Variable-Speed Rotor Helicopters: Performance Comparison Between Continuously Variable and Fixed-Ratio Transmissions

Abstract

Variable speed rotor studies represent a promising research field for rotorcraft performance improvement and fuel consumption reduction. The problems related to employing a main rotor variable speed are numerous and require an interdisciplinary approach. There are two main variable speed concepts, depending on the type of transmission employed: Fixed Ratio Transmission (FRT) and Continuously Variable Transmission (CVT) rotors. The impact of the two types of transmission upon overall helicopter performance is estimated when both are operating at their optimal speeds. This is done by using an optimization strategy able to find the optimal rotational speeds of main rotor and turboshaft engine for each flight condition. The process makes use of two different simulation tools: a turboshaft engine performance code and a helicopter trim simulation code for steady-state level flight. The first is a gas turbine performance simulator (TSHAFT) developed and validated at the University of Padova. The second is a simple tool used to evaluate the single blade forces and integrate them over the 360 degree-revolution of the main rotor, and thus to predict an average value of the power load required by the engine. The results show that the FRT does not present significant performance differences compared to the CVT for a wide range of advancing speeds. However, close to the two conditions of maximum interest, i.e. hover and cruise forward flight, the discrepancies between the two transmission types become relevant: in fact, engine performance is found to be penalized by FRT, stating that significant fuel reductions can be obtained only by employing the CVT concept. In

Aeroelastic eigenvalue analysis of a variable speed rotor blade with an applied compressive load

“…The rotary wing sector strives towards developing aircraft with increased performance capability and reduced emissions. One concept to achieve this is variable speed rotors; which have been shown to require up to 70% less power [2], expand flight envelopes [3], reduce noise emissions [3,4] and reduce fatigue (reducing operating costs and allowing for relaxed maintenance schedules [5]). This concept is utilised in the Boeing A160T Hummingbird [6] which holds the endurance record for new and experimental technologies at over eighteen hours [7], partly due to its ability to reduce its rotor speed by up to 40% [8].…”

Variable rotor speed and active blade twist for civil rotorcraft: Optimum scheduling, mission analysis, and environmental impact

“…Several authors have examined the topics of variable rotor speed and active blade twist in the existing literature [1,2,3]. However, previous researchers have approached these concepts either strictly from an aero-mechanical standpoint [6,9,14,19], or solely from an "isolated-rotor" point of view [5,11,13].…”

“…Although some recent references have tackled the problem from an "aircraftengine integration" perspective [2,3], the effectiveness of these technologies at "mission level" where assessments are carried out under realistic operational procedures instead of idealized trim conditions, has not been investigated. Thus, the implications associated with the optimum scheduling of variable rotor speed and active blade twist during a complete mission when the aircraft trim-state and gross weight vary continuously with time, have not been examined in the literature.…”

Resonance avoidance for variable speed rotor blades using an applied compressive load

“…The rotary wing sector strives towards developing aircraft with increased performance capability and reduced emissions. One concept to achieve this is variable speed rotors which have been shown to require less power [1,2,3], expand flight envelopes [4,5], reduce noise emissions [4,6] and reduce fatigue , thereby reducing operating costs and allowing for relaxed maintenance schedules [7]. This concept is utilised in the Boeing A160T Hummingbird [8] which holds the endurance flight record for new and experimental technologies at over eighteen hours [9]; partly due to its ability to reduce its rotor speed by up to 40% [10].…”

“…Whilst variable speed rotors can achieve significant performance improvements, they encounter substantial dynamics complications [4,12]. A rotor operating with insufficient separation between natural and excitation frequencies of the blade will incur resonance which leads to increased vibratory loads that negatively affect the blade's structural performance.…”