How can engine positioning affect flutter tendency?
To better understand what flutter is, see the structure as a mass-spring system like a spring pendulum that has a damper attached to it. The aerodynamic forces both make the spring stiffer and dampen the movement. Crucially, they will act with a delay to the motion of the spring pendulum. Since we are dealing with oscillatory phenomena here, this delay causes a phase shift. The amount of damping increases with dynamic pressure and the phase shift angle decreases with flight speed.
Things start to get interesting when the phase shift angle approaches π. Now the damper will not work against the movement of the spring pendulum but will support it. Instead of reducing the amplitude of the movement, it will increase it - damping will become negative*. Flutter happens.
There are several ways to shift that point outside of the flight envelope:

Reduce the mass or inertia of the mass-spring system: Now the eigenfrequency of the system will be higher, so the phase shift will approach π at a higher speed. Moving the engines closer to the fuselage will increase the bending eigenfrequency of the wing, so any flutter that involves wing bending will be shifted to higher speeds. A typical case would be the flutter resulting from the coupling of the fast-period mode with the wing bending mode in swept-back flying wings.

Make the spring stiffer: This again raises the eigenfrequency of the spring, so a stiffer structure will shift the flutter speed up. This is no panacea, however: When the Rockwell Commander 112 was developed, flight test revealed flutter of the horizontal tail. A professor from Kansas recommended stiffening of the tail spar and was so convinced that this would cure the flutter that he joined the next test flight, only to crash with the aircraft. As it turned out, the stiffening shifted the flutter speed up such that the forces involved broke the spar while flutter was non-destructive with the softer spar at a lower speed (like in this video).

Change the Eigenmode of the spring: If the motion of the bending wing will at the same time add a torsion moment which twists the wing, resulting in a lower local angle of attack and, consequently, lower lift on the up-moving part of the cycle and vice versa, the aerodynamic damping can be increased. By shifting the engines forward, the engine mass will introduce such a torsion moment into the wing. Of course, when placed further outboard, the engine mass will cause a larger torsion angle with the same torsion moment, so shifting the engines out towards the wingtips makes this more effective.

Shift the resonance speed below minimum flight speed: This might sound silly, but works sometimes: When flying fast enough, the phase shift can be small enough to turn aerodynamic damping back into the positive region, and the plane will not flutter with that particular flutter mode. But it might still flutter at a harmonic …


Generally, to achieve positive damping, the center of gravity of a part should be ahead of its elastic axis. In the case of control surfaces, their center of gravity should be in or ahead of their hinge line. That is the reason for balancing masses!

* to be precise: As long as damping retards the motion, the damping term is mathematically negative and turns positive when it starts to support it.