J-20... The New Generation Fighter III

Quickie said:

The FBW has to know that the aircraft is taking off so that the lift of the aircraft can be maintained at the optimum value for take off, irrespective of the other forces caused by the undulating surface. The undulating forces will be gone by itself once the aircraft starts to leave the ground.

Click to expand...

Undulating forces are sensed through the use of Inertial Measurement Units. Specifically, linear motion is sensed by accelerometers while angular motion is sensed by gyros. These sensors cannot differentiate between forces generated by uneven ground and forces generated by aerodynamic disturbances.

One purpose of FBW is disturbance rejection, and tiny control surfaces movement while the aircraft is taxing is just the FBW doing what it is supposed to do. Undulating forces will still be present after the aircraft left the ground, but the sources of these forces will change.

Quickie said:

That was during takeoff for a number of seconds from the start position. I seem to remember all the control surfaces, excluding the canards, was moving at that moment. It could also be due to a particular takeoff condition on that day e.g. sudden cross wind, but this is inconsistent with the stabilizers moving intoe and outtoe repeatedly which I seem to remember. Can't remember where I saw that video.

Click to expand...

You are probably referring to the rudders then. Rudder pedals control both nose wheel steering and rudders, so rudders would move whenever the pilot uses the pedals for steering.

Engineer said:

Undulating forces are sensed through the use of Inertial Measurement Units. Specifically, linear motion is sensed by accelerometers while angular motion is sensed by gyros. These sensors cannot differentiate between forces generated by uneven ground and forces generated by aerodynamic disturbances.

One purpose of FBW is disturbance rejection, and tiny control surfaces movement while the aircraft is taxing is just the FBW doing what it is supposed to do. Undulating forces will still be present after the aircraft left the ground, but the sources of these forces will change.

Click to expand...

Those sensors won't be able to tell what actual forces are at play. That's why the FBW would need human input depending on the actual condition like whether it's a takeoff, a landing or cruise flight.

You are probably referring to the rudders then. Rudder pedals control both nose wheel steering and rudders, so rudders would move whenever the pilot uses the pedals for steering.

Click to expand...

For the J-20, rudder and stabilizer are the same thing. It was taking off, so the nose wheel should be pointing straight ahead.

Quickie said:

Those sensors won't be able to tell what actual forces are at play.

Click to expand...

Hence why I said FBW is responding to uneven ground. To the FBW, the disturbances caused by uneven runway is indistinguishable to aerodynamic disturbances.

Quickie said:

That's why the FBW would need human input depending on the actual condition like whether it's a takeoff, a landing or cruise flight.

Click to expand...

No! It doesn't work that way. FBW lessens the pilot's workload, not increases it. The FBW runs regardless of whether it's on the ground and in the air, hence you see movement of control surfaces in respond to uneven ground.

Quickie said:

For the J-20, rudder and stabilizer are the same thing. It was taking off, so the nose wheel should be pointing straight ahead.

Click to expand...

The aircraft goes straight ahead because the pilot keeps it aligned by steering, the same idea that a car remains on the road because the driver is actively steering it.

The automatic control system uses the measured air speed to calculate the control surface deflection necessary. So at low air speed these deflections will be large and very visible. But it might also feel that the undercarriage is rolling over the runway and therefore desist from these deflections. This refinement has apparently not been applied perhaps because a false signal that the aircraft is on the ground would lead to the loss of the aircraft.

What is the flame like thing? :

no_name said:

What is the flame like thing? :

Click to expand...

Like I already noted ...

Deino said:

Very nice !!! Did You notice at 0:30/0:31 the PU/JFS exhaust flame !

Click to expand...

Deino

delft said:

The automatic control system uses the measured air speed to calculate the control surface deflection necessary.

Click to expand...

Flight control system uses more than that. The basic inputs are up-down-sideway acceleration, angular rates in pitch-roll-yaw, angle-of-attack, side-slip angle, airspeed, static pressure, and position of the flight control surfaces.

delft said:

So at low air speed these deflections will be large and very visible. But it might also feel that the undercarriage is rolling over the runway and therefore desist from these deflections. This refinement has apparently not been applied perhaps because a false signal that the aircraft is on the ground would lead to the loss of the aircraft.

Click to expand...

Air data computer can detect the airspeed as being in single digit, but that does not mean the FBW uses that speed as input. There's no point in making a FBW to work that far below the stall speed of the aircraft. At such slow speed, the FBW is just going to work as if the plane is flying at stall speed.

Engineer said:

Hence why I said FBW is responding to uneven ground. To the FBW, the disturbances caused by uneven runway is indistinguishable to aerodynamic disturbances.


No! It doesn't work that way. FBW lessens the pilot's workload, not increases it. The FBW runs regardless of whether it's on the ground and in the air, hence you see movement of control surfaces in respond to uneven ground.

Click to expand...

No. You miss my point! I'm saying the FBW will behave differently during takeoff, cruise flight or landing and it's the job of the pilot to set the FBW to the task at hand.

During takeoff, for example, the FBW will maintain the elevon and leading edge slat at a set fixed position since this would give a more streamline flow with uniform lift and less drag, instead of varying the positions of the control surfaces to counteract the undulating movement and mess up the airflow and increase drag considerably. Of course, during a cruise flight, one would expect the FBW to respond differently to any undulating movements caused by changing air pressure for example. (Ok, time to go to sleep.)

Quickie said:

No. You miss my point! I'm saying the FBW will behave differently during takeoff, cruise flight or landing and it's the job of the pilot to set the FBW to the task at hand.

Click to expand...

I get your point completely, and I explained it doesn't work that way. FBW has to be transparent to the pilot, and your assumption that the pilot gets to select takeoff, cruise, and landing mode violates that transparency. How the FBW behaves at different stages of flight is the responsibility of the designers, not the pilot, and the designers have to work in the confine of sensor data that is available to the FBW.

Quickie said:

During takeoff, for example, the FBW will maintain the elevon and leading edge slat at a set fixed position since this would give a more streamline flow with uniform lift and less drag, instead of varying the positions of the control surfaces to counteract the undulating movement and mess up the airflow and increase drag considerably. Of course, during a cruise flight, one would expect the FBW to respond differently to any undulating movements caused by changing air pressure for example. (Ok, time to go to sleep.)

Click to expand...

It doesn't work that way. The purpose of FBW is to counteract disturbances. If that means the flight control surfaces move slightly during aircraft take off, then so be it. As for drag, its effects is negligible in comparison the rolling friction from landing gears.

Once again, the FBW system cannot differentiate between undulating movements caused by uneven runway surface and aerodynamic disturbances. Thus the FBW will correct disturbance regardless of origin, leading to the observed slight movement in control surfaces in respond uneven ground. This observation shows your idea of how FBW works is a flawed one, as if the FBW doesn't respond the disturbances due to uneven ground, we wouldn't have observed the movement in flight control surfaces such as canards.

Engineer said:

Flight control system uses more than that. The basic inputs are up-down-sideway acceleration, angular rates in pitch-roll-yaw, angle-of-attack, side-slip angle, airspeed, static pressure, and position of the flight control surfaces.


Air data computer can detect the airspeed as being in single digit, but that does not mean the FBW uses that speed as input. There's no point in making a FBW to work that far below the stall speed of the aircraft. At such slow speed, the FBW is just going to work as if the plane is flying at stall speed.

Click to expand...

Quite, still larger control surface deflections won't contribute anything.