The pressure on a wing is lower than the surrounding air over most of the top surface and higher than it on most of the underside, and in steady level flight the average difference across the wing equals the aircraft’s weight divided by its wing area. For the 400 kg, 13 m² example here that average is 302 Pa, about 0.3% of atmospheric pressure, but the suction is concentrated near the leading edge on top and is several times the average there. The pattern moves as the angle of attack changes, and that movement drives both the structural loads and the trim.
Part of Aerodynamics and Flight
Where along a wing section the air pushes and pulls, how to read the pattern with Bernoulli’s relation, and what it demands of the structure.
The Average Pressure Difference Is Weight Over Area
In steady level flight at 1 g, the wing carries the whole weight. Total force = average pressure difference × wing area, so the average difference is weight ÷ area. Take the example aircraft: 400 kg × 9.81 = 3,924 N on 13 m² (a 10 m span with a 1.3 m chord, inside the carrier’s ranges). 3,924 ÷ 13 = 302 Pa. Atmospheric pressure at sea level is about 101,000 Pa, so the whole lift of the aircraft is a variation of about 0.3% of the pressure around it. The force is large only because the area is.
In a manoeuvre at load factor n the average difference is n times larger.
Reading The Pattern With Bernoulli
The pressure coefficient Cp = (local pressure − free pressure) ÷ (½ × density × free speed²). With Bernoulli’s relation, Cp = 1 − (local speed ÷ free speed)². Where the local air is faster than the free stream Cp is negative (suction); where it is slower Cp is positive. At the stagnation point on the nose the air stops and Cp = 1. This is the legitimate use of the relation (Bernoulli’s Principle): the speed changes are consequences of the flow around the shape, and Cp is a way to read them.
What The Pattern Looks Like And How It Moves
- Top surface: suction over most of the chord, strongest a short way back from the leading edge, and tapering toward the trailing edge, where pressure recovers to near ambient.
- Bottom surface: close to ambient or modestly above it.
- As angle of attack rises, the suction peak on top grows and moves toward the nose, and the pressure behind the peak must climb back up over a shorter distance. That adverse gradient is what eventually lets the flow break away and stall (Boundary Layer, Stall Dynamics).
- Centre of pressure: on a conventionally cambered section it moves forward as the angle increases, which makes the nose want to rise further, and it moves back sharply at stall. A symmetrical section’s centre of pressure stays almost fixed. The tail has to cope with both (Tail Assembly).
Real figures for the peak suction and the centre of pressure at a given angle come from published section data, not from this article.
What The Structure Feels
The top skin is pulled outward by suction, not just the bottom pushed upward. Fabric, ribs and fastenings on the top surface are loaded in tension away from the rib, so a covering held on only by glue at the edges can lift off (Fabric Covering). The ribs pass the load to the spars, and the spars carry it to the fuselage (Structural Spars).
Seeing It Without A Wind Tunnel
Short lengths of wool or thread taped to the surface lie along the flow when it is attached and flutter, reverse or stand away from the surface when it separates. They show the places where the pattern has failed, not its values.
Common Mistakes
- Treating the lower surface as the only working surface — most of the force on a typical wing comes from suction above, so damage to the upper covering matters as much as to the lower.
- Assuming the pattern is fixed — it changes with every angle of attack, flap setting and speed.
- Quoting the average as the peak — a rib that is sound for 302 Pa average can still fail at a local peak.
Safety
Covering and ribs fail where suction concentrates
A top-surface covering that lifts at the leading edge changes the section and can stall that part of the wing. Check the leading-edge covering and rib attachment before every flight.
What’s Next
- Learn next: Boundary Layer, why the pattern ends in a stall
- Combine with: Structural Spars, where the load ends up