Fuel contents gauging

[AVOCET]

T

 

If I don't turn the fuel off before shutdown the temp fluctuations of a day night day can cause the fuel in the header tank push out the breather onto the wing, turn the fuel off after landing and run it down a smidge and no troubles at all

that carn't happen on jab wing tanks , the vent from the header vents into the wing tank , then the wing tank vents to out side , except if tanks are brimmed or uneven ground

 

 

[Dafydd Llewellyn]

The mark 1 eyeball is a very good gauge of whether the stick or gauge is correct...even if it requires a ladder..

Let's see you do that on a Bonanza - unless it's almost full. The dihedral takes the fuel completely out of sight until the tank is almost full. Same with many low-wing types.

 

 

[Dafydd Llewellyn]

Won't work , dihedral creates air pocket

air pockets

You can put the filler at the inboard end of the tank - on a glider, which can have its wing tip on the ground when you're filling the tank. Bit of a problem for an aircraft that can't do this.

 

 

[AVOCET]

You could have the filer cap inboard and let the tire down every time you filled or dipped !

 

 

[sfGnome]

There's another problem with the belite fuel sensor. It works on the pressure (or weight) of fuel, which varies depending on fuel type. If you always use mogas or avgas, then no worries, but if you swap between the two depending on availability, then you'll never be sure. I guess in that case you could calibrate with the heavier fuel then the lighter fuel will under-read - ie there will be more fuel than indicated which has to be better than the alternative...

 

Hmmm... Here's a thought. You know if you blow across the top of a bottle, then the sound changes depending on how much fluid is in it. Likewise if you tap a glass. The resonant frequency of the container changes with the volume of the contents. So, if you set up a sensor to measure the resonant frequency of a rigid tank (wouldn't work on bladders), that should give you a measurement that is independent of both the fuel type and tank shape, and it should be independent of contaminants too. Just like the Belite pressure sensor, the instantaneous reading would vary wildly, but filtered with a time constant of something like a minute, it should read very well. I wonder??

 

 

[robinsm]

no problem for me, look out the corner of my eye and note level in tanks behind the seat through the clear Perspex divider between the seats. Marked the tanks with 5 ltr increments. Easy as.

 

 

[AVOCET]

There's another problem with the belite fuel sensor. It works on the pressure (or weight) of fuel, which varies depending on fuel type. If you always use mogas or avgas, then no worries, but if you swap between the two depending on availability, then you'll never be sure. I guess in that case you could calibrate with the heavier fuel then the lighter fuel will under-read - ie there will be more fuel than indicated which has to be better than the alternative...Hmmm... Here's a thought. You know if you blow across the top of a bottle, then the sound changes depending on how much fluid is in it. Likewise if you tap a glass. The resonant frequency of the container changes with the volume of the contents. So, if you set up a sensor to measure the resonant frequency of a rigid tank (wouldn't work on bladders), that should give you a measurement that is independent of both the fuel type and tank shape, and it should be independent of contaminants too. Just like the Belite pressure sensor, the instantaneous reading would vary wildly, but filtered with a time constant of something like a minute, it should read very well. I wonder??

10/10 for thinking .

I was thinking sonar , which is sound ,

 

With today's sensor hardware ,

 

,software, that could be worth a look . ?

 

 

[geoffreywh]

How big is the header tank on a Jabiru? Is it full when in motion? Would that Belite gadget tell you the level is falling? I have a flag on a float in my header tank, right in front of my nose. ( it wobbles constantly so no chance of sticking) Downside? I have a lap full of petrol if the tanks bursts on a turn-over................. oh well there's always a catch.....

 

 

[AVOCET]

I've been discussing this at home and what if ; a strain type of sensor were placed at the bottom of the tank ( out side) that measured the deflection of the tank and could be calibrated to read the weight of the fuel ?

 

Any thoughts ?

 

I would imagine there could be a small section of the tank that deflected slightly . Would work in 1 g ok , would need an accelerometer to adjust for

 

Turbulence ect .

 

These days you can get sensors for just about any thing .

 

 

[AVOCET]

How big is the header tank on a Jabiru? Is it full when in motion? Would that Belite gadget tell you the level is falling? I have a flag on a float in my header tank, right in front of my nose. ( it wobbles constantly so no chance of sticking) Downside? I have a lap full of petrol if the tanks bursts on a turn-over................. oh well there's always a catch.....

Yes , the header tank on jabs is full if its been installed properly , vent outlet at the top .

 

Q. How do you seal the tank in the cockpit ? Seems to me that if fuel comes out on roll over , how the hell did you get past the inspection ?

 

Just asking .

 

Mike .

 

 

[Captain]

Dafydd's post # 1 is spot on.

 

 

 

In addition, I believe that talk penetrations for a gauge should be avoided where possible and on my 230 I used a Dynon fuel flow gauge. That and once you get used to the sight glasses in the cockpit, plus maintained a liberal safety margin of fuel remaining, worked well for me.

 

 

 

But that is only meaningful if the tanks are filled, the aircraft must be on level ground and it must then be flown coordinated.

 

 

 

The cross-feed between the J wing tanks means that any slope and full tanks is a waste of money and you can pump half a tank out of the breather if flying uncoordinated immediately after take-off with full tanks.

 

 

 

But, in my view and experience, an accurate fuel flow gauge is way preferable to having an additional hole in each tank for a float of other type of level sensor, which are suss anyway in long shallow wing tanks.

 

 

 

Regards Geoff

 

 

[Dafydd Llewellyn]

There's another problem with the belite fuel sensor. It works on the pressure (or weight) of fuel, which varies depending on fuel type. If you always use mogas or avgas, then no worries, but if you swap between the two depending on availability, then you'll never be sure. I guess in that case you could calibrate with the heavier fuel then the lighter fuel will under-read - ie there will be more fuel than indicated which has to be better than the alternative...Hmmm... Here's a thought. You know if you blow across the top of a bottle, then the sound changes depending on how much fluid is in it. Likewise if you tap a glass. The resonant frequency of the container changes with the volume of the contents. So, if you set up a sensor to measure the resonant frequency of a rigid tank (wouldn't work on bladders), that should give you a measurement that is independent of both the fuel type and tank shape, and it should be independent of contaminants too. Just like the Belite pressure sensor, the instantaneous reading would vary wildly, but filtered with a time constant of something like a minute, it should read very well. I wonder??

Now THAT one is worth some serious research!

In regard to losing fuel out the vents due to crossfeed, there's a way to prevent that, but . . .

 

The way is to run the vent line for the stbd wing tank across the aircraft so its outlet is further outboard than the outer end of the port wing tank, and vice-versa. The lines need to be as high as possible within the wing before they turn downwards to exit below the wing.

 

The "but . . ." to that is that there will be a low point - or possibly a pair of low points - in each vent line where it crosses the fuselage; and every such low point must have a drain line and drain valve, otherwise the low point can collect water and freeze. (and you can't combine them to a single drain valve, or it crossfeeds again). Also, if the system is to be "all on", it needs a larger-bore intercommecting vent between the upper outboard corners of the wing tanks, to equalise the pressure in the airspaces - and that needs its low-points drained, too. You can end up with a dozen or so drain points - look at the most recent Cessna 172. The fuel system design in recreational aeroplanes does not, in general, inspire admiration, so far as I am concerned. The very simple fuselage tank setup in the early Jabirus was actually very good, and although the prospect of 60 litres in the cockpit looked alarming, it didn't happen in practice.

 

 

[AVOCET]

E

 

Dafydd's post # 1 is spot on. 

 

In addition, I believe that talk penetrations for a gauge should be avoided where possible and on my 230 I used a Dynon fuel flow gauge. That and once you get used to the sight glasses in the cockpit, plus maintained a liberal safety margin of fuel remaining, worked well for me.

 

 

 

But that is only meaningful if the tanks are filled, the aircraft must be on level ground and it must then be flown coordinated.

 

 

 

The cross-feed between the J wing tanks means that any slope and full tanks is a waste of money and you can pump half a tank out of the breather if flying uncoordinated immediately after take-off with full tanks.

 

 

 

But, in my view and experience, an accurate fuel flow gauge is way preferable to having an additional hole in each tank for a float of other type of level sensor, which are suss anyway in long shallow wing tanks.

 

 

 

Regards Geoff

excuse my ignorance , whats a talk penetrations ?

Sounds like what happens in the bedroom ?

 

 

[sfGnome]

Now THAT one is worth some serious research!

Another advantage would be that, as you are measuring the resonant frequency of the container, there should be no need to penetrate the tank. A sensor attached to one wall would do the job. However on further thought, I have my doubts that the frequency wouldn't change with orientation; ie with a long flat tank, that there wouldn't be a change when the fuel moved to the back of the tank (pitch change) or to one side (roll) or both (yaw).

Sadly, it won't be me who is going to research it; it seems that when I'm not at work () or maintaining the Domestic Harmony Quotient (), then the only thing I'm researching is the inside of my eyelids ()!

 

 

[Dafydd Llewellyn]

Ideal job for an undergraduate thesis. I wonder whether the thing will resonate, given that it has to have vents.

 

 

[Guest Andys@coffs]

Isnt that what Finite Element Modelling answers? ANSYS?

 

Andy

 

 

[AVOCET]

Forget about resonance freq.

 

Would it be simpler to weigh the tank , measure the deflection , like the ones used on the "jab fuselage" when doing

 

strain calculations ( or whats it called)

 

thats hooked to a computer ?

 

 

[Dafydd Llewellyn]

No, it's simpler to install a flowmeter.

 

 

[AVOCET]

No, it's simpler to install a flowmeter.

Doesn't reading an accurate "weights and measures" calibrated system ( if it were available ) trump a system that still needs human input every time you add fuel ?

Most pilots that I've seen with fuel flow meters just read the fuel flow figure and don't bother doing the program bits either not knowing or trusting the system .

 

Also don't large jets and the like weigh their fuel ?

 

 

[djpacro]

An electronic fuel gauge system that I fitted to a prototype aeroplane some years ago was interesting in that:

 

1. it could take two sensors per tank so one inboard and one outboard
    
    
   
2. it could also use a capacitance sensor which could be made long and inclined diagonally
    
    
   

 

 

Personally, I only fly aeroplanes with stubby tanks and the simple gauge systems are very reliable and accurate (although a little vague). With one I have a safe endurance of 1 hr 50 mins max so quite critical.

 

 

[Guest Andys@coffs]

Avocet yes, you are right, given the choice of being able to determine an exact quantity of fuel over merely measuring fuel use that's what you would do, but you cant because of the physical limitations of the tank and therefore monitoring use is a very reasonable alternate. Monitoring use I can determine I have a problem before quantity remaining shows that I had a problem and now have a whole different one to contend with

 

Andy

 

 

[AVOCET]

Avocet yes, you are right, given the choice of being able to determine an exact quantity of fuel over merely measuring fuel use that's what you would do, but you cant because of the physical limitations of the tank and therefore monitoring use is a very reasonable alternate. Monitoring use I can determine I have a problem before quantity remaining shows that I had a problem and now have a whole different one to contend withAndy

Monitoring use is the way i do it also ,

I don't see how the shape of tank matters if you weigh it .

 

But the word" carn't " is a swear word

 

around here .

 

Don't jets weigh the fuel & monitor fuel flow as well ?

 

Mike.

 

 

[Dafydd Llewellyn]

Doesn't reading an accurate "weights and measures" calibrated system ( if it were available ) trump a system that still needs human input every time you add fuel ?Most pilots that I've seen with fuel flow meters just read the fuel flow figure and don't bother doing the program bits either not knowing or trusting the system .

Also don't large jets and the like weigh their fuel ?

It depends upon the nature of the hypothetical system. A flowmeter depends upon a simple mechanical calibration, and it just counts pulses, so electronically it's stone-axe simple, and not particularly susceptible to interference. The main potential for inaccuracy is the miniaturisation of the impellers that is necessary to measure the small flows involved.

The big jets use pressure refuelling from an underground hydrant system; the fuel truck for that system carries a reel of large-size hose and a large and very accurate flowmeter; it hooks into the nearest hydrant and the aircraft, and the operator zeros the meter and turns the tap on - just like any service station, but about five times the size. He gives the meter reading to the pilot, who enters it into his fuel log, and the aircraft uses its own flowmeters from then on. This is the reason the domestic airlines run a "hub-and spoke" pattern of operations. They can of course, use overwing refuelling if the airport is not equipped for pressure refuelling; but not if they can avoid it.

 

The fuel quantities are normally figured in weight units, but I'll let one of our ex-airline pilots speak on how that is figured from the volume - there are "standard" fuel densities that are required to be used for weight & balance calculations.

 

Some military types may have load cells built into their undercarriages, but I'm not aware of this system being generally used for fuel management.

 

For certification flight testing, it is not uncommon to weigh the aircraft, with the test crew aboard, before and after each flight.

 

 

[fly_tornado]

Andy why don't you drop James from Belite an email and explain the Jabiru condition maybe he can re-jig the meter to work with Jabirus. Jabiru tank can't be much short of that 6" minimum height

 

 

[Dafydd Llewellyn]

It might be that, if you mean the total head from the fuel at the outboard end of the wing, to the drain valve location at the inboard end - provided the aircraft is laterally level. However the actual tank depth is around four inches as I recall, so the "slosh" program won't work properly if the thing is hitting the lower limit of the pressure transducer.