Boundary Layer Control

[eyusuf]

Firstly I noticed too tips have disappeared with the latest update. Can anyone else confirm.

 

As for the 'sps' system I noticed it makes no difference whether its swithched on or off. It makes no difference to the handling characteristics at take-off and for generall low speed handling.

 

Can someone else please confirm this.

Edited September 25, 2014 by eyusuf

[Buzzles]

What throttle setting are you using? From what's been posted, it only works >50% throttle (LND setting if you're in the English pit).

[Corrigan]

Also flaps need to be fully deployed, right?

[Justin Case]

I tried it just a few minutes ago by flying level at 350 km/h and then turning the system off using the switch, I'm positive I saw the expected effects of turning it off and then on again.

[Fishbreath]

And you need to be going slow enough that aerodynamic pressure isn't pushing the flaps up (< 375kmh or so), if I read right.

[Joki]

I tried it just a few minutes ago by flying level at 350 km/h and then turning the system off using the switch, I'm positive I saw the expected effects of turning it off and then on again.

 

+1

 

Maintaining level flight using the autopilot at 350 km/h with flaps full down, the AoA rises by about 2 degrees when you flip the SPS switch down.

[Vita_CZ]

SPS works for me.

[eyusuf]

Well considering that boundary layer control is linked to engine power; and since engine power is stated to be below spec, I have to wonder if the linkage between the two is causing the sps to under-perform as well.

 

The sps and engine are linked after all.

 

Of course only the designers of the software will be able to answer this.

[Tango]

Depending on how far they modeled bleed air, then it might, though all that could happen is it simply require a higher power setting to compensate.

 

As it is, I find I'm around 90% N1 on final, but whether this is symptomatic of the other issues we are seeing I don't know.

 

This is why it would be really useful to know typical power settings, so it can be determined if there is a problem.

 

Best regards,

Tango.

[Joki]

Depending on how far they modeled bleed air, then it might, though all that could happen is it simply require a higher power setting to compensate.

 

As it is, I find I'm around 90% N1 on final, but whether this is symptomatic of the other issues we are seeing I don't know.

 

This is why it would be really useful to know typical power settings, so it can be determined if there is a problem.

 

Best regards,

Tango.

 

Tried flicking the SPS switch at different power settings today, using the SAU and keeping an eye the AoA. At N1 as low as 57 % there is a noticeable difference (the AoA increases by 2-2.5%) when you turn the switch off (down). At around 57 % the BLC seems to cut out even if the switch is in the on position. You can try moving the throttle back and forth between 55%-60% N1 while watching the AoA.

 

The test was done with full flaps below 350 km/h at altitudes 500-3000 m.

[Tango]

I think my problem is I do not approach slow enough (I approach around 380 kph.

 

Best regards,

Tango.

[Joki]

The SPS switch seems to make a difference all the way up to 450-460 km/h IAS, when full flaps can no longer be maintained (at sea level).

[effte]

I'd say there's unlikely to be a direct link to engine power (or lack thereof). It's linked to the compressor compression ratio over the stages preceding the bleed air ports used for the BLC system - entirely different from the thrust output of the engine.

 

If they actually model the pressure rise in the engine, there could be a link. More likely, it is merely a function of RPM and atmospheric conditions. You'd have to dive into the code to know.

 

I find it unlikely for the system to contain a valve for abruptly cutting the bleed air supply for the BLC as pressure drops. This means that you won't find a single RPM below which you have no BLC and above which the system is in full operation. The loss of functionality will be gradual, at an unknown rate, with reducing RPM. I would expect there to be a pressure regulator capping the output pressure so it will only increase with RPM up to a certain point though.

 

(Advisory for those who lack the ability of critical reading: The above contains assumptions.)

[Joki]

I'd say there's unlikely to be a direct link to engine power (or lack thereof). It's linked to the compressor compression ratio over the stages preceding the bleed air ports used for the BLC system - entirely different from the thrust output of the engine.

 

If they actually model the pressure rise in the engine, there could be a link. More likely, it is merely a function of RPM and atmospheric conditions. You'd have to dive into the code to know.

 

I find it unlikely for the system to contain a valve for abruptly cutting the bleed air supply for the BLC as pressure drops. This means that you won't find a single RPM below which you have no BLC and above which the system is in full operation. The loss of functionality will be gradual, at an unknown rate, with reducing RPM. I would expect there to be a pressure regulator capping the output pressure so it will only increase with RPM up to a certain point though.

 

(Advisory for those who lack the ability of critical reading: The above contains assumptions.)

 

Maybe they've just made a simplification when implementing it.

But maybe the BLC cuts out at a certain N1 to ensure that the engine gets enough air for safe operation? Just speculating. :)

[eyusuf]

Tried flicking the SPS switch at different power settings today, using the SAU and keeping an eye the AoA. At N1 as low as 57 % there is a noticeable difference (the AoA increases by 2-2.5%) when you turn the switch off (down). At around 57 % the BLC seems to cut out even if the switch is in the on position. You can try moving the throttle back and forth between 55%-60% N1 while watching the AoA.

 

The test was done with full flaps below 350 km/h at altitudes 500-3000 m.

 

Yes, but the question still stands unanswered.

 

Is boundary layer control linked to engine power. If it is (and in a detailed sim like this I would think it should be [it is cutting out at around 57% as you stated]) it would follows logically that BLC will also be under-performing. Perhaps a sim-coder could shed some light.

[Tango]

Is boundary layer control linked to engine power.

 

Fundamentally, yes. It takes bleed air from the engine, and reduces the thrust output as a result. This is why it is necessary to increase RPM slightly when it is in operation, to restore the pressure lost as the result of the additional flow.

 

What we can't ascertain is the minimum RPM to restore this pressure, and ensure the BLC has the required system pressure to operate.

 

The system does have a valve, which appears to be linked to the flap position. When the flaps are not in LANDING position (either due to airflow or the pilot) the BLC system is shutoff.

 

Best regards,

Tango.

[Joki]

Yes, but the question still stands unanswered.

 

Is boundary layer control linked to engine power. If it is (and in a detailed sim like this I would think it should be [it is cutting out at around 57% as you stated]) it would follows logically that BLC will also be under-performing. Perhaps a sim-coder could shed some light.

 

Alright then.

 

Well, since the AoA change corresponds to what the real flight manual says: "with the BLC system operating normally, the angle of attack by the YYA-1 indicator diminishes by 2 to 2.5 degrees as compared to the angle of attack at the same speed with the BLC system not used."

 

, I wouldn't say the BLC is under-performing, unless there is something else wrong with the relation between AoA and lift in the flight model.

 

EDIT: Of course that's just the relative change, it might be worse with BLC than it should be and so on...

Edited September 27, 2014 by Joki

[Joki]

Fundamentally, yes. It takes bleed air from the engine, and reduces the thrust output as a result. This is why it is necessary to increase RPM slightly when it is in operation, to restore the pressure lost as the result of the additional flow.

 

What we can't ascertain is the minimum RPM to restore this pressure, and ensure the BLC has the required system pressure to operate.

 

The system does have a valve, which appears to be linked to the flap position. When the flaps are not in LANDING position (either due to airflow or the pilot) the BLC system is shutoff.

 

Best regards,

Tango.

 

Also found this in the real flight manual:

 

"In the process of flareout, as the aircraft is nearing the ground, retarding the throttle to not below the BLC gate (which corresponds to the LP rotor rpm of 50 to 52%)"

 

Assuming LP means low pressure?

[effte]

Affirm on the LP. I'm curious about that gate myself. Perhaps that is something which should be simulated, as I take it to be a physical gate akin to a flight idle stop.

[Joki]

I think the sim models the BLC as such quite well. At least after what iv'e read in the real world flight manual (the one in english with the blue cover).

 

However it had something interesting to say about performing go-arounds with the BLC on:

 

"WARNING:

 

1. The aircraft must not be accelerated to the speed of 360 km/h at a low altitude, because automatic cutoff of the BLC system takes place at the speed, entailing aircraft sinking by 25-30 m and lateral oscillations.

 

2. Watch the speed when going around again with the BLC system on

 

3. Premature recovery of the aircraft from gliding may result in major loss of speed and altitude.

"

 

As it is now this is not modeled by the sim. Maybe it deserves a seperate report?

 

(Sorry for going on and on about this, but it really caught my interest.)

 

Affirm on the LP. I'm curious about that gate myself. Perhaps that is something which should be simulated, as I take it to be a physical gate akin to a flight idle stop.

 

Maybe. There's another "lever" like the one for idle cutoff on the frontside of the throttle. But that might also just be for the afterburner.

[Fishbreath]

Is boundary layer control linked to engine power. If it is (and in a detailed sim like this I would think it should be [it is cutting out at around 57% as you stated]) it would follows logically that BLC will also be under-performing. Perhaps a sim-coder could shed some light.

 

It doesn't follow logically, necessarily—the high altitude/cruise performance concerns are almost certainly not caused by a lack of engine power modeled across all altitudes, nor is it a certainly that the BLC power model is tied to the output thrust rather than the throttle setting, nor is it certain that the amount of air going to the BLC system rises and falls with the engine power setting in the first place.