F-16 Fuel Quantities (was Power generation & "environmentalism")
-
both reservoirs are 480 lbs +/-30.
Normally fuel pumps are not the primary supply method to the reservoirs, they are a supplement to the siphoning action from the reservoirs. This method does depend on the reservoirs being full though.
Fuel pumps feed equally from both reservoirs to the FFP and from there to the engine.
It does seem to suggest in the emergency section that you can get flameouts if the reservoirs are not full and you select high fuel flow rates over 6000pph.
Thanks for the update
It sounds logical since high flow rates out of the reservoir may suck in some air too which disturbs the engine so it might literally blow out the flame. I know a similar behaviour with other liquids than fuel (coolants, for example). When you have a high flow rate, it sucks in some air too in some circumstances. Depends on where the pipe is fitted, the height of the liquid above the pipe, viscosity of the liquid and so on.
To sum it up: When fuel is out, the engine will be dead
-
split & moved
-
Emergency fuel has a fixed definition - I believe its FAA or something. Definition being : minimum fuel needed to get straight to the next runway and land with the Final Fuel Reserve.
In the F-16, I would make sense that the final fuel reserve is the reservoir feeding the tank indeed : the slightest G you get (in any direction), the fuel goes around everywhere and you risk to have a pump stall. Especially with negative Gs, as you said earlier Blu3wolf.
Well, to sum it up, seeing as we now have a new thread:
From the manuals I have here, it would seem to me that you should be able to fly the aircraft under positive G down to 0 fuel on the totalizer +/- 100 lbs so long as fuel flow is under 6000 pph.
The manual has a long series of warnings and cautions written in it, which detail any area that could affect flight safety. There is no hazard written regarding chances of flameout with say 500 lbs remaining, and there IS a section which states that the totalizer only displays usable fuel, and that unusable fuel is not included in the totalizer values.
There IS a warning about high fuel flow, and about negative G. As a reminder to all, WARNINGs in the -1 series of T.O.s indicate “Operating procedures, techniques, etc., which could result in personal injury or loss of life if not carefully followed.”
Limit fuel flow to the minimum required to
sustain flight while the cause of the fuel low
light(s) is determined. Avoid negative g flight
when either reservoir is not full .I would argue that based on this, a fuel low light is not likely to result in a flameout immediately so long as those guidelines are followed. That said under 150 to 100 lbs I would start expecting a flameout any time soon, and under 50 lbs reported I would expect one imminently.
The dash does make comment of ‘air ejectors’ which ensure the reservoirs remain full of fuel at all times, but they certainly would not work as desired with no fuel in the tanks to draw from. With the reservoirs less than completely full, the low fuel lights still do not come on immediately either, but wait until they are about half full (less than 250 lbs in the AFT reservoir, and less than 400 lbs in the FWD reservoir - reversed figures for the D model). This further leads me to conclude that you should get normal operation under normal flight conditions… but then negative G is apparently a problem with a low fuel light. What about high skid/slip? what about high roll rates? Questions not answered by the dash, and for which logical solutions may not be correct either (at least going by previous ‘logical’ guesstimates which have been put into falcon incorrectly).
-
split & moved
Well, to sum it up, seeing as we now have a new thread:
From the manuals I have here, it would seem to me that you should be able to fly the aircraft under positive G down to 0 fuel on the totalizer +/- 100 lbs so long as fuel flow is under 6000 pph.
The dash does make comment of ‘air ejectors’ which ensure the reservoirs remain full of fuel at all times, but they certainly would not work as desired with no fuel in the tanks to draw from. With the reservoirs less than completely full, the low fuel lights still do not come on immediately either, but wait until they are about half full (less than 250 lbs in the AFT reservoir, and less than 400 lbs in the FWD reservoir - reversed figures for the D model). This further leads me to conclude that you should get normal operation under normal flight conditions… but then negative G is apparently a problem with a low fuel light. What about high skid/slip? what about high roll rates? Questions not answered by the dash, and for which logical solutions may not be correct either (at least going by previous ‘logical’ guesstimates which have been put into falcon incorrectly).
Personnally, I think that as soon as the tanks are not full, you have the risk of a pump stall - especially, as you said, under negative G. Basically, even with the ENG FEED on NORM, if remaining fuel gets pushed away from the pump, you have a pump stall and air gets into the pump, its flameout. Probably recoverable though, if you get back to straight and level.
With straight and level flight, with low fuel flow, you should get pretty close to 100 -150 lbs without too much of an issue, i agree. But as soon as you get some lateral, axial, or negative G (especially), fuel might get pushed away from the pump, and you risk a flameout. I think the 500 lbs is a fair threshold under which you consider the limitations in Gs are too important to be neglectable anymore for safety of flight.
-
Which of course brings up the question of whether positive G maneuvering is a problem ( I would guess it is, but plenty of wrong guesses have made their way into the sim before ), and if so, crucially, HOW MUCH is a problem.
As soon as the reservoirs have some air in them, if all the boost pumps from the reservoirs start sucking air, it follows that shortly afterwards (a few seconds?) the engine will lose power and flameout (and maybe stall as well?). Question is whether the engine can run with less than 3 boost pumps supplying fuel. Answer is clearly yes, as the aircraft can run under positive G with the ENG FEED knob in OFF.
So I guess the next question becomes how little fuel can you have and still get enough fuel… I think advanced liquids simulation inside the tanks is probably more advanced than the sim should get until after it has advanced fluid simulation for the airflow (per packet basis). So, in a simple system, how much side loading can you get and still have adequate fuel supply for half full reservoir, for a quarter full, etc… Notably, high positive G is not likely to be a problem at all, and coordinated flight (automatically provided by ARI) should limit side loads on the aircraft except during rolls.
so to sum that up, I reckon positive symmetric G should be fine, asymmetric G should be a problem, high speed rolls should be a problem, maybe high speed decelerations should be a problem? Negative G should be right out.
-
so to sum that up, I reckon positive symmetric G should be fine, asymmetric G should be a problem, high speed rolls should be a problem, maybe high speed decelerations should be a problem? Negative G should be right out.
That would be my guess too. Then there is the issue of fuel flow :IMO, the higher, the more likely problems (flameout) can arise.
-
Id agree. In fact the dash makes specific reference in the emergency section, to reduce fuel flow to under 6000pph if either fuel low light illuminates. It also says not to use AB with totalizer fuel under 2000 lbs remaining.
Interestingly, I found a section of the dash which says that the totalizer reports actual fuel in the tanks, AND that fuel in the tanks is almost equal to usable fuel. I imagine this is due to the fuel transfer system detailed in section 1 of the dash, which comments about the fuel pumps scavenging fuel to minimise unusable fuel.
Anyway, seems like we agree on what should constitute a problem for low fuel conditions. Afraid Im not a coder, so you wont see a working prototype of this from me anytime soon mebbe we need to bug someone who can make it happen
-
Anyway, seems like we agree on what should constitute a problem for low fuel conditions. Afraid Im not a coder, so you wont see a working prototype of this from me anytime soon mebbe we need to bug someone who can make it happen
Well it wont happen in 3-4 weeks But who knows what the future can yield
-
I see you are also a fan of Dos Gringo’s tale of how they flew the whole mission on alt hold…! XD
DROPPIN’ JDAM!
Holy crap this stuff is hilarious
-
Check out the wingmen song too, its gotta be heard if you fly someone’s wing in BMS…!
-
BUMP!
Interesting thread…
One thing I don’t see mentioned here is fuel tank construction. Fuel slosh is not the same in a fuel tank as it would be in a drum or tub. Fuel tanks are equipped with baffles and in most cases a well for the pick up point (siphon or pump intake). Baffles are helpful in that they keep the weight transfer in check when the fuel is subject to changes in pitch of the tanks. Imagine banking hard 45* and all of a sudden the fuel hits one side of the AC.
The well is usually a trap of sorts that is at the lowest point of the tank and typically has it’s own supply volume. Fuel is allowed in by trap doors, but not out of the well.
The biggest factors I can see with low fuel level, are gravity and inertia problems. As long as G forces stay above 1.0 and less than 1.3(???-just guessing) and bank angle is not sustained for excessive (based on fuel state) periods of time, the risk of a flame out should stay relatively low as long as there is a few hundred pounds of fuel left.
That being said…
I would love to see random flame-outs occur with dangerously low fuel levels. I am constantly abusing my fuel quantity and it is not out of the ordinary to come home “on fumes”. It is almost like an addiction at times. I know I shouldn’t do it, but if the risk is low, I WILL DO IT EVERY TIME. :mrgreen:
-
Definitely. Something to think of for the future : doing random flameouts when fuel is below the minimum. This way V-pilots would understand the need for fuel tanks
… At least four years I am waiting for this! … Maybe, maybe in … (?) …