Uneven fuel burn

tshort · Post by **tshort** » Wed Aug 23, 2006 12:35 pm

The other thing I have noticed when fueling the '48 is that you need to fill the R tank(s) and then wait a bit - the tank with the fuel cap will equilibrate with the other tank on that side and then you can fill more. I usually do this a couple times and get 7+ gallons over the initial "fill"

Thomas

jrenwick · Post by **jrenwick** » Wed Aug 23, 2006 1:13 pm

George wrote a very thorough discussion of this topic 3 years ago, here:
http://cessna170.org/phpBB2/viewtopic.php?p=8110#8110.

My 170 always uses more fuel from the left tank while on the "both" setting, and my vented cap is on the right. I've wondered if this could be because, in flight, the air pressure above the caps is lower than atmospheric pressure, and there's actually a partial vacuum in the air space over the fuel in the right tank, which would give it a reduced head pressure at the valve. The fuel level in the vented-cap tank would then have to be higher than the level in the non-vented-cap tank to flow at the same rate.

Another way to look at it is from the point of view of the vent line that goes between the two tanks. It receives ram pressure from the vent on the roof, which then splits into the line to each tank. Air in the vent line to the left tank will not be moving, but air in the vent line to the right tank will. That means the left tank (with the non-vented cap) will be receiving static ram air pressure from the vent, but the vented cap will be bleeding that off to some extent from the right tank. The left tank gets more of a positive pressure "push" from the vent than the right tank does.

This makes sense to me, but there are a lot of things in physics that are counter-intuitive, and physics was never my best subject. So I'll stand by and wait for somebody to tell me where I'm messed up!

Best Regards,

John

Post by **Bruce Fenstermacher** » Wed Aug 23, 2006 1:40 pm

John I believe the vented fuel cap has, when it's valve is fresh and working properly, a one way valve allowing air in the tank but not out. It's just a rubber flapper cap. So any ram air pressure from the vent tube would be equal to both tanks.

I'm still thinking about the head pressure thing........

jrenwick · Post by **jrenwick** » Wed Aug 23, 2006 1:50 pm

N9149A wrote:John I believe the vented fuel cap has, when it's valve is fresh and working properly, a one way valve allowing air in the tank but not out. It's just a rubber flapper cap. So any ram air pressure from the vent tube would be equal to both tanks.

Thanks, I didn't know there was a check valve. I wonder if it's working right on mine....

I'm still thinking about the head pressure thing........

Here's a reference, chosen at random from a Google search: http://hyperphysics.phy-astr.gsu.edu/hbase/pflu.html

Best Regards,

John

tshort · Post by **tshort** » Wed Aug 23, 2006 2:04 pm

Both my fuel caps are the same - is there a requirement for vented caps on the '48?
(I'm not so sure that either of them is all that airtight - when I blew into the vent the other night I could hear air escaping on both sides).

T.

cessna170bdriver · Post by **cessna170bdriver** » Wed Aug 23, 2006 3:10 pm

N9149A wrote:Miles and BL ... Hmmmm. So if I had a pond at a given altitude that was 5 feet deep and covered 1 acre, the dam at the end could be the same size as a like pond that covered 10 acres. The pressure at the dam would be the same. Hmmm... So many things to study and think about. Hmmm....

The altitude doesn't matter either, only the depth and the density of the fluid. Doesn't matter if it's 5 acres or a hundred square miles, sea level or 10,000 feet, the pressure at the bottom of 5 feet of water (on this planet

) is about 312 pounds per square foot (2.16 psi). If that same pond were full of 6 lb/gallon avgas, the pressure at 5 feet would be a bit over 224 pounds per square foot (a bit over 1.5 psi), the same as the pressure at a carburetor 5 feet below the level of the fuel in a gravity feed system.

Miles

Post by **Bruce Fenstermacher** » Wed Aug 23, 2006 9:40 pm

This is very interesting Miles, we learn so much here at the forum.

John that is a very interesting and informative site.

Thanks

Post by **GAHorn** » Thu Aug 24, 2006 3:54 am

The AFM stipulates that the fuel selector valve be in BOTH for takeoff and landing.

flat country pilot · Post by **flat country pilot** » Tue Oct 24, 2006 12:39 pm

tshort wrote

I know the high wing Cessnas are well known for uneven burn, but mine has changed recently and I'm looking for reasons why.

Why do high wing Cessnas have an uneven fuel burn?

Does this neccessarily mean the fuel system has a problem?

Bill

N2255D · Post by **N2255D** » Tue Oct 24, 2006 4:31 pm

George wrote a very thorough discussion of this topic 3 years ago, here:
http://cessna170.org/phpBB2/viewtopic.php?p=8110#8110.