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.