A hybrid airship is a powered aircraft that obtains some of its lift as a lighter-than-air (LTA) airship and some from aerodynamic lift as a heavier-than-air aerodyne. A dynastat is a hybrid airship with fixed wings and/or a lifting body and is typically intended for long-endurance flights. It requires forward flight to create the aerodynamic lift component. A rotastat is a hybrid airship with rotary wings and is typically intended for heavy lift applications. Its rotary wings can provide lift even when hovering or manoeuvring vertically, like a helicopter. No production designs have been built, but several crewed and uncrewed prototypes have flown. The term "hybrid airship" has also been used to describe an airship comprising a mix of rigid, semi-rigid, and non-rigid construction.
Features Conventional airships have low operating costs because they need no engine power to remain airborne, but are limited in several ways, including low payload/volume ratios and low speeds. Additionally, ground handling of an airship can be difficult. Because it is floating, in even a light breeze it is susceptible to wind buffeting. On the other hand, heavier-than-air aircraft, or aerodynes, especially rotorcraft, require the constant use of power to generate lift, and conventional airplanes also require runways. The hybrid airship combines the airship's aerostatic lift, from a lighter-than-air gas such as helium, with the heavier-than-air craft's dynamic lift from movement through the air. Such a hybrid craft is still heavier than air, which makes it similar in some ways to a conventional aircraft. The dynamic lift may be provided by helicopter-like rotary wings (the rotastat), or a lift-producing shape similar to a lifting body combined with horizontal thrust (the dynastat), or a combination of the two. Hybrid airships are intended to fill the middle ground between the low operating cost and low speeds of traditional airships and the higher speed but higher fuel consumption of heavier-than-air craft. By combining dynamic and buoyant lift, hybrids are intended to provide improved airspeed, air-cargo payload capacity and (in some types) hovering capability compared to a pure airship, while having longer endurance and greater lifting capacity compared to a pure aerodyne. Hybrid aircraft technology is claimed to allow a wider range of flight-performance optimizations ranging from significantly heavier than air to near buoyant. This perception of uncommon dynamic flight range when coupled with an appropriate landing system is claimed to allow ultra heavy and affordable airlift transportation.
Design Compared to a conventional airship, the hybrid can be made smaller and does not need to carry ballast for altitude control, while compared to a heavier-than-air craft the hybrid requires either a smaller rotor or a shorter runway. Where the dynastat is seen as more promising in the longer-distance passenger and freight roles, the rotastat is anticipated to be more suitable as a "flying crane" able to lift heavy external loads for shorter distances. Some airships employ thrust vectoring, typically using pivoted ducted fan propulsors, to provide additional lift when the engine thrust is no longer needed for forward propulsion. Once airspeed is gained, the craft can use body lift to help carry a load greater than its aerostatic lift capacity alone. However, such airships are not usually regarded as hybrids.
Dynastats The dynastat obtains additional lift by flying through the air. Configurations studied have included using deltoid (triangular), lenticular (circular), or flattened hulls, or adding a fixed wing. Some early airships were fitted with wing planes, with the intention of providing additional dynamic lift. However, the added lift of planes can be less efficient than simply increasing the volume of the airship. At low air speeds, of 60 mph (97 km/h) or less, the increase in lift obtained by the use of planes on an airship would require a disproportionate increase in engine power and fuel consumption compared to increasing the size of the gas bags. Moreover, the attachment of flying surfaces to the airship's envelope would require significant structural strengthening, with attendant weight gain. Conventional airships often make use of aerodynamic lift by using their elevators to set a nose-up attitude so that the main body of the airship provides some lift as it flies along; however, this is typically done to counteract minor out-of-trim conditions, and it is as likely that the nose may need to be pointed down to reduce lift. Some Hybrid designs, such as the Lockheed Martin LMZ1M (follow on to the Lockheed Martin P-791 test vehicle), use a flattened or multi-lobed hull to increase the aerodynamic lift obtainable. The aerodynamic approach is similar to that of a lifting body aircraft, although the airspeeds involved are much lower. Attainable dynamic-lift-to-drag ratios are significantly below those of efficient fixed wings, in part because induced drag increases with decreasing aspect ratio. As a result, the lift comes at a higher drag penalty than when using wings. On the other hand, compared to a helicopter, the dynastat has better fuel efficiency within a given speed range. Another issue arises during takeoff and landing, when, in calmer conditions, the airspeed may be too low to provide sufficient aerodynamic lift. For this reason, the dynastat is often conceived of as a STOL rather than VTOL aircraft, requiring a shorter runway than a conventional airplane.
Rotastats The rotastat obtains additional lift from powered rotors, similarly to a helicopter. Single-, twin-, and four-rotor designs have all been studied. Early examples in the inter-war period included designs by Oehmichen and Zodiac. These used the rotors for vertical control only, with additional powered propellers for forward flight, as in the gyrocopter. In more recent times, the experimental Piasecki PA-97 "Helistat" attached four helicopter airframes to a helium blimp, while the SkyHook JHL-40 remains a project. Typically, aerostatic lift is sufficient to support the weight of the craft itself, while, when a load is carried, the rotors provide additional lift as required.
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