Super Hornet inherently unstable?

Military aircraft - Post cold war aircraft, including for example B-2, Gripen, F-18E/F Super Hornet, Rafale, and Typhoon.
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CFIT

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Unread post12 Oct 2004, 22:32

Haven´t thought much about this until today.

The standard F/A 18A/B/C/D is inherently stable right?

How about the Super Hornet? Has Boeing made it unstable to make it more agile ( something that most agree on that it needs)?


If so, how much (relative to the F-16 / Gripen / Typhoon)?
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KarimAbdoun

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Unread post14 Oct 2004, 12:41

Wooh!!! the F-15,16,18 including the E & F and 117 and the EF 2000 Typhoon are the aircraft that I know of that are suffering from an unstable design, they don't appear to be due to on board computers.
The fighter is not what counts, it's the one who's flying it that matters!
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Zephyr

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Unread post14 Oct 2004, 17:57

Almost all fighter a/c are built unstable(from a pure aerodynamic standpoint).
i.e the resultant lift component is infront of the weight component
this gives the aircraft a high nose(pitch) authority which it needs.
The only thing preventing the nose from snapping up is the chip onboard.
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habu2

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Unread post14 Oct 2004, 18:39

KarimAbdoun wrote:Wooh!!! the F-15,16,18 including the E & F and 117 and the EF 2000 Typhoon are the aircraft that I know of that are suffering from an unstable design, they don't appear to be due to on board computers.

Suffering from an unstable design? I don't think so....

The correct (and preferred) term is "relaxed static stability" - not "inherently unstable". This is an intentional design feature, one which enhances maneuverability.
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KarimAbdoun

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Unread post17 Oct 2004, 07:29

One which enhances maneuverability.

Yes my point exactly
The fighter is not what counts, it's the one who's flying it that matters!
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ram816

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Unread post21 Oct 2004, 15:23

This is the most well-known principal of Tactical Aircraft design.

In order to increase manuverability, an aircraft will have to sacrifice stability.

For example, Airliners are the most stable aircraft in the world because their flight envelope doesn't require any sharp manuvers to do it's job... just truck it from Point A to Pont B.

Pretty much all Tactical Combat aircraft are "inherently unstable." Those that aren't don't do very well in a Knife-Fight. The F-16 itself, for example, is designed to be very unstable in order to let it pull 9G turns. Of course, this can be dangerous if not controlled properly (hence the old "lawn Dart" reputation) but the advent of Digital FBW Flight Control Systems in modern Combat Aircraft minimize the dangers of pushing the plane over the edge of its envelope and into a departure.


As Habu has pointed out, "Relaxed Stability" is the magic word in the art of tactical aircraft design.

As always, though, the best way to overcome ain aircraft's weaknesses is to strap in a stick jockey who knows what the machine can and cannot do and how to use both to the maximum advantage.
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Obi_Offiah

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Unread post04 Nov 2004, 09:56

CFIT wrote:Haven´t thought much about this until today.

The standard F/A 18A/B/C/D is inherently stable right?

How about the Super Hornet? Has Boeing made it unstable to make it more agile ( something that most agree on that it needs)?


If so, how much (relative to the F-16 / Gripen / Typhoon)?


Hi All! :D

The F/A-18A-D and F-15 are stable aircraft that can be flown without the use of computers. The E-F is described as being statically neutral to slightly unstable (relax static stability) and has no mechanical backup.

Obi
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ximeno

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Unread post04 Dec 2005, 10:44

PLus a MUX bus and 2 flight computers
F-16A,B,C,D in AF
built AV8-b, F-15E, FA-18C/D/E/F/G, T-45.C-17
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falconfixer860261

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Unread post05 Dec 2005, 17:14

The Viper was designed with negative static stability as are most fighters that are fly-by-wire. As in the X-29 - you lose all the computers and the jet will flip ends.....
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Guysmiley

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Unread post05 Dec 2005, 17:26

Isn't the Viper only negatively stable below M1? I thought the center of lift shifts aft as the jet passes through transonic?
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falconfixer860261

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Unread post05 Dec 2005, 17:34

My understanding is that the jet is unstable at any speed without the benefit of the FCC's. Same goes for the Raptor. I also seem to remember that most (if not all) jets become unstable in the transonic range due to the aft shift.
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zero-one

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Unread post19 Jan 2015, 01:35

So for quite some time now I've been getting into discussions with other enthusiast claiming that the Rhino is a stable design.

I thought to my self that this could't be. Relaxed static stability is perhaps the most important reauiremnt to make an aircraft very maneuverable.

So i did some digging. This was the most solid link that I found
https://books.google.com.ph/books?id=ZT ... ty&f=false

It confirms that the F/A-18, atleast the E/F/G variants were designed as unstable aircraft.

Another interesting note is that the E actually weighs around 31,500 lbs empty.
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spazsinbad

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Unread post19 Jan 2015, 02:37

Interesting that 'Google' would not allow me to view the page at URL above. Sucks to google because the Super Hornet NATOPS may be downloaded here:

NATOPS FLIGHT MANUAL NAVY MODEL F/A-18E/F 2001

http://info.publicintelligence.net/F18-EF-000.pdf (22.5Mb)
OR
http://www.scribd.com/doc/34302299/A1-F ... per-Hornet
RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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johnwill

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Unread post19 Jan 2015, 04:14

Relaxed static stability or negative stability does in fact make an airplane more maneuverable, but not in the way most people here describe it. Any airplane, positive or negative stability, can be designed to pull 9g or whatever g is required. So why go to the trouble of negative stability? Because there are other benefits available.

1. Reduced trim drag. Obvious benefits are longer range, reduced fuel cost, higher sustained g etc.
2. Quicker response to pilot command. Not just 9g capability, but reaching desired g faster.
3. Lighter structural weight. Stable airplane has down tail load, up wing load. Wing must be designed to overcome that down tail load in addition to lifting airplane mass. Unstable airplane has up tail load and wing load. Tail helps to lift airplane, so wing structural is load smaller, thus lighter structure. Aft fuselage load smaller due to lifting tail, so lighter fuselage structure.
4. Lower angle of attack required. Induced drag is smaller, still higher sustained g capability.
5. As an alternative to (4), a smaller wing can be used. Less drag, higher sustained g.

Mention was made of the effects of aft lift shift in supersonic flight, and it is true the negative stability becomes less negative or even positive. But other things are done to regain at least some of the benefits. On the F-16 for example, the leading edge and trailing edge flaps are raised 2 degrees in supersonic flight. Both of those actions move wing lift forward, similar to subsonic flight, thus regaining some of the benefits. Other fighters have similar ways of improving performance.
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tbarlow

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Unread post19 Jan 2015, 08:21

The B-2 is the same way. If it were not for the computers, it would flip end over end like the the problem in the YB-35/XB-49. Edwards AFB is named after Capt Glen Edwards who was killed in a XB-49 crash.
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