Aircraft Stability & Flight Control System

[shbaziz]

I understand the concept of stable aircraft. But certain terms like relaxed stability, static stability and augmented stability are confusing. I d be glad if someone could explain these to me.

Flight Control Systems:
As far as the little knowledge I have about flight control systems, they are as follows;

1) Mechanical
2) Hydro-Mehanical
3) FBW
(a) Analog FBW
(b) Digital FBW

My question is about the FCS of JF-17 Thunder. What FCS it uses in roll and yaw axis and as i have written above, what do conventional controls with stability augmentation mean? Also what flight control systems do 3G fighters use. Thanks alot

 

[VCheng]

See if you can get a hold of this book:

Pratt, Roger W. (editor). Flight Control Systems - Practical Issues in Design and Implementation. Published by Institution of Engineering and Technology, 2000.

ISBN 978-0-85296-766-9

 

[Birbal]

I believe the JF-17 Thunder uses a fully digital FBW system for all controls except maybe throttle (roll, yaw, and pitch).

Conventional controls with stability augmentation generally means a hydro-mechanical system with either mechanical or electronic damping. This damping system makes the aircraft feel more stable during non-maneuvering flight. The Mig-29, for example, has an analog electrical stability augmentation system.

The difference between stability augmentation and most FBW systems is subtle. In a FBW system like that of the F-16, the flight computer looks at the control input and translates that into a target g-load/pitch rate, yaw rate, roll rate, etc., and compares this with the actual g-load/pitch rate, yaw rate, and roll rate. It then adjusts the control surfaces to try to get the aircraft pitch/roll/yaw to match the desired rate based on the input. Thus, control of the aircraft is much more abstracted and moving the joystick doesn't directly translate into control surface movement.

With a stability augmentation system, on the other hand, moving the control stick results in a direct mechanical movement of the aircraft control surfaces. However, there is also some movement added in to keep the pitch, roll, and yaw rates from changing too rapidly, thus making the aircraft easier to fly.

 

[shbaziz]

I believe the JF-17 Thunder uses a fully digital FBW system for all controls except maybe throttle (roll, yaw, and pitch).

first off.. thanks buddy for ur time and information. I ve only read that "JF-17 has digital fly-by-wire in pitch axis and conventional controls with stability augmentation in roll and yaw axis."... could u plz tell about ur source of information or give me some online link.

Also plz tell if the stability augmentation is some feedback control system, which senses and feeds back the position of control surfaces and/or stability parameters. what sort of signal processing takes place in stability augmentation.
[/QUOTE]

 

[VCheng]

.............. I ve only read that "JF-17 has digital fly-by-wire in pitch axis and conventional controls with stability augmentation in roll and yaw axis."... could u plz tell about ur source of information or give me some online link.

.................

That information is confirmed by Jane's All the World's Aircraft.

For the role of signal processing as part of feedback to augment stability, you really should read that book I referenced above.

 

[gambit]

I believe the JF-17 Thunder uses a fully digital FBW system for all controls except maybe throttle (roll, yaw, and pitch).

Conventional controls with stability augmentation generally means a hydro-mechanical system with either mechanical or electronic damping. This damping system makes the aircraft feel more stable during non-maneuvering flight. The Mig-29, for example, has an analog electrical stability augmentation system.

The difference between stability augmentation and most FBW systems is subtle. In a FBW system like that of the F-16, the flight computer looks at the control input and translates that into a target g-load/pitch rate, yaw rate, roll rate, etc., and compares this with the actual g-load/pitch rate, yaw rate, and roll rate. It then adjusts the control surfaces to try to get the aircraft pitch/roll/yaw to match the desired rate based on the input. Thus, control of the aircraft is much more abstracted and moving the joystick doesn't directly translate into control surface movement.

With a stability augmentation system, on the other hand, moving the control stick results in a direct mechanical movement of the aircraft control surfaces. However, there is also some movement added in to keep the pitch, roll, and yaw rates from changing too rapidly, thus making the aircraft easier to fly.

Stabaug is desirable whether the aircraft is in maneuvers or not, if anything, stabaug is more desirable during maneuvers, so stabaug exists in all flight regimes. Fly-by-wire FLCS includes stabaub.

 

[gambit]

I understand the concept of stable aircraft. But certain terms like relaxed stability, static stability and augmented stability are confusing. I d be glad if someone could explain these to me.

In the early days of aviation, the goal is to create a stable aircraft -- no matter what. What we found out was that increasing speed effectively put a sort of an aerodynamic 'clamp' on the aircraft, making it increasingly difficult to maneuver. So we began to make aircraft designs less stable. Note: Do not confuse aerodynamics with stability. A brick is stable but hardly 'aerodynamic'. Anyway, in making the aircraft less stable in subsonic speed to make it more maneuverable in supersonic, we ended up making it more difficult to fly, as in difficult for the pilot to compensate for turbulence, large or small, noticeable or not.

Entered more advanced flight control system:

Flight Control Systems:
As far as the little knowledge I have about flight control systems, they are as follows;

1) Mechanical

WW I to WW II.

2) Hydro-Mehanical

Korean and Vietnam Wars and up to the F-15.

3) FBW
(a) Analog FBW
(b) Digital FBW

F-16A/B are analog. F-16C/D are digital.

 

[Birbal]

first off.. thanks buddy for ur time and information. I ve only read that "JF-17 has digital fly-by-wire in pitch axis and conventional controls with stability augmentation in roll and yaw axis."... could u plz tell about ur source of information or give me some online link.

Sorry looks like I was incorrect. You're correct. The roll and yaw axis are conventional controls while the pitch axis is digital fly by wire. This is similar to the Su-27, which had conventional controls in roll and yaw and fbw in pitch (though that one might be analog).

Also plz tell if the stability augmentation is some feedback control system, which senses and feeds back the position of control surfaces and/or stability parameters. what sort of signal processing takes place in stability augmentation.

The stability augmentation, or damper, to use a simpler term, would try to keep yaw and roll rates at 0. Thus, it would sense any roll rates and yaw rates and try to counter them by turning the aircraft control surfaces in the opposite direction. This damping response is added to the pilot's input. Of course, if the pilot is trying to turn the aircraft, then you don't want the damper to keep fighting him. Thus, the damper uses a washout filter. The math behind the washout filter is pretty complex (using transfer functions). However, the basic idea is that it will stop trying to damp the yaw and roll rates if they persist for longer time periods.