Cayley's Whirling Arm and the Aerofoil

The insight costs nothing and changes everything that follows.

1. On paper, list what a flying machine must do: hold itself up, move forward, and stay pointed the right way.
2. Sketch a bird’s wingbeat and try to mark which part of the stroke does which job.
3. Now sketch a fixed wing, a separate propulsor and a tail, and mark the same three jobs.

In the bird sketch the three jobs are hopelessly entangled; in the second they are three separate components you can design, test and improve independently. That is the whole of Cayley’s advance, and it is a design principle far beyond aviation: when one mechanism is doing three jobs, you cannot improve any of them without disturbing the others.

A rotating beam turns a wind tunnel you cannot afford into one you can build in an afternoon.

1. Make a rigid arm about 1 m long from a hardwood or aluminium strip, balanced on a low-friction pivot — a ball bearing pressed into a wooden block works well.
2. Fit a counterweight at one end and a mount for the test surface at the other, with the mount free to be set at a measured angle.
3. Drive it by a falling weight on a cord wound round the spindle, so the driving force is known and constant.
4. Mark the arm so you can count revolutions, and time them.

A falling weight is the key detail: it gives a known, steady input rather than the variable push of a hand. Cayley used exactly this, because the whole point is quantitative comparison. If you drive it by hand you will produce beautiful spinning and no data.

Establish the baseline everyone assumed was the answer.

1. Mount a flat plate — thin balsa or card, roughly 100 × 25 mm — on the arm.
2. Set it to 0° and release the driving weight; measure the lift with a spring scale on the mount, or by how much counterweight is needed to restore balance.
3. Repeat at 2°, 4°, 6°, 10°, 15° and 20°.
4. Plot lift against angle.

Lift rises roughly in proportion to angle, then stops rising and collapses. That collapse is the stall — the airflow has stopped following the upper surface and separated into a turbulent wake. Note the angle where it happens; it is one of the most important numbers in aviation, and you have just measured it rather than read it.

This is the measurement that founded the aerofoil.

1. Make a second surface of identical area, but curved — a shallow arc, roughly 1 part depth to 15 parts chord.
2. Run the identical series of angles.
3. Plot both curves on the same axes.
4. Compare the maximum lift, and the angle at which each stalls.

The curved surface produces more lift than the flat plate at every angle, and it goes on working to a higher angle before stalling. There is no obvious reason it should: it has the same area and is made of the same stuff. But a cambered surface turns the airflow more smoothly and keeps it attached longer, and the flow stays attached because it is not asked to negotiate a sharp change of direction at the leading edge.

This single result is why aircraft wings are curved and not flat, and it was found by measurement, not derivation. Cayley did not have a theory of circulation — that arrived a century later with Lanchester and Kutta. He had a spinning stick and a scale, and he was right.

Lift has a magnitude, which you have measured, and a location, which decides whether the machine is controllable.

1. Suspend a test wing from a thread and find the point where it balances in still air.
2. Now run it on the arm and note whether it wants to pitch nose-up or nose-down at each angle.
3. Move the mounting point forward and back until the pitching tendency is minimised, and record where that is along the chord.

The point where lift effectively acts — the centre of pressure — MOVES as the angle changes, and on a cambered surface it moves forward as the angle increases. That is a vicious property: a small nose-up disturbance increases the angle, which moves the lift forward, which pitches the nose up further. The wing is unstable by itself and will diverge.

The tail is the answer, and now you can see why it is not decoration. A surface set well behind the wing at a slightly lower angle produces a restoring moment: pitch up, and the tail’s angle increases, its lift increases, and it pushes the nose back down. Cayley’s cruciform tail was not copying a bird’s feathers — it was a stability device, and he understood it as such.