Factors Influencing Performance Degradation

The performance of a helicopter can be degraded under different circumstances. The accumulation of dirt and dead insects on the leading edges of the blades can degrade both main and tail rotor performance, although these effects are relatively mild. Estimates of such effects on rotor performance can be obtained by increasing the value of in
accordance with measured effects of roughness on 2-D airfoil sections (see Section 7.9.1). The operation of the helicopter in dusty or desert environments can lead to blade erosion, and this can have much more deleterious effects on rotor performance. А С^0 allowance of up to 50% can be used to represent such effects in rotor performance estimation. Aerody­namic degradation of the rotor performance from battle damage can be an issue for military helicopters, but such effects are much more difficult to predict (see Section 7.14). Never­theless, an assessment of degraded effects on rotor performance is essential for assessing operational risks to military helicopters in a combat environment.

Icing is a particularly significant factor that can degrade rotor performance. For good utility, helicopters must be designed to operate in an all-weather environment, of which flight in extremely cold conditions are not uncommon. However, icing can occur under other atmospheric conditions, and there is a risk whenever visible moisture is present in the air. Under certain conditions, ice accumulation on the rotor and airframe may occur. Icing characteristics of helicopters are similar in some respects to airplanes, and ice may accumulate on windshields, engine inlets, the leading edges of the empennage, antennas, Pitot probes, and so on. Icing can result in a serious performance degradation and may also affect flight control capability. The field as applied to helicopters is comprehensively reviewed by Flemming (2002).

Ice accumulation may increase the parasitic drag of the airframe. Engine inlet icing may reduce engine power and in extreme cases can cause engine surge-stall. However, airframe, engine inlet, and windshield icing can be dealt with effectively by electrical deicing. Em­pennage icing and the associated aerodynamic degradation is usually less important for helicopters than for airplanes because icing may only affect trim, and on most helicopters the rotor itself has ample flight control capability to compensate for this. Icing on the rotor, however, can have serious consequences, with a significant degradation in aerodynamic capability – see Flemming et al. (1987). Because the type and accumulation of ice normally varies in a 3-D manner along the blade span, and may be different from blade to blade, icing can be accompanied by significant rotor vibrations. Modem helicopters may be ca­pable of flight into icing conditions with suitable deicing protection, but most helicopters are not certified for flight onto known icing conditions because of the amount of flight certification tests and high costs incurred by the manufacturer. Icing protection on the rotor is provided by electrically heated deicing mats that are molded into the composite blades during manufacture [see Rauch & Quillien (2003)], but Coffman (1987) discusses other concepts.

Steady 2-D airfoil testing in special icing wind tunnels has allowed measurements of ice accumulation at the leading edge of rotor airfoils, which have shown substantial degradation on aerodynamic performance – see Flemming & Lednicer (1985). Unsteady tests on airfoils, which are designed to more accurately simulate the time-varying flow environment at the rotor, have suggested different ice shapes may accumulate, with smoother profiles because of the continuously varying AoA. Tests with subscale rotors in special icing tunnels has been conducted to more accurately assess the susceptibility of helicopters and their rotor systems to icing – see Korkan et al. (1984) and Flemming & Saccullo (1991). These icing shapes can be used to cast molds to modify the blades of other model rotors, which are then tested in dry-air tunnels in an attempt to measure the effects on rotor performance. Scaling issues (such as Reynolds number effects and heat transfer) that are associated with ice accumulation on model rotors are, however, not well understood. Therefore, confidence levels in extrapolating wind tunnel results to estimate the impact of icing on the full-size rotors is not yet acceptable. Various mathematical models have also been developed to assess the effects of icing on rotor performance both empirically [see Korkan et al. (1984)] and by using modem CFD approaches [see Narramore et al. (2002)]. However, validation is lacking because of the limited amount of existing flight test data, and so predictive confidence levels with CFD alone remain lower than required to satisfy the certification authorities. Actual in-flight icing tests are often augmented by artificial tests to cover the full flight envelope of the helicopter – see Ramage (2004). Computational methods continue to be developed, however, to enable better predictions of the aerodynamic characteristics with ice accreted on the entire rotorcraft with the ultimate goal of reducing significantly the amount of required certification flight testing [see Narramore et al. (2003)].