel to be monitored by observing the level and behavior of the fuel in the vent line (fuel seeks a common level). Prior to take-off, this tube should be inspected. Any fuel level noted in the vent tube below the level of the outlet from the wing tank is a concern. This would mean that either there is no fuel in the wing tanks or fuel flow has stopped flowing to the header tank.

This Service Letter will be followed-up by a detailed Service Bulletin containing information and recommendations, when testing is completed.

If you have any questions concerning this Bulletin or procedures, please feel free to call our Technical Support Dept. at (208) 466-1711.

[Oupa-G]

I wrote in building the BOSVARK my concern of the fuel flow and the header tank. I also spoke about adhesion and cohesion and explained this . I fitted the in line fuel pumps and then when you want to slip or do whatever you like you should have motion potion till you are empty. The Bosvark also has return lines to the tank where you can see the return to tank when the pump is running. If you have purged the line at the first flight of the day you will also have checked your fuel level because the LED's that show the fuel level in the tanks are connected to the in line fuel pump. The fuel pump is from Facet and just on gravity she has a fuel flow of 45liter per pump.

Cheers Oupa-G

[Tailspin]

Here is a rough diagram of how i have setup the Sunbirds Fuel system.
This is how i think it should look as i got it all in pieces with no piping so if it is wrong then please Tell me.
The vent pipe theorectically should keep the pressure in the main tank and the headder tank more or less the same, so thereshould not be any airlock in either of the tanks.( at least with this setup)

[justin.schoeman]

Here is a rough diagram of how i have setup the Sunbirds Fuel system.
This is how i think it should look as i got it all in pieces with no piping so if it is wrong then please Tell me.
The vent pipe theorectically should keep the pressure in the main tank and the headder tank more or less the same, so thereshould not be any airlock in either of the tanks.( at least with this setup)

Hi Tailspin, I have two comments on your installation (no idea how accurate they are, but these are problems I encountered plumbing the water tank in my 'cruiser):

1) Run both fuel pickups in the top tank independantly to the header tank. If you lay the pipes out as in your diagram, if you climb steeply enough to unport the front pickup, the fuel line from the back pickup will be running up-hill - and seeing as the front one is drawing air, you won't have any siphon effect...

With gravity feed, it is best to try and make sure that every fuel line runs continuously downhill, at any pitch angle expected in flight.

2) Not sure what will happen if you fill the main tank with enough fuel to cover the header tank vent line? Or if pitch angle changes enough to cover the vent tube in flight?

-justin

[Morph]

The header tank must not be vented at all. This will allow the tank to draw air, and an equal pressure in the fuel tank and header might stop the flow of fuel to the header. You want the fuel to move from a high to low pressure system.

In the Bush Baby, there is a vent valve in the top of the header tank, that is opened to allow the header tank to full up and bleed the air out. Once this full this valve it closed. The header tank now remains full drawing fuel from the available fuel tank as the motor is drawing fuel out of it.

If it does happen to suck a bit of air you can easily see the level in the header has dropped slightly during your pre-flight. It is then a simple process of opening one of the fuel taps, opening the valve and letting the header tank refill. Our tanks are 5liters and I have never seen more than a few milileters of air in the tanks.

[Tailspin]

Hi Justin

The headder tank has enough fuel in to hold about 1 hour of flying, so unless i am climbing steeply for that amount of time there should not be time for air to fill the headder tank completely.
if i do fill the vent pipe it will only be when both tanks are completely full. Then a slight vacum will be created in the header tank, as soon as the aircraft is in level flight and the fuel drops sufficiently about 1/2L then the vent will be open again and then the tanks should balance themself out properly.

If i have my theory correct i would have to climb out at a 45Deg angle with the top tank 50% full to have that problem. But i get what you are saying and i will definately look into it. I am also putting a drain valve on the rear tap so any water can be drained off.

[Tailspin]

The header tank must not be vented at all. This will allow the tank to draw air, and an equal pressure in the fuel tank and header might stop the flow of fuel to the header. You want the fuel to move from a high to low pressure system.

Ah .

So Morph you are telling me i should remove the vent pipe from the header tank

[Oupa-G]

Just some comments I agree with Justin if you have two feeders from the tank you must have two separate pipes to the header tank. There is no reason why the header tank cannot breath into the main tank as long as it is a separate pipe that breaths to the highest part in the tank . A fuel line that works on syphoning must be uninterrupted. If the fuel syphon's into the header tank then the fuel level will push up in the breather line to the level of the tank. I put fuel pumps in line to make sure that my syphon is airlock free. The Bushbaby's are known to have interuption in fuel flow on long descends and circuit work the header tank takes care of that. A fuel pump on for take-off and landing is standard practice in many other aircraft. When I did the fuel system of the Bosvark I gave a lot of thought to this flow condition.

Cheers Oupa-G

[Morph]

yes, the header tank must be part of the airless fuel system. It is there for two things, 1. to act as the last stage of water trap, and 2. catch air bubbles coming through and giving the motor the a constant feed of fuel.

Here is my header. the two inlets are on the sides, water drain at the bottom (I have a hose from there out the side of the plane to a brass tap.) The bleed valve on top and the fuelout goes though the firewall to the engine

[justin.schoeman]

If you are designing your header tank in such a way that you expect to draw air into it on occasion, then it must be vented. Personally I would try to vent it in such a way that the vent tube can never draw fuel - even if that meant adding a separate vent tube to the top of the main tank.

With vent lines, it is very important to make sure that there are NO low points. It must run continuously uphill from the top of the header to the top of the vent tube. Any dip that can possibly fill with fuel, will fill with fuel, and the vacuum pressure in the header may not be enough to move that fuel.