14 ms·

Grumman X-29: An impossible fighter jet with inverted wings

73 points by nwrk 7y ago

jaytaylor 7y ago
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 7y ago
  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.
  - onion2k 7y ago
    Is that actually a thing? Landings would be terrifying.
    - Stevvo 7y ago
      It's not a thing, however acrobatic aircraft often have symmetrical airfoils, such that they can fly inverted.
      - brandmeyer 7y ago
        > such that they can fly inverted. Make that, such that inverted flight has the same stall margins as normal flight.
        Stevvo 7y ago
        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.
- Someone 7y ago
  FTA, second sentence after the title: ”Its astonishing forward-swept wings were just one of its many bold innovations.” I count 8 occurrences of “forward-swept”, and 4 of “inverted wing”. I think they used ‘inverted wing’ as a synonym to prevent repetition of a fairly long term. It also is quite possible that the editor, not the writer of the article, chose the article’s title, picking the shorter and, arguably, ‘easier’ term.
- djsumdog 7y ago
  I guess there's not much information on the Su47 since it was military, USSR, etc. The X-29 required several flight computers to make micro adjustments to keep the plane stable. I wonder if the Russians had to design something similar with the X-29 .. and if not, how did they keep it stable?
  - baybal2 7y ago
    > how did they keep it stable? I don't see anything unstable there. It's surely very long, and the wing is way behind the centre of mass, so the centre of aerodynamic forces will be behind it too. Cannards look to be put way ahead of centre of mass too.
    - nwallin 7y ago
      They're fairly unstable. The trouble is that at the wingtips, if the wing twists up, aerodynamics forces tend to push it more up. So there's a positive feedback loop. Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force. Again, positive feedback loop.
      - Bendingo 7y ago
        > Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force According to geometry, this problem must also occur with conventional backward swept wings.
        p_l 7y ago
        The geometry makes forward swept wing destabilize further, while classic design self-stabilizes (or oscillates). That said, slight forward swept had been used for years in many planes, it's the more extreme angles that get the fun mentions.
- sdinsn 7y ago
  The Su-47 is discussed in the article.
turk73 7y ago
I had a GI Joe toy based on this in the 1980s. What's the point of bringing this up now other than if it's not in the Internet it didn't exist at all? Again, this is 1980s aerospace tech that didn't really go anywhere.
agurk 7y ago
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.
GordonS 7y ago
Maybe I'm missing it, but does the article actually say what the benefits of this are?
- djsumdog 7y ago
  I don't think it's mentioned in the article, but it's to reduce shockwave formation, generating more lift, efficiency and maneuverability. I posted a link to a real engineering video in another comment that goes into a lot more detail.
  - GordonS 7y ago
    Kind of strange to write an article about a particular design feature and not say what the point of it was?!
    - pafje 7y ago
      It's not very explicit but it's there (around mid-point of the article): "The Hansa Jet wings are also swept forward by just a few degrees, compared to 33 degrees in the X-29. Such a radical adjustment meant trading stability for maneuverability, because to maneuver quicker, a plane must be inherently unstable to start with." ... "At the time, this maneuverability was believed to be absolutely essential to fighter superiority. If your airplane is going to stall before mine, I can shoot you out of the sky in a heartbeat."
- na85 7y ago
  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.
djsumdog 7y ago
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.
mysterydip 7y ago
"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.
- hollerith 7y ago
  How is when it first flew relevant to how unstable it is?
  - EdwardCoffin 7y ago
    Because it is an example of a totally unstable plane which flew three years before the plane under discussion.
    - imtringued 7y ago
      The X-29 is an even more totally unstable plane.
    - hollerith 7y ago
      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 7y ago
        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.
dvh 7y ago
https://www.youtube.com/watch?v=34de_aDKnQ8 https://www.youtube.com/watch?v=34de_aDKnQ8
smacktoward 7y ago
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).
- duxup 7y ago
  Makes sense as the F16 really seemed to be a successful system cost to utility wise.
  - smacktoward 7y ago
    Yeah, plus it had been a huge success in the foreign military sales market. So any F-16 2.0 that got traction was a product they could potentially sell to dozens of countries, even if the U.S. Air Force never bought it.
  - nostrademons 7y ago
    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.
NKosmatos 7y ago
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/EC93-41012-3.html https://www.dfrc.nasa.gov/Gallery/Photo/Fleet/HTML/EC93-4101...
mcarmichael 7y ago
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?
V_Terranova_Jr 7y ago
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.html https://www.nasa.gov/connect/ebooks/sweeping_forward_detail.... 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-and-faculty-member-dr-norris-j-krone-jr-passes-at-88 https://aero.umd.edu/news/story/distinguished-umd-alumnus-an... 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.

73 points by nwrk 7y ago

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 7y ago
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.
- onion2k 7y ago
  Is that actually a thing? Landings would be terrifying.
  - Stevvo 7y ago
    It's not a thing, however acrobatic aircraft often have symmetrical airfoils, such that they can fly inverted.
    - brandmeyer 7y ago
      > such that they can fly inverted. Make that, such that inverted flight has the same stall margins as normal flight.
      - Stevvo 7y ago
        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.
Someone 7y ago
FTA, second sentence after the title: ”Its astonishing forward-swept wings were just one of its many bold innovations.” I count 8 occurrences of “forward-swept”, and 4 of “inverted wing”. I think they used ‘inverted wing’ as a synonym to prevent repetition of a fairly long term. It also is quite possible that the editor, not the writer of the article, chose the article’s title, picking the shorter and, arguably, ‘easier’ term.
djsumdog 7y ago
I guess there's not much information on the Su47 since it was military, USSR, etc. The X-29 required several flight computers to make micro adjustments to keep the plane stable. I wonder if the Russians had to design something similar with the X-29 .. and if not, how did they keep it stable?
- baybal2 7y ago
  > how did they keep it stable? I don't see anything unstable there. It's surely very long, and the wing is way behind the centre of mass, so the centre of aerodynamic forces will be behind it too. Cannards look to be put way ahead of centre of mass too.
  - nwallin 7y ago
    They're fairly unstable. The trouble is that at the wingtips, if the wing twists up, aerodynamics forces tend to push it more up. So there's a positive feedback loop. Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force. Again, positive feedback loop.
    - Bendingo 7y ago
      > Another problem is that if you yaw slightly, the yawed backward wing gets more straight on airflow, and the forward wing gets less, causing a stronger yaw force According to geometry, this problem must also occur with conventional backward swept wings.
      - p_l 7y ago
        The geometry makes forward swept wing destabilize further, while classic design self-stabilizes (or oscillates). That said, slight forward swept had been used for years in many planes, it's the more extreme angles that get the fun mentions.
sdinsn 7y ago
The Su-47 is discussed in the article.

turk73 7y ago

I had a GI Joe toy based on this in the 1980s. What's the point of bringing this up now other than if it's not in the Internet it didn't exist at all? Again, this is 1980s aerospace tech that didn't really go anywhere.

agurk 7y ago

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.

GordonS 7y ago

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

djsumdog 7y ago
I don't think it's mentioned in the article, but it's to reduce shockwave formation, generating more lift, efficiency and maneuverability. I posted a link to a real engineering video in another comment that goes into a lot more detail.
- GordonS 7y ago
  Kind of strange to write an article about a particular design feature and not say what the point of it was?!
  - pafje 7y ago
    It's not very explicit but it's there (around mid-point of the article): "The Hansa Jet wings are also swept forward by just a few degrees, compared to 33 degrees in the X-29. Such a radical adjustment meant trading stability for maneuverability, because to maneuver quicker, a plane must be inherently unstable to start with." ... "At the time, this maneuverability was believed to be absolutely essential to fighter superiority. If your airplane is going to stall before mine, I can shoot you out of the sky in a heartbeat."
na85 7y ago
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.

djsumdog 7y ago

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.

mysterydip 7y ago

"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.

hollerith 7y ago
How is when it first flew relevant to how unstable it is?
- EdwardCoffin 7y ago
  Because it is an example of a totally unstable plane which flew three years before the plane under discussion.
  - imtringued 7y ago
    The X-29 is an even more totally unstable plane.
  - hollerith 7y ago
    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 7y ago
      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.

dvh 7y ago

https://www.youtube.com/watch?v=34de_aDKnQ8 https://www.youtube.com/watch?v=34de_aDKnQ8

smacktoward 7y ago

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).

duxup 7y ago
Makes sense as the F16 really seemed to be a successful system cost to utility wise.
- smacktoward 7y ago
  Yeah, plus it had been a huge success in the foreign military sales market. So any F-16 2.0 that got traction was a product they could potentially sell to dozens of countries, even if the U.S. Air Force never bought it.
- nostrademons 7y ago
  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.

NKosmatos 7y ago

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/EC93-41012-3.html https://www.dfrc.nasa.gov/Gallery/Photo/Fleet/HTML/EC93-4101...

mcarmichael 7y ago

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?

V_Terranova_Jr 7y ago

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.html https://www.nasa.gov/connect/ebooks/sweeping_forward_detail.... 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-and-faculty-member-dr-norris-j-krone-jr-passes-at-88 https://aero.umd.edu/news/story/distinguished-umd-alumnus-an... 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.