170A/B fuel tank quantity

[Bruce Fenstermacher]

I know our 170A and B is supposed to have 2- 21 gallon tanks.

But today after reinstalling one of my tanks that was bone dry I completely filled the tank stopping at 5 gallon increments to double check the accuracy of my fuel dip stick. I fully expected to put 21 gallons in the tank but was only able to get 20.1 gallons in till the gas started up the gas cap tube.

OK maybe I could have trickled in another .4 gallons for a total of 20.5 but I'd never get 21 gallons in.

Plane was of course in a 3 point attitude and on what I'd consider level ground.

Anyone else had this experience?

[lowNslow]

Bruce, I had the same thing happen when I filled my empty tanks. I was able to get in about 20.4 gals. Best I can figure is either the pump wasn't that accurate or due to the dihedral of the wings there is a bubble on the outboard part of the tank.

[jrenwick]

Bruce,

What size are your tires? I have 800s on my 170, and they seem to raise the nose enough to create some air space ahead of the fuel filler necks. I wouldn't be a bit surprised if I'm not getting 21 gallons into my tanks either.

John

[170C]

Bruce, although my plane is a conventional geared 172, the wings, fuel tanks, etc are identical to the B model 170's. Several years ago I too filled my left tank after pulling the sump valve out to make sure it was empty and on what I felt was a level area by the pump (at an airport here in Fort Worth) I was able to put in only 19.6 gallons. A week or so later, I repeated the same procedure and again 19.6 gallons. That is with 600 tires. I too think there is air space ahead of the filler neck that would allow some more fuel to be added if the tanks were in true level (fore & aft as well as left to right configuration). Probably what we all need is to seal off the filler neck and put one out at the outboard end of the tank. You may remember our member's 170 from Chicago at Petit Jean (don't have his name handy now as I am out of town) had installed larger capacity fuel tanks in his plane when he put the IO-360 Continental in his plane and those tanks had the fillers at the outboard tank location. Still probably had some area at the front of the tanks that couldn't be totally filled, but certainly more than had the fillers been in the stock location. As I recall his total tankage was around 50 gallons. Maybe he will chime in with more specific details if he reads these posts.

[Bruce Fenstermacher]

I've got 8x6 tires. Hard to believe there is an airspace at the top head of the tank.

Now let me think about this. They are 21 gallon capacity tanks that you can't get 21 gallons into. Then how can they be 21 gallon capacity tanks?

May this is the real reason Cessna put a nose wheel on the aircraft. So you could really get 21 gallons in the 21 gallon tank. Hmmmm

[johneeb]

Bruce, although my plane is a conventional geared 172, the wings, fuel tanks, etc are identical to the B model 170's. Several years ago I too filled my left tank after pulling the sump valve out to make sure it was empty and on what I felt was a level area by the pump (at an airport here in Fort Worth) I was able to put in only 19.6 gallons. A week or so later, I repeated the same procedure and again 19.6 gallons. That is with 600 tires. I too think there is air space ahead of the filler neck that would allow some more fuel to be added if the tanks were in true level (fore & aft as well as left to right configuration). Probably what we all need is to seal off the filler neck and put one out at the outboard end of the tank. You may remember our member's 170 from Chicago at Petit Jean (don't have his name handy now as I am out of town) had installed larger capacity fuel tanks in his plane when he put the IO-360 Continental in his plane and those tanks had the fillers at the outboard tank location. Still probably had some area at the front of the tanks that couldn't be totally filled, but certainly more than had the fillers been in the stock location. As I recall his total tankage was around 50 gallons. Maybe he will chime in with more specific details if he reads these posts.

Frank it is probably me you are referring to, I do live near Chicago.

Moving the filler neck out is part of Harry Delagar's mod to add 7 gallons to original stock 21 gallon 170A & B tank. Harry's theory was that due to wing dihedral the stock tanks are hard to fill completely and the issue would be worse with the extension he adds to the tank. Harry's solution, cap off the original filler and put a filler in the outboard end of the newly modified tank. The capacity after the modification is 28 gallons with 25.5 usable.

If you have the bladder for it Harry also offers a bigger tank modification, can not remember the numbers on this modification.

Both of the above mods are approved and come with paper work.

[bradbrady]

Bruce,
Maby you don't want to say anything, but when was the fuel distribution system last calabrated? If you just got .4 gal. fuel for nothing you just saved 1.60 filling your tank! (at 4.00 / gal)
brad

[blueldr]

Are you guys sure your tanks have no dents in them? Airplanes with a fuel pump can collapse a tank if the vent plugs up.

The main drawback to moving the filler necks outboard is the difficulty fueling without a stand or a ladder.

[jrenwick]

I'm not sure I was clear enough about what I experience with the 800x6 tires. When I fill each tank, the fuel comes up into the filler neck, and after that I still seem to be able to put more fuel in the tank if I pump it slowly enough. I usually run out of patience before I've got the tank completely full, so I always have the feeling I could have put another pint or three in the tank if I'd taken more time.

Where the tires come in: The top of the tank is curved to match the camber of the wing. The bigger tires should be raising the nose slightly, and it looks to me as if the highest part of the tank is forward of the filler neck. Maybe if I had the original 600x6s on it, the nose would be lowered enough so that the filler would be right at the top of the curve, and it would truly be full when the fuel was up into the neck. I haven't tried the standard tires to see if this is true, but I don't remember having this experience with my previous '55 170B (N2947D) that had them.

[GAHorn]

The airplane's total fuel capacity is 42 gals. This is divided between a left and a right system, and includes tanks, lines, valves, etc.. This is usually referred to as "two, 21 gallon fuel tanks", although that is only a nominal capacity.
The aircraft actually has only 37 gallons of useable (all attitudes) fuel, which is 18.5 gallons each side. If, due to maintenance or other method of totally emptying the fuel system (tanks, valves, lines, etc.) one is capable of pumping more than 18.5 gallons into each side, this is due to a certain amount (or all) the unusuable fuel having been drained and replaced.

[jrenwick]

So maybe the 37 gallons useable is taking into account that you may not be able to put 21 gallons into a completely empty tank in the first place....

John

[bsdunek]

The airplane's total fuel capacity is 42 gals. This is divided between a left and a right system, and includes tanks, lines, valves, etc.. This is usually referred to as "two, 21 gallon fuel tanks", although that is only a nominal capacity.
The aircraft actually has only 37 gallons of useable (all attitudes) fuel, which is 18.5 gallons each side. If, due to maintenance or other method of totally emptying the fuel system (tanks, valves, lines, etc.) one is capable of pumping more than 18.5 gallons into each side, this is due to a certain amount (or all) the unusuable fuel having been drained and replaced.

So, the 21 gallons is TOTAL capacity. If these guys can get 20-20.5 gallons in the tanks, that leaves 0.5-1.0 gallons in the lines, carb, etc. Seems like a lot, but maybe. I suppose the number is rounded off. Interesting things we find out about our 50+ year old flying machines!

[Bruce Fenstermacher]

George in my case the tank was empty but the line down to the fuel valve probably still had fuel in it. I don't think that would make up the difference though.

John you might be on to something. I did trickle about .4 gallons in to just get to 20.1. Perhaps I could have trickled in another .9 but I doubt it.

I thought I'd be able to get 21 gallons in with out to much care and in fact thought perhaps a pinch more and that the 21 gallons was a conservative number.

Turns out in fact and at least John has confirmed that 21 gallons is a optimistic number. I don't think many if anyone, plans that close for flight planning purposes but if you for all practical purposes are only putting 20 gallons a side, thats 12 pounds of useful load we may not be using.

That could be critical when you and your wife are packed for a convention and someone along the way gives you a few cases of beer to take. What do you think George.

[blueldr]

In my '52 model flying the tank dry, with the wing slightly up and the ball down, 'till the engine stops, the right tank will take 21.1 gallons and the left tank will take 21.3 gallons right up to the overflow point.
Airplane had 700x6 tires and lady legs gear on a level ramp using a pretty accurate Gasboy pump and meter.

[Bruce Fenstermacher]

That's interesting BL. The plot thickens.