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Discussion in 'Firewall Forward / Props / Fuel system' started by wsimpso1, Dec 14, 2018.
The header tank in this scenario is acting like the middle man in a mattress negotiation.
With due respect to Ross' earlier answer, I think from a consumer's perspective the answer is "yes". Yes, the ECU has the capability to calculate the fuel flow based on the number of injectors and the duration each injector is open (and a bunch of other magic). the information is presented as a Kfactor that emulates the output of a mechanical flow transducer. So "yes" the ECU can emulate the fuel totalizer, but you cant use the information without a third party display.
Just a thought Ross - how hard would it be to display this info on the ECU programmer? Would be a nice addition to the Gauge page.
I see 2 distinct issues being discussed here – the first is running a tank dry in a “normal” aircraft fuel system. There are parties who will routinely run a tank dry on max effort cross country flights as a conscious fuel management and risk reduction methodology. There is plenty of precedent to support such an action from military and commercial ops all the way down. Then there is an equally strong opinion that doing so is careless, reckless and borderline insanity. A quick search on VAF will provide hours of entertainment reading through the impassioned responses of both sides. Regardless of where you fall on this issue, overwhelming evidence suggests that a carb or mechanical FI system does not suffer much harm, provided you have sufficient altitude to get the fire lit again. Issue 2 is running an EFI system dry. Clearly, the typical EFI pumps are far less tolerant of dry/semi-dry operation than the diaphragm Lycoming pump, and the rail loop/regulator configuration of EFI adds less certainty than the typical deadhead, constant bleed Bendix. I think for the sake of this thread we have to state that running the EFI all the way dry is bad. However, with a header tank the only time this should happen is when the absolute last drop of fuel has been expended. At that point, nobody is going to care about the lifespan of the pumps.
We've had FF readout for about 15 years available on our display based on some different math. With the advent of glass engine engine monitors, nobody seemed very interested in that any more so we haven't had offered it for a while. Easy enough to do that using the new math which is more accurate than the old math. Might even be able to add a totalizer window too.
At most of our ages here....the two hour bladder is probably why we would never run a tank try.
For systems with backflow check valves.
If you have say four fuel tanks #1, #2, #3, and #4 and you are feeding an engine from #1 but you want to change to #3 you turn on the #1 boost pump on low boost and observe a rise in fuel pressure above the engine driven pump pressure and then turn on the #3 boost pump on high boost then open the #3 fuel selector valve(s) and note an additional increase in fuel pressure this shows positive fuel flow from #3 to the engine you may now close the #1 fuel valve(s) and then turn off the #1 boost pump while observing no change in fuel pressure then turn off the #3 boost pump and observe a return to engine driven pump pressure.
If say you want to empty #1 fuel tank and you are on the #3 fuel tank turn the #3 boost pump on low observe the fuel pressure rise and note the fuel pressure then turn the #1 boost pump on high open the #1 fuel valve(s) and observe an additional increase in fuel pressure, you are now feeding the engine from the #1 fuel tank and it is backed up by positive pressure from the #3 fuel system on low boost. When during your scan you notice that fuel pressure has dropped to the noted #3 low boost pressure #1 tank is empty you can turn off the #1 fuel selector valve(s) and turn off the #1 boost pump and you are back on only #3 turn off the #3 boost pump and observe a return to normal engine driven fuel pressure.
How exactly does a header tank improve on this?
Appropriate sized hose and a pitot tube, done.
Yep. I'm adding fuel capacity so I can go 5+ hours nonstop. The bladder relief issue is solved by any number of ways, and has been for a very long time.
Seating comfort... now that's a harder nut to crack.
A header allows a lot of time for distraction or other cockpit management chores before the empty wing tank becomes a potential engine starvation issue. In theory you can ignore the blinking "WING TANK EMPTY" indication for many minutes to an hour before getting around to switching to the next tank
In an RV, I agree. In a COZY MKIV - 6.3 hours, no problem. Could have napped the whole way .
Your system requires active management to avoid starvation. A header buys you time.
Glass is popular, but not ubiquitous just yet (IMHO). A stand alone FF readout in your existing display would (I think) be a low cost/high value feather in your cap.
There have been times on the way back from a particularily "active" weekend: Just the right bit of turbulence, warm temps and a partied out physiology and it makes for "nap time"... Makes me glad for the autopilot and iron maiden like RV seats.
Sorry Bill... I'm screwing up your thread!
I haven't seen a non-glass panel in a new build RV for a long time now and I was just perusing VAF last week and copying panel pix for a product evaluation. That being said, I'll add this feature to the EM-6 wish list. It might help a few folks out who are on a budget or are working on a plane with an older panel.
Hey! I am 62. Grin. My wife and I routinely fly four hour legs when they suit us. True, I might be a two hour leg guy by the time I am flying this thing...
There are bottles and stuff for those legs where you are in the sunshine on top, and a stop along here will mean a descent through wet and potentially icy clouds to an instrument approach, then an instrument departure and climb back up through the potentially icy clouds...
? Mattess Negotiation? Maybe this joke is from a different era.
You are joking right? All that monkey motion and memory stuff while you are supposed to be flying the airplane? Ugh. I would rather have way less things to remember how to run them and then do them while trying to keep it right side up in turbulence and circumnavigating something nasty…
With a header tank system I am describing, the transfer pump runs all of the time, the header tank stays full until the wing tank goes dry, then the header tank takes an hour to become empty.
If the lights are all green and the fuel levels are where you think they should be, you are good. Wing tank is showing near empty with gauge and/or light, you switch tanks and finish the flight. You can even get busy with traffic and re-routes and weather and diversions to the point where the transfer pump is sucking air, then depending upon installed equipment, the warning switch at the top of the header tank OR the fuel flow indicator on the line to the header gives you a heads up to switch to the other wing tank.
Looking at Failure Mode Management, if the warning lights malfunction, you still have the wing tank level gauge showing empty and the header level gauge dropping when it shouldn’t be doing that, and you have an hour to catch that indication before the EFI goes dry. When you do catch it, you switch transfer pumps and look for the header level to go up. If the fuel level indicators go bad, you still have the level switches to give you a heads up, and if they all go (we are getting into pretty low levels of probability here), we should know times to empty for each tank. If the transfer pump has been dry for a while and you are concerned about the pump maybe being damaged (Facet vibrator pumps, probably OK, but maybe you damaged it), you push the transfer pump switch from A to B and motor on.
Remove the header tank, and if you miss the transfer time, the engine goes quiet RIGHT NOW. Yank the carb heat, check fuel pumps are on, switch tanks, and wait 10 seconds, 15 seconds. I am here to tell you, I had hair turn grey waiting for the restart. No thank you.
Not to disagree with you unnecessarily, but you are making it both a big learning discussion and entertaining, so NOT screwing up my thread... Thanks!
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