Density Altitude, Part Three: Gyroscopics

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A tumble or gyroscopic maneuver has been called an “out of control” maneuver. While it is not purely aerodynamic, it is not out of control. A gyroscopic is a physics-based maneuver combining aerodynamics, momentum, gravity and the very predictable forces associated with a gyroscope, in our case the prop.

When we practice our gyroscopics at sea level, we balance these forces to accomplish a consistent entry and exit from what appears to be an “out of control” maneuver. The forces at our disposal are generated primarily by the flight controls of the airplane. We can adjust the gyroscopic force by changing RPM, but — for the sake of this discussion – let’s assume RPM is constant.

When we move from sea level to high density altitude (DA), RPM and the resulting gyroscopic force is unchanged. Lift and drag on the aircraft are also unchanged if we start the maneuver at the same indicated airspeed. Gravity is also unchanged. Unfortunately, that’s where the similarity ends.

Engine power is reduced at high DA, meaning less airflow over the control surfaces. True airspeed and the resulting kinetic energy and momentum are substantially higher. Reduced engine power leads to less thrust, thereby reducing vertical penetration and less altitude gain. The control forces available are less at high DA. The rate of deceleration is higher. The altitude gain on the upline is lower. All this leads to a completely different maneuver than that which was perfected at sea level.

While it is mathematically possible to define the entire maneuver at sea level and the resulting differences at high DA, the reality is that the only way to safely perform gyroscopic maneuvers at high density altitude is to go up where the air is rare and practice.

If you are fortunate enough to be invited to the high country, plan on arriving a few days early. Get in the air and find out exactly what your airplane will do and what it won’t do. Plan your maneuvers to be aborted. As those who fly above 4000 MSL know, regardless of your planning and practice, sometimes it just doesn’t feel right. At high DA keep it simple. Keep it clean. Give yourself an extra margin for every maneuver.

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