AOPA Hover Power

Since helicopters land in areas that have not been previously approved, the pilot must make some last minute decisions regarding the landing site. One of these is the slope of the land where the helicopter will be touching down. Depending on the model helicopter the flight manual might have published limits.

The Bell 206 Jetranger is one helicopter that does not have slope limits listed in the limitations section of the flight manual. Bell’s approach is that slope landings are a function of available cyclic margin. In other words, if the pilot determines that the limit of cyclic control (close to or at the physical stop) will be reached before the helicopter is completely seated on the slope, then the slope is too steep and the landing should be aborted. (The proper technique to execute a slope landing is another discussion coming up.)

However, in the case of Eurocopter’s AS350 AStar the helicopter’s flight manual contains limitations on the amount of slope (in degrees) depending on the direction the pilot wishes to land. This is due to stress placed on the mast when landing on a lateral slope greater than 8 degrees.

The maximum slope when the ground is sloping down is 6 degrees. The shallower slope limitation in this direction is due to a 2 degree forward tilt that is built into the rotor mast. 

Also, the 2 degree tilt allows the maximum slope when the ground is sloping upwards to be 10 degrees 

Trying to determine the exact angle of a slope while hovering is difficult at best, however, with enough experience in a making off airport landings in a specific helicopter a pilot can become fairly good at judging the safety of landing on sloped terrain.

When a helicopter starts to move forward from a hover another aerodynamic condition (in addition to effective translational lift that was discussed previously) that occurs is transverse flow effect. This condition involves a differential airflow between the front and rear parts of the rotor system.

Moving forward from a hover, with no wind, the edge of the rotor system over the nose moves into clean air while the rear portion moves into air that has already been accelerated downward. This causes the angle-of-attack of the blades passing over the nose to increase, producing more lift. Because of gyroscopic precession, the maximum reaction occurs on the left side of the helicopter causing the rotor disc to tilt to the right. To continue moving straight the pilot must compensate with left cyclic.

Transverse flow effect can be recognized by an increased vibration of the helicopter at airspeeds around 12 to 15 knots and can be produced by forward flight or from the wind while in a hover. This vibration happens at an airspeed slightly lower than effective translational lift (ETL). The vibration happens close to the same airspeed as ETL because that’s when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.

A hovering helicopter can require a lot of power. However, as it moves forward the horizontal flow of air across the rotor system improves the efficiency by changing the induced flow, and therefore the relative wind, which increases the blades’ angle of attack. This added efficiency is called translational lift. The forward motion also causes other aerodynamic issues with the rotor system, like dissymmetry of lift and transverse flow effect (a later discussion).

Wind can also create translational lift. Trying to hover at a constant altitude in gusty winds requires the pilot to constantly add or reduce power to compensate. Gusty winds can affect the tail rotor and power changes require pedal input as well. Holding a precise hover in these conditions is challenging.

With no wind, translational lift starts with any amount of airspeed and continues to develop as the helicopter’s speed increases. However, somewhere around 50 knots (it varies between different helicopters) induced drag increases to the point where it overtakes the gain in efficiency from translational lift.

Effective translational lift (commonly referred to as ETL) is a term used to describe the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter’s rotor disc moves completely out of its own downwash and into undisturbed air. Depending on the helicopter this occurs between 12 and 18 knots of airspeed. The pilot will recognize effective translational lift on departure when the helicopter begins to have a noticeable tendency to climb and on approach when the helicopter starts to sink as the airspeed drops below ETL.