Variable-Geometry Wing Question

the first issue I can think of is when it is in high lift mode, it would be struggling to keep the nose up, and in high speed mode, it would be much less stable. I think to get it to work, it would require the wing root to travel in the fuselage, instead of pivoting on fixed points.

to spell it out, the center of lift and drag would move forward when you swept the wings, and back when you opened them, if the wings simply pivoted.

but I think there was an experimental scissor wing aircraft.

www.nasa.gov/aeronautics/thinking-obliquely-tells-story-of-nasas-scissors-wing-airplane/

There are a number of problems with forward swept wings.

To start, the forces of the air going over the wings is constantly pushing out the wings, basically the wings are constantly trying to rip themselves away from the aircraft as it flies. On rear swept wings, the same forces are pushing the wings towards the fuselage. Metals generally are stronger in compression than tension.

Next, the leading edge of the wing will naturally direct air along the leading edge. On rear swept, the air is pushed along the edge until the wing ends and then it will not be a problem. On forward swept, the air is pushed towards the fuselage. This air combines with the air that is flowing around the body of the airplane. So that point where the root of the wing connects with the fuselage, has a LOT more forces on it than you would have on a normal airplane. WHat makes it worse, is because you have two different airflows coming together, you run a risk that at higher speeds, while the plane may not be flying supersonic, the amount of air going over that wing root may get pushed up to transsonic or worse, supersonic speeds and pressures. The once you start getting sonic shock waves forming, the lift generated after those shock waves is way less. So you get a lot of heat, forces, and pressure on that joint, and you loose lift for the aircraft when that happens.

I'll post more on this later today.

The next issue with forward swept wings is Yaw control (side-to-side movement of the airplane). Back to the idea of the forces of the wings trying to rip themselves off the airplane. As long as the airplane is flying straight, the forces acting on each wing are even, so they are trying to rip themselves off at a balanced rate. However, once the plane starts a turn, the angle of the wings hitting the oncoming air changes. There will be more wind load on the wing on the inner side of the turn than they outer wing because the inner wing has more surface area exposed to the oncoming air. This means that once a turn starts, it will want to keep turning more and more. The best you can do is to manage it with control surfaces. This problem won't go away. And it's not just a problem when the airplane is turning. any time you get a crosswind or hit an air pocket, it you will get the uneven forces and it will want to turn on you.

So either this make the airplane much more difficult to fly, or you need a bunch of extra systems to counter how this thing flies. Thrust vectoring has been proposed as, but I don't know if anyone actually built a plane like that. You can also have software controls to manage the plane, but ask Boeing how it goes using software to make up for poor airplane design performance. It can be done, but is very tricky to do right.

So to the idea of a variable wing geometry with forward swept wings...

Variable geometry wings are very difficult on a good day. Wings need to be super strong to withstand the forces of flight. Normally there is one beam that the wings are built around. Really you build the wings around the central spar and the rest of the airplane is basically just bolted to that. IN most airplane, the spar is a single large piece of metal or as close to a single piece as possible. That gives the best combo of strength and lowest weight. With variable geometry airplanes, that spar is intentionally broken. That joint takes a huge amount of stress. Huge stress means you have to over engineer the hinge which drastically ups the weight. That is why variable geometry is typically only used in military applications where you can make up for added weight by having larger engines and fuel/cost efficiency isn't a big concern. That hinge assembly is almost an Achilles' Heel. if it gets damaged or breaks, it can disable the whole aircraft. So it has to be over engineered to protect the plane from the rigor of combat flight. more metal, more weight, therefore larger engines.

So now we take all the risks and engineering challenges of variable geometry and add a bunch of new engineering problems. Not only do you have a major structural week point with the hinges, that point is right where the forward swept wings put a whole lot of additional stress on the aircraft. So big stress and lots of weight right at the point where you risk loosing lift in your wings.

You may actually be able to help address the yaw problem with the variable geometry wings. You could change the angle of each wing independently as the plane flies/turns to minimize the differences in forces on the wings. I don't think a human could control them in addition to everything else needing attention on such an airplane. That bit would almost have to be software controlled to be able to work.

Considering how advanced flight computers are getting these days, it actually wouldn't surprise me if we have that technology in a generation or two.