Quick question on unsuable fuel

[hikouka]

Gidday Gidday all,

Anyone know for sure how much unusable fuel there is on standard 18 Gal (68l) tanks?

By the way went out on a limb and made a dipstick but I think it only shows the top 13 Gal (48l). What I mean is when a tank is pretty low the dipstick reads nothing. Then slowly putting in fuel and measuring from I see the first trace on the dipstick up until the tank is fully fueled I can only mark 48 litres on the stick so presumbly there's another 20 litres wallowing round in the bottom of the tank. I just want to know for sure how much of that is useable.

With my useless fuel gagues I know when a) a tank is fully gassed up & b) when I have between 21-68litres in a tank.

The unreadable bottom of the tank is a bit of a mystery . . .

 

[StewartB]

If you drain your tanks by allowing your gascolator to drain until it stops, you've removed all your usable fuel. Unusable fuel is that which remains in the fuel system after the gascolator stops running. If you begin your dipstick calibrations from that point, you'll be measuring usable fuel. I never worried about the first 5 gallons not showing up on a dipstick. I wouldn't begin a flight with only five gallons in each tank. My first mark on the dipstick is 10 gallons.

Stewart

 

[hikouka]

Thanks for the clarification on the unusable fuel Stewart. Wish I could measure from when the gascolator stop running with the dipstick its just put her in the tank and see what she shows.

Once the airport I went to said they had a problem with their fuel and try next door. After dipping the tanks one showed 10 liters and the other 0. I knew I had 30 in one (10 on the stick plus 20 below) and something under 20 in the other. The flight back to the neighbouring airport was only 20 minutes (11 liters) so it was no problem but it still was a strange feeling not knowing for sure how much was in the tank with nothing on the dipstick and exactly how much of the 30 in the dipable tank was usable.

Hmmmm, as embarrassing as this sounds I still wonder the exact quanity of useable fuel there is in each standard 18 gal tank. I think I've heard 1 gal a tank unusable but can't remember for sure and couldn't find it in the flight manual.
What do you reckon?

 

[StewartB]

My personal fuel minimums are higher than those specified by the FAA. Imagine leaving a strip in marginal but okay weather with 60 minutes of fuel on board to make a 25 minute flight. Approaching the destination you find the airport closed and are required to call for a special VFR arrival, which means you'll be spending some time and fuel orbiting around waiting for your entry call. Or....you can turn around and return to where you came from and you might even make it, if the weather doesn't drop. Do you stay with radar or turn and leave it?

Been there, done that. Carrying extra gas is better. It buys you time. Decisions are easier to make and to manage when you have time.

My dipstick is a go/no go indicator for fuel, and that's all it is. As long as I have a known quantity and that quantity exceeds what I need plus a reserve, all's good. If not...I don't go. Period.

Stewart

 

[SuperCub MD]

You have about 1/2 gallon in each header tank in addition to the 18 in the wing, (unless you have done something foolish like removing the headers). If you are stretching it far enough that you need every last drop, skid the plane a little nose and wing that you are feeding from high. This should get all the fuel out of the wing tank.

 

[StewartB]

Wow, I hope that's theoretical. I'd never reach that point. I'd have landed at least 15 minutes earlier and called a friend to bring me gas. Especially before crossing the Turnagain or Knik Arms of the Cook Inlet returning to Lake Hood. Yikes!

Stewart (the conservative )

 

[hikouka]

Agree whole heartily with you Stewart "carrying extra gas is better" but can't help feeling that as a PinC I leave a lot to be desired not knowing the exact useable fuel I carry.

See even though we have two tanks I guess one could crap out or run out which makes it all the doubly more important to know as much about the one remaining tank as possible (not sure if that last sentence makes sense or not but hey its 3:30am now )

Its shocking how little I know but, ummm, lets lets not tell the passengers that. I just want to know more every aspect of the Cub. Especially those things that I as an ignorant pilot have some control over, like the fuel. That's why the comments from you guys are so valuable. I've heard rumours that there's only about a dozen (civillain) taildraggers here and there isn't really anyone to talk to about the details of Cub ownership or flying to the limits.

For example SuperCub MD's header tank/skidding advice, which was news to me - thanks MD, could maybe make the difference one day especially for those of us flying over water all the time. Hey after all, half a gallon is half a gallon and at say 8gal an hour at 100mph well there's another 3minutes 48seconds or 6.3sm of flying time.

Would much rather fly 6 miles and glide gently down onto a golden beach with welcoming Yukuta clad lasses than have to tire myself swimming those same long six miles miles through the octopie (?) invested waters for want of knowledge about my fuel system. Knowing how many of them I eat each week I'm sure Japanese octopuses (?) have definately got it in for me and the Yukuta lasses would never forgive me if I washed up at their feet all exhausted and covered in octupussy (?) bites

Time for this rambler to hit the futon . . .

 

[RedEye]

fuel

SB,
Speaking from experience, there's no excuse not to know how much fuel you "dont" have !!!! A calibrated guage may tell you apprximately how much is left in the tank, but there is nothing quite like the silence of a starved engine telling you how much you dont have !! (especially over Rockford at night) !!!!

 

[RedEye]

fuel

Oh ya, by the way its all useable ! Trick is getting it all to the carb !!!!!

 

[mvivion]

The other, equally important issue is knowing how much fuel your engine is burning at several power settings. Granted, the Cub engine doesn't use huge quantities of fuel, but then again an hour's fuel remaining is only 7 or 8 gallons.

Distribute that fuel between two tanks and you've got a very little fuel sloshing around out there in a lot of space.

Knowing how to get it all, and knowing how to manage fuel is key to survival.

By the way, does anyone worry about using only the left tank for takeoffs, landings and descents?

I'm curious.

I'm with you, Stewart, being low on fuel is just too spooky to mess with. I've been there a few times, and constantly asked myself how I'd explain this one.

MTV

 

[Yard Dart]

Just burn the tank out then swicth to the other tank.

 

[hikouka]

Gidday Gidday all,

Ahh here I'm back from a day in the trenches all eager to log in and find out the answer to "Anyone know for sure how much unusable fuel there is in a standard 18 Gal (68l) tanks?" but knock me down with a feather no ones come up with the definate answer. Sigh . . .

Ok if its answers we're after Mvivion I'll take the bait on "does anyone worry about using only the left tank for takeoffs, landings and descents?"

Yes I do. For me its always:
Start -LEFT TANK ON - (time stamp1)
Takeoff - STAY ON LEFT TANK (time stamp 2)
Leaned out Cruise - CHANGE TO RIGHT TANK (time stamp 3)
Joining checks - CHANGE TO LEFT TANK (time stamp 4)
Landing - STAY ON LEFT TANK (time stamp 5)
Shut down - STAY ON LEFT TANK (time stamp 6)

So my left tank is always for takeoff and landing and my right tank is for leaned out cruise. I do this because of
a) the comments I heard about the advisablility of always using the left tank for t/o & ldg due to the header tank arrangement;
b) so I can cruise as far as possible and even run the right tank dry knowing that all the remaning fuel will be in the left tank;
c) so I don't need to worry about 30 minute fuel tank changes and 'balancing the fuel' doesn't seem to make much difference to me and after two or three legs of a flight my fuel papers always seem to get mesed up;
d) so I can try to accurately measure my cruise rate of consumption by just using the right tank. When I land I always gas up and by checking the amount put in the right (leaned cruise tank) against the time flown on that tank (time stamp 4 - time stamp 3) I am trying to slowly build up a database of cruise consumption figures. For example last weekend one flight on the right tank was leaned 2350 rpm for 45 minutes which took 22 litres. Thus I know that when fully fueled and flown solo in a leaned cruise my rate of consumption is 30 liters per hour.

However that still doesn't tell me how much is unusable in a standard 18 USG tank

David glad to hear that you got back from your silent engine Rockford experience.

 

[Carey Gray]

I get 16.8 usable without getting creative. How much more fuel can you get in your tank if you have 6.00s and the airplane sits more level than it does with 31s?

 

[Mikey]

If I drain a tank 'empty' from the gascolator, I can only get about another .5gal max from each tank sump drain. (PA-12 w/ no header tank)
I just keep close track of my hobbs /vs fillup times and calculate a conservative burn rate. It is a little disconcerting to takeoff w/ the wing sight guages not floating at all, but like yesterday, there was still 16gal remaining when I filled up :15 min and 2 takeoffs later. If I am unsure about my numbers, I'll put the tail up on a stand to get close to a level attitude and see what the sight guages are saying. I would be hesitant to rely on a dip stick because varying the angle it is inserted slightly could make a big difference on how deep it went into the tank and how far up the same amount of fuel showed on the stick.
Carey, I never saw much difference in how much fuel I could get in the tanks w/ 8:00x4's vs 8:50x6's and v's 29" BWs
Chris

 

[S2D]

Anytime I've run a tank dry, I've been able to put 18 back in it. (usually plus a little more.)
that is the nice thing about the original fuel system. you can run one tank dry and find out how much it really takes to fill it back up.

 

[palhal]

Top off and take off on the right tank and keep track of the time it takes to run out of fuel. Now switch to the left and you now know how much time it will take to run it out. Simple! Palhal

 

[cubdrvr]

My method for extended trips at same pwr setting:
T/O & climb on left for 1 hr. burn
Switch to right and run dry ( keep track of time)
Back to left, subtract 1 hr. from the right tank burn and that is your air time left.

 

[kase]

I take off and climb on the left tank to burn a little fuel so it dosent vent out the caps very bad. Then I switch to my right tank and fly 2:42 till its out, then switch back to my left tank and its good for 2:20 at 2500 rpm. That was when I had 18 gallon tanks.

 

[cubdrvr]

2:42 Kase? I only get 2:40:36 on mine

 

[kase]

Its the altitude

 

[paibre]

Timex vs Rolex