Fuel Flow Discussion, Moved from Float Mounting

[zkelley2]

Generally nothing.

[Nev]

Haha better memory than me!

[alaskabearhawk]

On my installation, the port for the cross vent it at the top and outboard side of each tank. http://www.mykitlog.com/users/displa...=280695&row=19

[Bcone1381]

"4-8-1-3 green - flight idle - low - beacons on - parallel- starting engine check - clear capped flat on the right - clear capped flat on the left."

[Nev]

Jared, that's exactly what we think was happening. Eventually after a lot of head scratching, we were able to repeat it consistently on both sides. Specifically we think a bubble was being trapped between the electric pump and the Gold Cube transducer right behind the firewall (we were able to release it by untorquing the AN fittings). Changing the fuel selector to another position cleared it every time and it then flowed from the original position as well. We tested with the opposite tank empty which then probably acted as a vent. When it later contained fuel it probably sufficiently increased the head pressure.

[jaredyates]

I have a question about cross venting. If the plane is parked on a slope and the fuel selector is set to isolate the tanks, what is to keep the high side from draining to the low side through the cross vent? A ball valve?

[jaredyates]

Nev, in this case, imagine if an air bubble was trapped in the left line near the fuel selector. It was trying to rise up the line and there was enough head pressure from the 3 gallons to keep it trapped from rising, but not enough pressure to purge the air out through the firewall end. Selecting both allowed the head pressure from the right tank to overpower the small pressure on the left, thus burping the bubble back up into the left tank. I haven't experienced this type of thing with the fuel system but have with gravity-fed water systems as we collect lots of rainwater here on the farm.

[Nev]

One fuel flow test I'd be interested to see that had me stumped. Start with an empty L tank. Pour a small quantity of fuel (say 3 gallons) into the left tank. Then disconnect the line where it exits the firewall and select L on the selector. Does any fuel come out ? If not, select both, the L again. Any change ? On mine it only started flowing after making another selection then back to the original tank.

[Nev]

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

More likely when only feeding from one tank when lower on fuel. I would think the 6/4 is more likely to unport if feeding from the L tank and the ball well out to the Left. Problem is that eventually the individual tank selected will contain LESS fuel because it's been burn off, and unporting is even more likely if flying uncoordinated.

Using the 9/1 as a more extreme scenario to illustrate, with only the R tank selected and the ball out to the right indicating a skid/slip situation, the fuel in the right tank will be at the right hand (outboard) end of the tank away from the ports so unporting is very likely. Fuel from the left tank in this situation is not available because only the right tank is selected. If the BOTH position was selected, the fuel in the left tank would be at the right hand (inboard) end of the left tank covering the ports, and available to feed the engine.

This effect can be observed inflight by putting the aircraft out of balance. If you look at the sight gauges, one gauge will read very full (it has fuel at the inboard end of the tank), and the other gauge will read very low (fuel is at the outboard end of the tank). Both tanks might actually contain the same amount of fuel. What is happening is that the fuel in both tanks is moving in the same direction as the ball and occupying that end of the respective tanks. Hence the recommendation to normally run with BOTH tanks selected.

In my mind, whether fuel can transfer from one tank to the other is not a question, I've seen it happen often. It results easily from flying in an uncoordinated situation and the fuel will always flow in the direction of the ball. The Bearhawk can be challenging to fly in balance initially so it makes sense to me that this could be happening and would result in more fuel being consumed from one tank, and more fuel remaining in the other tank. But importantly it also causes the APPEARANCE of more fuel in one tank due to the sight gauges indicating a fuel imbalance situation that may or may not exist.

Try it next time you go flying, With about half fuel in each tank, push the left rudder pedal and observe the ball move to the right. Then look at the fuel sight gauges. The left sight gauge will suddenly be indicating alot more fuel. The right sight gauge will suddenly be indicating less fuel. Then put the aircraft back into balanced flight and the fuel sight gauges will indicate the same fuel in both tanks. All that has happened is that the fuel has moved to a different position in each tank, either towards the sight gauges (and ports) or away from the sight gauges (and ports).

Another way to observe this is to park on a slope with say 5 gallons each side. The downhill sight gauge will show empty while the uphill gauge shows full. The engine will start in the BOTH position, and the Uphill tank position. But select the downhill tank, and it'll likely be completely unported. Leave the fuel selector in BOTH while you go and have lunch and see where the fuel is when you come back. It flowed to the downhill tank through the fuel selector and while it did this there was fuel flowing back up the lines and into that tank. Block either tank vent before you go to lunch and the fuel will likely stay exactly where you left it.

When inflight in these out of balance situations, we tend to then select what appears to be the fullest tank in an effort to rebalance the fuel. The tanks may actually contain the same amount of fuel, but by selecting the tank the appears fullest we are lowering the amount of fuel in that tank more than the other side.

Eventually, we will select the opposite tank when it shows a large enough real fuel imbalance. At this point (if we're still flying uncoordinated to the same side) a very large fuel imbalance may have developed that has overcome the tendency for the sight gauges to show the "uphill" tank as being the fullest. Incidentally, if the fuel imbalance had been caused by a blocked/restricted vent issue for example (instead of flying uncoordinated) we would now be selecting the tank that is most resistant to feeding fuel to the engine (if that was how it ended up with more fuel in it). In a more extreme out of balance situation (with BOTH selected) the fullest tank may even have a fuel flow into that tank (up the lines instead of down the lines) as the fuel flows across from the other tank. The question in my mind is what happens if we now select that (fullest) tank - is it able to adequately feed the engine ?

Just to clarify as Kevin mentioned above, we're not talking about a conventional side slip to land here. In that situation the aircraft is normally high on approach, and engine at idle, so fuel demand is very low.

Incidentally, in the case of the Metroliner, it was a twin engine aircraft and the cross feed was a separate line to the engine supply. So having fuel flowing into one tank didn't impede the fuel supply out of that tank to the engine.

[Redneckmech]

All good! After you commented I realized that my sig was confusing so I updated it. I figured out who you were talking to! haha

[schu]

Unporting a tank and sucking in air instead of fuel.
My feeling is that this is more likely to occur when a selector is in either the L or R position, and the aircraft is being flown out of balance. My understanding is that the aircraft is designed to be normally flown in the BOTH position to prevent this.

I think that unporting a tank and sucking air are two different issues. With gravity and almost certainly bendix/returnless, the system is gravity to the inlet of the pump, and as long as the engine isn't using more fuel than gravity provides, I imagine they are the same, and that unporting a tank isn't a huge deal, if it's brief. In the case of a returning fuel system, it's possible to have the pump pulling more fuel than gravity can supply and I suspect we are now firmly in the realm of sucking air.

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
My current thinking is that it's not having a severe imbalance that's causing the issue, rather the other way around. Flying out of balance when in the BOTH position is leading to a severe fuel imbalance, and the fuel flowing through the selector may be causing an issue at the gascolator/selector. Many years ago I flew Metroliners and if we ever had a fuel imbalance we would simply open the cross flow valve (similar to our BOTH position) and fly out of balance for a few minutes. The fuel would transfer from one side to the other really quick. Sometimes we forgot to close the valve for a few minutes and would quickly develop an imbalance to the opposite side.

My own experience is that I found the Bearhawk particularly difficult to fly in balance initially. More recently I am getting much better at keeping it in balance, and as a consequence I seldom get more than a couple of liters of fuel imbalance between refueling. I mention this not to knock the Bearhawk, but to reiterate that it did get flown out of balance alot when I was new to it. Other pilots found the same difficulty when flying my aircraft. When I refueled I would always find that I pumped far more gas into one tank than the other - though not always the same tank. MattS mentions in post 27 that his Bearhawk with floats was more demanding to fly in balance, and that after the mishap they found 55 liters in the Right tank and only 10 liters in the Left tank. This was after feeding off the Right tank for 30 mins in an attempt to balance the fuel.

I think it's probably very important to maintain balanced flight (side slipping excluded ), and (for my own bearhawk), to use the BOTH position of the fuel selector whenever possible.

Given you evidence that imbalanced fuel load is due to flying in a slip gives a lot of credibility to Bill's theory above. If imbalance is due to one wing flying more or less than the other, then it seems that putting the vent in the cap and not having a vent is for sure a factor.

I stand by my original thought that having a cross vent probably goes a long way to dealing with this, and that single tank operation also gets around this because balancing is explicit and the tanks are discrete systems.

In regard to the metroliner, who knows how that worked. I suspect the vents aren't in the caps, which changes everything.

Thanks for the dialog!
schu

[Bcone1381]

Ops....Sorry....Bill, not Troy. (Bill from Troy, ID)

[Gerhard Rieger]

The Cessna 210 has L&R and a header tank, it was working for Cessna in this advanced aircraft for more than 40 years. For the Bendix system you need large vent tubes by double venting the header tank making sure it will always fill in no time. Having left and right only allows you to isolate a problem, what I did is venting my aux tanks to the main tanks, the header tank is vented to the left and right main tanks by 1/2 inch lines, they are on the upper right and lower left of the header tank both on the upper limit of the header tank. This will also vent out any air from the tank no matter what the attitude of the aircraft is. If the main tank vent block, it will vent from the aux tank. There are one risk to manage, the fuel selector, to make sure that its either on left or right. Taking off with the fuel selector in the off position will allow you to be air born and running the header tank dry, exactly why its important to use checklists and have operating procedures in place, even on a light aircraft like the Bearhawk. The main issue is to have a functional and proven fuel system. We all test our fuel systems to determine flow to the engin, many believe fuel under the floor is a huge risk, no more a risk than your gascolator ripped off in a crash and fuel pouring out by the gallons.

[Nev]

Very interesting Kev. What is happening in the out-of-balance test to stop the fuel from either or both tanks re-establishing normal feed to the selector ?

[AZBearhawk272]

Very interesting thread and I have been following and re reading for a considerable time.
Just about to perform my fuel flow tests and was wondering what tests I can do while I am in the testing mode to gather additional information about modes and scenarios.

Configuration is: Two main tanks, gravity feed down to 2 gascolators, one for each tank located at the lowest point on the fuselage bottom just inboard of each wing strut.
Gascolators feed fuel to the Newton Fuel Valve. R, L, B, OFF. OFF is off with no cross feed. L,R isolate. Fuel valve feeds a pass thru electric boost pump to the Continental engine driven mechanical pump. Mechanical pump has a Vapor / Excess fuel return to the left tank via a check valve.

That said, I am making a test matrix that isolates various system elements, vents, gascolators, Tank levels, Mechanical fuel level sensors, Redcube fuel flow transducer.
With all the data collection through the Dynon I should be able to have a pretty good picture of baselines and influences of changes.


Fringe technical comments: With respect to aircraft and most fuel systems in general. Very few fuel pumps are designed to " Suck or have Suction" on the inlet to the pump. The desired baseline is to have a flooded inlet with positive head pressure on the inlet of the pump. " Suction" at the inlet will cause fuel to vapor lock and create bubbles and possibly associated cavitation. The pumps are designed to move a volume of liquid from inlet to outlet. Any air, entrained or otherwise will cause a substantial decrease in the pumps ability to move fuel from inlet to outlet.
Pressure seen at the outlet of the pump is the result of trying to force the liquid fuel through a downstream restriction. Pumps do not make pressure, pressure is from a restriction to flow. If there is not flow, there is no pressure.

With respect to slipping with low fuel causing the fuel to move away from the tank outlets and pickups becoming un-ported.
When slipping to landing, fuel burn is ounces per minute. Very little flow consumed by the engine. At altitude performed numerous full cross controlled slips, engine idle with less that 1\4 tank
and the engine never faltered while in BOTH. With the un-ported tank selected it generally took 3 minutes of sustained slip before engine roughness and it smoothed out within 7 seconds of wings level coordinated flight.

Now it gets interesting, this is a ground based test you can safely perform yourself. Have the left tank less than 1/4 full. Select left tank, and taxi at a nice fast walking pace in a fairly tight right turn... take a Dramamine first and bring a lunch. When the engine finally quits, stop the plane and immediately try to start the engine. If it does not start wait a minute and try again. If it still does not start, switch to BOTH, If it still does not start, walk back to the hangar and get the truck and a tow strap......... Been there. Done that. Now the question is why would it not start with no airfow over the wings and exactly the same tank air vent pressures and conditions??????

More will be revealed..........

Kevin D

KCHD