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Fuel Dipstick Accuracy?


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Mine does even out over a few days. The fuel shutoff makes absolutely no difference. Hangar floor or landing gear?

 

Ours does not level out, no matter how long the airplane sits in the hangar, even though the ball is centered on the turn coordinator. The left tank is usually lower than the right tank, often by a 2:1 ratio.

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For those who have unequal fuel depletion during flight or while sitting with one tank fuller while in the hangar, try measuring the fuel flow next time wings are removed. Before emptying fuel from the tanks to remove the wings, pinch off the fuel line on one tank and measure the flow from the other wing thru the gascolator. Remove the clamp and do the same for the other side. Compare the flow. If the flow is not equal, do some detective work to find out why. I would first check the vent in the caps (CTSW) or at the wing tips (CTLS). Do both vents have the low points of their skirts towards the trailing edge of the wings and are these clocked in the same position if a CTSW?

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For those who have unequal fuel depletion during flight or while sitting with one tank fuller while in the hangar, try measuring the fuel flow next time wings are removed. Before emptying fuel from the tanks to removing the wings, pinch off the fuel line on one tank and measure the flow from the other wing thru the gascolator. Remove the clamp and do the same for the other side. Compare the flow. If the flow is not equal, do some detective work to find out why. I would first check the vent in the caps (CTSW) or at the wing tips (CTLS). Do both vents have the low points of their skirts towards the trailing edge of the wings and are these clocked in the same position if a CTSW?

 

Interesting suggestions. So, if I read you correctly, if the vents are implicated simply removing the filler caps for awhile should equalize the tanks, provided the airplane is sitting on a level surface.

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Interesting suggestions. So, if I read you correctly, if the vents are implicated simply removing the filler caps for awhile should equalize the tanks, provided the airplane is sitting on a level surface.

I'd think so, but I'd want something to cover it so no dirt, bird feces or other junk got in.

 

 

 

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I'd think so, but I'd want something to cover it so no dirt, bird feces or other junk got in.

 

Well sure, but just unscrewing the covers and letting them sit loosely on top of the openings for awhile should be enough to allow the air pressure to equalize. Wouldn't you think?

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Oh yes. Murphy's law is in full effect here. The only time a bird would fly in one's hangar and take a dump directly onto the filler is when it has been removed for a flow test :excl:

 

Another thought is to make sure the ball is correctly centered when the wings are level. Based on my CTSW, I don't have a lot of confidence that the wings are level when the ball is centered due to the "mushroom" not being "true" to the airplane. I suspect that there are a few CT's that do not have good agreement between the ball "center" and leveled wings due to this. My mushroom can be moved and the ball goes along for the ride with this. The result of this is that the ball indicates that my wings are level when in fact, they aren't. I have another independent instrument, a TruTrak attitude indicator, which runs on it's own gyro's and this provides a true picture of my attitude.

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The vents on the filler cap are open to the atmosphere. The cap on top is open around the edges. It shouldn't be any different with the cap on or off if it sits over night. Air pressure equals very fast if there is a slight differential. That's how we breath is pressure differential, we don't suck air in.

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The vents on the filler cap are open to the atmosphere. The cap on top is open around the edges. It shouldn't be any different with the cap on or off if it sits over night. Air pressure equals very fast if there is a slight differential. That's how we breath is pressure differential, we don't suck air in.

The question was to try it with the vent cap completely off as one way to exclude the vent from being part of the failure fo the fuel to balance. It's a simple, free and easy test that is harmless as long as no debris gets in. I suppose I could just grab the air hose and put 125 lb thorough the vent. Better take it off, first, thoough, so I don't distend the tank. :)

 

 

 

 

 

 

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If 1) the tank vents are clear, 2) the connection between the tanks is not plugged, and 3) the plane has been stationary for a while (I suspect more than one day), then the level of fuel in the tanks is equal. That doesn't mean that the amount of fuel in the tanks is the same, although, if the plane is level and the airframe not badly bent, they will be close. As Dick mentioned, a centered ball does not guarantee a level plane. I checked the level of my plane by putting a 4 foot straightedge on my wings and measuring their inclination (the manual says 1.7 degrees of dihedral per wing). When the wings have the same inclination, I conclude that the plane is level. Many CTs have a poorly leveled ball, and fly in a subtle slip with ball centered. This can be reviewed in books on airplane rigging.

 

Bottom line, unless something is defective (see #1-#3, above), the fuel system is a "water level" (see http://en.wikipedia.org/wiki/Water_level) with equal fuel levels between the tanks. If the fuel level between the tanks is not equal, odds are good that physics and gravity have not failed but that something is out of plumb or some tube is plugged.

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Sorry Roger, but when we breath, we do suck air in. We do this by creating a pressure differential between the inside of the chest and the outside of the chest. These are not opposing concepts as you imply, they are simply different descriptions of the same phenomenon.

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I did the vent research back in 2007 for fuel flow. I tried several different things and even pressurized then with a forward facing vent into high pressure air and it had no effect. I still have those special vents. What I have found is the short fuel hose at times have a slight bend with a reduced radius on one wing feed, the mushroom is not square to the plane so the ball is off. These are the two most common.

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Hi Fred,

 

We breath because of pressure differential and in our normal breathing and it is a passive operation. You can suck air in, but that's a conscious effort not our normal pattern. I was a medic for 30 years and taught Advanced Cardiac Life Support (ACLS) to Doc's, nurses and medics for 27 years. Your intercostal chest muscles lift the chest and the diaphragm moves downward to help increase chest volume. The lungs adhere to the chest wall with a gooey surfactant. As the muscle lifts the chest wall and the diaphragm moves downward with the lungs adhering the lung volume expands which causes a slight negative pressure in your lungs and with the out side air at a higher pressure the air rushes into the lungs to equalize. As the chest reduces size the lungs reduce there size and the pressure inside increases and forces out the air. Our fuel tanks work on a similar basis as far as air transfer from a higher to a lower pressure. The 1/8" hole in the vent tube won't make any difference compared to the cap being removed. The air will transfer with pressure differential.

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. What I have found is the short fuel hose at times have a slight bend with a reduced radius on one wing feed

 

A bent hose stopping the fuel from seeking it's own level doesn't make any sense. It may slow it down, but it would not stop it.

 

 

The discussion on how we breath is very educational. I look forward to it's development.

 

 

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Roger, thanks for the lesson. My small point was that you seem to think that sucking air into the lungs is somehow a different process than creating a pressure differential. They are different words for the same phenomenon. Consider this example - if you pull the plunger on a syringe, it sucks air in. Yes, of course it does so by by creating a pressure differential. There is no difference. I objected to your suggesting that one explanation was right and one explanation was wrong. Such an approach is, in my opinion, dogmatic and unhelpful.

 

For what its worth, I am board certified in internal medicine and occupational medicine. I am professor of pulmonary, critical care, and occupational medicine. I have been teaching pulmonary physiology to physicians, PAs, med tech, nurses, and other in major US medical centers for decades. I have been treating patients with lung disease for decades. I still do. I actually do understand how we breath. I have no doubt that you do, too. I don't expect you to suddenly drop your opinion because of my credentials. Why do you think I should do that when you tell me your credentials?

 

Of itself, this difference of opinion about breathing (active vs passive, sucking vs pressure differentia) is irrelevant. But, I see this discussion of breathing as an example of a problem that persists on this list and diminishes its value. In particular, I believe that this list would be better served if the expertise of its members was valued rather than being regularly discounted by the official "experts". This phenomenon is not limited to discussions of pulmonary physiology, but, for me, this discussion has most certainly illustrated it.

 

I do not doubt your expertise and I value the energy you put into the list. I have learned much from it. I would like to see it grow.

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Hi Fred,

 

 

The only reason I brought up my back round was to let you know I had that medical backround. I didn't know you had one too. So I was just trying to be clear if I was talking about an anatomy and physiology issue to a lay person who may not have had that training. I didn't want you as a possible lay person to think I was just pulling the physiology out of the air. It was just a qualifier. I meant nothing by it.

I probably should have left the lung example out.

My original point was that our tanks transfer air by pressure differential, that was it and with the hole size in the vent removing the cap shouldn't make any difference in that differential as the air can move freely. I have tried several different vents, negative and positive pressuring. I have completely different vents than any other CT. Each plane has its own little quirks on fuel transfer while sitting. In the air they pretty much behave the same and through research I have found that the flat wing tanks and mushroom mis-alignment are the main causes for fuel drain.

un-equality which can be either the left or right tank depending on each plane. I think the left is the more common tank to drain faster. These planes are all hand assembled and even though a jig is sometimes used they are all individuals. The way I found my

mis-alignment was to put an 8" piece of yarn in the center bottom part of the windscreen. When my yarn is centered the ball is out 1/2. If I fly like that my fuel drain is equal.

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The propeller generates a rotational air flow that can displace a yaw string mounted behind it to one side even when the plane is flying straight and coordinated. It's better if the yaw string is clear of the prop blast. As a glider pilot, where a yaw string is normal, I wouldn't put much stock in it behind an engine without some corroboration.

 

Again, the discussion of removing the vent cap over night was to address the possibility of the vent being blocked. No one thinks the vent orifice is so small as to affect the fuel finding it's own level so long as the vent is clear.

 

I agree 100% with your conclusion that the mushroom can be off.

 

 

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I did that air flow test too. That was questioned in 2007 during the original testing. Went to altitude and put the string in a yaw and then another time straight and shut the engine completely off each time. The string never moved during shut down or start up going fast or slow.

 

It's interesting to watch these discussions re-cycle after a few years. Charlie Tango can remember these. There may be about 6 people still active and here that can remember.

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From a practical standpoint how the pressure differential is described makes no difference, From a scientific view it is akin to talking about heat - one always talks about heat transfer, not cold transfer. In another vein, hen we talk about high and low pressure systems, we don't usually talk about the low sucking air from the high, the air flows from high pressure to low. All this is a bit like talkig about which terminal electrons flow to or from in a circuit. There is a standard in electronics that is the opposite of the physical reality.

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Jim, can you explain this? I must have missed something amidst all the back-and-forth about lungs.

 

I'll let Roger or FredG discuss it, as it has been covered quite extensively here and I know they both have more direct experience than I do. Mine was off, but I was not as intimately involved in fixing it, whereas I know FredG has and of course Roger has checked many. It's a long-known problem with some installations.

 

 

 

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The mushroom or entire black instrument panel may not be and in many cases not perfectly square to the plane and it's true direction. Then when the instruments are put in place in the panel they are off just a tad. Not way off, but enough to have let's say the ball out by 1/4 to 1/2 since this is what we are talking about. You may not even notice it, most don't. Couple this with flat wing tanks and you have an uneven fuel drain. Remember the fuel will follow the ball. A few have taken the trouble to adjust their panels. If you push on our instrument panel you will also find it is easy to move and not a real solid mount to the forward firewall. This really bothers some and doesn't bother others. It isn't a major concern and knowing this it makes fuel management pretty easy. This is also why some have a right side drain or a left side drain faster and some are even. Each aircraft has it's own personality since they are hand assembled. This was a huge discussion like it is here back in early 2007. We aren't the only ones like this and it is spread throughout other aircraft.

 

Even sitting fuel transfer as discussed above has several factors that will affect static transfer. Some are obvious and noticeable and some aren't.

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I'd like to add my solution to this, if I may. Having had the same problem with unequal fuel usage and flying "one-wing-low," my AI/A&P in Stanton, MN and I got together and discovered the problem. We found the flattest piece of asphalt in the area. Then we took a five-foot bubble level and put it on top of the overhead plexiglass extending out toward each wingtip. We moved the plane a bit until we had the bubble exactly centered: now the plane was truly level.

 

Then we took a six-inch digital level and placed it under the EFIS and discovered that it was tilted 2.5 degrees from level. We decided to rotate the EFIS within the panel rather than trying to jack up the entire panel on one side.

 

Significantly retrimmed, the plane flys with wings level and I no longer lose fuel out of the right tank when I fill it over 14 gallons. Initially, it appears to burn fuel equally from both tanks as well. Of course, I'd have gotten the same results if I had just flown the plane with the ball 3/4 of the way out of the "goalposts". Who da thought?

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