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.
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.
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?
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.
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.
> 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.
”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.
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.
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), 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).
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.
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.
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.
"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.
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.
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.
- 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.
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.
> 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.
The loss of aileron control isn't hugely dangerous in and of itself. What's dangerous is the incipient spin. There is a yaw-roll coupling in most aircraft such that you can use the rudder to bank if you have to, even with zero aileron authority.
There’s generally a tradeoff in control systems between stability and performance. Which is to say: by definition, the more stable a system is, the harder it is to get that system to move away from its setpoint. The classic response to a program manager who wants the system to be “as stable as possible” is that a boulder sitting on the ground is stable, but you probably don’t want it.
If you want an aircraft that’s incredibly responsive, then you probably want an aircraft that’s just this side of unstable. Or, in the X-29’s case, an aircraft that actually is unstable and being actively stabilized by the flight computer.
My pet peeve is that while I want any mechanism I use to be "responsive", everybody seems to market things as being "adaptive", and it doesn't seem to be appreciated that being adaptive is fundamentally opposed to responsiveness.
I wonder if the military gets suckered into buying things that are "adaptive"?
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.
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."
For reference, see the Russian Su-47.
https://en.m.wikipedia.org/wiki/Sukhoi_Su-47
https://en.m.wikipedia.org/wiki/Forward-swept_wing