'Destroyed in Seconds': X-31

[Simpo Two]

Just watched a recording of 'Destroyed in Seconds' and found the clip on Youtube. The speed sensor on the fly-by-wire aircraft ices up (cos somebody forgot to plug the heater in) and the computers do the rest...

No manual override of course... when will they learn?

Slightly spooky seeing the computers trying to recover the (unmanned) aircraft on the way down.

www.youtube.com/watch?v=817w5ps7HwI



(Reminded me of the Air France affair)

[Pints]

Incredible the way it's able to turn and seemingly stall in mid-flight.

A few years ago that sort of flight would have only been attributed to something like a UFO.

[anonymous-user]

Manual Override?? what use exactly would that be for a plane that is unstable in at least one axis?

(i.e. if the computer can't fly it, neither can you...... ;-)


There are a fair number of early crashes of both prototypes and in-service military FBW aircraft of all types put down to air data errors (blocked pitot's, wrongly calibrated gyro's etc) Even human errors like connecting the PFC's backwards (for example A-12 #929)


More modern aivionics are more robust to such basic errors, with enough computaitona power built in for redundancy, selfchecking, and alternate flight laws to be implemented etc. Still occsionally get it wrong however:

http://www.youtube.com/watch?gl=GB&hl=en-GB&am...


ooops.....

[Manicminer]

Pints said:

Incredible the way it's able to turn and seemingly stall in mid-flight.

A few years ago that sort of flight would have only been attributed to something like a UFO.

It first flew more than 2 decades ago! Incredible considering the technology available at the time.

[Simpo Two]

Max_Torque said:

Manual Override?? what use exactly would that be for a plane that is unstable in at least one axis?

(i.e. if the computer can't fly it, neither can you...... ;-)

Point is, the computer was receiving faulty airspeed data due to icing so it thought the aeroplane was doing, say, 50kts when it was doing 500kts - which caused it to panic somewhat. The machine itself was (as I interpret it) perfectly flyable. So you ping the switch to 'off' and fly the bd down by hand... gently...!

Edited by Simpo Two on Sunday 16th December 19:41

[Manicminer]

Wasn't this the first of the new breed that are basically so unstable that they are impossible to fly manually?

Thus making them extremely manoeuvrable.

[Simpo Two]

I know the computer allows you to do things which were otherwise impossible, but is 'straight and level' impossible too? I'm not sure which is worse, being out of control or having a computer that's trying to get you into orbit!

But there has to be a way to intervene between 'forgot to plug the thingy in' and 'smoking hole in the ground' (other than an ejector seat)...

[anonymous-user]

Simpo Two said:

I know the computer allows you to do things which were otherwise impossible, but is 'straight and level' impossible too?

The aerodynamics of a plane that is deliberately designed to be "unstable" in one or more axes are complicated to say the least. An airframe with conventional stability will experience a "negative feedback" from any pilot input, that acts to neutralise that input. For example, pull back on the stick, the nose rises, the AOA increases, the airframe climbs. Centre the stick, and the rate of climb is maintained wihtout any further pilot input. For an airframe that is say unstable (not critically damped) in pitch, the same input causes the airframe to continue to pitch up, because the gain of the system is positive not negative. To prevent the airframe from doing a "backflip" the stick must be pushed forwards to halt the nose up rotation. This makes it virtually impossible to fly by a human, because it has rate gains so high that control surfaces must be controlled with a very high bandwidth (hundreds of movements per second).

Imagine trying to drive a car, where to turn left you had to turn left, then immediately turn right to stop the car continuing to turn left! Then, take this and add in another 2 axes of instability. Even "straight and level" flight is just about impossible to maintain for any length of time with such and airframe.

The upside of course, of this positive gain system, is that maneouvers just not possible with a conventionally stable airframe become possible. Add in Thrust vectoring, or alternate systems (canards, or forward lift fans for example) and the airframe can be made to follow all sorts of incredible vectors.

It also makes the control parameters and interrelated control blending very complicated. For example, take a look at the F35 taking off with assistance from the lift fan, the elevators actually move the "wrong" way at low speeds!

A human pilot would have a big job on their hands to remember and implement/deal-with that sort of control input non linearity under anything but ideal conditions.

Edited by anonymous-user on Sunday 16th December 22:11

[Ginetta G15 Girl]

Simpo Two said:

I know the computer allows you to do things which were otherwise impossible, but is 'straight and level' impossible too?

On a totally unstable FBW a/c the answer to that is, basically, yes.

Without getting too technical you can consider that there are 3 modes of Stability/Instability in a/c. These are 1. Unconditionally Stable, 2. Conditionally Stable, 3. Unstable.

In serial 1 anything that deflects the a/c from its flightpath will result in the deviation gradually disappearing over time. Ie damping occurs.

In serial 2 no damping occurs, the deviation continues in a sinusoidal sense. Ie it neither diminishes nor increases.

In serial 3 the reverse of Serial 1 occurs. Ie the deviation increases and continues to increase. In other words, deviation from the flightpath becomes rapidly uncontrollable.

Simpo Two said:

But there has to be a way to intervene between 'forgot to plug the thingy in' and 'smoking hole in the ground' (other than an ejector seat)...

There is. It's called having 3 computers monitoring the situation and each other. Ie a Fail-Safe situation. However, if it all goes totally pear shaped you have no other recourse but a Martin-Baker let down.

[FunkyNige]

There's an interesting site about a fair few of the "x-plane" crashes - http://www.thexhunters.com/ - the site is about a group of people who went looking for debris of the crashes but it's interesting none the less.

[Mojocvh]

(Reminded me of the Air France affair)


hows that then?

[Simpo Two]

Max_Torque said:

stuff

Sound info, thanks.

Ginetta G15 Girl said:

Without getting too technical you can consider that there are 3 modes of Stability/Instability in a/c. These are 1. Unconditionally Stable, 2. Conditionally Stable, 3. Unstable.

In serial 1 anything that deflects the a/c from its flightpath will result in the deviation gradually disappearing over time. Ie damping occurs.

In serial 2 no damping occurs, the deviation continues in a sinusoidal sense. Ie it neither diminishes nor increases.

In serial 3 the reverse of Serial 1 occurs. Ie the deviation increases and continues to increase. In other words, deviation from the flightpath becomes rapidly uncontrollable.

Nicely put. In a mickey mouse example this explains why Attack on Pearl Harbour is difficult to play after its predecessor - it's (2) and not (1)!

Ginetta G15 Girl said:

There is. It's called having 3 computers monitoring the situation and each other. Ie a Fail-Safe situation.

How would that work when a sensor fails, eg ice in the pitot head? They will all fail together surely?

Mojocvh said:

(Reminded me of the Air France affair)

hows that then?

Frozen pitot head = false or no speed reading = computers go wibble. In the AF case the pilots went wibble as well, unfortunately.

[Ginetta G15 Girl]

Simpo Two said:

In a mickey mouse example this explains why Attack on Pearl Harbour is difficult to play after its predecessor - it's (2) and not (1)!

Sorry, I have absolutely no idea what that means!

Simpo Two said:

How would that work when a sensor fails, eg ice in the pitot head? They will all fail together surely?

You have independent sensors for each channel for each computer.

[Simpo Two]

Ginetta G15 Girl said:

Sorry, I have absolutely no idea what that means!

Sorry,flight sims Essentially (1) (your serial 1) damps control movements out which makes it nice and easy to control, while the other (2) (your serial 2) seems to exacerbate them, so it's much twitchier, easy to over-input (is that a word?) and then over-compensate. You go all over the place. Of course once you've adapted to 2 then 1 seems very odd.

[Apache]

Simpo Two said:

Ginetta G15 Girl said:

Sorry, I have absolutely no idea what that means!

Sorry,flight sims Essentially (1) (your serial 1) damps control movements out which makes it nice and easy to control, while the other (2) (your serial 2) seems to exacerbate them, so it's much twitchier, easy to over-input (is that a word?) and then over-compensate. You go all over the place. Of course once you've adapted to 2 then 1 seems very odd.

over input = PIO?

[dr_gn]

Max_Torque said:

Simpo Two said:

I know the computer allows you to do things which were otherwise impossible, but is 'straight and level' impossible too?

The aerodynamics of a plane that is deliberately designed to be "unstable" in one or more axes are complicated to say the least. An airframe with conventional stability will experience a "negative feedback" from any pilot input, that acts to neutralise that input. For example, pull back on the stick, the nose rises, the AOA increases, the airframe climbs. Centre the stick, and the rate of climb is maintained wihtout any further pilot input. For an airframe that is say unstable (not critically damped) in pitch, the same input causes the airframe to continue to pitch up, because the gain of the system is positive not negative. To prevent the airframe from doing a "backflip" the stick must be pushed forwards to halt the nose up rotation. This makes it virtually impossible to fly by a human, because it has rate gains so high that control surfaces must be controlled with a very high bandwidth (hundreds of movements per second).

Imagine trying to drive a car, where to turn left you had to turn left, then immediately turn right to stop the car continuing to turn left! Then, take this and add in another 2 axes of instability. Even "straight and level" flight is just about impossible to maintain for any length of time with such and airframe.

The upside of course, of this positive gain system, is that maneouvers just not possible with a conventionally stable airframe become possible. Add in Thrust vectoring, or alternate systems (canards, or forward lift fans for example) and the airframe can be made to follow all sorts of incredible vectors.

It also makes the control parameters and interrelated control blending very complicated. For example, take a look at the F35 taking off with assistance from the lift fan, the elevators actually move the "wrong" way at low speeds!

A human pilot would have a big job on their hands to remember and implement/deal-with that sort of control input non linearity under anything but ideal conditions.

Edited by Max_Torque on Sunday 16th December 22:11

So...What's the point of designing an unstable aircraft purely for the gain in maneuverability, when a stable aircraft (even without fly-by-wire) can be made to easily exceed the physical limits of the pilot by simply adjusting control gain?

[Eric Mc]

The X-31 was by no means the first unstable, computer controlled, fly by wire aircraft.

And it wasn't a fighter. It was a pure test aircraft - partly funded and built by MBB in Germany.

[Apache]

dr_gn said:

So...What's the point of designing an unstable aircraft purely for the gain in maneuverability, when a stable aircraft (even without fly-by-wire) can be made to easily exceed the physical limits of the pilot by simply adjusting control gain?

Agilty. A useful analogy is to take the example of holding a broomstick, hold it by the tip of the handle and try to move it quickly, it has inertia and takes time to move and settle. Now try balancing it in the palm of your hand, it takes lots of input to keep it vertical but when you want to move it rapidly its there immediately.

[dr_gn]

Apache said:

dr_gn said:

So...What's the point of designing an unstable aircraft purely for the gain in maneuverability, when a stable aircraft (even without fly-by-wire) can be made to easily exceed the physical limits of the pilot by simply adjusting control gain?

Agilty. A useful analogy is to take the example of holding a broomstick, hold it by the tip of the handle and try to move it quickly, it has inertia and takes time to move and settle. Now try balancing it in the palm of your hand, it takes lots of input to keep it vertical but when you want to move it rapidly its there immediately.

That wasn't the question.

[Simpo Two]

OK try it this way. Move the rudder of a boat from the front to the back - now the nose is not only much more responsive/pointable but any control input has to be actively countered, it won't self-centre.

(I am quite good with boats )