No Su-27 auto trim, can it really be true?

[GGTharos]

It can, but it does not. Again, it was a philosophical choice :)

[21stCenturyPilot]

I hear you but maybe it is a soviet russia 'design philosophy' if you understand what I mean.

 

It is logical to decrease pilot workload and pilot fatigue wherever possible in fighter design as it increases combat effectiveness. No?

[Ironhand]

It's interesting.

I've looked at the manual and it does seem to me to have some conflicting info.

 

This is what it says in my DCS Su-27 manual.

The fly-by-wire system is a trajectory control system. In other words, if the control stick remains in the same position, the system will hold the set flight path. This is why the reduction of airspeed (for example) and resulting decreased lift and increasing angle of attack, will lead to the aircraft attempting to hold the initial flight path and prevent the aircraft from descending. This will result in the airspeed stability degradation up to neutral stability.

The longitudinal control system includes an airspeed trimming law that generates a signal proportional to the ram air. When the indicated airspeed is increased, the FCS causes the stabilizers deflection (up to 5 degrees) nose down. When decelerating, it causes the nose to come up. This imitates the airspeed stability of the aircraft, which is neutral in the presence of a g loading feedback signal. The airspeed stability imitation allows the pilot to use the control stick like he or she would with a stable aircraft.

I think you're referring to what is in the 2nd paragraph, especially. It's a bad translation. Where it says: "nose down" read "leading edge down". And read "leading edge up" for "nose up". Reading it that way makes sense in the context of what's in that and the following paragraphs about allowing the pilot to fly like he's flying "an airspeed-stable aircraft".

Edited March 20, 2015 by Ironhand

[GGTharos]

Pilots had a lot of input in this design. They had their reasons, and perhaps what you think is logical might not have been practical or may not have been a factor for them.

 

I hear you but maybe it is a soviet russia 'design philosophy' if you understand what I mean.

 

It is logical to decrease pilot workload and pilot fatigue wherever possible in fighter design as it increases combat effectiveness. No?

[esb77]

The proliferation of these Su-27 pitch trim/nose down inversion threads finally inspired me to try a flight with the FSC turned off. Starting with the taxi.

 

It took four tries to get a good takeoff and landing.

 

 

It feels sort of like skating on still wet Zambonied ice with skates that are a bit dull.

 

ANY input you give the plane will cause it to react with every bit of maneuverability you asked for. This includes things like operating gear, flaps, and the airbrake.

 

There are basically two rules the pilot needs to follow at all times.

 

1. Unless you wish to die, NEVER move the stick forward rapidly.

 

2. Do not over-control the plane.

 

Follow them, and refrain from giving the plane any new aerodynamic inputs, and it's not hard to reach equilibrium. Give it the slightest change in input though, and you find that the equilibrium is not at all stable.

 

It's also a very high pilot workload. I'm semi-tempted to do it some more though. As difficult as it is, it's also sort of like the plane is directly controlled as part of your body. It's no easier than learning how to walk or how to ice skate (there's a high probability of falling down a lot), but you get a feel for how the plane is flying and what its true capabilities are in a way that almost completely disappears when you turn the FCS back on.

 

Long story short, with the FCS on at least 3 pilots worth of workload is magically caused to vanish. The trim is mild, predictable, and allows you to get away with handling the stick like a staggering drunkard.

 

The weirdest thing was that with the FCS off I had to move the stick back surprisingly far to rotate on takeoff. It was very reluctant to get off the ground below 370 km/h. It makes life interesting, because it's easy to pitch up too much and stall (and crash and die), but once you come off the ground with a lot of back stick, you can't move it forward rapidly to recover (because you invert, stall, crash and die).

Su-27_FCS_off.trk

Edited March 21, 2015 by esb77