Control systems and the 4th and 5th generation fighter

The fighters 4th o 5th generation have in common several skills.

• Stealth.
• Super cruise.
• Super agility.
• Full equip digital electronics and communications.

In this post where I try to analize the relationship of two points that have this fighters

• Super agility.
• Digital electronics.

Introduction

Since 1960 the electronics development is on fire, the seed is in the WWII but the increase of the develop is in the 1960 with the Space Race, when change the electronic valves for the silicon technology.

The third stage of this rocket is the domestic electronics, PC, FPGA, systems, that it allow that and people could develop a lot of technologies, and with internet share with the rest of the world. (iOS and Android mainly is develop for the users)


Application in fighters

The electronic techology is applying in all fields of the fighter such as RADAR, ECM, weapons, navigation, .... and Flight Controls System (FCS).

The first step of FCS is in the F-16 like fly by wire. This system limit the actuation the of the pilot, and always system is into of the flight envelopes.

The system show some great potential, FCS is more precise than human pilot and it can flight into the limit envelopes that the pilot could not flight.

This introduce the Super agility


The base of control of flight by design is:

More stable less agility, but increase the agility the system may be can unstable.

For definition with point of sight of flight we have 3 cases

• Stable: The airplane try to keep a flight straight and balance, if you extract the airplane of this profile of flight and then you free controls the airplane try to come back the same straight and balance that it had before. This is to seek for airline airplane or cargo airplane.
• Neutrum: if we follow the example, the neutrum airplane when you free controls the airplane don't try to come back the same straight and balance, and this new position is the new "stable" position. (this is not practical due to is only a point of the control field and always the systems are stable or unstable).
• Unstable: it is Super agility, the airplane don't like flight straight and balance, always try turn and this provide a lot of agility, and unstable have the properties of the "Caos theory" try turn but depend a lot of varibles of the radius turn, and this turn is erratic, and this provide a unpredictable path, and this complicate form example a missile computers and try to follow and destroy.

I can see that the unstable airplane will be a good fighter.

For example of a unstable is the same when we try to keep equilibrium a vertical broom in above of our hand.

This for a human pilot could be stressing, or imposible, for example the Eurofighter or a flight straight and balance, it needs 200 corrections per second, therefore the FCS is mandatory, beause the human pilot can't flight the airplane without this aid.

The problem of the unstable system are two, our electronic control is necesary and the error isn't pardonable.

• The electronic fault of our computer is solve for redundance, with several control systems in parallel, and monitor system.
• The nature of the unstable can't change, therefore only we can design a good and robust system without problems

The last is easier to say than to make.

As I marked before, the unstable behaviour have properties of the "Caos theory", and the system control theory only can control if you knows the system perfectly. Therefore if the system don't have the behaviour that we expect, we have a problem, we can see any examples.

The SAAB JAS39 Gripen had an accident in a Stockholm Air show due to FCS fail, and into out of control envelope flight.

The press presentation of F22 Raptor, due to the FCS can't correct the ground effect without gear deployed.

Arianne V fail in the first launch, due to the control (Arianne IV control) can't control the launcher

When our smartphone or dishwasher hangs is beacause the system control into in any part unknow for them, but you make a reset, and all problems are solved, but in airplane is more critical.

In the stable airplane we pilot could flight due to the air planbe follow stable and the critical time to act is minutes, in unstable the time to act is millisecond, and the inputs must be into of a tolerance very small. The problem of the actuation in system unstable

• Time to response, unstable system are very quick, for that we chose this system
• Force to response, we have actuator for control unstable system, but this only can control into of limit envolope. For exampe, into of tha stall we knows perfectly the actuation the airplane, but the control surfaces barely have effect.
• Knows to response, all this ever system knows and acting well, and this could provide problems source.

Operation of control system The operation of the system is very easy, and very difficult too. The easy part is understand that system is a box where I introduce one input and receive one response, which it could depends of the other inputs and previous inputs.

For example, we have one system and we want that the value of system output will be 10. We are controlling the inputs, and we shall put a several values in the input for the output is 10. Our system only multiply the input for 2

Therefore our control system knows this and if we want a 10 in the system the control take our desired 10 and divide for 2.

The problem is when the dinamic of the problem change:

Our system think that our exit is 10 when in the real output is 12. Note This is a easy example, this type of error is very easy to correct with ouput monitor, but this want to show the error in the knowledge of system provides unknown exits, and it can follow the system out of envelope flight. Changes in the dynamics is very easy in one fighter, for example change of weapons configuration, but existed other types of unexpected changes, for example if the fighter have damages for a combat, and the system to control isnt easier than multiply for 2, for example the error of the F22 raptor, that it crash for only the control system are not awareness of the ground effect.

Unstable airplane In flight if we have damages, normally the airplane increase the unstable nature, and if airplane have unstable nature may be any damages increase the unstable nature. The system unstable evolution, increase divergence between the initial point (our control point) and the next points that airplane follow. If we want keep the divergence to zero, and this we can get with our controls but the control have one problem

We have the control very limited, and the normal the unstable system increase the K factor, therefore damages are going to limited our control or directly we cannot control de airplane, the controls have not effect. For example, if we follow the broom equilibrium and we make a sum of the forces, we have the inverted pendulum.

The necesary force to keep the angle, is the same the weight of the pendulum multiply for tan of angle.

• 0º the force F is 0N.
• 26.56º the force F is middle of the weight the pendulum.
• 45º the force F is the same of the weight the pendulum.
• 63.43º the force F is double of the weight the pendulum.
• 90º the force F is infinite.

In this example I do not apply other forces such as inertia that incresase the force applied. Therefore, if the control system is in the limit of the control with knowledge of the airplane, if we introduce others values such as damages, the control system could not flight the airplane, due to system control cannot provide the enought force to correct the flight path. One example is the Eurofighter in Lybia, where the Tornado select the targets and the Eurofighter only flight like a bomb cargo. But RAF denies this idea. Obviously this article is only based in supposition without real and truthful information due to is this secret and it isn't published the algorithm of control, of test made for the controls.