An aerofoil is described by its chord (leading edge to trailing edge), its camber line (the curve midway between the top and bottom surfaces), its thickness and its leading-edge radius, and it is flown at an angle of attack measured between the chord and the oncoming air. Camber shifts how much lift appears at a given angle, but the angle is what a pilot changes to change lift, which is why a flat plate flies. The coordinates of a real section such as the Clark Y must be copied from published tables, because an invented profile has unknown stall behaviour.
Part of Aerodynamics and Flight
The vocabulary and shape of a wing cross-section, and the difference between the angles that are easily confused.
Four Angles Get Confused, And Confusing Them Kills
- Angle of attack is between the chord line and the direction the air is coming from. It decides lift and stall.
- Angle of incidence (rigging angle) is the fixed angle at which the wing is mounted to the fuselage. It is built in.
- Pitch attitude is the angle of the fuselage to the horizon. It is what the pilot sees.
- Flight-path angle is the direction of travel against the horizon.
In a climb the nose is far above the horizon, yet the angle of attack can be modest, and in a hard pull-up the angle of attack can reach the stall while the aircraft is moving upward. A pilot who equates “nose high” with “angle high” will misjudge both.
Shape Terms
- Chord: the straight line from leading to trailing edge. Wing area for a rectangular wing is span times chord (Aspect Ratio).
- Camber line: the line halfway between the upper and lower surfaces. A symmetrical section has a straight camber line; a flat-bottom section has a curved one.
- Thickness: the distance between surfaces, quoted as a fraction of chord. The carrier gives 11-12% for the Clark Y recommended for a first aircraft.
- Leading-edge radius: the roundness of the nose. A blunt nose tolerates a higher angle before the flow separates; a sharp one stalls sooner.
- Trailing edge: it must be thin and sharp, because the smooth departure of the flow from the trailing edge is what sets up the circulation described in Bernoulli’s Principle.
Camber Offsets The Line, The Angle Moves Along It
Plot lift against angle of attack for any section and you get a nearly straight rising line until near stall. Camber moves the line upward: a cambered section still makes lift at zero angle, a symmetrical one makes none, and a flat plate makes none at zero angle yet flies when tilted. All three are controlled by tilting the wing. Slope and stall angle differ between sections, and they must be read from tunnel data for the section at the Reynolds number it will fly at (Boundary Layer).
Getting The Section Right Without Inventing It
Step 1: Take ordinates from a published table. A section is defined by a table of upper and lower surface heights at stations along the chord. The corpus holds no such table and this article will not print one: an error of a millimetre at the nose changes where the flow separates.
Step 2: Scale to the chord. Multiply each ordinate by the chord you have chosen. A rectangular wing needs a single template for every rib.
Step 3: Make a master template and check it. Cut the template from hard sheet, check each station with a straightedge and a feeler, and read Precision Measurement for the resolution you can honestly claim.
Step 4: Check the finished rib against the template. The nose and the trailing edge matter most; sag between stations matters least.
A Worked Check Of The Carrier’s Section
For the 1.2-1.5 m chord and 11-12% thickness named in the carrier, the maximum section depth runs from 0.11 × 1.2 = 0.132 m (thinnest section on the shortest chord) to 0.12 × 1.5 = 0.18 m (thickest on the longest). That is the depth available for a spar to fit inside; a spar deeper than the local thickness of the section is not possible, and a spar that nearly fills it is the most efficient (Structural Spars). The carrier’s description of where the thickest point and the flat undersurface lie is not repeated here, because it should be confirmed against the section’s published coordinates before a template is cut.
Common Mistakes
- Taking “more curve” as “more lift” — more camber lowers the angle needed for a given lift, and also changes pitching moment and stall.
- Rigging angle mistaken for angle of attack — a wing set at a fixed incidence still meets the air at a different angle in every phase of flight.
- A rounded or thick trailing edge — fabric and dope that leave a blunt edge destroy the sharp exit the section depends on.
Safety
An inaccurate section is a stall you have not met yet
A profile cut from memory or from a drawing in a book may stall earlier, more abruptly, or on one side first. There is no ground test for that, only a flight. Copy a published section exactly and say in the aircraft’s log which table it came from.
What’s Next
- Learn next: Pressure Distribution, what the section does to the air
- Combine with: Wing Twist, where the angle varies along the span