Search the Community

Every so often someone asks a question about the advantages of forward-swept wings, and usually they get a shitty half-assed answer about how they somehow improve maneuverability and stuff. I will attempt to provide a fully-assed answer. The short version is that forward swept wings do roughly the same thing as conventional aft swept wings; they increase critical mach number. I found an excellent video explaining transonic effects, so watch that first if you don't already know what that is. Typically, a straight wing starts experiencing shock wave buildup at around mach .7. These effects are generally bad; control surfaces lose effectiveness, the aircraft's center of lift moves, stability can decrease, and drag greatly increases. It's generally desirable to delay the onset of this badness. The critical mach number is strongly affected by the thickness to chord ratio: So, critical mach number could be increased by having really thin wings. The F-104 does this, but at the expense of having ridiculously tiny wings that generate barely any lift and no internal volume for fuel storage. Critical mach number could also be delayed by having wings with a normal thickness, but very long chord. This would improve the supersonic performance of the wing, but subsonic drag would be negatively affected, because the wing would have a large amount of induced drag, and additional wetted area that would cause more drag. Finally, the wing could be swept. This would increase the chord length relative to the airflow, but would not give the wing undue surface area and thus subsonic drag. In theory, the critical mach number could be increased by a factor equal to the inverse of the cosine of the sweep angle (much like calculating the LOS thickness of tank armor, and for the same reason), but secondary effects mean that it's less effective than this. The practical effect of sweep on drag coefficient looks about like this: (from Design for Air Combat) This, incidentally, is why the ME-262 doesn't really have swept wings. The change in Mcr is basically negligible for any leading edge sweep under thirty degrees. Note that this logic applies whether the wings are swept forwards or backwards; as far as delaying and reducing the transonic effects, forward or rearward sweep should be equally effective. There are some secondary effects that make forward-swept wings more desirable. One of these is spanwise flow: In any swept wing, the air isn't just flowing over the wing, it's flowing across them as well. This means that while pulling Gs the tips of the wings will stall first. Since the tips aren't producing lift anymore, but the rest of the wing is, the center of lift of the wing moves forward, which means that there's more pitch-up torque on the plane, which means that the nose goes up even more and the stall gets worse. This is known as the "sabre dance," as the F-100 displayed this undesirable property. With the wings forward swept, the root of the wings would stall first (although in practice, forward swept wing aircraft tend to have the wings attached well aft, so the CL still shifts forward during a stall) To make matters worse, the air spilling out sideways and the early stall interfere with the effectiveness of the ailerons, which means that the aircraft can lose roll control effectiveness as it increases AOA. This is a particularly alarming behavior during landing, as speed is low, AOA is high, and keeping the aircraft level is of paramount importance. Additionally, the air spilling out outwards towards the wingtips reduces lift. Reducing this bad behavior increases lift coefficient, therefore. So, forward swept wings are a little more efficient, aerodynamically than aft swept wings. Why aren't they more popular? The problem is something called aeroelastic divergence; which is engineer-speak for "the goddamn wings try to tear themselves off." I will attempt to illustrate with the finest MS pain diagrams: The amount of lift that a wing generates is a function of the angle of attack. The wing will generate more lift the more inclined it is relative to the airflow. Wings in the real world are, of course, not perfectly rigid, so when they generate lift in order to pull the weight of the fuselage through the sky, they bend slightly. In swept wings, the wings aren't just bending, they're twisting as well because the center of lift is not aligned with the structural connection between the fuselage. In an aft-swept wing, the force of the lift tends to twist the wings downwards. Increasing the angle of attack will increase the lift, which will increase this downward twist, which is a naturally self-limiting (negative feedback) arrangement. In a forward-swept wing, it's exactly the opposite. When the angle of attack increases, lift increases and the wings twist themselves upwards, which increases lift even more which increases the twisting... This is why forward-swept wings had to wait until magical composites with magical properties were available.