SPS switch / system does nothing

[bkthunder]

When landing with full flaps, engine rpm >80%, turning SPS on/off doesn't resutl in any change in lift or rate of descent, so either the switch doesn't work or the SPS system is not simulated (it was definitely working in some earlier versions).

 

Another thing, with throttle <80% amd SPS on, there should be unpredictable wing drops because the bleed air being directed to the flaps doesn't have a constant flow. This is also not simulated but it used to be in earlier versions of the FM (form some years ago).

[bkthunder]

BUMP!

[-Rudel-]

Where are the log, track, flight data files at?

[Hiromachi]

I'm not sure if I understand you correctly, but SPS operation is limited around 50 % of the RPM. According to Polish manual, during landing, throttle should not be pulled back to less than 50 - 52 % of its range due to SPS limiter. So regarding 2nd point, the 80 % range is not something I can relate to anything in manuals I have.

[bkthunder]

I'm not sure if I understand you correctly, but SPS operation is limited around 50 % of the RPM. According to Polish manual, during landing, throttle should not be pulled back to less than 50 - 52 % of its range due to SPS limiter. So regarding 2nd point, the 80 % range is not something I can relate to anything in manuals I have.

 

Not sure about the 5%0 vs 80%, years ago in your very module, it used to be 80%. Given the manual provided in the module is not very detailed, the description of this system is not giving any such details.

 

Regardless of that, let's go back to my OP and focus on the main point:

 

turning SPS on/off doesn't result in any change in lift or rate of descent, so either the switch doesn't work or the SPS system is not simulated

 

Let's make the following assumptions:

 

1. We all agree that at >80% RPM and below 360 Km/h IAS, the Boundary Layer Control system should be active. In my test I was within these parameters.

 

2. The SPS (BLC) system should provide extra lift, which for a given IAS, flaps setting and RPM setting, results in a rate of descent that is lower compared to what it would be, if the SPS system was not active.

 

 

Now, what I reported in the OP is this:

 

if I switch the SPS off, by clicking with the mouse and placing the "SPS" switch on the left wall panel in the "OFF" position, there is abosultely no change in rate of descent compared to when the switch is placed in the "ON" position.

 

Ergo, either the switch is not actually turning the SPS off, or the SPS system is not simulated.

 

Which of these two possibilities is the one at hand, I don't know, so I am simply reporting that the SPS system isn't behaving according to what would seem logical AND according to how it used to operate in previous versions of your product.

[Hiromachi]

Alright Bkthunder, I finally had some time to sit and try it out.

I tried to replicate your parameters, all in landing configuration and in my experience there is a small difference in rate of descent, which however can be compensated (in SPS off situation) with about 5-10 % throttle advancement. Based on the manual there should also be about a 2 deg AoA decrease noticeable on the UUA but I couldnt observe that. However I could feel the greater tendency to fall (not sure if I'm using correct word here, but I hope you can follow).

I'll try to search for better references for that.

 

As for the %, well if it was 80 % than based on the manuals I have it is not correct, as system limitations are fairly well described in original document, thank you for reporting.

[bkthunder]

Thanks Hiromachi, I didn't notice a difference as you reported, but I'll try it again.

As you said, no AoA difference.

 

I hope you can improve it! :thumbup:

[Dr_Arrow]

As for the %, well if it was 80 % than based on the manuals I have it is not correct, as system limitations are fairly well described in original document, thank you for reporting.

 

I think that your documents are correct, you are probably only confusing throttle lever travel and engine's RPM, i.e. it can mean that at 50-52% of the throttle lever travel you get 80% N2 engine RPM. So you should not move the TL lower than 50-52% (there is a SPS detent) of its travel as it will cause the engine's RPM to drop below 80% and SPS system will shut down.

[m4ti140]

I'm not sure if I understand you correctly, but SPS operation is limited around 50 % of the RPM. According to Polish manual, during landing, throttle should not be pulled back to less than 50 - 52 % of its range due to SPS limiter. So regarding 2nd point, the 80 % range is not something I can relate to anything in manuals I have.

 

It's not "50-52%", 50% of throttle travel does not correspond to 50% RPM, since when does 0% of throttle axis coreespond to 0% RPM? It's not 1:1, it's not linear even. There's an "СПС" position on the throttle with a physical detent that you can see next to the throttle (and which is described in said manual, I assume that you refer to the izd. 75A manual provided by the Polish Aviation Museum in Kraków), you've even animated it. And it doesn't seem to correspond to the actual SPS cutoff setting anymore - though it should, because it's the only reliable indication of whether it's on or off. If it should correspond to 50% RPM, then the throttle should be calibrated so that this detent corresponds to 50% RPM. For reference, the detent is at around 10-20% of throttle travel.

 

 

As for the AoA - the manual says there's an increase of 2-2.5 degrees for the same airspeed (and vertical speed I presume, need to look into it again) - it won't pitch your nose because you're trimmed for that AoA, you will get that nubmer of 2deg once you reestablish on glideslope/level flight at same speed you had before you switched the system on/off.

 

 

 

I think that your documents are correct, you are probably only confusing throttle lever travel and engine's RPM, i.e. it can mean that at 50-52% of the throttle lever travel you get 80% N2 engine RPM. So you should not move the TL lower than 50-52% (there is a SPS detent) of its travel as it will cause the engine's RPM to drop below 80% and SPS system will shut down.

 

 

@Dr_Arrow 80% is even more ridiculous. As I said, look at the throttle quadrant, there's a physical detent to the left of the throttle lever that limits its operation to the range enabling SPS use until a paddle is pressed to lift it. Move your PoV in cockpit and pay attention to the throttle mechanization.

Edited July 20, 2019 by m4ti140

[Dr_Arrow]

 

@Dr_Arrow 80% is even more ridiculous. As I said, look at the throttle quadrant, there's a physical detent to the left of the throttle lever that limits its operation to the range enabling SPS use until a paddle is pressed to lift it. Move your PoV in cockpit and pay attention to the throttle mechanization.

 

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.

[m4ti140]

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%?

[Dr_Arrow]

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.

[m4ti140]

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.

 

 

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.

 

 

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.

Edited July 21, 2019 by m4ti140

[Dr_Arrow]

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.