JF-17 Thunder performing during Dubai Air Show 2017

[Raider 21]

Actually the other way around. Fighters are designed to be stable. It's fly-by-wire that makes them dynamically unstable. No fighter can be designed to be inherently unstable; they would find difficulty taking off and landing.

That's a good one.

 

[Mrc]

I dont know why but commentator seems to be bit negative about thunder ...

British are usually negative about every one and every thing except themselves.....
Americans for probably that reason brought in their own commentator

 

[Trango Towers]

Actually the other way around. Fighters are designed to be stable. It's fly-by-wire that makes them dynamically unstable. No fighter can be designed to be inherently unstable; they would find difficulty taking off and landing.

Joe trust me they are designed to be unstable.

 

[Shane]

Actually the other way around. Fighters are designed to be stable. It's fly-by-wire that makes them dynamically unstable. No fighter can be designed to be inherently unstable; they would find difficulty taking off and landing.

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[GDP Adil Khan Niazi]

Some of the glimpses of Dubai Airshow, including a couple of pictures of Pakistani Super Mushak and JF-17 Thunder as well.

 

[Joe Shearer]

Joe trust me they are designed to be unstable.

Umm...weight, lift, drag and thrust notwithstanding?

OK, I trust you. Do you promise to hold that football in place?

No, seriously, you are right, in that design is unstable when it can be controlled; control without computer assistance is almost - not totally, but almost - impossible. These two readings may help.

First, the question:

I'm told that in order to be more maneuverable fighter jets are designed in a way that makes them impossible for a human to control without the help of a flight computer. Is this actually true? Would a modern fighter (like an F-22 or Su-35) crash if the stability computers died?

Further, if this is true, what design techniques are making them unstable and how do they help with maneuverability?

Then the answer:

My short answer:

• Stability is reduced by shifting the center of gravity aft.
• Shifting it past the neutral point makes the airplane unstable, so movements away from the trimmed state are accelerated. This increases maneuverability.
• Flight computers are multiple redundant, if one dies the others take over.
• Slow unstable airplanes can be flown by a human pilot, but not fast unstable airplanes.

For the long answer, let me first clarify terms:

Static stability is the tendency of a system to return to it's old state after being disturbed. Take a pendulum: If you pull it to one side, it will return to the middle. Eventually.

Dynamic stability is the tendency of an oscillating system for the oscillations to die down over time. Take the same pendulum: It will swing from side to side, and friction will ensure this happens with ever smaller amplitude.

Now we need to add dimensions, all three of them: Pitch, roll and yaw. An airplane can be stable in one dimension and unstable in a different one. I understand your question such that you ask about the static pitch stability (or longitudinal stability) of fighter aircraft.

The Wright Flyer was longitudinally unstable (see here for more). Once aircraft designers learned that aircraft can be made to fly stable, and learned that this is of immense benefit in pilot training, static stability became a requirement for new aircraft. When war in Europe broke out, the British forces were equipped with a superb training aircraft, but it was so stable that it took effort and time to convince it to change course. They were shot down in droves.

From now on, low stability was a prime requirement for fighters and aerobatic aircraft. Static stability is proportional to the control forces (more precisely: To the hinge moment of the respective control surface), so reducing stability gave pilots more response for the same effort. Longitudinal static stability is measured as the relative distance between neutral point (NP) and the center of gravity (CG). See here for more. Longitudinal static stability is achieved by placing the CG ahead of the NP. Shifting the CG back gives you a more responsive airplane, but also one which is more easily disturbed by gusts.

This is the design technique you asked about. Pretty simple, right?

Once you shift the CG aft of the NP, stability is lost and the airplane will increase deviations from the trimmed state. This can be helpful if you want large angle changes, and quickly. An unstable aircraft only needs a small kick and will do the rest of the maneuver all by itself.

This is how it helps in maneuverability. But it is even more helpful to reduce the inertias, especially around the roll axis, for a faster response. That is why all combat aircraft have their engines close to the center.

Of course, negative stability is not acceptable when you need to take your hands off the stick to get a map out or to pee on a long flight. So without computer control, the limit was a CG position near, but not aft of the NP.

With supersonic aircraft, things got more complicated. Now the aircraft operates in two flight regimes, one where lift acts at the quarter chord of the wing and one where it acts at mid-chord. Aircraft with low static stability become very stable in supersonic flight, and the tail surface has to create a high down-force so that the sum of all lift stays where the CG is. Creating lift always incurs a drag penalty, and in supersonic flight it has to be payed twice: One for the excess lift on the wing (which is needed to compensate for the tail's down-force) and one for the down-force on the tail.

Using a flight control computer offers the possibility to allow the pilot to let go of the stick without the aircraft going off course. Now the stick does not command elevator deflection, but pitch rate, and the CG can be moved back from maybe 12% of MAC (mean aerodynamic chord) to -2%. If you compare the wing areas of stable and unstable jets (Jaguar and Mirage F-1 are prime examples), you will see how much is achieved just by going back with the CG by a few percent of wing chord.

Both configurations have the same airfield and combat performance!

Can a human still fly such an aircraft? In glider competitions, the more daring pilots fly with relaxed static stability and have no problem to keep the aircraft under control. Even the Wright brothers could handle their unstable airplane, and handling improved when they moved the CG further back (If you want to know why, please post a new question. This answer is getting too long already!). However, the speed of an aircraft's pitch response is proportional to flight speed (and inverse to the pitch moment of inertia), so faster aircraft are harder to control. You can compare dynamic pressure with the stiffness of a spring: A stiffer spring moves the eigenfrequency of a spring-mass system up, and the same is true for the eigenvalues of the equations of motion of an aircraft. Given that the reaction time of a good pilot is at least 0.1 s (and more if he/she is tired), it is impossible to counteract motions with frequencies of more than a few Hertz. The lag means that the reaction comes too late and will support the motion. See this YouTube clip how that works out in practice. This crash was due to wrong signal gains, not a classic instability (after all, the flight computer was still working, but produced too strong elevator deflections).

I would venture to say that a human can still barely fly an unstable jet at low speed (after all, Tom Morgenfeld almost got the YF-22 under control), but once he firewalls the throttle, he will be always behind the plane, and will crash it soon.

Size helps: Bigger planes have lower eigenfrequencies, and light, big vehicles are easy to control, regardless of stability. All Zeppelins were completely unstable in yaw and above the critical airspeed of an airship (again, please ask for a more detailed answer on this aspect) also in pitch, but with one person each for the vertical and the horizontal control surfaces, and enough people on board to rotate them after 2 - 4 hours, nobody felt the need to make Zeppelins naturally stable.

If one computer dies, the others take over. Most unstable configurations have four parallel computers which cross-check their result to catch any malfunction. The Dassault Rafale uses only three, but adds safety by clever algorithms for checking the results.

-------------------------------------------------------------------------------------------------------------------------------

Yes, the first such airplane was the F-16. It was designed as inherently aerodynamically unstable, which allows it to respond superbly in combat. This was made possible in that it is a fly-by-wire aircraft. Maneuverability is increased, because by definition it is the ability to change states. Stability is the resistance to change. The more stable you are, the harder it is to turn/pitch quickly in a dynamic situation.

And yes, a pilot would not be able to land these aircraft if the fly-by-wire systems became inoperative. There are instances where F-16 pilots have lost their computer and have died because of Pilot Induced Oscillations - the condition where the pilot isn't correcting for their aircraft's instability at a rate fast enough to maintain control.

Other such unstable aircraft are the B-2, F-22, F-35, Eurofighter, etc. All modern fighters need to be inherently unstable to be competitive.

 

[Raider 21]

British are usually negative about every one and every thing except themselves.....
Americans for probably that reason brought in their own commentator

Commentary didn't have anything negative other than not satisfying a fan's ears. They were quite delighted to have had the JF-17 on display. Americans were represented by their own tactical demo teams other than the F-22 as that had flown in from Abu Dhabi for high speed passes.

 

[GriffinsRule]

From all the performances I have seen so far, including F-16E, Mir 2000, F-18E, Gripen and the J-10s, none of them performed a complete loop, so I am not sure what is this Indian fascination with JF-17s airshow routine.

 

[Mrc]

Commentary didn't have anything negative other than not satisfying a fan's ears. They were quite delighted to have had the JF-17 on display. Americans were represented by their own tactical demo teams other than the F-22 as that had flown in from Abu Dhabi for high speed passes.

for a commentator in a major prestigious air show, where less than 10 aircraft performed flying demos, he certainly was extremely poorly informed ... he did not even know that jf 17b exists...

his technical knowledge was sound.....

I would suggest easy on booze and bitchs and some more reading time a night before

 

[Raider 21]

for a commentator in a major prestigious air show, where less than 10 aircraft performed flying demos, he certainly was extremely poorly informed ... he did not even know that jf 17b exists...

his technical knowledge was sound.....

I would suggest easy on booze and bitchs and some more reading time a night before

Or the PAC/PAF representatives didn't inform him well. PAC does have a huge PR issue that needs to be looked at. Technical knowledge would be good as he's a pilot. I would take your suggestion to him and maybe you'd get invited to provide commentary as the last time when PAF provided commentary at an air show in the UAE it was worse than booze-and-bitches-and-more-reading-time-at-night scenario. The part about the invitation is to be taken lightly, just throwing in some slack.

 

[Mrc]

Or the PAC/PAF representatives didn't inform him well. PAC does have a huge PR issue that needs to be looked at. Technical knowledge would be good as he's a pilot. I would take your suggestion to him and maybe you'd get invited to provide commentary as the last time when PAF provided commentary at an air show in the UAE it was worse than booze-and-bitches-and-more-reading-time-at-night scenario. The part about the invitation is to be taken lightly, just throwing in some slack.

you are presuming that he was poorly informed (he is a professional) and omissions were not intentional....and I never say no to b and b a night before

 

[Darth Vader]

Commentator Missed alot and one main point he said there is no twin seat version while already a twin seat model is out and being tested
And there was no discussion in English on the capabilities of the plane beside few points in Arabic,
it falls on PAF to give proper details so other can represent it properly

 

[BATMAN]

Stability is reduced by shifting the center of gravity aft.

Center of gravity is center of gravity, you can shift it by design... but than you have new center of gravity!
Perhaps you meant its not intentionally kept at middle of the structure!

 

[chaklala2]

I do not think there is any specific reason for not doing it (I am assuming you are referring to a vertical loop here). Its a routine flight demonstration that they fly and each maneuver requires the next one to be lined up seamlessly for the demonstration. They may feel that bringing in a loop may not showcase the flight control capabilities as well as the other maneuvers they execute in their program. In the past, our F-86s and F-6s (with the earlier Sherdils), with lower thrust to weight ratio used to do vertical loops back in the late 60s and early 70s demonstrations (when there air shows were a very common occurrence in Pakistan). If you look up some of the videos of the recent flight shows over in Clifton, Karachi, you may see the JF-17 carrying out a loop.

No the issue is not that if JF-17 can do the vertical loop or not because you are right that T-37s can also do a loop as the sherdils have shown...the issue is to do this in a very tight radius. The sherdils and T-37s go very high up 2-3K ft whereas F16 can close the loop in a very tight envelope...that is also what Tejas show. This is a critical aspect of any fighter that needs to be demonstrated..so why aren't they showing it? Is it because it cannot do this as well as the F-16? I am thinking unfortunately that may be the answer. Also I have seen all videos of Jf-17 including the ones from Karachi but there is no vertical loop. There are manuevres more like the Cuban 8 but that is not really a loop as the aircraft breaks the loop as it heads down. Can any expert who has any inside knowledge comment on this?

 

[Hakikat ve Hikmet]

Leave everything aside, where is the mighty Tejas?

Preparing for the top notch "Bahrain Air show"?

What would the commentators say???? Here's the aircraft neiher IAF nor IN wants!!!!! However, it has some Vedic staffs in it!!!!