Aeroelasticity — where it appears
Named by 8 essays across 2 fields — each of them below, with the objects they name alongside it.
The shake that is not resonance
A steady airstream contains no oscillation at any frequency, so nothing is driving anything. What happens instead is that the aerodynamic forces couple two structural motions that were independent, drag their frequencies together, and turn one damping negative — and the wing takes the energy out of the air itself.
The control that works backwards
Divergence is the static aeroelastic failure everybody names, and a wing with its elastic axis at its aerodynamic centre cannot diverge at any speed. It can still reverse — deflect the aileron down above a certain dynamic pressure and the aeroplane rolls the other way — because the aileron's own nose-down moment is there whatever the elastic axis is doing.
The speed where the damping is exactly zero
Flutter is an algebraic condition on a quartic: one root crosses the imaginary axis, at one speed, exactly. And the quantity that locates it is so nearly flat there that the standard way of finding it from flight test overshoots by a quarter.
The lag that makes flutter possible
This site's own flutter model set the lift deficiency to one and recorded in its notes that doing so throws away the lag which stabilises the torsion mode. Putting the lag back moves the flutter speed from 80.8 metres a second to 131, and removes the need for the structural damping that was covering the artefact.
Two lifts at one incidence
A wing pitched up and down through the stall does not retrace its own lift curve. At twelve degrees it carries 0.22 more lift going up than coming down, and the loop that opens between the two is the work the airstream does on it — which is where the energy for a stall flutter comes from.
A wing that twists with its speed washes out the wrong way
A fixed washout is right at one lift coefficient, and a flight that spends its time at four of them pays for the spread. A wing whose twist changed in flight could follow — but the twist a real wing gets for free is driven by dynamic pressure, and it adds washout as the aeroplane speeds up, exactly where less is wanted. On a trainer's sortie a degree of it at cruise raises the induced energy by a third of a point. The best twist driven by dynamic pressure washes in, and only one that follows the incidence itself removes the spread entirely.
Plating that gives way slows the water that slams it, but only on a flat bottom
The slamming pressure on a hull's bottom is a ridge that rides at the edge of the wetted width, and how fast that edge moves depends on how far the water has to climb. A panel of plating that bends inward under the load gives the water further to climb, and while it is moving away it slows the climb itself. Fed back into Wagner's condition, that takes a seventh off the peak stress of a panel on a four-degree bottom loaded to the edge of what aluminium can bear, and almost nothing on a deep V — because what matters is the plating's deflection against the bottom's own rise across the panel.
A pivoted wingtip follows the lift only with no spring stiffness and no camber
The washout a trainer's sortie wants grows in proportion to the lift coefficient, and the twist a wing gets for free follows the dynamic pressure instead. A tip panel free to pivot on a hinge ahead of its own aerodynamic centre can do better, and on one condition it does exactly what is wanted: held by a constant torque and carrying no camber moment, its own lift coefficient stays in proportion to the wing's, and every segment of the sortie flies at its own best twist. Every unit of spring stiffness gives part of that back, a cambered section's moment turns the panel into the wrong law altogether, and the free panel sheds a quarter of a gust's lift for nothing.
Named alongside it
The objects these essays reach for when they reach for this one.
Model limitFlutterReduced frequencyDivergencePitching momentStabilityDampingEigenvalueInduced dragLifting lineMeasurementMemory kernel