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Old 07-20-2019, 09:27 PM   #11
m4ti140
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Quote:
Originally Posted by Dr_Arrow View Post
I was referring to a speed around 80% of the N2 (hi pressure turbo-compressor) rotational speed of the engine, this is where the SPS detent is (at 50% throttle travel). SPS system needs 2.5 kg/s of air to operate.

The detent is NOT at 50% throttle travel, it's around 20%. Also please tell me, how do you even put a turbojet engine in a steady state where the HPC is at 80% and LPC is at 50%?
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Old 07-21-2019, 09:21 AM   #12
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Originally Posted by m4ti140 View Post
The detent is NOT at 50% throttle travel, it's around 20%. Also please tell me, how do you even put a turbojet engine in a steady state where the HPC is at 80% and LPC is at 50%?
It is an old engine, where LPC is NOT controlled by the throttle! - I did not write anything about LPC revs as they will be dependent on N2 and outer conditions (temperature, velocity, pressure, humidity, etc). At ground they and zero velocity N1 will be at around 75% with N2 at 80%. SPS detent is NOT at 20%, there is the are of "Maly Gaz" - idle, it is in the range around 40-50%, and this is how it is correctly modeled in DCS.
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Old 07-21-2019, 06:20 PM   #13
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Quote:
Originally Posted by Dr_Arrow View Post
It is an old engine, where LPC is NOT controlled by the throttle!
Of course it isn't, where did anyone say it does? And what does it even have to do with the age of the engine? Or with the topic? The throttle controls the fuel flow to the combustion chamber, which controls the rate of combustion, which influences the mass flow rate through the engine, which influences the RPM of both HPT and LPT, and by extension HPC and LPC. The reason RPM is used as a reference is because it's the most convenient quantity to measure and other quantities like the mass flow rate, the thrust etc. can be calculated if necessary. And the relationship, in particular, between the RPM on either stage and the flow rate is rather close - much closer in fact than between the throttle and the RPM.

The engineers who wrote the flight manual calculated (or more likely checked experimentally) that the required flow rate for SPS operation occurs at a minimum of 50-52% of N1 RPM (notice it's a range of values, it can fail anywhere between 50 and 52 depending on other conditions). Why did they choose the LPC as a reference if the bleed air valve is past HPC? I don't know, ask the Mikoyan engineers responsible. Maybe the interval was smaller. It doesn't matter from pilot's perspective, all they need to know is that if they don't retard the throttle past the detent and the N1 RPM is above 52% everything will be fine.


Quote:
Originally Posted by Dr_Arrow View Post
I did not write anything about LPC revs as they will be dependent on N2 and outer conditions (temperature, velocity, pressure, humidity, etc).

I wrote about it, because that's what the flight manual uses as a reference rather than N2. Nowhere does it state you need 80% N2.


Quote:
Originally Posted by Dr_Arrow View Post
At ground they and zero velocity N1 will be at around 75% with N2 at 80%. SPS detent is NOT at 20%, there is the are of "Maly Gaz" - idle, it is in the range around 40-50%, and this is how it is correctly modeled in DCS.
Here's the throttle resting on М. ГАЗ. detent (which is lifted with a separate lever aft of the throttle). The physical throttle is barely ~5%-10% forward. Take note of the aft edge of the throttle lever as that's your physical constraint - it won't go any further than the point where the aft edge hits the throttle quadrant frame - that's the Stop detent. The axis position of this setting in DCS is 0%.



Which corresponds to approximately 35% N1 and 51% N2



This is the throttle moved partially through the СПС detent. I circled it for easier reference. When advancing the throttle, the stop slides over the detent, when retarding the throttle the paddle (also circled in pressed in position) needs to be pressed to lift the stop, to prevent accidental retraction past the detent during flare and subsequent loss of lift due to disengagement of SPS. Now of course I could be wrong and this could not be the SPS detent, in which case 1. Where is the SPS detent mentioned in the manual then? 2. What is this and what purpose does it even serve?



Here is the throttle resting on the СПС detent. The axis reading is below 10%, physical position is 15-20%.



Which corresponds to ~40% N1 and ~55% N2. Way too low, but 1. We're stationary - I will later check it in flight. 2. The military detent isn't synced either - afterburner engages when the throttle starts moving into second stage (progressive detent, it gradually increases resistance rather than stopping the throttle) while the max detent (opposite one to the SPS, it requires lifting the stop to advance the throttle past it) is at around 85-90% RPM (~60% axis) - which means that the animations might just not be synced at all, which isn't a huge issue to be fair - as long as we're given some way to know whether SPS is running. There doesn't seem to be any, other than the assurance that it runs when N1 is at 52% - the aircraft doesn't feature one in reality, there were plans to retrofit it, but they never got around to it. Still, some debug message would proof useful.



Here is what happens when you stabilize the engine at 50% N1 on the ground. The N2 RPM is at 62%, the throttle axis is at 25%. At 52% N1 the N2 RPM is at ~63-64%. This is all static, I will later check readings in flight.





For reference - this is what the throttle looks like for the above



Here's 80% N2. The N1 RPM stabilized at 75%, just as you said. This corresponds to 55-60% throttle. The physical animation is past SPS marker (they don't correspond to detent locations, they're orientational). Again though, the animations are not synced and the conditions we checked it in might be different.



Now, I don't know where you got the value from, but the flight manual for MiG-21bis with R-25-300 engine states 50-52% N1. Not 75%. Not 80% N2.

Last edited by m4ti140; 07-21-2019 at 06:24 PM.
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Old 07-21-2019, 09:01 PM   #14
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m4ti140: nice analysis, thank you, you're right. If SPS operates from 50% N1 for R-25 engine then it indeed corresponds to around 20-25% of throttle travel dependent on approach speed.
I was under false assumption that it operates above 80% N2 as previously written by others in the thread, sorry.
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