Noise would be reduced without the weight and other disadvantages of shrouds.
The terms “inherently ducted propfan” (IDP) and “inherently ducted bi-prop” (IDBP) denote members of a proposed class of propfan engines that would be quieter and would weigh less than do other propfan engines that generate equal amounts of thrust. The designs of these engines would be based on novel combinations of previously established aerodynamic-design concepts, including those of counter-rotating propfans, swept-back and swept-forward fixed wings, and ducted propfans.
Heretofore, noise-reducing propfan designs have provided for installation of shrouds around the blades. A single propeller surrounded by such a shroud is denoted an advanced ducted propeller (ADP); a pair of counter-rotating propellers surrounded by such a shroud is denoted a counter-rotating integrated shrouded propeller (CRISP). In addition to adding weight, the shrouds engender additional undesired rotor/stator interactions and cascade effects, and contribute to susceptibility to choking.
An IDP or IDBP would offer some shielding against outward propagation of noise, similar to shielding by a shroud, but without the weight and other undesired effects associated with shrouds. An IDP would include a pair of counter-rotating propellers. The blades of the upstream propeller would be swept back, while those of the downstream propeller would be swept forward (see figure). The downstream blades would have a geometric twist such that their forward-swept tips could act as winglets extending over the tips of the upstream blades. In principle, the resulting periodic coverage of the upstream-blade tips by the downstream-blade tips would suppress outward propagation of noise, as though a short noise-shielding duct were present. Furthermore, it is anticipated that an IDP would be less susceptible to some of the operational limitations of a CRISP during asymmetric flow conditions or reverse thrust operation.
An IDBP would be based on the same principles as those of an IDP, except for one major difference: In an IDBP, to enhance structural integrity, pairs of the blades of the downstream propeller would be connected by the winglets. This arrangement is particularly suitable for high solidity installations and can reduce overall weight and drag as compared to a rotating shroud concept.
This work was done by M. A. Takallu of Langley Research Center. For further information, contact the Langley Innovative Partnerships Office at (757) 864-4015. LAR-15031-1