What's with the slow stabilizer lately?

Since the last few versions of DCS the Su-27 stabilizer moves too slowly in pitch. I timed it with the aircraft on ground and a full nose up deflection (that is a 20deg angle) takes 1.1s. This means the available rate is about 18deg/s. I don't believe for a second this is realistic :D

I searched youtube and found several adequate videos and timed how long it takes IRL. Concluded that a full nose up deflection of the stabilizer takes about 0.65-0.7s. This means the available rate is about 30deg/s. So in simulator we have an available rate that is more than 1.5 times slower than IRL.

This slowness you can really feel in flight. When the rate was reasonably accurate (a few months ago, I can't say precisely) the pitch "nimbleness" was already pretty mediocre due to size and weight of the aircraft. Now it is rather poor and I think stabilizer slowness in pitch is the cause.

Quick question. How did you manage to time it that accurately?

There is a damper in the stick that limits the rate at which you can pull. Hold the wheel brake to override it.

In the air, the control system severely clamps pitch and G onset rate to prevent overshooting angle of attack or G limits.

 

I searched youtube and found several adequate videos and timed how long it takes IRL. Concluded that a full nose up deflection of the stabilizer takes about 0.65-0.7s. This means the available rate is about 30deg/s. So in simulator we have an available rate that is more than 1.5 times slower than IRL...

That 1/2 to 3/4 sec sounds about right for when either the landing gear are extended or the aircraft is in direct control mode. Was that the case? Just curious about the situation at the time of measurement.

Quick question. How did you manage to time it that accurately?

I made the timing with time acceleration set to 1/16, then divided the result by 16.

That 1/2 to 3/4 sec sounds about right for when either the landing gear are extended or the aircraft is in direct control mode. Was that the case? Just curious about the situation at the time of measurement.

Measurement was made with aircraft on runway just before takeoff, not in direct control mode.

There is a damper in the stick that limits the rate at which you can pull. Hold the wheel brake to override it.

 

In the air, the control system severely clamps pitch and G onset rate to prevent overshooting angle of attack or G limits.

In this video at 51s time the pilot performs a final flight controls check on the runway just before takeoff (as RL manual recommends). He does a full nose up deflection of the stabilizer, and such a deflection rate is certainly impossible in current DCS version. Obviously the flight control system was in "takeoff-landing" mode. The AOA/G limiter works only in "flight" FCS mode (IRL and in simulator). Switching between "takeoff-landing" and "flight" mode is done with the landing gear handle. So the AOA/G limiter doesn't work with gear down and does not limit the stabilator rate in any way in the test I done with aircraft on the runway.

Other videos where stabilizer deflection rate in pitch is visible:

Other videos where stabilizer deflection rate in pitch is visible:

 

Is this what you're referring to? Real time:

Wheel brakes on.

^^^ Yes, with W key pressed the stabilizer moves fast. By pressing W we tell the simulator that we are applying an additional 15Kgf to the stick to overpower the AOA/G limiter.

The problem is in the RL videos the stabilizer can be moved in pitch at a rate that is not available in simulator with W unpressed, but it should. The pilot in video links on this page moves the stabilizer at a faster rate than what is possible in DCS without overpowering the limiter because the limiter is OFF anyway due to gear down. He also doesn't press the W key because in Cyrillic alphabet there is no W letter :)

^^^ Yes, with W key pressed the stabilizer moves fast. By pressing W we tell the simulator that we are applying an additional 15Kgf to the stick to overpower the AOA/G limiter.

 

The problem is in the RL videos the stabilizer can be moved in pitch at a rate that is not available in simulator with W unpressed, but it should. The pilot in video links on this page moves the stabilizer at a faster rate than what is possible in DCS without overpowering the limiter because the limiter is OFF anyway due to gear down. He also doesn't press the W key because in Cyrillic alphabet there is no W letter :)

It's my understanding that, with the gear extended, only the G-limiter is off. But I could certainly be wrong about that. I can't claim to truly understand all the finer points of the system. But, anyway, the range of motion isn't limited with gear extended. You're complaint is with the speed of motion.

It's my understanding that, with the gear extended, only the G-limiter is off. But I could certainly be wrong about that. I can't claim to truly understand all the finer points of the system.

From real flight manual, the limiter works only in FCS mode "flight". With gear down the FCS is in "takeoff-landing" mode. The limiter is a stick motion restrictor using inputs of AOA, G, AOA rate, G rate, pitch rate and whatever. It's not like the limiter has 2 parts for G and AOA and one or the other works. It is a single stick motion restrictor that works to limit the AOA or G, whichever happens to be above a certain value.

And even if the limiter worked with gear down too, it needs AOA or G input to be above a certain value. With aircraft on the runway I am giving it an input of 1G and zero deg AOA. In such a situation the limiter does not restrict or slow the stick motion in any way.

But, anyway, the range of motion isn't limited with gear extended. You're complaint is with the speed of motion.

Yes!

I've asked Yo-Yo to look in on this thread, if he has time. He'll be able to clarify the issue.

In game, it seems that the FCS is even MORE restrictive in AoA with gear down.

Don't you think the pilot in the video would have the wheel brakes on?

(Forgive me if i'm missing something)

IRL the pilot doesn't activate the wheelbrakes to override the limiter, it's a DCS feature. IRL the pilot override using brute force on the flightstick.

IRL the pilot doesn't activate the wheelbrakes to override the limiter, it's a DCS feature. IRL the pilot override using brute force on the flightstick.

Oh, thank you, and all this time I thought it was real :doh:

Since the last few versions of DCS the Su-27 stabilizer moves too slowly in pitch. I timed it with the aircraft on ground and a full nose up deflection (that is a 20deg angle) takes 1.1s. This means the available rate is about 18deg/s. I don't believe for a second this is realistic :D

 

I searched youtube and found several adequate videos and timed how long it takes IRL. Concluded that a full nose up deflection of the stabilizer takes about 0.65-0.7s. This means the available rate is about 30deg/s. So in simulator we have an available rate that is more than 1.5 times slower than IRL.

 

This slowness you can really feel in flight. When the rate was reasonably accurate (a few months ago, I can't say precisely) the pitch "nimbleness" was already pretty mediocre due to size and weight of the aircraft. Now it is rather poor and I think stabilizer slowness in pitch is the cause.

What were the conditions of the test? By that I mean did you measure the rotation rate immediately following a cold start, or after a hot start, or under other conditions? I ask because I still think there's something going on with the hydraulic systems for a couple of minutes immediately after engine start

Immediately after engine start the controls can be slow to respond, particularly mechanical device actuators such as the flaps and speed brake. Increasing RPM for a couple of seconds, e.g. to start a taxi, appears to normalise hydraulic function. It's almost like simulation of pressure build-up in the hydraulic system following engine start. I don't know if the Su-27 being an FC3 plane actually models that, but empirically the effect is there.

The limiter seems to be slowing down the pitch rate of the flightstick while sitting on the rwy.

I think that's the issue at hand here. If you override by holding the wheelbrake there seems to be no problem.

Dunno if the limiter should be active while sitting on the rwy IRL.

Anyway, it's probably not much of an issue since it engages when you takeoff anyway.

What were the conditions of the test?

Test was done selecting "Takeoff from runway" from ME.

Dunno if the limiter should be active while sitting on the rwy IRL.

Please read post #11.

From real flight manual, the limiter works only in FCS mode "flight". With gear down the FCS is in "takeoff-landing" mode. The limiter is a stick motion restrictor using inputs of AOA, G, AOA rate, G rate, pitch rate and whatever. It's not like the limiter has 2 parts for G and AOA and one or the other works. It is a single stick motion restrictor that works to limit the AOA or G, whichever happens to be above a certain value.

 

And even if the limiter worked with gear down too, it needs AOA or G input to be above a certain value. With aircraft on the runway I am giving it an input of 1G and zero deg AOA. In such a situation the limiter does not restrict or slow the stick motion in any way.

 

 

 

Yes!

So I've been perusing the flight manual. Isn't the "W" key mimicking the use of the AoA/G limiter switch on the left hand panel? This is not the famed "S" key.

Edit: I had always been under the impression that the "W" key in this instance was mimicking an actual switch.

There is as well indication bars behind the stick, one for each hydraulic channel and also the wheelbrake(s)

Test was done selecting "Takeoff from runway" from ME.

Hmm you should have been good to go in that case. The only thing effecting the control surface rate in that case would be the fact that with wheels down the ACS would be in takeoff / landing mode rather than flight mode.

As far as I'm aware the "W" key (as well as working the wheel brakes) simulates the AOA / G limiter disable / enable switch which is part of the ACS in the real Su-27.

So, the question then is whether the ACS operates with different rate limits and/or different maximum deflections in 'takeoff / landing' mode than it does in 'flight' mode. Pretty sure that it does. I don't read Russian so can't comment on the actual Su-27S manual, but empirically this would make sense in order to provide best stability under low / slow / higher-AOA conditions (takeoff or landing).

Isn't the "W" key mimicking the use of the AoA/G limiter switch on the left hand panel?

I don't think it is. If it was, you would have to only tap W key and the limiter would be switched off. Probably would also be an animation of the switch on the left panel. But instead you have to hold W key. Since overpowering the limiter is the only other way to exceed limits, this must be what holding W does.

So, the question then is whether the ACS operates with different rate limits and/or different maximum deflections in 'takeoff / landing' mode than it does in 'flight' mode. Pretty sure that it does.

Whether it does or does not, the fact remains that the stabilizer in simulator moves too slowly. I posted 3 videos in this thread with Su-27 aircraft just before takeoff where a stabilizer deflection rate more than 1.5 time faster than the one available in the sim is observed.

In simulator the available rate is 18 deg/s (see first post). Visually I can tell you right away this is a ridiculously slow rate for any fighter aircraft from any era.

MiG-29 stabilizer has an available deflection rate of 38deg/s (source, MiG-29 practical aerodynamics manual). That is more than 2 times the rate available for Su-27 in simulator. Besides being a larger and heavier aircraft than MiG-29, Su-27 also has relaxed longitudinal static stability. Fast stabilizer deflection rate is very important, if not vital. I would expect a deflection rate for Su-27 at least equal to that of MiG-29. It's engineering common sense. I timed the Su-27 deflection rate from videos and came to 30deg/s (post #1). But that is not necessarily the max rate available for the real aircraft. A pilot making a final check of the controls right before takeoff on the runway, probably does not very often check pitch control by pulling the stick fully as fast as he can.

As I have already explained in this thread, the rate available in the test I performed on runway before takeoff has nothing to do with the AOA/G limiter (post #11). I repeat that even if the limiter was working in the test discussed here, the limiter is a stick stop and not some sort of a "stick damper" that slows the stick along its entire motion range. The fact that on the runway deflection test holding W key suddenly you have a rate of more than 40deg/s available does not make any sense IMO. What would make sense is if such a rate would be available without holding W key.

It doesn't matter how the things move on the ground because they move according to a whole set of rules in flight. You would destroy the jet in real life if they moved full deflection just because you move the stick full deflection.

Flankers strong airframe holds together two extremely powerful engines (in its class) and thus has lot of hydraulic power available from it's sources. Max angular speed (rate of elevator rotation) is constant and only diminished by hydraulic-aerodynamic pressure battle results on elevator surfaces. Wont broke off either in case of over-deflection because it has been balanced in wind tunnel tests to feather itself on no input and also must sustain very high loads for a very long time. Elevator axle is last to broke off from plane (same as undercarriage) so when looking all aspects of plane systems Flanker should deflect its surfaces safe and crazy - in flight.