Fuel Flow in a 7GCBC

T

TwinCessna

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Dallas, TX
Hello All! I just flew a Citabria 7GCBC from Virginia to it's new home in Texas. My stops along the way home were roughly 1.5 hour legs. Did it this way to play it "safe" on fuel consumption and performance. Here's my question/concern/observation.... At every fuel up, I would put about 50% more in the right tank as I would in the left tank to bring the tanks full. 15 gals total fuel would be roughly 5 in the left and 10 in the right, etc. Is this normal? Doesn't seem normal to me, but I also don't have the years of experince that many of you have. Any thoughts/suggestions are appreciated. Thanks!
 
Yes, it's normal, or at least common. My 8KCAB seems to even out when it gets to around 5 gallons left in each tank.

I've done a lot of XC in the last year. I plan on 2.5 hour legs and have a hard stop at 3 hours, which leaves a 1 hour reserve. I plan for 10 GPH and find that to be pretty accurate.
 
is your fuel tank vent overhead on the left side or under the wing? are the underwing vents on the left side too? i forget.
 
If the fuel vents and fuel caps are all working properly the fuel will burn from either side depending on the trim of the airplane. You can make it burn from either side by holding the rudder pedal on the opposite side.
The most common cause of uneven fuel burn is a leaking fuel cap gasket. You can check for this by swapping caps side to side and see if the uneven burn switches tanks.
 
I have never heard of a fuel starvation incident where uneven feeding was cited as a cause for our type. If that were possible there would be an AD. So if you are worried about running out of gas earlier than planned because of it, don't be.

The only negative consequence can be a heavy wing in cruise, or so it feels sometimes. I found that getting my rudder trim tab and power settings dialed in so I am cruising with the ball perfectly centered with feet flat on the floor makes a big difference in even feeding.

If you only have one gauge, then uneven feeding does complicate interpreting it for in-flight confirmation of your burn calculations.
IMO best practice is to assume the non-gauged tank is empty. So for example when my gauge reads 1/4 full, I assume I have 5 gallons or 30 minutes, and I look for a place to land immediately. In theory I could have as much as 25 gallons, but I found from experience that the tank levels are more even once the gauge gets below 1/4. Landing and refueling with the gauge at 1/4 usually results in 25 to 30 gallons added, so I have always had at least 1 hour of flight time in reserve. One time I ran it to 1/8 trying to stretch, and only had 4 gallons when I landed. Won't do that again.

YMMV
 
the vent system is the culprit. location of the vent tube, the vent check valves at the tanks, and gas cap gaskets will affect the positive pressure being fed to the tank. Also, with fluids converging, the side that is already in flow will continue to flow until the energy of the flow is affected. You can slip the plane, to 'push' fuel, but not necessary. Once the tank not flowing gets enough head pressure to overcome the other tanks flow, it'll take over. This will continue until there is no fuel. Sorry if this sounds confusing.

In other words, the tanks will take turns feeding the engine until they are both empty

This is a common characteristic of single vent systems
 
Ed's comment made me reflect. My Dec is about the same vintage, only one gauge for two 20 gal tanks. A three hour flight puts my gauge at O (I guess as opposed to zero) but leaves me ten gallons. Timing is everything in light planes - know your fuel burn and wear a wrist watch.
 
Thanks everyone! All great things to review/consider! Bartman - only the left tank is vented, gas caps are non-vented types. I will spend some time flying locally and accuratley triming & watching/adjusting power settings to see how that affects flight. I also likely drain the tanks (since they are nearly empty now), then top off to see exactly how much is going in (want to know exactly what I'm starting with). Then - fly some exact times, like taxi, takeoff, land, park at exactly one hour to see what my real fuel burn is. Just need to get confident with the real world fuel flow of this plane, not with what the books say. Bob Turner - perfect with "know your fuel burn and wear a wrist watch"! I WILL be doing that. Looking forward to getting to know this plane! Again, thank you all for the responses. Really digging this forum too - tons of great info!
 
Correct!! Sorry - newbie mistake. The vent is on the left wing and also crosses over to the other tank. Which is the tube running across the top of the cockpit. Thanks for pointing that out. Learning everyday!
 
I fly a couple of long x-countries between Tucson and Bozeman (1000 mi) each year in my 7GCBC. It's three 3.0 to 3.5 hour legs depending on wind. (Makes a long day in the saddle!) Each leg burns +/- 24 gallons cruising at 2450rpm and 10000 ft. GPS speed in this configuration is 115mph +/- wind.

My 7GCBC has a vent on the left wing that vents both tanks and fuel gages in each wing root. The fuel caps are not vented. A leaky fuel cap gasket will cause uneven fuel flow from the tanks. WagAero sells the fuel caps quite reasonably and I keep a spare in the hanger as it's easier to replace the whole cap than to replace the gasket.

Also, with my plane, it's pretty easy to screw the fuel cap on without it seating properly. This causes fuel to vent from the cap and can empty one tank in an hour or so. Checking that the fuel caps are seated correctly is part of my pre-flight and also checked after someone else fuels the plane.
 
Thanks Desert7GCBC for the info! I always double-check the caps for secureness/seating, but did notice a questionable seal on one of them. I'll be ordering two new caps. I know I could just replace the seals, but it's not a great expense to just replace the whole cap - even both sides.
 
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