C172 Fuel Selector

arnoha said:
Why does the fuel pump matter? If the port goes dry, the pump can't fix that.
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I thought you were referring to the vapor lock. The un-porting of the fuel tanks in a low-wing is less likely due to wing dihedral causing fuel to naturally stay near the ports. The high-wings don't normally have dihedral and sometimes are anhedral, which doesn't help fuel porting. Since low-wings are always using a fuel pump to feed the engine, you aren't relying on gravity to feed the header tank, either.
 
My high wing plane has dihedral. You can look at most high wing Cessnas and see the dihedral also. Which common high wings do *not* have dihedral? I'd bet most of them are aerobatic.
 
bullwinkle said:
My high wing plane has dihedral. You can look at most high wing Cessnas and see the dihedral also. Which common high wings do *not* have dihedral? I'd bet most of them are aerobatic.
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Agree, but that said, it is true that low wings would have far more dihedral, which helps. Also...header tank. I thought you only see header tanks with akro planes? Or only low wings (which would also explain why they don't care about slipping)? I don't think my 172S has one.
 
bullwinkle said:
You can look at most high wing Cessnas and see the dihedral also. Which common high wings do *not* have dihedral? I'd bet most of them are aerobatic.
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Cessna 150/152 has 1 degree of dihedral; the other Cessna strut-braced piston singles since the 170B are 1.7 degrees. Cardinals are 1.15 degrees. Among low-wing types, Bonanzas are 6 degrees and Piper PA-28s are 7.

The Cessna 170A's wings are so flat it looks like something is broken. Dihedral was increased on the 170B.

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My GPS tells me to switch tanks after 45 mins, so up 'high' I do. It makes me feel like I'm flying a more sophisticated airplane than a '66 172. :) I also use it to balance the load as mine likes to drop a wing slightly depending on configuration. I've never thought about using it to prevent fuel crossfeed on the ground. One night the FBO that fueled me up refused to put me in a hangar with an approaching storm because fuel was dripping from the vent.
 
arnoha said:
Agree, but that said, it is true that low wings would have far more dihedral, which helps. Also...header tank. I thought you only see header tanks with akro planes? Or only low wings (which would also explain why they don't care about slipping)? I don't think my 172S has one.
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Your 172S has a header under the copilot's feet.
 
bullwinkle said:
My high wing plane has dihedral. You can look at most high wing Cessnas and see the dihedral also. Which common high wings do *not* have dihedral? I'd bet most of them are aerobatic.
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The 210 has minimal dihedral and a serious unporting problem because of it. Cessna recommends not letting the tanks get too low.
 
The single-tank operation on old 172s was a result of some engine power loss events that Cessna blamed on vapor lock in the fuel lines under certain conditions of altitude, temperature and humidity. What humidity does I have no idea. Anyhow, they had two service bulletins on in the late '60s and a service kit to fix it. That kit involved installing tees in the vent crossover tube above the headliner behind the front spar carrythough and a couple more in the fuel lines above the doorsills where the fuel moves out of the tanks and back and down behind the aft doorposts. The idea was to let any vapor bubbles that formed to escape into the vent line.

Cessna tested the original system using clear plastic lines and a movie camera to try to spot how the vapor formed and what it did to the fuel flow. They weren't able to get vapors to form on their own, so they had to introduce some air and see what it did. They designed the fix based on that. They never were able to confirm that the power loss complaints were due to vapor lock in that area, but that was the most likely spot.

From their info in the service bulletins, I inferred that vapor might form there if hot and high enough, since the head pressure is at a minimum there. A bubble in the line might want to move upward at the same rate as the fuel is travelling downward, so it is, in effect, a partial blockage in the line. That, I think, is why it was OK in climb (lots of fuel flow at high power that carries the air to the carb where it is vented through the bowl vent) or in single-tank operation in cruise (maintains a good rate of fuel flow in the line, twice that it would be if on Both). Cessna installed the modified system in 172s from 1969 and on, IIRC.
 
If I were flying for more than an hour, and were alone, I would draw fuel from the right tank until the wings came level then switch back to both.
 
Dan Thomas said:
Your 172S has a header under the copilot's feet.
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Ah, you are correct. It's right there on the diagram, marked as a "FUEL RESERVOIR TANK" coming just after the selector and it vents into the crossover vent line. I should pay more attention!
 
PilotRPI said:
While on a longer flight, do any of you use individual tanks, or just keep it on both the whole time?
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The only time I put the selector on a 172 or 182 to anything other than BOTH is when it is being fueled so it doesn't crossfeed while fueling is underway. Otherwise, BOTH works just fine. But, also, all the club's planes are 1974 or later (but not much, 1980 is the newest).

This assumes, of course, that a leak hasn't developed during flight. :D
 
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