Why stealth aircraft is not stable on all it's three axis?

Question

Lockheed F-117 A Night hawk and Lockheed SR-71 A are said that they are not stable on all there three axis and a pilot has to constantly handle the aircraft with fly-by- wire system. Lockheed F-117 A Night Hawk is said to have low speed and low service ceiling, but Lockheed SR-17 A has 80,000 feet servise ceiling and maximum speed up to 2000 miles per hour at 80,000 feet.

Answer 1

it does have nothing in common with operational limitations, but with aerodynamics. F117 is not aerodynamic at all, it is just an coincidence that that fazzeted shaped piece of stuff flies.

seriously, the [in]stability around axis is inherited from the design. inbred instability provides excellent maneuverability, but requires constant control. that is why the fly by wire was introduced - to ease the workload of a pilot. Typical example of an unstable aircraft is F-16.
Another nice example is the B-2 design. the first attempts to construct this flying wing design date back to the 40ties of the 20 century. Northrop, and German Blohm-voss tried but they lacked the computers to control this definitely instable thing.
that is why the design was ressurected only in the 90ties..

Answer 2

I am sorry, is there a QUESTION in there somewhere?

F/A-117 Nighthawk is NOT stable in flight due to stealth concerns. It's just the way it's designed. A flight computer makes constant corrections while taking into account pilot input, making it possible to fly such crafts. Same with B-2, as a flying wing is not that stable without tail stabilizers.

I guess what you're asking is what is "inherent stability". In an airframe design, usually you want the airframe to "self-correct". Say the a normal airplane was hit by an updraft. The plane pitches up, then down, but if the pilot keeps his hands on the stick and doesn't panic, the oscillations will damp out, and he'll resume normal flight. The plane is said to be "inherently stable".

However, it is perfectly possible to build an UNSTABLE plane. Such plane will be able to turn, spin, and roll faster than a "stable" aircraft, but will be MUCH more difficult to fly (unless a computer helps you with the minor corrections). These airframes are inherently UNstable, and are only feasible after the advent of flight computers.

SR-71 is stable enough once you get it up to the proper speed (above the speed of sound).

Answer 3

* instability does not have anything to do with stealth
you can make a naturally stable stealth aicraft

* SR-71 is a first generation stealth aircraft. The "not so stealth" design is only its chine circular air intake and circular nozzle. everything else is pretty stealthy. Note the stealthy sawtooth design at wing's leading edge.

* I believe SR-71 is a naturally stable aircraft and dont have FBW (FlyByWire) systems. And at SR-71's time, FBW was fine tuned to be mass produced. I believe 71 can go over 2000 mph and reach 90000+ ft (some said it can even go to 100.000 ft)

* F-16 has only 1 axis instability (pitch)

* F-117 uses FBW (FlyByWire) systems similar (if not the same as) to F-16, but modified to serve 3 axis. F-117 has max operational speed of around mach 0.8-0.85, but many believed it can go to Mach 0.95-1 in a dive and still control it well. It also can fly up to 40000+ ft (classified)

* Pilot in a aircraft that has FBW system, control/handle the aircraft just like any other airplane. When the pilot move the stick backwards, the airplane pitches up, just like any other airplane. It's the FBW system that constantly controls/move/adjust the control planes (aileron, flaps, rudder, elevator, elevon, ruddervators). Every second, FBW moves the control planes up to 10-30 times !!

* Lockheed ( now L.Martin) engineers had to make an object the absorb while deflect radar waves, PLUS make it flyable (aerodynamic). even though F-117 is made of flat panels, without any curvature at all, it's pretty aerodynamic !! it has been tested thousand of hours in enclosed wind tunnel in total secrecy. Without FBW, it's impossible to fly F-117. About ten yrs ago, they install a "panic button" so when in the event F-117 goes berzerk, the pilot just press the "panic button" and let the flight computer stabilize the plane.

* due to the lack of hispeed computers back then, so engineers came up with an object with faceted surfaces, this design results in an aerodynamic instability, so they put in the FBW system.

* The logic behind FBW in a stealth plane is, you don't want put bombs & nukes in an expensive marvel aircraft, and fly it into the warzone would you ?

- pyotr.baranovich (firefox)

Answer 4

Hey Tina,

I think Kasey C was on the right track but I would just add that there have been many, many unstable aircraft that saw LOTS of service long before computer controls and fly by wire systems were created.

As Kasey said, stability generally means the plane will fly hands off. (you can get in deep with the different types of stability and their goals but lets keep it simple)

Stable planes are not maneuverable planes and vice versa. Any *good* fighter plane from any era is unstable. Any *good* Aerobatic design is unstable.

Before engineers had a good grasp of aerodynamics very few planes were stable. The Wright Flyer was terribly unstable. Almost all the early bi-planes were unstable. A Ford Tri-motor was and is unstable yet it still fly’s commercially to this day! :)

Fly by wire merely made it so that an unstable plane *feels* stable to the pilot. He can point where he wants it to go and the computer will keep it there for him making continuous corrections for the pilot. Not that long ago the Pilot was the one making continuous corrections. :)

Answer 5

Firstly, the SR71 was NOT a stealth aircraft. Every inch of it was designed for speed, and radar can see it coming from a LONG way off. They just couldn't catch it with anything. The SR71 is actually pretty stable according to the pilots I've talked to.

The "Sawtooth" along the edge of the wings has to do with the manufacturing issues concerning titanium in the early 1960's. Not stealth. Again, it is NOT a stealth aircraft.

The F-117 was designed around the principle of electromagnetic deflection, which is why you see all the odd angles in the aircraft design. Those odd angles however, are not naturally aerodynamic at all, and require a computer to help the pilot with the flight controls.

Answer 6

Although instability is a direct result of the radical design. All modern fighters are designed to operate on the edge of instability to allow them to be more agile. This was not possible before the advent of the digital computer and fly by wire flight controls.

Answer 7

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