[FIXED] Flight Controls Hydraulic System

[AH_Sid]

I've been running a few test comparing the before taxi 'Flight Controls Hydraulic System Check' against what we have simulated for our F86F-35. Generally it's modelled well, but I've noted a few differences and problems.

 

For reference here's the relevant pages from the F-86F Flight Manual.

 

click for larger versions

 

 

The below tests were carried out at Senaki-Kolkhi using the default 'Cold Start' mission (Altitude 39 Feet, Temp +18°c), using V1.5.2.48162.126 of DCS.

 

 

The very first note tell us to perform these checks with the throttle at IDLE. It doesn't mention any altitude or temperature restrictions, so I assume these checks could be made at all airfields under all conditions, but this becomes relevant performing the first check.

 

 

Normal System Check

 

 

 

1. Works Ok, with the warning light Out and pressure steady at around 3000 psi.

2. Works Ok, with the pressure returning at a reasonable rate.

3. Works Ok here, but with some effort. The change-over happens correctly at around 700 psi, but the controls need quite a lot of movement to bleed the pressure down to that amount.

 

As mentioned above the aircraft is at IDLE (around 30% RPM), but as stated later in the manual, idle rpm varies with altitude and temperature.

 

 

This increase in idle speed seems to be correctly modelled, for example:

 

Senaki-Kolkhi, Altitude 39 feet, Temp +18°c gives at idle of 30%

 

Beslan, Altitude 1772 feet, Temp +15°c gives at idle of 33%

 

McCarran Intl, Altitude 2051 feet, Temp +21°c gives at idle of 35%

 

Groom Lake, Altitude 4459 feet, Temp +30°c gives at idle of 40%

 

But as the idle speed increases, so does the output of the engine driven 'normal' hydraulic pump, meaning with the idle speed much above 30%, the normal system simply can't be 'bleed-down' sufficiently to perform the change-over. I think the amount the normal systems drops when the stick is being moved rapidly like this needs increasing so the check can be completed at all airfield altitudes and conditions.

 

 

Alternate System Check

 

 

 

1. Works Ok, with the warning light On and pressure around 2750 psi, but the pressure is meant to be 'Cycling' between 2550 - 3200 psi here, not maintaining a constant pressure.

 

There's no description of how fast the cycling would have been, but bear in mind this was a before taxi check not a maintenance check, so I wouldn't have expected the system to take 5 mins to drop from 3200 to 2550 and then build back up again. Personally I would have thought something in the order of a 3-4 seconds for the pump to push the pressure up from 2550 to 3200, then maybe 10-15 seconds for 'designed leakage' to drop the pressure back to 2550, ready for the pump to kick back in and pump it up to 3200 psi again, so a cycle every 15-20 seconds would seem reasonable if you're asking the pilot to check it at this stage before taxi.

 

2. Works Ok, proper control movement.

3. Works Ok, the 'bleed-down' is much easier on the alternate system and correctly changes-over at around 750 psi.

 

But note how long the system takes to rebuild the pressure back up! The NORMAL system takes around 12 seconds to rebuild the pressure, the ALTERNATE system takes 67 seconds. That strikes me as a huge difference for two systems that where both tasked with doing the same job. I haven't seen technical specs for pumps, but I'm surprised there's this huge difference even given the fact the Normal system pump was engine driven and the Alternate pump electrically driven.

 

4. Works Ok, the light comes On.

5. Works Ok, the light goes Out, and the NORMAL pressure is around 3000 psi.

 

 

Emergency Override System Check

 

 

 

1. Works Ok, Flight control switch can be held at RESET.

2. Works Ok, Emergency override can be handle pulled Out, pressure rises correctly to 4000 psi (Non-Cycling).

3. Works Ok, Alternate warning light is Out.

4. Works Ok, proper control movement, pressure drops.

 

But again note how slow the pressure is to rebuild (250 - 4000 psi takes 82 seconds)

 

5. Works Ok, Flight control switch released to NORMAL, Alternate warning light comes On. Pressure remains at 4000 psi (Non-Cycling).

6. Emergency override handle In, pressure incorrectly remains at 4000 psi. Due to the 'designed leakage' the pressure should drop back to 2550 and begin 'Cycling' between 2550 - 3200 psi again.

7. Works Ok, system resets to normal, light Out, and NORMAL pressure Ok.

 

 

Load Meter

 

All the above tests were carried out with the engine running, battery switch on, ground power on, and the generator selected on (but with the idle speed below 45% it shouldn't have had an output anyway).

 

 

 

At no stage did the Load Meter appear to show any load. I'm not sure of the circuitry with the ground power on (does the Load Meter show a load at this stage?). But even in flight, if the generator is turned off (switch on the right forward console) and the Emergency Override pulled (pump constantly running, connected directly to the battery), the Load Meter doesn't show this heavy load.

 

 

Summary of differences and problems.

 

1. The NORMAL hydraulic system needs to 'bleed-down' at a higher rate, so the change-over can be checked even at higher altitude airfields on a hot day (but not so high the system changes over during 'normal' flight though).

 

2. The ALTERNATE hydraulic system needs to cycle between 2550 - 3200 psi when not under load.

 

3. Possibly increase the ability of the Alternate system pump to rebuild the pressure to something more similar to the Normal system pump.

 

4. After the Emergency Override Handle has been pushed back in, the system should return to cycling between 2550 - 3200 psi when not under load.

 

5. Should the Load Meter show a heavy load whenever the Alternate system pump is running.

Edited December 18, 2015 by =AH=_Sid

[Corrigan]

Awesome post, good job buddy. BST should count themselves lucky to get these kind of tests/reports.

[cofcorpse]

Summary of differences and problems.

 

1. The NORMAL hydraulic system needs to 'bleed-down' at a higher rate, so the change-over can be checked even at higher altitude airfields on a hot day (but not so high the system changes over during 'normal' flight though).

 

2. The ALTERNATE hydraulic system needs to cycle between 2550 - 3200 psi when not under load.

 

3. Possibly increase the ability of the Alternate system pump to rebuild the pressure to something more similar to the Normal system pump.

 

4. After the Emergency Override Handle has been pushed back in, the system should return to cycling between 2550 - 3200 psi when not under load.

 

5. Should the Load Meter show a heavy load whenever the Alternate system pump is running.

 

1, 2, 4, 5 - we wil adjust bleed rate and other things.

 

3 - Alternate system pump output is correct now, it is indeed less powerful than Normal system pump

Edited December 16, 2015 by cofcorpse

[AH_Sid]

Many thanks cofcorpse

 

Great to know the Alternate system pump output is correct as it now, thanks for confirming this (although it still surprises me there is such a big difference).

Edited December 16, 2015 by =AH=_Sid

[AH_Sid]

Load Meter

All the above tests were carried out with the engine running, battery switch on, ground power on, and the generator selected on (but with the idle speed below 45% it shouldn't have had an output anyway).

 

 

At no stage did the Load Meter appear to show any load. I'm not sure of the circuitry with the ground power on (does the Load Meter show a load at this stage?). But even in flight, if the generator is turned off (switch on the right forward console) and the Emergency Override pulled (pump constantly running, connected directly to the battery), the Load Meter doesn't show this heavy load.

 

I've just been looking at this a bit more, it would appear the load meter is taken from the generator, so in my tests above where ground power is connected, it quite rightly shows a load of zero. The same with the airborne test with the generator turned off (switch on the right forward console) and the Emergency Override pulled (pump constantly running, connected directly to the battery), again the it quite rightly shows a load of zero.

 

 

 

The load meter is only going to show the load the generator is supplying.

 

So the load caused by the Alternate Hydraulic Pump is only going to show when Ground Power is disconnect, and the Generator is Online (Generator Light Out).

 

 

Of course the load will vary as the Alternate Hydraulic Pump switches on and off as it cycles.

 

Edited December 16, 2015 by =AH=_Sid

[AH_Sid]

I see this is all functioning correctly now in the latest public build (V2.0.1.49921), many thanks, I can now do a meaningful 'Flight Controls Hydraulic System Check' from the real manual as part of my pre-taxi checks now.

[Corrigan]

You're a machine, Sid. Good work.

[AH_Sid]

Credit goes to Belsimtek for doing the hard work fixing it, I just pointed out it wasn’t working ;)