The International Cessna 170 Association

n2582d wrote:Here's a question -- probably a stupid question -- for you engineers. When working on one tank that requires it to be emptied could one transfer fuel from one less than half full tank to the other tank, also less than half full? Reading about the recent issue with the '48 with the probable fuel cap leak made me think about this possibility. My thought was to place the fuel valve on "both" and then either pressurize the tank to be emptied or place a vacuum on the tank to be filled. This would eliminate the safety and avgas armpit issues that accompany draining fuel. The issues one would face might include deforming tanks with too much pressure or vacuum. I would think one would have to pressurize to something like two psi. Creating a vacuum by sucking fuel vapor into a Shop Vac would be an explosive experience so that would not be good. I have a vacuum tool for getting air out of auto coolant system that might work in an application such as this. It uses shop air to create the vacuum. Capping the overhead vent lines would probably make this idea impractical. Thoughts?

I would be extremely cautious about doing that. The vacuum differential pressure needed to collapse a tank would be very small. I’m guessing one or two psi.

I would NEVER creat a vacuum in a Cessna fuel tank . I have seen horrible results when fuel tanks have had a vacuum situation. They are designed to have a slight positive pressure but never a vacuum.

johneeb wrote:Gary,
while doing your experiment wouldn't you have to raise the wing with the tank you want to empty so as to not un-port the cross cabin vent line?

John,
Yes, that's why I added the last sentence, "Capping the overhead vent lines would probably make this idea impractical." But that's what puzzles me about these reports of '48 models transferring fuel to the tank with the leaking cap. If the cross-cabin vent line is not clogged shouldn't there be equal air pressure in both tanks? The line isn't long enough that there would be a differential pressure. Residual fuel collecting in the cross-vent which doesn't allow the air pressure to equalize between tanks?

ghostflyer wrote:I would NEVER creat a vacuum in a Cessna fuel tank . I have seen horrible results when fuel tanks have had a vacuum situation. They are designed to have a slight positive pressure but never a vacuum.

Isn't a vacuum being created with a leaking cap?

I would never pressurize or pull a vacuum on the gas tanks. They are not rated for either.

Say the tanks are 30” x 30”. That’s 900 square inches times 2psi is 1800 pounds force, on the top and on the bottom. I think you would have a balloon and a busted up wing.

I also think any measurable vacuum would collapse the tanks, they have no internal structure.

n2582d wrote:

johneeb wrote:Gary,
while doing your experiment wouldn't you have to raise the wing with the tank you want to empty so as to not un-port the cross cabin vent line?

John,
Yes, that's why I added the last sentence, "Capping the overhead vent lines would probably make this idea impractical." But that's what puzzles me about these reports of '48 models transferring fuel to the tank with the leaking cap. If the cross-cabin vent line is not clogged shouldn't there be equal air pressure in both tanks? The line isn't long enough that there would be a differential pressure. Residual fuel collecting in the cross-vent which doesn't allow the air pressure to equalize between tanks?

Sorry Gary I read that last line to fast and missed the plural "lines" and made the assumption you meant the exterior (above the cabin) vent opening.

Seems to me that the USAF blew up (vacuumed up) a KC-135 when doing this incorrectly. I’m fairly certain more than a couple of planes have been destroyed when fuel tank work was carried out incorrectly.
When I was thin and stupid, (free shot here for everyone) I used to do fuel tank work. I pretty much knew the insides of 747, Dc-10 and 767’s wings and center sections and we did use negative pressure to locate leaks. We would be sealed in the tank and coloured dye would be sprayed on suspect areas on the outside while we were inside looking for the spot where the leak originated.
I was somewhat fearful of mistakes and my survival. I think as much as it might be a fun experiment to use vacuum on a Cessna, positive pressure might be easier to control if I was to try inventing a fun experiment.

hilltop170 wrote:I would never pressurize or pull a vacuum on the gas tanks. They are not rated for either.

Say the tanks are 30” x 30”. That’s 900 square inches times 2psi is 1800 pounds force, on the top and on the bottom. I think you would have a balloon and a busted up wing.

I also think any measurable vacuum would collapse the tanks, they have no internal structure.

Not that I'm about to test it but based on CAR 3.441(a)(1) I think the tank should be able to handle 2 psi.

Let us know how that works out Gary.

Gary, that is Pressure not Vacuum. It’s been 20 years since, but when I was last flying them, we were warned the 400 series Cessnas had accidents occur when the vent system was plugged and the wet-wing tank imploded / collapsed with a big “BANG”. Thin-Jim would be inside a tank with much thicker walls and stressed for such things... I’m pretty sure a vacuum in our 170 tanks is not a good idea.

If your seeking a method to move 10 gals from one tank to the other I think it’d be safer and not too time consuming to siphon the fuel into a 5-gal jug, then pour it into the opposite tank.

Agree with George, gravity is pretty safe.

hilltop170 wrote:Agree with George, gravity is pretty safe.

It is just a theory though. Wanna go skydiving?

Too thick Jim remembered it wrong it was a positive fuselage px check not tank test http://www.discity.com/kc135/ It might have occurred at McConnell AFB
Anyway I think Gary was just pondering which made me think which is a good thing for me.

Slim-Jim, the “luth-ty thavage”

GAHorn wrote:I’ll be it flies GREAT!

I’m glad you’ve joined up, and look forward to enjoying your airplane with you!

Soo I bought the Fuel Hawk 19 gal dip stick and cut the bottom off at 0.

Today I emptied my tanks to clean them an it filled two 5 gal jerry cans exactly (10 gal total). However, I dipped the tank prior to draining and the Fuel Hawk said 12 gal. Troubles me it wasn't accurate at all.

I ensured it was against the aft portion of the filler neck and all the way at the bottom.

ALSO

One of my fuel tanks has baffles inside of it right below the filler neck, but the other tank does not. **One of the wings was replaced with a 180 wing at some point from an accident.

What's going on here? I can't seem to get an accurate reading.

I am not one of the old timers on here who will likely answer your question in detail.

But, in reading your post, I have several questions right off the bat. The dip tube as you have modified it, works incredibly well on Cessna 170A and 170B wings and tanks. I have been using one for several years now and it is calibrated for usable fuel when placed exactly against the baffle.
Yet, you mention one wing is a 180 wing. You have told us one fuel tank has a baffle, and one doesn't. This fuel dipstick requires that you are dipping it in a stock 170 tank, with the baffle in the correct spot. If you are not able to place it at the baffle, you won't get an accurate reading.

You tell us you dipped "the tank" (singular) and it read 12 gallons. I am assuming you dipped both tanks, and it added up to 12 gallons?

The stock 170 tank holds 21 gallons with 2.5 of that unusable. A stock 180 wing and tank holds 30-42 gallons depending on the year. And the baffle likely is not in the same place?

Things to consider in your quest to measure accurately. I'm sure you will get a good knowledgable answer in short order.

Best,
Bill