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Old 03-26-2020, 10:41 PM   #1
vstolmech513
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Default Manual Fuel/Water Injection

When you select Man Fuel and put the water switch into either LDG or T/O, you should NOT get a rise in RPM. The whole reason behind this is by selecting MAN FUEL, you are taking the jet out of Electronic Fuel Control (EFC) which is controlled by the DECUs and the position of the throttle tells the FMU (Fuel Metering Unit) how much fuel to put into the combustion chamber.



Currently, if you select MAN FUEL and put the switch into either T/O or LDG you will get the normal rise in RPM.
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Old 03-27-2020, 01:53 PM   #2
Captain Orso
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Is this documented in NATOPS or somewhere?

I'm not sure how this should work otherwise, because if the DECU is basically setting rmps from idle to full throttle to X(rpm) at idle and Y(rpm) at full (depending on engine temps etc.), if turning water injection on raises Y(rpm) to +Z, the most logical results would be to scale the throttle from idle from X(rpm) to Y(rpm)+Z.

What it currently does in the RAZBAM Harrier is to add Z to the entire throttle setting range, so that even at idle there is a jump in actual rpm, when from my understanding it should be analog to how far the throttle is pushed forward.

In other words, when idling on the Tarawa, getting ready to takeoff, when you flip H2O to T/O you actually shouldn't have an rpm jump, but after takeoff when you have reached flight speed and you back off the throttle and turn H2O off, you should have a noticeable rpm drop, depending on throttle settings.
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Old 03-27-2020, 07:19 PM   #3
vstolmech513
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Default DECS Limiting

Quote:
Originally Posted by Captain Orso View Post
Is this documented in NATOPS or somewhere?
All of my information comes from the A1-AV8BD-290-000/100/200/300/310/320 series as well as current and former NATOPS manuals that unfortunately I cannot post due to forum rules. I will however shoot you a PM.

Quote:
Originally Posted by Captain Orso View Post
I'm not sure how this should work otherwise, because if the DECU is basically setting rmps from idle to full throttle to X(rpm) at idle and Y(rpm) at full (depending on engine temps etc.), if turning water injection on raises Y(rpm) to +Z, the most logical results would be to scale the throttle from idle from X(rpm) to Y(rpm)+Z.

What it currently does in the RAZBAM Harrier is to add Z to the entire throttle setting range, so that even at idle there is a jump in actual rpm, when from my understanding it should be analog to how far the throttle is pushed forward.

In other words, when idling on the Tarawa, getting ready to takeoff, when you flip H2O to T/O you actually shouldn't have an rpm jump, but after takeoff when you have reached flight speed and you back off the throttle and turn H2O off, you should have a noticeable rpm drop, depending on throttle settings.
This is definitely one of the more advanced systems on the Harrier and it takes many years to understand it fully, let alone being able to test and troubleshoot it accordingly. I will try my best to summarize this system with the basics and not get too far down the rabbit hole:

The electronic fuel control (EFC) system operates by electronic control of the DECUs. Only one DECU is 'in control' at a time and it is completely random and the pilot never knows which DECU 'lane' s/he is in, unless they physically switch the lane from 1 to 2, or 2 to 1. Now both DECUs have a Datum plug inserted into them that tells the DECUs the limitations of each of the four datums. There is Short Lift Wet (SLW), Short Lift Dry (SLD), Max Thrust and Combat. The datums are determined by physical aircraft configuration including JPTL switch position, gear position, nozzle position, water injection switch position, air data computer (ADC) airspeed, and combat mode switch position.

With gear down, or nozzles greater than 16°, the short lift wet or dry datum is selected depending on the water arming switch position, thereby giving the pilot 113.5% RPM and 780°C JPT dry limits or 120% RPM and 800°C wet (currently modeled with 116.8% corrected fan speed max RPM limitation, although this is not correct). Max time limit is 15 seconds, should hear an audible "15 seconds, 15 seconds" from Betty if time limit is reached.

With gear up, nozzles aft (<16°), or airspeed >250 knots, the maximum thrust datum is reached with max RPM being 109% RPM and 710°C JPT. Max time limit is 15 minutes of flying around at 109%. No audible warning for exceeding this limit but could cause engine to fail.


Combat mode is achieved by being in maximum thrust datum (With gear up, nozzles aft (<16°), or airspeed >250 knots) and the pilot pushing the combat enable 'switch' or more appropriately named 'push button'. The combat datum gives the pilot up to 111% and 750°C JPT for 10 minutes. No audible warning for exceeding this limit but could cause engine to fail.


Easy stuff, right? So now that we know these 'limits' what does the jet do with them? Well that's where the term limiter comes from.

Quote:
Originally Posted by Captain Orso View Post
I'm not sure how this should work otherwise, because if the DECU is basically setting rmps from idle to full throttle...

No, the DECUs do not set RPMs from idle to full throttle. There is another component called the Pilot Lever Angle Assembly/Unit (PLAA/PLAU) that actually tells the DECUs where the PILOT wants the RPMs at in terms of fuel flow. The PLAA turns mechanical linkages into an electronic signal that the DECU can interpret as how much fuel should the Fuel Metering Unit (FMU) send to the engine. The DECUs see what the pilot wants and gives him what he can get based off of those limits we talked about earlier. So, if the pilot wants as much fuel as he can get, i.e. full throttle, the DECUs will say sure, but only until the RPMs get to their limit or after a bit if the JPT gets hot enough, it limits the fuel and thereby causes the JPT to be reduced along with RPM. You can tell when you are near the limiter as you will see a Power Hex in the HUD with each side of the Hex being either 1% RPM or 10° JPT, with the final side going up and diagonally across the HUD from low left to high right.

Hope this makes sense.
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Old 03-30-2020, 05:37 PM   #4
Shrike88
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Great find Vstol. Unfortunately the devs lack of participation on the forums is never really a surprise. Please PM decoy and hopefully this will get passed to Zeus to fix
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