Can you explain how a forward swept wing affects stablity?

It is well known (if I'm not making this up) that a highly backward swept wing is more stable in yaw and more inherently structurally pitch incidence stable than a forward swept wing. The yaw reason is to me, obvious (if I'm way off the mark, be kind, I'm an electrical type). I believe it has to do with apparent airflow frontal area changes in yaw which feed back negatively. I wonder if pitch has something to do with flow patterns, or the positively fedback incidence changes that occur with angle of attack changes.

From : Don Stackhouse

You're right, the aft-swept wing shows more apparent frontal area on the forward-yawed side (in other wards, the more foward wing stucks out further to the side), causing a drag increase on that side that tries to yaw the aircraft straight again. On the other hand, the forward-yawed wing also has more effective span, which tends to reduce its induced drag, som sometimes you can get some weird instabilities from that. Most of the time, though, that first effect dominates and the aft-sweep improves yaw stability. Forward swept wings show the opposite effect, so they tend to be de-stabilizing in yaw.

The infamous pitch divergence in forward swept wings is STRUCTURAL in nature. The key phrase in your question is pitch INCIDENCE. Incidence refers to the angle of a flying surface relative to the rest of the structure.

This phenomenon falls into the same general category as flutter, something called "aeroelasticity" . There is no such thing as a perfectly rigid structure. When the air loads on an aircraft's structure cause it to change shape enough to significantly change its aerodynamic behavior, it enters the realm of aeroelastic phenomena.

In this particular case, it's because most wing structures behave like what we engineers call a cantilever beam. When wings bend under lift forces, they usually do it without twisting significantly, and in a plane running vertically and along the wingspars. If you were at the root looking out towards the tip, you would see little or no change in washout at the tip as it bent upwards under load. In unswept wings the air sees essentially the same thing, no angle of attack change (and therefore no additional "g" force or lift change) as the wing bends from an initial lift change.

The trouble comes when the wing is swept. Imagine a straight wing with no dihedral. Now bend it up. It now looks like it has polyhedral, like a typical 2-channel sailplane. Next, while stil keeping it bent, sweep it aft. Now it looks like a polyhedral wing that's been yawed by a rudder input. With the aft sweep plus the bending, the airflow is now striking it more from above, causing a reduction in angle of attack and a corresponding loss of lift. If the initial bend was due to a lift increase, this loss of lift will tend to let the wing relax back towards its original un-bent condition. This is what happens with an aft-swept wing.

With forward sweep, the opposite occurs. As the wingtip bends up, it starts acting like a polyhedral wing that has been yawed forward. The air is now catching it UNDER the tip, increasing its angle of attack and therefore its lift. If it was just a yawed unswept wing, the other tip would be losing lift right now and the aircraft would be rolling away from the forward yawed wing. Unfortunately this is an aircraft buit with forward-swept wings, and the other wing is seeing a similar increase in lift right now too. The aircraft doesn't roll, instead it sees an increase in lift on both wings, causing the tips to develop a bit more wash-in. This adds to the bending force on each wing, causing it to bend up and wash-in even more. This causes a further increase in lift, which causes additional bending, which causes additional lift, which causes.... At this point the wing spars usually say "ENOUGH!!" and cause a sudden but very effective decrease in wingspan! Problem goes away. No more wings, therefore no more excessive bending loads. Can you say "Lifting Body"?

There are generally two ways to fix this. One is to make the wings so stiff that they can't bend far enough under load to significantly change their angle of incidence. They are still not perfectly rigid, but they are rigid enough for all practical purposes from the air's point of view. For very small forward sweep angles this may be an acceptable solution, but as the forward sweep angles increase, the weight penalty for the extra wing structure rapidly gets worse. Before the advent of modern composites, with their highly directional stiffness properties, this was about the only available solution.

With composites we can focus the stiffness of the structure in specific directions. It then becomes practical to build a wing that naturally washes OUT as it bends upward. For example, in the X-29 they do this with a combination of spar and wing skin design. The forward spars are titanium, while the rear spars are much more flexible aluminum. The top and bottom wing skins between them are carbon fiber, and have the fibers oriented such that they naturally want to twist as they bend. The combination of all these features cause the wing to naturally increase washout when it bends up, which reduces the angle of attack at the tip and stops the runaway bending from occurring.

Don Stackhouse @ DJ Aerotech