
Grumman X-29: An impossible fighter jet with inverted wings - nwrk
http://edition.cnn.com/style/article/grumman-x-29-nasa-darpa-fighter-plane/index.html
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jaytaylor
Is this really called an "inverted wing", I've always heard such
configurations referred to as "forward swept wings".

For reference, see the Russian Su-47.

[https://en.m.wikipedia.org/wiki/Sukhoi_Su-47](https://en.m.wikipedia.org/wiki/Sukhoi_Su-47)

[https://en.m.wikipedia.org/wiki/Forward-
swept_wing](https://en.m.wikipedia.org/wiki/Forward-swept_wing)

~~~
sandworm101
Ya, inverted wing means something else, a wing tuned for inverted flight... at
which point it becomes just a normal plane with the pilots seat upside down.

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onion2k
Is that actually a thing? Landings would be terrifying.

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Stevvo
It's not a thing, however acrobatic aircraft often have symmetrical airfoils,
such that they can fly inverted.

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brandmeyer
> such that they can fly inverted.

Make that, such that inverted flight has the same stall margins as normal
flight.

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Stevvo
Right, even a Cub with its flat-bottomed USA-35B will fly inverted, but in
terms of angle-of-attack you would have no room for error, and the gravity-fed
fuel system is going to cause you some problems too.

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smacktoward
An interesting footnote: the proposal Grumman beat to win the DARPA contract
that led to the X-29 came from General Dynamics, who were pitching a forward-
swept wing variant of their ubiquitous F-16 called the F-16 SFW.

[http://www.f-16.net/f-16_versions_article26.html](http://www.f-16.net/f-16_versions_article26.html)

GD really wrung as much mileage out of the F-16 airframe as they possibly
could, coming up with all sorts of weird proposed spinoffs from it. They bid
on the fighter-bomber contract that was eventually won by the F-15E Strike
Eagle, for instance, with a delta-winged F-16 they called the F-16XL
([https://en.wikipedia.org/wiki/General_Dynamics_F-16XL](https://en.wikipedia.org/wiki/General_Dynamics_F-16XL)),
and they attempted to jam thrust-vectoring into the platform with the F-16
VISTA
([https://en.wikipedia.org/wiki/General_Dynamics_F-16_VISTA](https://en.wikipedia.org/wiki/General_Dynamics_F-16_VISTA)).

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duxup
Makes sense as the F16 really seemed to be a successful system cost to utility
wise.

~~~
nostrademons
Same with the F/A-18.

I remember being really into naval aviation when I was a kid, and I'd look at
carrier air wings. There would be F-14s for air superiority, A-7s for light
attack, A-6s for heavy attack, EA-6Bs for electronic warfare, KA-6Ds for
tankers, S-3Bs for anti-submarine warfare, etc.

Now I look at a modern carrier air wing, and it's F/A-18E/Fs for air
superiority, F/A-18E/Fs for light attack, F/A-18E/Fs for heavy attack,
F/A-18E/Fs for anti-submarine, F/A-18E/Fs for tanking, and EA-18Gs for
electronic warfare. Same for the Marine Corps and a bunch of foreign nations.

For that matter, the X-29 was itself a variant of the F-5, another
lightweight, low-cost design that got _a lot_ of mileage in different roles,
including being the starting point for the F/A-18 design. Sometimes it pays to
be cheap.

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djsumdog
The Real Engineering show has a really great and detailed video on these types
of inverted wing designs:

[https://www.youtube.com/watch?v=RN6vGxyMcVU](https://www.youtube.com/watch?v=RN6vGxyMcVU)

It has a lot more detail about the aerodynamics and advantages than this
article. It's worth the watch if you're interested.

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agurk
An interesting fact about this aircraft is that as the wings are positioned
behind the center of gravity of the plane they typically induce the plane to
pitch down. This is in contrast to a more normal layout that will typically
pitch up.

On the conventional layout this means that the horizontal stabilisers at the
back need to produce a downwards/negative lift to stabilise the plane.

The X-29 has canards at the front that have to produce positive lift to
balance the plane. This means it's actually more efficient as both surfaces
are producing useful lift and avoiding the penalty of the negative lift of
traditional horizontal stabilisers.

This is of course more complex in practice, with the CoG not being in a fixed
position and careful positioning of wings to reduce the pitch-up tendency.

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mysterydip
"But its highly experimental design made it the most aerodynamically unstable
aircraft ever built."

I'm not sure if that's true. The F117 was aerodynamically unstable in all
three axes due to its design prioritizing stealth over all else, needed
constant corrections by the fly-by-wire system, and it first flew in 1981.

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hollerith
How is when it first flew relevant to how unstable it is?

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EdwardCoffin
Because it is an example of a _totally_ unstable plane which flew three years
before the plane under discussion.

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hollerith
Ah, I see. You want to argue that since the F-117's instability did not
prevent its becoming a success, despite using fly-by-wire computers that were
3 years older (and consequently more primitive) than the X-29's, then
instability cannot be the root cause of the failure of the X-29.

~~~
EdwardCoffin
The person you replied to was expressing _skepticism_ that the claim that the
X-29 was "the most aerodynamically unstable aircraft ever built", on the
grounds that the F-117 first flew in 1981 (which implicitly predated the X-29
which first flew three years later in 1984), and was unstable on all three
axes.

You asked of what relevance the date first flight was, and I made the relation
of first flights explicit, which answers your question of what relevance it
is. That is all. Your claim about what I want to argue exists only in your own
mind.

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V_Terranova_Jr
The technically-minded will enjoy Prof. Bill Mason's inside detail regarding
his time as one of the principal aerodynamicists on the X-29:
[http://www.dept.aoe.vt.edu/~mason/Mason_f/AnX-29StoryV3.pdf](http://www.dept.aoe.vt.edu/~mason/Mason_f/AnX-29StoryV3.pdf)

Also, NASA's e-book on the program is far more useful than these superficial
enthusiast articles:
[https://www.nasa.gov/connect/ebooks/sweeping_forward_detail....](https://www.nasa.gov/connect/ebooks/sweeping_forward_detail.html)

For anyone wondering, one of the main reasons forward-swept wings never became
commonplace, the configuration is at odds with low radar observability.

Dr. Norris Krone, the principal DARPA instigator behind the program, recently
passed away: [https://aero.umd.edu/news/story/distinguished-umd-alumnus-
an...](https://aero.umd.edu/news/story/distinguished-umd-alumnus-and-faculty-
member-dr-norris-j-krone-jr-passes-at-88)

Some other nitpicks:

\- The article indicates thrust-vectoring obviated the need for FSW, but
you'll note no thrust-vectoring on western jets outside of the F-22 and
experimental aircraft.

\- They make a big deal about the degree of instability, but talk about that
with respect to longitudinal static stability, not dynamic stability or
stability about other axes.

\- Both TACIT BLUE and HAVE BLUE were quite unstable as well, and HAVE BLUE,
unstable in all three axes, flew well before the X-29.

I'm not going to go find numbers, just pointing out the article sounds very
hyperbolic. Artificial stability requires vastly less computational power than
the breathless words tend to imply. We should be more impressed with control
systems theorists, and flight control design has come a long way since then.

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mcarmichael
I suppose it is literally true that the X-29 is "an impossible fighter jet",
in that it was never put into production and fitted with weaponry.

The phrase "inverted wing" is more troubling, because of the potential for
confusion with actual wing design terminology:
[https://en.wikipedia.org/wiki/Gull_wing#Inverted_gull_wing](https://en.wikipedia.org/wiki/Gull_wing#Inverted_gull_wing)

Perhaps this link could be migrated to style.ycombinator.com?

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NKosmatos
Looked up an interesting photo from the article: Dryden “Research Aircraft
Fleet on Ramp, X-15, F-18, SR-71, X-31, X-29” link:
[https://www.dfrc.nasa.gov/Gallery/Photo/Fleet/HTML/EC93-4101...](https://www.dfrc.nasa.gov/Gallery/Photo/Fleet/HTML/EC93-41012-3.html)

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dvh
[https://www.youtube.com/watch?v=34de_aDKnQ8](https://www.youtube.com/watch?v=34de_aDKnQ8)

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GordonS
Maybe I'm missing it, but does the article actually say what the benefits of
this are?

~~~
na85
When aircraft pull too much angle of attack, the wing abruptly suffers a loss
of lift anywhere the maximum lift coefficient has been exceeded. If your
entire wing loses lift all at once this can be very dangerous. As a result,
most wings are designed with what's called Washout, wherein the angle of
incidence at the root is greater than at the tip. This causes the wing root to
stall before the tip, which allows a pilot to maintain control as the onset of
stall appears.

Using a forward-swept wing causes the spanwise (along the wing's length) flow
to be net inward, rather than outward which is the case in a conventional
aircraft. With the right mix of composite materials to control aeroelastic
twist this causes a similar effect to washout wherein the wing root will stall
before the tips, but greatly reduces wingtip vortices and thus reduces total
aerodynamic drag. As someone else pointed out, you also get an induced drag
benefit because both the canards and main wings are producing upward moments,
so the total magnitude of lift force required for a given weight is less.

The "so what?" of all that is it lets you build a smaller, faster, more agile
aircraft at the cost of significantly increased aerodynamic and control
complexity.

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Gibbon1
> This causes the wing root to stall before the tip, which allows a pilot to
> maintain control as the onset of stall appears.

I think I've read and been told that also you want the root where the flaps
are to stall before the middle where the ailerons are. Because stalling and
losing your aileron control during a turn is bad news.

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Gravityloss
If one of your wing tips stalls, you will have a very fast roll and there is a
high probability of entering a spin.

Also if a part of the wing that is far aft stalls, then that can cause
pitchup, making the stall worse.

An aircraft with benign behavior stalls so that the nose stays pointing
straight ahead with no roll or yaw but it drops (pitch).

