round_peg Posted May 29, 2012 Report Posted May 29, 2012 Can't believe we are the first to encounter this... we recently drained the tanks for fuel line work, measuring what we took out into the jerry cans we use for refueling. According to the dipsticks we should have had around 16 gallons in the tanks, but we measured out closer to 13 gallons. We scratched our heads over this discrepancy, setting it aside until we were ready to refuel. We then measured 19.5 gallons in the cans and poured them into the dry tanks. The dipstick came up with approximately 22 gallons. So it appears to us that the dipstick is reading between 2-3 gallons high. Has anyone else tested this? We have the L/R style dipstick, if that makes any difference.
Runtoeat Posted May 30, 2012 Report Posted May 30, 2012 Mitch, you definitely aren't the first to notice this and you will find discussion on the topic elsewhere here. Some CT's have L/R stick but others have a stick that shows gallons on one side and liter on the other so just one sided. Personally, I've found that my L/R dipstick reads fairly accurately if I use just the Right side of the stick for both tanks on my CTSW. Best to calibrate the dipstick by the method you used, which is, fill a known amount in the tanks and mark the stick. I marked the fuel level on my fuel sight tubes with paint marking pen to show 3 gallons remaining on each side to insure I know when I get to my 6 gallon minimum.
round_peg Posted May 30, 2012 Author Report Posted May 30, 2012 Mitch, you definitely aren't the first to notice this and you will find discussion on the topic elsewhere here. Some CT's have L/R stick but others have a stick that shows gallons on one side and liter on the other so just one sided. Personally, I've found that my L/R dipstick reads fairly accurately if I use just the Right side of the stick for both tanks on my CTSW. Best to calibrate the dipstick by the method you used, which is, fill a known amount in the tanks and mark the stick. I marked the fuel level on my fuel sight tubes with paint marking pen to show 3 gallons remaining on each side to insure I know when I get to my 6 gallon minimum. I searched but did not find any discussions of this specific issue, but I'll try again. Another thing we noticed when filling the tanks with a measured amount is that reading both tanks with the dipstick "backwards" results in the same total fuel load. Perhaps this explains why later CTs are shipped with a non-directional stick. I have a Fuel Hawk from a 172 that I am tempted to try and recalibrate but at this point I'm not sure I understand the asymmetrically of the CT tanks well enough to have confidence in the results. It also seems like the method would be painstaking, as in, a real pain.
Jim Meade Posted May 31, 2012 Report Posted May 31, 2012 How much fast does the fuel cross level? I'm guessing the fuel valve must be turned off to calibrate each tank accurately. I guess that is two separate observations. Cross level speed, anyone know?
round_peg Posted May 31, 2012 Author Report Posted May 31, 2012 How much fast does the fuel cross level? I'm guessing the fuel valve must be turned off to calibrate each tank accurately. I guess that is two separate observations. Cross level speed, anyone know? I don't believe that closing the fuel valve would have much if any impact on the syphoning effect since the valve is downstream of the Y-connection between the tanks. So you'd have to block the flow where the lines come down from the tanks and through the firewall. This seems like an inherently messy operation, and one you'd think FD would have done when it calibrated the dipstick, so we don't have to. It's annoying to have it indicate wrong (especially since it measures 2-3 gallons high). At this point we've filed this information under things the pilot should know. The solution really needs to come from FD.
opticsguy Posted June 1, 2012 Report Posted June 1, 2012 I'll attest to the extra 2 gallons/side on the dipstick reading. After hundreds of hours comparing what my sight tubes, Dynon, and the dipsticks say, 2 gallons high is about right in the middle range on the dipstick. Full is still full, and low readings (<2gal) are pretty close, but if the dipstick says 10 gal, it's probably 8 or less.
207WF Posted June 2, 2012 Report Posted June 2, 2012 my calibration of the dipsticks is also similar to what you find. WF
Duane Jefts Posted June 2, 2012 Report Posted June 2, 2012 Is it possible that FD thinks there is 2 gal usuable fuel downstream from the bottom of the tanks measurement and they compensated for that "outside the tanks fuel"? This dosen't seem reasonable but there must be some explanation folks think the dipsticks read 2 gal high. A known stick calibration into a known tank capacity should be simple technology. I think we have been using that since the day of the first fuel tank. I will keep a log and pay particular attention to readings and quantities during the Alaska trip this summer.
207WF Posted June 4, 2012 Report Posted June 4, 2012 When I did the calibration I drained all of the downstream fuel, so I agree that explanation does not work. WF
round_peg Posted June 4, 2012 Author Report Posted June 4, 2012 Agreed, all of the fuel downstream of the valve couldn't amount to much more than a few ounces.
Mike Koerner Posted June 12, 2012 Report Posted June 12, 2012 We have a right-left dipstick which has been shown to be very accurate across the full range of the tanks. Note that the right and left scales differ by more than a gallon. Obviously if the fuel is sloshing around at all, which it does, the dip stick will read high. I wait for the fuel to settle before taking a reading, and then do it quickly to eliminate the high fuel reading bias. Finally the bottom of our tank is not flat right under the fuel port. It seems some excess epoxy or sealant dripped in from the front of the opening during manufacture. I slide the dip stick down along the back edge. Mike Koerner
rookie Posted August 29, 2012 Report Posted August 29, 2012 Dick was right, Since I had to drain the fuel to do the wing check I decided to check actual vs. dipstick reading. Adding fuel in 2 gal increments a made a chart of actual vs. reading. The left reading was worthless, no, less than worthless because it read HIGH! The right however was generally pretty close for both tanks. This finally explains some very odd fuel flow rate anomalies I've experienced. I now have my 5 gal. mark on both sight tubes and only use the right indicator dip stick. Good call Dick, al meyer
Runtoeat Posted August 30, 2012 Report Posted August 30, 2012 Hi Al. Good information. Knowing when I am to a comfortable reserve remaining is key information. For me, this is when either sight tube shows 3 gallons. Before getting to this level of fuel, I'm looking for the nearest airport to land and refill my tanks. Completely draining the tanks and filling with a known amount and then marking the sight tubes leaves no doubt how much fuel is on board. This is easy to do when the tanks are drained at the required wing removal inspection.
round_peg Posted August 30, 2012 Author Report Posted August 30, 2012 Another consideration here is that fuel quantity measurement is not precise under the best of circumstances. Fuel is denser in cold weather. If you filled your tanks in hot weather and measured the quantity again the following morning you'd have less by volume. Not sure how much of a factor this could be, but two gallons (the measuring difference some of us have found on the dipsticks) into 30 gallons is about 6%.
FredG Posted August 30, 2012 Report Posted August 30, 2012 According to the engineeringtoolbox.com, the volumetric expansion coefficient of gasoline is 0.00056 per degree F. So, a 100 degree change in fuel temperature results in a 5.6% change in gasoline volume. The relationship is linear, so a 10 degree change in fuel temperature gives a 0.56% change in fuel volume, etc. Real world maximum temperature changes are around 40 degrees F, resulting in a change of 0.67 gallons per wing with 30 gallons of fuel on board.
round_peg Posted August 30, 2012 Author Report Posted August 30, 2012 According to the engineeringtoolbox.com, the volumetric expansion coefficient of gasoline is 0.00056 per degree F. So, a 100 degree change in fuel temperature results in a 5.6% change in gasoline volume. The relationship is linear, so a 10 degree change in fuel temperature gives a 0.56% change in fuel volume, etc. Real world maximum temperature changes are around 40 degrees F, resulting in a change of 0.67 gallons per wing with 30 gallons of fuel on board. So I take it from your calculations that temperature might account for about a gallon or so?
FredG Posted August 30, 2012 Report Posted August 30, 2012 Actually, I made an error. Its 0.67 gallons change total (with a starting volume of 30 gallons). The per wing change would be half of that. See formulas, below. Change in fuel volume = 0.00056/deg F x temperature change in deg F x starting volume in gallons Change in fuel volume = 0.00056 x 40 degrees x 30 gallons Change in fuel volume = 0.67 gallons (note, that is total, not per wing). For a change of one gallon in 30, temperature has to change by 60 degrees F. Still, that's only 0.5 gallons per wing, which is hard to measure with any precision at all. A one gallon change per wing would require a 120 degree F change in temperature (and 15 gallons to start, per wing). Overall, except for very uncommon circumstances, given that the precision of the dipstick is about one gallon, such temperature related changes in fuel volume are unmeasurable unless many repeated measures are obtained and averaged every time the dipstick is used.
Al Downs Posted September 5, 2012 Report Posted September 5, 2012 All of this talk about the fuel level makes me think about the different levels in the two tanks. I know it has been discussed before but I still don't understand how the levels can become so different in the two tanks after filling up. It has been said that flying with one wing slightly lower than the other one is the cause. Well then why doesn't it even out when back in the hangar. I have checked the fuel level after sitting in the hangar for a week and one side will have 3,4,or even 5 gallons more than the other side and the fuel shutoff was open. Why doesn't the fuel seek a level that is the same in both tanks? Given the way it is plumbed I would think it would even out.
Doug G. Posted September 5, 2012 Report Posted September 5, 2012 Mine does even out over a few days. The fuel shutoff makes absolutely no difference. Hangar floor or landing gear?
FredG Posted September 6, 2012 Report Posted September 6, 2012 Roger summed it up. Given enough time to equalize, the level of fuel in the two tanks will be identical. But, that is identical fuel level with respect to the horizon (or the center of the earth), not necessarily with respect to the wings (or fuel tanks). If the fuel levels in the two tanks are unequal after sufficient time for equilibration (don't know exact time, but a day should be more than enough), then the plane (the wings) is (are) not level. Other explanations are obscure and hypothetical, such as a defect in the bottom surface of the tank on which the bottom of the dipstick rests.
NC Bill Posted September 6, 2012 Report Posted September 6, 2012 I measured my hangar floor at the front of the main wheels - dead level. Landed Monday with 8gal left side - 14gal right. Wednesday same measurement - 8 left and 14 right. :wacko:
Jim Meade Posted September 6, 2012 Report Posted September 6, 2012 I would be curious if mine didn't transfer sitting for a day or so. Why would it not? One wing higher (could level the plane to verify). Vent restricted? Hmmm, not sure. Could check them. One of the course wing filters gooped up? Naww. Could you prop a wheel up a couple of inches over night to force a transfer (be careful to not overflow the down wing) and see if it works? Just thinking out loud. If the plane would not transfer in an expected manner, I'd like to know before I took off on a long trip or one where I thought I'd get down to the last hour of fuel.
Ed Cesnalis Posted September 6, 2012 Report Posted September 6, 2012 My tanks generally equalize overnight but sometimes it remains un-balanced. I think there is some tension somewhere that has to be overcome in order for the siphon to begin.
round_peg Posted September 6, 2012 Author Report Posted September 6, 2012 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.
Runtoeat Posted September 6, 2012 Report Posted September 6, 2012 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?
Recommended Posts
Archived
This topic is now archived and is closed to further replies.