How it works

Dissymmetry of lift and translational lift

In forward flight the advancing blade sees its own rotational speed plus aircraft airspeed, while the retreating blade sees rotational speed minus airspeed. Without correction the advancing side would produce more lift than the retreating side and roll the aircraft. On an articulated rotor this is solved by blade flapping; the H145's bearingless, hingeless main rotor achieves the same result through blade flexure in the composite root, which acts as a 'virtual hinge' without mechanical flapping or lead-lag bearings.

Translational lift is the marked increase in rotor efficiency as the aircraft accelerates out of the hover, because the rotor moves into clean, undisturbed air instead of recirculating its own downwash (vortex ring in the hover becomes a steady inflow). It typically becomes noticeable around 15-20 kt and is felt as a slight pitch-up and a reduction in the power needed to hover, sometimes called 'effective translational lift' (ETL).

Retreating blade stall and advancing blade compressibility

At high forward speed the retreating blade must fly at a higher angle of attack to make up for its lower local airspeed, and can stall near the blade root and trailing edge. This shows as low-frequency airframe vibration, a pitch-up tendency and reduced roll response, and is one reason VNE is reduced with weight, altitude and temperature. The advancing blade tip, meanwhile, can approach transonic speeds at high forward speed and high rotor RPM, producing compressibility drag and noise; the H145's blade tip design and NR range are chosen to keep margin from both limits across the flight envelope.

Vortex ring state (settling with power)

In a steep, low-speed descent with power applied, the rotor can re-ingest its own tip vortices, causing lift loss, increased sink rate and reduced control effectiveness despite increased power -- the classic 'power settling'. Recognised by high rate of descent, low airspeed, and vibration; recovery is to lower collective and gain forward airspeed to fly out of the recirculating downwash, not to pull more power, which only deepens the vortex ring. HEMS and offshore approaches that combine steep profiles with low speed are the typical exposure.

Dynamic rollover and ground resonance

Dynamic rollover can occur if one skid (or wheel) is held on the ground as a pivot -- for example by a skid caught on a surface obstruction, or lateral cyclic held in during a sliding takeoff/landing on a slope -- while the aircraft rolls about that pivot point faster than the cyclic can arrest it. Critical rollover angle reduces with crosswind, slope and sideways drift; the defence is to keep the aircraft level, limit lateral cyclic near the ground, and lift off/set down vertically.

Ground resonance is a self-reinforcing oscillation between an unbalanced lead-lag rotor and the airframe on its landing gear, classically associated with articulated rotors with lead-lag dampers and oleo struts. The H145's hingeless/bearingless rotor and skid gear make classical ground resonance far less of a concern than on articulated, wheeled types, though a correctly serviced rotor and undamaged skid gear are still required to avoid airframe-coupled vibration on the ground.

Technical detail -- blade element view of dissymmetry of lift

Consider a blade element at 0.7 radius: on the advancing side its airspeed is rotational speed plus aircraft speed; on the retreating side it is rotational speed minus aircraft speed. At typical cruise speeds the retreating blade root can see reversed flow (air striking the trailing edge first) inboard of the point where rotational speed equals forward speed, which is one reason blade root sections are not optimised purely for lift there -- they are built to survive reversed flow aerodynamically rather than produce useful lift in it.