Whirl Mode

Whirl Mode is a divergent, low-frequency vibration of the engine and mount which can result in separation of the engine from the airframe. Whirl Mode is an issue which is often ignored when designing the engine mounting structures used to install V8 engines on aircraft.

As of this writing (2003) FAR Part 23 does not currently require whirl-mode analysis for piston powered aircraft. However, with a high-powered V8 installation, there is the potential for generating a Whirl Mode oscillation. This section explains why.

Whirl Mode is a divergent conical oscillation of the engine and mount about an axis more or less parallel with the powerplant static thrustline. It occurs when the application of a gyroscopic moment to the powerplant provides an excitation of a bending resonant frequency of the powerplant / mount system.

The divergent oscillation occurs because of a positive feedback loop created by the 90° phase offset between the airframe motion generating the gyro-moment and the bending response of the structure. It usually begins when a large, sudden gyroscopic moment (such as from a sharp gust when flying in "chop") is applied to the engine mount structure which, although it may be sufficiently strong, the structure is not sufficiently stiff. That sudden load causes a relatively quick bending deflection 90° from the triggering maneuver. For example, a propeller rotating clockwise (viewed from behind) when subjected to a sudden nose-right yaw, produces a pitch-down moment which is proportional to the velocity of the yaw.

The feedback loop occurs when:

1. the initial bending deflection is quick enough to generate a large, new gyroscopic moment, which again, is applied 90° from the generating deflection,
2. when the excitation frequency is sufficiently close to a resonant bending frequency of the engine-and-mount system, and
3. the horizontal and vertical resonant frequencies are insufficiently separated.

If this divergent oscillation begins, the prop-end of the powerplant can begin to whirl around the static prop centerline, describing a horizontal cone of ever-increasing base diameter until something breaks off.

For example, suppose that an aircraft has an engine mounting structure with insufficient STIFFNESS (as differentiated from strength). Suppose that aircraft encountered a violent gust which caused it to pitch-up rapidly. If the prop rotation is clockwise, the prop gyro-moment would try to bend the engine mount structure to the right. If the mount was sufficiently flexible, it would deflect (yaw) rapidly to the right, which would generate an upward gyroscopic moment on the engine mount. If the flexible mount deflects upward quickly, it causes a strong yaw-left moment. The 90°-out-of-phase excitation continues, and if these excitations and deflections occur at or near the natural frequency (or a harmonic thereof) of the engine-and-mount system, the deflections can develop into a whirling deflection of the engine structure of increasing amplitude until some disaster occurs.

Early in the days of turboprop-powered aircraft, there were several inflight breakups caused by engine nacelle departure from the aircraft because of whirl mode. Generally, it happened because the engines were much lighter and much longer than the radial engines of comparable (or less) power which they replaced. The longer mounting structures, which were designed to carry the g-loads appropriate for the powerplant weight were relatively more flexible (less stiff) than those designed to carry the heavier and shorter radial engines.

A similar scenario is possible with a high-powered, relatively light V8. An engine mount which has suitable strength to bear the g-loads might not have sufficient stiffness (resulting in a low resonant frequency) to prevent whirl. In addition to high stiffness, it is also essential that the horizontal and vertical stiffnesses of the mount be separated by an appropriate margin, in order to assure that the horizontal and vertical resonant frequencies are substantially different.

Whirl can be less likely to occur with low mass-moment-of-inertia (MMOI) composite props, but a higher probability exists with a large MMOI metal prop.

Whirl mode is an important flight-safety subject that demands analysis for each specific airframe-engine-propeller installation.