TIL the Su-27 has an AOA/G limiter override

[rami80]

I don't know how common this is, but if you hold the speed break(W) the plane's limiter goes off(I.e. you can pull the stick back fully) while keeping the FBW on.

[Stuge]

Yes, this is an important secret that should definitely be mentioned in the documentation. Almost seems like they don't want it to be found :D

[ekg]

What does this do exactly? I thought W is the wheel brake.

[SilentGun]

TIL the Su-27 has an AOA/G limiter override

 

What does this do exactly? I thought W is the wheel brake.

 

 

Yes but it allows you to pull more g's in a turn

[Bushmanni]

Yes but it allows you to pull more g's in a turn

 

It also allows more AOA.

[DarkFire]

I wonder if this is a bug or whether activating the wheel brake has an effect on the hydraulic control augmentation system that provides a bit more turning ability and slightly increases the AOA limit.

[ekg]

Is this the same as using the 's' key to disable fbw?

[DarkFire]

Is this the same as using the 's' key to disable fbw?

 

No. Disabling the FCS with the S key will give you direct control which does allow you to pull fantastically tight turns but at a G-level that would in reality cause catastrophic airframe failure.

 

What's being discussed here is that in a maximum-G turn, holding down the wheel brake button (W) will for some reason increase the G loading on the aircraft (by between 1 and 1.5 G) and will also relax the AOA limiter.

[Svend_Dellepude]

Probably a workaround for the way it works in the real thing.

IRL you just have to pull more to pull through the limiter.

[SinusoidDelta]

No. Disabling the FCS with the S key will give you direct control which does allow you to pull fantastically tight turns but at a G-level that would in reality cause catastrophic airframe failure.

 

What's being discussed here is that in a maximum-G turn, holding down the wheel brake button (W) will for some reason increase the G loading on the aircraft (by between 1 and 1.5 G) and will also relax the AOA limiter.

 

Well that's interesting...I'm assuming it's not documented anywhere. How much does it actually improve sustained turn rate?

[Stuge]

Well that's interesting...I'm assuming it's not documented anywhere. How much does it actually improve sustained turn rate?

 

Doesn't improve sustained turn rate at all, since the engines don't provide enough thrust for a sustained maximum G turn even without it. And such high sustained G level would result in blackout anyway :)

 

It does give a nice boost to instantaneous turn rate. As someone mentioned it is approximately 1 G extra. Very useful for an initial vertical turn after a merge. Tight, but doesn't bleed all your speed like the S button does ;)

[Svend_Dellepude]

Or let you pull 20+ G's?

[Stuge]

Or let you pull 20+ G's?

 

No, for 20G you need to hit the S button!

[DarkFire]

It would be interesting to see a hydraulic system schematic for the Su-27. I'm wondering if the wheel brakes are in fact on the same system as the flight controls and that by applying the wheel brakes you are in fact reducing the total effective volume of the hydraulic system and therefore slightly increasing the system pressure. That would explain the higher control surface deflection and the higher G load.

 

AFAIK in the real aircraft, applying a certain extra force to the control column (I vaguely remember it being either 9 or 19kg extra force) allows the pilot to exceed the normal AOA limiter.

[SinusoidDelta]

It would be interesting to see a hydraulic system schematic for the Su-27. I'm wondering if the wheel brakes are in fact on the same system as the flight controls and that by applying the wheel brakes you are in fact reducing the total effective volume of the hydraulic system and therefore slightly increasing the system pressure. That would explain the higher control surface deflection and the higher G load.

 

AFAIK in the real aircraft, applying a certain extra force to the control column (I vaguely remember it being either 9 or 19kg extra force) allows the pilot to exceed the normal AOA limiter.

 

I don't really follow your logic regarding the hydraulic system. I find it hard to believe a modeling error would result in a usable, helpful feature like this. It'd be really nice to hear from the developers if this function is intended and if so, what part of the FBW system it is simulating?

[DarkFire]

I don't really follow your logic regarding the hydraulic system. I find it hard to believe a modeling error would result in a usable, helpful feature like this. It'd be really nice to hear from the developers if this function is intended and if so, what part of the FBW system it is simulating?

 

My bad, I should have been clearer. I'm not certain that it is a modelling error, it may be intentional. I'm thinking that if activating the wheel brakes closes a valve then the effective volume of the rest of the hydraulic system would be lower than the total volume, therefore potentially providing increased hydraulic pressure in the rest of the system. This is purely conjecture on my part but I suppose it would explain the effect that's observed.

[ShuRugal]

I'm thinking that if activating the wheel brakes closes a valve then the effective volume of the rest of the hydraulic system would be lower than the total volume, therefore potentially providing increased hydraulic pressure in the rest of the system

 

not even a little.

 

operating the wheel brake -increases- the volume of the system because it extends the brake cylinders. However, the difference in volume would be negligible, even if each cylinder expanded 100cc, and there were 2 cylinders on each wheel, the system would only gain just over half a litre of volume.

 

This aside, you are mis-applying Boyle's Law:

For a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional.

First, and most importantly, hydraulic fluid does NOT behave as an ideal gas: it neither expands nor compresses with changes in pressure or temperature (within normal operating ranges). Even if it did, the amount of fluid in the active section of a hydraulic control system is not fixed: rather, a pump is used to move fluid into and out of the control cylinders, storing the excess in a non-pressurized reservoir. The pump and valves will also be designed to maintain a constant pressure in the working side of the loop.

 

The way a hydraulic control system applies Boyle's Law is to keep the pressure constant, and to control the volume by varying the quantity of fluid in the system.

[SinusoidDelta]

Great explanation ShuRugal! It may be worth mentioning accumulators are actually pressurized hydraulic reservoirs, basically energy storage. An example being the JFS on the F-15, no batteries needed! I think this is all beside the point though.

 

This isn't a study level sim but the systems or functions modeled must exist in real life. This is the reason trim reset was removed from the F-15, it doesn't exist IRL. Intentional or not, it's my opinion this needs to be addressed. Pressing W in a flanker actually alters the flight envelope and no one knows why.

[DarkFire]

not even a little.

 

operating the wheel brake -increases- the volume of the system because it extends the brake cylinders. However, the difference in volume would be negligible, even if each cylinder expanded 100cc, and there were 2 cylinders on each wheel, the system would only gain just over half a litre of volume.

 

This aside, you are mis-applying Boyle's Law: First, and most importantly, hydraulic fluid does NOT behave as an ideal gas: it neither expands nor compresses with changes in pressure or temperature (within normal operating ranges). Even if it did, the amount of fluid in the active section of a hydraulic control system is not fixed: rather, a pump is used to move fluid into and out of the control cylinders, storing the excess in a non-pressurized reservoir. The pump and valves will also be designed to maintain a constant pressure in the working side of the loop.

 

The way a hydraulic control system applies Boyle's Law is to keep the pressure constant, and to control the volume by varying the quantity of fluid in the system.

 

I agree that extension or retraction of a piston would probably be a negligible change in volume, but it may not merely be a piston. It's unfortunate that we don't know what sort of hydraulic system the wheel brakes on the Su-27 actually uses.

 

As for Boyle's law, I didn't apply that at all. As you noted Boyle's law is for ideal gases and does not take in to account phase changes or state changes, thus making it inapplicable to fluids of any sort.

 

What you want to use for fluid systems are the Navier-Stokes equations. As you noted hydraulic fluid is more or less incompressible at these sorts of temperatures and pressures, so under these conditions the deviatoric stress tensor can simply be expressed as a function of viscosity. Unfortunately there are no qualitative solutions for the incompressible-state Navier-Stokes equation, or at least none that apply to simple systems such as a pressurised hydraulics system, and without the exact dimensions of the system components and the characteristics of the hydraulic fluid, making any quantitative predictions is generally impossible.

 

Anyway, this is all largely academic. Hopefully the fact that applying the wheel brakes appears to affect control surface performance will be looked at.

[SinusoidDelta]

Wow this topic has really gone off the rails. I think it is confirmed hitting the brakes defeats the the stall inhibitor, increases instantaneous turn rate, makes no logical sense, is not documented, and puts flanker pilots unaware of this secret at a disadvantage.