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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?

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3 Answers 3

up vote 18 down vote accepted

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 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. conventional Jaguar and CCV version

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 is 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 below 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.

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Excellent answer! I wonder, though, if you could possible put a one paragraph tl;dr version at the top of the post? For people who aren't quite as enthusiastic for knowledge as I am? –  Jay Carr Aug 8 at 21:51

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.

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actually you have "Pilot Induce*d* O*scill*ations" when the pilot is just barely fast enough to keep up with the movement of the aircraft, but due to the delay into the brain-muscle reaction it is out of phase (i.e., he does the right thing at the wrong moment). –  Federico Aug 8 at 6:18
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Actually the principle design driver for the unstable configuration is for supersonic performance as an unstable design(subsonically) reduces the supersonic trim drag, it has little to do with manoeuvrability and there are arguments that it is detrimental. From a pilots perspective he wants to point the nose, the more unstable the design the harder it is to stop the nose at the required attitude. Finally this is NOT an example of a PIO, the clue is in the name, the oscillation is NOT induced by the Pilot, if the pilot let go of the stick the oscillations would not subside. ergo not PIO. –  Adrian Aug 8 at 9:39
    
RE Federico - thanks for the catch on the typo, corrected. I accept your explanation for PIO, but was trying to simplify the explanation. –  tmptplayer Aug 9 at 4:16
    
RE Adrian: PIO first - the oscillation is pilot induced, but not the initial deviation. The oscillation doesn't come from the unstable design, but from the pilot trying to correct a deviation from stability. Still PIO. Your statements regarding supersonic stability are also true, but I wouldn't say that it is more of a driver than maneuverability. –  tmptplayer Aug 9 at 4:19
    
If the system is unstable and the computers fails then the aircraft will depart whether there is a pilot in the loop or not. In a PIO it is the feedback loop around the pilot that unstable, so if the pilot can go open loop, i.e. he physically lets go of the stick then the aircraft will return to a stable state. For highly unstable aircraft if the FCS has failed then the aircraft will depart irrespective of what the pilot does, which is why this is not a PIO. It's also irrelevant because if the FCS has failed then it won't respond to any pilot input so the pilot can't induce an oscillation. –  Adrian Aug 11 at 9:13

Instability in pitch lowers trim drag for an aircraft with a tail. Stability refers to the relative positions of the center of lift (cl) and center of gravity, (cg). When the cg is ahead of the cl (stable) an aircraft that stalls can fall forward, increasing speed and recovering. When cl is ahead of cg this will not happen. However, when cl is ahead of cg the tailplane produces lift. When cg is ahead of cl (unstable), the tailplane produces a downward force, lowering efficiency. For a canard, stability lets both surfaces produce lift. However, the canard fighters tend to be unstable in order to increase pitch rate and lower supersonic drag. At supersonic speeds cl moves backwards meaning the canard would have to shoulder a larger part of the burden of lifting the aircraft. Since the canard is a less efficient lifting surface than the wing, this is undesirable.

Edit:

I'll add that as a fighter goes supersonic it goes from unstable to stable for both canards and tails.

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A stable pitch axis means that when the aircraft is disturbed, e.g. a gust then the resultant pitching moment counter-acts the disturbance, stall is more a loss of lift due to flow breakdown over the wing. –  Adrian Aug 8 at 9:54
    
The center of lift is always at the same lengthwise point as the center of gravity if the aircraft is trimmed in pitch. Probably you mean the center of lift of the wing-fuselage combination. Better yet, use the term neutral point. Also, the aircraft is longitudinally stable when the cg is ahead of the neutral point. You seem to think it is the other way around. –  Peter Kämpf Aug 8 at 12:23

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