IvanK Posted March 17, 2016 Posted March 17, 2016 Whilst playing around at max altitude engine flamed out. On descent I eventually lost all controls with both HYD1 and HYD2 lights on. IAS was 300Kias with 25% windmilling RPM at 30000ft. I would have thought with windmilling RPM the HYD system would still be producing usable pressure ? The instant I relit HYD1 and HYD2 came back.
Azrayen Posted March 17, 2016 Posted March 17, 2016 I would have thought with windmilling RPM the HYD system would still be producing usable pressure ? Agreed.
Badlego Posted March 18, 2016 Posted March 18, 2016 I don't think that the engine is able to produce enough work without fuel combustion. With low rpm and high intake airspeed the compressor-blades are producing high energy losses (choke-flow Effect) and not enough pressure for the turbine-blades behind the combustion Chamber.
Azrayen Posted March 18, 2016 Posted March 18, 2016 (edited) Yet despite what each of us may think, 25% RPM windmilling engine is more than enough for hyd pumps to pressurize both systems. ;) Edited March 18, 2016 by Azrayen
Eddie Posted March 18, 2016 Posted March 18, 2016 (edited) If that's the case I would be very surprised, it certainly isn't on any of the aircraft types I've worked on. Windmilling motors will potentially produce some pressure, but certainly not usable (i.e. supportive of full, safe, system operation) pressure. Don't forget that while the low pressure compressor may be spinning at a reasonable rate, it's normally the high/low pressure turbine stages that have the power take off to drive the accessory gearbox and on to the hyd pumps and generators. Of course, anythings possible given that it's French, who knows how they've hooked the system up. ;) Edited March 18, 2016 by Eddie
mvsgas Posted March 18, 2016 Posted March 18, 2016 (edited) This being a single spool engine, does it have N1 (low pressure compressor) and N2 (high pressure) or are they consider just n1 or just n2? RPM in the cockpit should show both since they spin together in a single shaft, right? Edited March 18, 2016 by mvsgas To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Azrayen Posted March 18, 2016 Posted March 18, 2016 Well, the M-53 is a single-shaft turbofan. There is no N1 and N2. Juste N. The gearbox that drives all external things is between LP compressor and HP compressor, i.e. "far ahead" of the turbine stages.
mvsgas Posted March 18, 2016 Posted March 18, 2016 (edited) Well, the M-53 is a single-shaft turbofan. There is no N1 and N2. Juste N. Ok, thanks The gearbox that drives all external things is between LP compressor and HP compressor, i.e. "far ahead" of the turbine stages. Same as all other engine I know, but ok Edited March 18, 2016 by mvsgas To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Eddie Posted March 18, 2016 Posted March 18, 2016 Fair enough. If it's a single shaft then you'll get something from the PTO. But even then you shouldn't be seeing a stable 2000+ psi. I'm not aware of the Mirage's normal hyd operating pressure, but I assume somewhere between 2 and 3 thousand psi) from it while windmilling, enough to build up pressure without demand on the system sure, but as soon as something starts pulling pressure (i.e. flight control actuation) it'll be diving below minimum pretty sharpish.
mvsgas Posted March 18, 2016 Posted March 18, 2016 Another thing I'm wondering, how is the FBW powered with the engine off? F-16 (USAF, block 40, circa 2012) has several generators (Main, standby, emergency) and finally the battery. If the engine is off, the emergency generator is turned by the Emergency Power Unit (EPU) which uses Hydrazine as fuel. To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Eddie Posted March 18, 2016 Posted March 18, 2016 I was wondering the same thing myself actually now I've started looking at the Mirage properly. I guess it could be powered from the battery via the inverter in a single FCC reversionary mode (how many FCCs does the Mirage have)? 1
Azrayen Posted March 18, 2016 Posted March 18, 2016 Both main hyd pumps provide 280 bar (4060 psi). Pump 1 driven through accessories relay Pump 2 driven directly by engine Emergency electro pump provides 190 bar (2750 psi), autostarts when hyd2 pressure falls < 160 bar (2320 psi). The old manual also mentions a second emergency EP, with one minute electrical power available. This one was designed to auto-start if 5%<N<15% <= that I used as data to think that above N=15%, we were "safe" to maintain hyd pressure. HYD1 or HYD2 caution lights come on if respective circuit pressure is < 190 bar (2/3 of what main pump provides).
Azrayen Posted March 18, 2016 Posted March 18, 2016 (edited) Another thing I'm wondering, how is the FBW powered with the engine off? If you ask about electrical power to the FBW, then from old RDM manual, page 47: Channel 1 & 2: each its own 350 VA alternator (not the same as main alternators) driven by hyd1 & 2 pressure. Channel 3: through electric converter from 28V DC bus (if not unloaded) Channel 4: through electric converter from Batt bus Channel 5 (last resort emergency): direct from battery Edited March 18, 2016 by Azrayen
mvsgas Posted March 18, 2016 Posted March 18, 2016 (edited) Eddie, Makes since. Azrayen, So the battery powers both the FBW and the hydraulic with the engine off? Additionally, can you move the flight control without power? Again, using the F-16 as a example since it is what I know, there is no physical connection to the flight controls actuators ( integrated Servo Actuators or ISA). So without power, you have no control. Edited March 18, 2016 by mvsgas To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Azrayen Posted March 18, 2016 Posted March 18, 2016 Uh? You mean engine off & seized (not windmilling) (or with both power-drive broken)? If yes then of course battery is the last energy source. Useful for brief time (a few minutes), to get to best ejection conditions (anyway with no engine you're not going far). If engine off but windmilling let's say at N=25% then you have hyd1 & hyd2 so you have FBW channels 1 & 2 powered as usual, channel 3 unpowered (unloaded DC bus), channel 4 powered by batt, channel 5 offline (as it should be if not needed). Let's hope you also have time/alt to attempt a relight, if not => eject.
Eddie Posted March 18, 2016 Posted March 18, 2016 Additionally, can you move the flight control without power? Again, using the F-16 as a example since it is what I know, there is no physical connection to the flight controls actuators ( integrated Servo Actuators or ISA). So without power, you have no control. That's another good point. Typhoon is the same, all the actuators are electrically controlled, so no power no control. Of course if there is power to the backup FCS channel, the actuators should be powered as well, at least in a reversionary fallback state.
mvsgas Posted March 18, 2016 Posted March 18, 2016 How much pressure and gallons per minute you get at 25% RPM compared to normal? What is the pressure and gallons per minute needed to operate VA alternators? To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Azrayen Posted March 18, 2016 Posted March 18, 2016 (edited) That's another good point. Typhoon is the same, all the actuators are electrically controlled, so no power no control. The above is true for the Mirage too. Full FBW = no mechanical backup. No power (at all) = no controls (at all). This is why I reacted several times when someone talk about "switching off FBW" in the Mirage. How much pressure and gallons per minute you get at 25% RPM compared to normal? What is the pressure and gallons per minute needed to operate VA alternators? I have no idea. No gallons in the Mirage anyway.:smartass: It's not "VA alternators" it's 350 VA alternators, that's their output. To compare, the main alternators output 20 kVA each (20,000 VA). My guess is: it doesn't require much ;) Edited March 18, 2016 by Azrayen
mvsgas Posted March 18, 2016 Posted March 18, 2016 I hope we can find more info. I agree with Eddie, most of our aircraft do not consider wind milling a reliable source of hydraulic pressure in an emergency because of the gallons per minute. As an example, it take about a minute of "windmilling" at 12% or more to fill JFS accumulators to 3000psi in a F-16. To whom it may concern, I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that. Thank you for you patience. Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..
Eddie Posted March 18, 2016 Posted March 18, 2016 I hope we can find more info. I agree with Eddie, most of our aircraft do not consider wind milling a reliable source of hydraulic pressure in an emergency because of the gallons per minute. As an example, it take about a minute of "windmilling" at 12% or more to fill JFS accumulators to 3000psi in a F-16. That's the key point. Yes just spinning the pumps will generate pressure all the way up to the standard max in theory, but in practice the pumps can't move enough fluid at low speed to maintain that pressure with any real demand on the system. So while you could move the flight control once or twice with charged accumulators, the pumps couldn't bring the pressure back up fast enough to prevent loss of hyd pressure and control lockup would occur as a result. Or course, there will be a minimum RPM at which usable pressure is stable and maintainable, and that could possibly be within the windmill RPM range depending on airspeed. You'd just have to have enough airspeed to maintain it.
Azrayen Posted March 18, 2016 Posted March 18, 2016 Yeah, I understand that but we're talking windmilling @ 25% N in an aircraft whose (ground) idle is a bit less than 50% N. Given that FBW tests are run at ground idle, I assume this regime is enough to have max pressure (and rebuild it nominally even with lots of control surfaces full actuations). I don't know if pumps are "linear"? I yes, we can assume pressure rebuilds itself at ~ half the normal rate when windmilling @ 25%. Considering pilots are not (that) dumb, and won't yank their controls from side to side while trying to make something of a dead engine aircraft, and that the aircraft has HYD system that light caution light when pressure is 2/3 of max, I guess there is room to work.;) Of course, windmilling @ 12% is not the same story.
Azrayen Posted March 19, 2016 Posted March 19, 2016 Well, tried to reproduce... found another thing. Shutting down the engine (stop button) in flight gives N=0 very quickly (and a shit load of alarms, including HYD 1&2). No windmilling. Should be a bug. Will have to try by starving it from fuel (G<0) or from air (tailslide) but that's for another day.
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