Hydraulic FLCS accumulators, weird behaviour?
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Flying both the C and D versions I have deliberately starved my plane of hydraulic pressure. Basically:
- Turned off generators by putting the main electrical switch to Battery.
- Used up all the hydrazine for the EPU by flying at idle for a while
- Cut off the engine and let it shut down completely midair.
Not sure at which percent the engines actually give enough pressure for FLCS or recharge its accumulators (I assume it’s around 25%, see below), but anyway, RPM went down to zero. So I expected a few seconds of flight authority, but that’s not what happened. I could glide down and move the stick like crazy, all surfaces moved perfecly for minutes. However, as soon as I opened the brakes, two things happened:
- Immediately lost all FLCS control
- Brakes remained in the open position.
I’ve tried the scenarion multiple times, I even restarted the engine midflight, and at around 25% RPM I could close the brakes and regain control.
Now, what I don’t understand. Aren’t the brakes kept in place by hydraulic pressure? And shouldn’t I have lost control much sooner without the need to open the brakes? Are the FLSC accumulators that big?! Looking at the schematics on page 72, the speedbrakes are not even connected to the FLSC accumulator, but direcly to the pumps, so basically only the reservoir main pressure accumulator. So how could they open anyway?
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We’ll have to do some testing with your cases you present… it’s very possible you have found “bugs” / unaccounted for cases in the coding of these systems. And it could also be some “incompleteness” in the modeling.
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@mirv said in Hydraulic FLCS accumulators, weird behaviour?:
We’ll have to do some testing with your cases you present… it’s very possible you have found “bugs” / unaccounted for cases in the coding of these systems. And it could also be some “incompleteness” in the modeling.
there is no bug in th efact that the F16 can glide a few mintues using remaining pressure to feed FLCS , because that the way i set it up. Of course the more angle the DEfelction are , the more pressure is used
the reality is that no data exists on how much time Hyd would be depleted with FLCS only
so this is totally estimated
for the speedbrakes, if you guys tell me that with ni more pressure speedbrakes must be retracted, then no problem
But TBH this is kinda useless fix since if no more HydPressure => it’s a garantee crash anyway (no more FLCS)
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@Mav-jp said in Hydraulic FLCS accumulators, weird behaviour?:
@mirv said in Hydraulic FLCS accumulators, weird behaviour?:
We’ll have to do some testing with your cases you present… it’s very possible you have found “bugs” / unaccounted for cases in the coding of these systems. And it could also be some “incompleteness” in the modeling.
there is no bug in th efact that the F16 can glide a few mintues using remaining pressure to feed FLCS , because that the way i set it up. Of course the more angle the DEfelction are , the more pressure is used
the reality is that no data exists on how much time Hyd would be depleted with FLCS only
so this is totally estimated
for the speedbrakes, if you guys tell me that with ni more pressure speedbrakes must be retracted, then no problem
But TBH this is kinda useless fix since if no more HydPressure => it’s a garantee crash anyway (no more FLCS)
It is very true, even in Soviet aviation. I remember a being in an mi-17 checking the autopilot on the ground connected to the UPG (static hydraulic pressure station) and move the cyclic and look at the main hydraulic pressure system gauge and see how much the needle deflected on consumption before stop the movement of the cyclic and the needle returned to 250kg/cm² at the working pressure. I remember that I used the reserve hydraulic system and since it is aligned it was not connected to the UPG so I simulated running out of hydraulic pressure and saw each movement of the cyclic as the main system was unloading and at the same time the cyclic became very rigid to the point of not being able to move. With this experiment I calculated and I was able to verify that with small movements the helicopter can be controlled without hydraulic pressure either in the main system or in the reserve system, but with each small movement it wears out until it is left without control. In the aeronautical theoretical base we study that the hydraulic systems are consumers and the hydroaccumulators without those in charge of maintaining the stable pressure and in case of failure leave pressure to be able to operate the systems until that pressure is exhausted. In general, the systems that consume the most pressure in Soviet aircraft are the extension and retraction of the landing gear, the controls and the airbrakes.
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@Mav-jp I wasn’t doing a simple glide, but wildly circling the stick and pedals. I wanted it to deplete, not trying to survive. Are you saying that it is indeed modelled? Even if estimated, how long does it work? I took the plane from 40000 feet to zero and it took a long time. Also there was no fade in the ability to maneuver.
Here’s what I found: https://quizlet.com/505148841/f-16-block-5-diagram/
Flight Control Accumulators: One accumulator for each hydraulic system Provides immediate fluid flow to all servoactuators when demand is higher than pump delivery Provides 3 seconds of hydraulic power if dual hydraulic system failure occurs, allowing EPU to come online
This makes sense since they are tiny:
And anyway, when opening the speedbrakes, the sudden complete loss of stick control is jarring. I might expect them to open immediately after pumps fail, but not after a full minute of stick tornado.
And… here’s your answer for why speedbrakes shouldn’t stay open:
Link is from here: https://www.f-16.net/forum/viewtopic.php?f=23&t=6630&start=15
Hope that helps.
So my proposed behaviour with no hydraulic pumps:
- no opening of speedbrakes
- if open, should close depending on airspeed. Say 100kts for full closure, linear interpolation down to 30kts where it has no effect. The bigger the speed, the faster they should close.
- stick should work for 3 more seconds of full deflection at say mach1 (all surfaces combined, ie max rudder, max aileron, max elevator). Hmm… I should test this in the current version, I’m curious. And if gentle and at medium speed, in no circumstance should operating anything else hydraulic (i.e brakes) should drain these accumulators, since they are isolated.
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Ok so I redit my test, finally managed to deplete the accumulator. Full deflection all the way down, lowered the emergency gear handle and quickly raised it back up (hmm… not sure how). Lost stick control without touching the brakes. Still, took ages, not 3 seconds
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@karamazovnew said in Hydraulic FLCS accumulators, weird behaviour?:
@Mav-jp I wasn’t doing a simple glide, but wildly circling the stick and pedals. I wanted it to deplete, not trying to survive. Are you saying that it is indeed modelled? Even if estimated, how long does it work? I took the plane from 40000 feet to zero and it took a long time. Also there was no fade in the ability to maneuver.
Here’s what I found: https://quizlet.com/505148841/f-16-block-5-diagram/
Flight Control Accumulators: One accumulator for each hydraulic system Provides immediate fluid flow to all servoactuators when demand is higher than pump delivery Provides 3 seconds of hydraulic power if dual hydraulic system failure occurs, allowing EPU to come online
This makes sense since they are tiny:
And anyway, when opening the speedbrakes, the sudden complete loss of stick control is jarring. I might expect them to open immediately after pumps fail, but not after a full minute of stick tornado.
And… here’s your answer for why speedbrakes shouldn’t stay open:
Link is from here: https://www.f-16.net/forum/viewtopic.php?f=23&t=6630&start=15
Hope that helps.
So my proposed behaviour with no hydraulic pumps:
- no opening of speedbrakes
- if open, should close depending on airspeed. Say 100kts for full closure, linear interpolation down to 30kts where it has no effect. The bigger the speed, the faster they should close.
- stick should work for 3 more seconds of full deflection at say mach1 (all surfaces combined, ie max rudder, max aileron, max elevator). Hmm… I should test this in the current version, I’m curious. And if gentle and at medium speed, in no circumstance should operating anything else hydraulic (i.e brakes) should drain these accumulators, since they are isolated.
Yes it is modeled
Where you miss something is that moving you stick all around does not mean the surface are deflecting much , the slower you move the more angle the surfaces need . Do the same at low speed and you will see your depletion rate is much. Higher
As I said o had no reference but yes it right now you can operate the plane and attempt a landing within a few minutes
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@Mav-jp Indeed, plus the FLCS does its own thing limiting control surface angles. Which is I was in external view most of the times. Those were big deflections, particularly from the ailerons. And yeah, the lower the speed the higher the angle, but also lower aerodynamic pushback. Wish I had more reference but the one I have (i.e the official F-16 manual) doesn’t mention being able to land the thing…
Each hydraulic system has an FLCS accumulator which is isolated from the main system by check valves. These FLCS accumulators serve a dual function. If demand exceeds the pump maximum flow rat e during rapid control surface movement , the FLCS accumulators provide additional hydraulic pressure. Also, if both hydraulic systems fail, the FLCS accumulators provide adequate hydraulic pressure to t h e flight controls while the E P U comes up to speed.
As a non-educated guess, from 200 bars (ups, 3000psi), without further pumping, I expect rapid fade in something like 30 seconds to something unusable even without touching the controls in that time. With active movement, I’m guessing a few seconds only.Ok fine, this is an extreme situation, most of us don’t find ourselves after taking a missile hit, but as the EPU is so lovingly simulated, it seems wrong to take away from its importance in keeping us in actual control of the plane. Nursing the plane home at high RPM to keep enough hydrazine for final landing is precisely what got me into this scenario in the first place. To quote the manual…
https://falcon.blu3wolf.com/Docs/HAF-F16.pdf page 332:Under some failure conditions, hydrazine may not be a vailable to the E P U or it may continue to deplete even with adequate bleed air. If a hydrazine malfunction or depletion occurs, landing must be accomplished using an engine thrust setting sufficient to maintain an adequate bleed air supply to the EPU. Failure to maintain minimum engine rpm can result in hydraulic pressure fluctuations or electrical bus cycling . Advance throttle as required to maintain adequate bleed air supply. ( If the E PU is the sole source of hydraulic pressure , the throttle should be set so as to keep the EPU run light on during mild flight control inputs.) This action may result in a thrust level tha t is higher than required for a normal straight in approach. Fully open speedbrakes or a shallower than normal approach may be required . A straight in approach followed by an approach end arrestment is recommended.
If PTO shaft or both hydraulic systems are failed , underspeed of t he EPU results in loss of control. Do not retard throttle completely to IDLE until after touchdown.
Then, on page 384:
When bleed air is no longer available to operat e the EPU, confirm that the EPU is operating on hydrazine ( EP U run and HYDRAZN lights on ) since the JFS alone does not provide adequate hydraulic pressure to land the aircraft. If the EPU is inoperative, maneuver the aircraft as necessary on JFS assisted hydraulic pressure to a more favorable ejection envelope and initiate ejection
Later, page 387
Starting JFS reduces load on EPU, conserves EPU fuel, and partially restores hydraulic system B. EPU fuel quantity should be at least 25 percent (20 percent with JFS running) at high key to insure adequate hydraulic pressure throughout landing
Interesting… during mid-air restart I did start the JFS but had no stick authority at all until the engine spooled up to around 40%.BTW, is there some console command I can use to stop my hydraulic pumps? It would save me a lot of time and allow me to test things with engine running as well.
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@Mav-jp
Ok, seems it seems the actual problem is about wording. The fact that the manual basically says “eject, don’t land” makes sense since it isn’t a sustainable pressure or predictable outcome. But indeed you have a couple of minutes of control, which doesn’t contradict either you or the manual. It seems I was taking things too literally.Anyway, I did a bunch of ground tests, easier to replicate:
Scenario 1: start any mission in taxi or runway, cuttof the engine. Wait until RPM goes to 0. Time to surface control collapse:- do nothing: 6 minutes (roughly)
- half pull of elevator: 2 minutes and a half (roughly)
- full pull of elevator: 2 minutes
- move elevator up and down fully (external view): 3 minutes
- move all surfaces like a madman: 1 minute
- open speedbrake immediatly without doing nothing else: 30 seconds
Scenario 2: take off with EPU in OFF mode, cutoff engine at altitude
- do anything except touch the speedbrake: between 1-3 minutes, since the FLCS does its own thing. This is what threw me off in the previous comments. I had to override the elevator to really force it and see a more replicable result.
- open speedbrake before doing anything else: 10 seconds (opens but only half closes back if you try to close it). The time makes sense since it would use up the main reservoir, but not the FLSC one. In my previous tests I had already moved my controls a lot before so I assume the FLSC reservoir was already used up so opening the brake meant isntant loss of control. In fact this points to the fact that the FLCS accumulator actually provide very few seconds of control alone, the rest of it came from the main accumulators. I did not take that into account before.
So all behaviour seems more than reasonable. I assume that should the actual main accumulator pipes fail, the FLSC accumulator would only have around 10 seconds of usefulness. However in the sim it’s impossible to replicate such a scenario. So call me satisfied on the actual name of this topic. I personally would’ve halved all times, as they seem very generous, but maybe they built the thing like a tank, who knows.
However… two slight issues remain:
- JFS seems overpowered and I can’t figure out why the manual forbids landing with it alone. I had full control, with brakes and emergency gear, very little sluggishness. My assumption is that although the recharging of the JSF/Brake accumulator is realistic, the size of the rest of the system is not taken into account, so the entire system is repressurised at the same rate? That would be easy to fix by scripting the flow from the JSF to drop to something proportional (say, 1/3?) when the accumulators are filled? Normal drain on the system would thus be unsustainable for anything more than short bursts of basic attitude control.
- I still can’t explain how the brakes would stay open against the airstream. Behaviour on the ground is logical.
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See we have done some homework on that front , now is it accurate ? Certainly not as we don’t have real reference , but the general outcome is acceptable