r4y30n wrote:Killratio wrote:Not so simple I am afraid..the "unburnt fuel" amount also depends on timing, cylinder/piston condition and efficiency, current RPM compared to power setting and a host of others (no doubt) that I don't know.
Yes, but Accusim knows how much fuel the engine is burning, all you have to do is factor in the flow rate of the fuel pump and subtract the fuel burn from it, any excess gets returned.
Not really...flow rate of the pump is certainly a factor on the intake side of the fuel system but on the outflow side it is how much that can physically pass THROUGH the system to "outflow" which will be the limiting factor (assuming the pump can keep up....and all else being equal, if it can't, then there won't be much unburnt fuel, at least not until combustion stops.) Flow rate is not the whole answer to the equation as you also need to take into account fuel pressure.
Let's say that at current efficiency 12 units of fuel can be burned in a given time period (1 per cylinder). The supply is capable of providing 18 units of fuel. This MAY lead to 6 units being returned (say at high RPM where everything is "working" and IF all cylinders are performing the same). In this example let's also accept that the "outflow" (return) system is capable of handling all 6 units...at a rate of 0.5 per cylinder. BUT Accusim tracks EACH cylinder and it's efficiency. If one cylinder is doing sterling work and burning 1.1 units, it will not be returning anything..and that fits your comments, Accusim KNOWS how much fuel is being burned, so fuel "returned" is now Flow 18 - ((11 x 1) + (1 x1.1) = 5.9. Good so far. But what if another cylinder is only burning 0.5? Is return now 6.4units?...obviously not, as the cylinder burning 0.5 is being supplied with 1.5 units (18/12 = 1.5) but is only burning 0.5 and can physically only return 0.5units. 0.5 is lost/exhausted/vented/evaporated etc etc So we have a burn of 11.6, a return of 5.9 and a waste of 0.5 Also, excess fuel flow on the "inlet" side may also not actually be physically capable of being passed into the system, building fuel pressure and not increasing flow.
Again, please understand that I am not saying that it couldn't be done, just that it is not as simple to calculate as "fuel flow - fuel burned".
r4y30n wrote:Killratio wrote:Once the reserve is full the system would normally port overboard, you can see from the diagrams that the reserve has a fuel vent on it to a fuel drain. (nice diagrams Ron
)
That's what I initially thought but it doesn't make much sense to throw away fuel just because the reserve is full. Rigged that way you would have to regularly switch back to the reserve tank to prevent it from overflowing... Is that what the pilots actually did?
Put simply..yes. Fuel management in aircraft is an active task. Some aircraft (Concorde springs to mind) actually become unflyable if fuel is just used from successive tanks. Even in a simple GA aircraft, you can often need to use, say, 20 minutes from one tank, then switch, alternating back and forward like this for the whole flight. If not done, this can not only affect trim/stability but even fuel supply to the engine. Many, many accidents happen due to "fuel starvation" with NO associated mechanical failure. (ie no fuel going to engine even though there is still fuel onboard, due to pilot error, not breaks, blockages etc). And a proportion of fuel "exhaustion" accidents (no fuel left onboard) happen when there was enough fuel for the trip but mishandling of tanks has led to venting. Most often this venting is caused by exactly the problem you describe..using the "non return tank" when the "return tank" is full. In fact I recall an incident some years ago where a pilot ran out of fuel. I don't recall the exact aircraft (I want to say a Piper) but he was experienced, with many hours flying model "1" and "1a". He took off on a long flight in model "2" which was almost identical in all but a few cosmetic details and a slightly bigger engine. He had a full compliment of passengers too. They had no worries until the engine suddenly stopped on approach. He managed to just make the threshold and coasted to a (rather sweaty) stop safely just off the runway. The tanks were BONE DRY. He KNEW he should have had a 45 minute fixed reserve left plus about 10 minute's variable. (much finer than I would like to cut a long trip!!!). He checked his calculations, the fuel records etc and all looked good..but there WAS no fuel.
The mystery was solved by a young L.A.M.E. who had just done "a course" on that aircraft model. The answer was in plain view in the POH, had the pilot not assumed he knew the type inside out. It was a very simple answer that could have had much more serious consequences than it did..no thanks to the pilot!!!:
In the "1" and "1a" fuel return went to the left tank...in the "2" it went to the right. He had followed his usual procedure and used fuel from the left tank first. Merrily venting all unburnt fuel overboard for the first segment of his flight.
best regards
Darryl
