By the end of this lesson, you will be able to:
What is a “Planform”? Planform: shape of the wing as viewed from above.
Different planforms
The sweep angle of a wing is the angle between a line drawn along the wing (usually at 25% of the chord from the leading edge) and a perpendicular line straight across the fuselage.
Aft-swept (sweptback) wings: the wingtips are behind the wing root. Think of almost every jet airliner.
Forward-swept wings: the wingtips are ahead of the wing root. This appears to make the wing look like it’s flying “backwards.”
The sweep angle dramatically changes how air flows over the wing, especially at high speeds, and that flow dictates both performance and structural challenges.
Reverse airflow-forward-swept wing vs Aft swept wing. On the forward-swept wing, ailerons remained unstalled at high angles of attack because the air over the forward swept wing tended to flow inward toward the root of the wing rather than outward towards the wing tip as on the aft-swept wing. This provided better airflow over the ailerons and prevented stalling (loss of lift) at high angles of attack.
When an aircraft flies, air flows chordwise (from leading to trailing edge) and spanwise (along the wing). On a sweptback wing, some of the air’s kinetic energy turns into spanwise flow — air travels outward toward the tip.
Transonic benefits: Near the speed of sound, the air over the top of the wing can reach supersonic speeds before the aircraft. A sweptback wing reduces the effective chordwise airspeed that “sees” a shockwave, delaying the formation of a drag-bursting shock. This is why airliners and fast jets have swept wings.
Stall characteristics: The spanwise outward flow thickens the boundary layer near the tip. This makes the wingtips stall before the roots. When tips stall, the centre of lift shifts forward, which tends to pitch the nose up (called pitch-up). A sudden pitch-up right as the wing loses lift can be dangerous, but designers manage this with fences, vortex generators, or slats.
