From Wikipedia,
The P-791 is an experimental aerostatic/aerodynamic hybrid airship developed by Lockheed-Martin Corporation. The first flight of the P-791 was made on 31 January 2006 at the company's flight test facility on the Palmdale Air Force Plant 42. According to press reports, the designation "P-791" has no particular meaning.

Hybrid airship design

The P-791 is an example of a hybrid airship. In such designs, part of the weight of the craft and its payload are supported by aerostatic (buoyant) lift and the remainder is supported by aerodynamic lift.

The combination of aerodynamic and aerostatic lift is an attempt to create a craft that is a "best of both worlds" combination with the high speed of aerodynamic craft and the lifting capacity of aerostatic craft. Critics of the hybrid approach have labeled it as being the "worst of both worlds" in that such craft require a runway for take-off and landing, are difficult to control and protect on the ground, and have relatively poor aerodynamic performance.[citation needed]

Proponents of hybrid designs claim that these shortcomings can be overcome through advanced technologies. In particular, it has been proposed that the use of various buoyancy control mechanisms can minimize or in some cases eliminate the need for a runway. However, to date, no effective buoyancy control mechanism has ever been demonstrated on a hybrid airship.

No hybrid airship design has ever been developed past the initial experimental stages. And, in spite of the fact that many such designs have been proposed over the years, very few proposed designs have ever flown. One example of hybrid airship design that did take flight was the Aereon 26.