175 Wing Fuel Capacity on 170?

How to keep the Cessna 170 flying and airworthy.

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alaskan9974
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175 Wing Fuel Capacity on 170?

Post by alaskan9974 »

Almost done with the repair and rebuild on my 175 wings, we are going to write the 337 and I see a few options on past 337s for the fuel capacity.

Some use the 170 data and say 18 per side is usable, another one uses 175 data and says 21 per side is usable, the other one stated 24 per side is usable. What is the limit factor on the 175 tanks to have so much unusable? I see the 172 LR tanks look similar, but have a different P/N

Which should I use and does it matter? My longest flight I regularly fly is about 350nm, just out of reach of the stock tanks with reserve at 85 knots, but doable with 175 tanks.
'53 B-model SN:25712
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GAHorn
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Re: 175 Wing Fuel Capacity on 170?

Post by GAHorn »

Useable fuel is fuel which is “available for all normal flight attitudes”. UNuseable fuel is that which can NOT be used in ALL normal flight attitudes.

The 175 tanks are not symmetrical and in a steep-climb about 5 gallons in each tank will not reach the engine for consumption, so the 52 gallon capacity is limited to 42 gal “useable”.

Why is this important? ... And why is it a problem since at the end of a long flight you will be descending and landing and instead of climbing (where all the fuel is unavailable)..?? Because WHAT IF YOU MUST GO-AROUND at the destination because some aircraft pulled out in front of you, etc etc? That steep climbing go-around may be when you find your engine cannot use that last ten-gallons remaining in your tanks. 8O

(But it does explain whey some 175 owners “pooh-pooh” the ten-gallon unuseable fuel placard.... because they’ve run the fuel quantity down below that level and did not attempt a go-around and experience the potential sounds of a sputtering and dying engine. :?

If it were ME.... if I converted my airplane to 175 tanks.... I’d use the 175 fuel placards. Someday my airplane will belong to someone else.... and I’d not want them to be mis-informed by MY error.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
counsellj
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Re: 175 Wing Fuel Capacity on 170?

Post by counsellj »

Has anyone added an additional fuel outlet to those 175 tanks to feed from the area of the trapped fuel?
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c170b53
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Re: 175 Wing Fuel Capacity on 170?

Post by c170b53 »

Not sure what the real answer is here but I have 175 wings on my plane. The tanks are 175 tanks with the fuel gauge portion of 170 tanks welding to the sides of the 175 tanks. Thus the mechanical fuel indication when the tanks get low is sketchy but somewhat consistent with the type. I use 5 unusable each side with a C/S combined fuel burn of 9 gal an hour for flight planning. I do not lean aggressively nor am I a LOP guy but if the number 9 is not the number then something is going on. (Makes me think of the prisoner). Its always 9. I cruise somewhere around 100 knots, at 2350-2400 RPM with a 80 inch, if my speed is lower than that, my belly is very dirty and it does a bit better when clean but not much. Its time that I have in my tanks and I typically plan for 4 hours of endurance
What I have difficulty with is, visualizing the low fuel climb out scenario. If anything, a long descent would be more problematic as there’s much more fuel tank area to hide the fuel in a descent than there is in a climb. The 175 tank doesn’t sit any further aft than standard tanks and the pick up is in the same spot between tanks types. XP standard tanks do have the pick- up at the rear portion of the tanks with an additional forward stub at the front of the tank for header tank return. Maybe those tanks would be the best to get every drop out.
My plane does climb out well, fully loaded, standard day sea level will easily see at 1000-1200ft. Cold weather will see almost 2000 but at 70 Knots the deck angle is not pointing straight up, so I think my fuel management numbers to be acceptable.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
alaskan9974
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Re: 175 Wing Fuel Capacity on 170?

Post by alaskan9974 »

So on paper it isn’t useable, but in real life it is accessible, say running one dry then switching to the other, or both?
'53 B-model SN:25712
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GAHorn
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Re: 175 Wing Fuel Capacity on 170?

Post by GAHorn »

c170b53 wrote: What I have difficulty with is, visualizing the low fuel climb out scenario..
I understand the difficulty as I originally did also, but it’s what was explained by none other than Cessna Chief Production Test Pilot Mort Brown when I asked about this. It apparently was experienced when the 175 was undergoing certification flights, and that unusable fuel is what they had to publish.
alaskan9974 wrote:So on paper it isn’t useable, but in real life it is accessible, say running one dry then switching to the other, or both?
I would not run on one tank if I were low on fuel, but as for what’s useable, that’s what some say. When the same amount of remaining fuel is in either 170 or 175 tank... the fuel is spread out over that larger area and not as “deep” in the 175 tank.... and not as reliably available to the tank outlets. It therefore makes sense that the 175 tanks would have a larger amount of unusable fuel. It is difficult to understand 10 gallons, but you can be sure Cessna didn’t just make it up because it serously hurts the airplane’s performance numbers.

The 170/172 with their C145/O300 engines were limited to 145 HP while the geared-version of that engine output 175 HP at it’s rated RPM.... meaning it’s fuel consumption was higher. It would have not been as marketable if it’s range were reduced in a trade-off for speed claims... yet CAR3 required specific fuel and oil be available for the claims. Yes, there’s paperwork fiction at work perhaps, but it would be difficult to explain at the off-airport landing site why one disregarded the placards and AFM.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
alaskan9974
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Re: 175 Wing Fuel Capacity on 170?

Post by alaskan9974 »

Are there any modifications to the 175 tanks I can make to increase useable, or will later 172 LR tanks fit the 175 wings? My A&P is willing to write the field approval as needed.
'53 B-model SN:25712
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GAHorn
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Re: 175 Wing Fuel Capacity on 170?

Post by GAHorn »

Just FYI, in 2009 I investigated replacing OEM 170B tanks with 175 tanks (altered by installation of original 170B gauges) into my airplane and the cost in ‘09 was estimated to be $7K if I supplied the tanks. It would take approx $3K in additional parts (hat sections, ribs, metal for replacement upper tank cover, etc)... and $4K labor. I’d not count on that being applicable to 2021.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Karl Towle
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Re: 175 Wing Fuel Capacity on 170?

Post by Karl Towle »

Alaskan9974, were you able to finalize your paperwork and call your wing-swap complete? I did the same swap back in 2014. Chris at Del Air in Porterville, Calif. was very helpful in obtaining a field approval for my project. Just to add some data to the discussion on unusable fuel, here are some numbers from the C175 Type Certificate 3A17.
The C175 was certified with 9 gallons of unusable fuel (52 total, 43 usable).
The C175A & B were certified with 10 gallons unusable (52 total, 42 usable).
The C175C & P172D were certified with 10.5 gallons unusable (52 total, 41.5 usable).
The R172E (USAF T-41B, C & D) were certified with 6.0 gallons unusable (52 total, 46 usable). (Same TC for these other models)
Did everyone catch that last entry? Those airplanes used one of the Continental IO-360 variants, developing between 195 and 210 HP (which is obviously more than the GO-300's 175 HP). If we can assume the wing structure and tank geometry are all the same, this begs the question: Why only 6 unusable gallons with a 210 HP engine? I was unable to determine if the R172E & T41 were equipped with a header tank - one possible reason for the difference in usable fuel. Anyway, there are the numbers I found.
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GAHorn
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Re: 175 Wing Fuel Capacity on 170?

Post by GAHorn »

Karl, I believe you have hit on it….the IO-360 was fuel injected and required a header tank.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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falco
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Re: 175 Wing Fuel Capacity on 170?

Post by falco »

Header tank on the R172K is below the floor, 3qts if I recall.
On the XPMods (or whatever he calls it now) STC with the TCM IO-360, the header tank is a Maule part, also 3qt. With the tank at shin level forward of the left door post.

With either setup, when you get low enough on fuel to intermittently unport either main tank, there still should be enough fuel in the header tank to delay the inevitable for a short time.

But I still call it and plan for 37 usable with my stock 170 tanks.
alaskan9974
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Re: 175 Wing Fuel Capacity on 170?

Post by alaskan9974 »

Karl Towle wrote:Alaskan9974, were you able to finalize your paperwork and call your wing-swap complete? I did the same swap back in 2014. Chris at Del Air in Porterville, Calif. was very helpful in obtaining a field approval for my project. Just to add some data to the discussion on unusable fuel, here are some numbers from the C175 Type Certificate 3A17.
The C175 was certified with 9 gallons of unusable fuel (52 total, 43 usable).
The C175A & B were certified with 10 gallons unusable (52 total, 42 usable).
No not yet, I did pull a few copies of other 170's 337's with the approved swaps and will file it as 52/43. I bought some digital fuel gauges and senders, will probably throw a totalizer in just for fun. My typical nonstop flight is 350nm when I am not out in the backcountry and it will be nice not to have to stop for fuel.
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Karl Towle
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Re: 175 Wing Fuel Capacity on 170?

Post by Karl Towle »

Digital fuel level sensing turns out to be a wonderful thing. I ended up with CiES CC2840220620 lifetime guaranteed fuel senders, and a single Aerospace Logic FL202D gauge. Carefully following the calibration process ensures the gauges are accurate throughout their entire range. The conversion was a bit spendy, but it definitely solved a list of frustrations with analog gauges in my case. I have to add that CiES makes a digital sender to replace the 170's mechanical gauge in the end of the tank, so stock 170 owners can go digital electric as well.
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