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 manned and unmanned prototypes have flown.
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.
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 HTA type.
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.
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.
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, 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 take off 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.
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.
Gliding under gravity
If an airship does not have enough lift, it will sink under gravity. By angling the nose down, this can lead to a gliding forward flight. If an airship has excess lift, it will rise. By angling the nose up, this can also lead to forward movement. In this way, an airship can gain aerodynamic forward thrust like any other glider, by periodically alternating its buoyancy between positive and negative while adjusting its attitude accordingly. Thus, flight proceeds in a leisurely vertical zig-zag pattern. Because no energy is consumed directly in creating thrust, the principle allows for flights of long duration, although at slow speeds. The proposed Hunt GravityPlane is a hybrid airship designed to take full advantage of gravity gliding.
The principle also works underwater, and is used operationally in the underwater glider.
Gliding under gravity dates from the period during and shortly after the American Civil War, when Solomon Andrews built two such airships. The first of these, Aereon, used three individual cigar shaped balloons rigged together in a flat plane; the second, Aereon #2, employed a single "lemon-shaped" balloon. Andrews' Aereons were propelled by angling the balloons upward and dropping ballast, then process was then reversed with the balloons being angled downward and large quantities of lifting gas being vented.
In 1905 Alberto Santos-Dumont conducted various experiments with his first airplane, the Santos-Dumont 14-bis, prior to attempting to fly it for the first time. These included hanging it from a steel cable and towing it, and subsequently hanging it beneath the envelope of a previously built airship (Number 14) - akin to learning to swim with "water wings". The combined craft was unusable, and was broken up, being referred to as "a monstrous hybrid". After these "rehearsals" were completed, Santos-Dumont made the first public demonstration of a heavier-than-air aircraft in Europe.
In 1907 the British Army Dirigible No 1 (named Nulli Secundus) first flew. It used aerodynamic surfaces for attitude control in flight, and for its first flight was also fitted with large wings amidships. The wings were intended to aid stability rather than provide lift and were removed for all subsequent flights. The use of dynamic lift by pitching the nose of the airship up or down was also recognised and practised on this airship.
In June 1907 Alberto Santos Dumont constructed his No. 16, described by l'Aérophile as an appareil mixte. This had a 99 m3 (3,500 cu ft) envelope but was too heavy to fly without supplementary lift supplied by a 4 m (13 ft) wing surface. It was tested without success on 8 June 1907.
The Aereon 26 was an aircraft which made its first flight in 1971. It was a small-scale prototype of the hybrid Airship Aereon Dynairship and part of the "TIGER" project. But it was never built due to lack of market for a hybrid airship.
The 1986 Piasecki PA-97 Helistat experimental design combined four helicopters with a blimp in an attempt to create a heavy-lift vehicle for forestry work. It broke up at the end of its first flight possibly due to inadequate structural strength
The SkyCat or "Sky Catamaran" vehicular technology is a hybrid aircraft amalgamation; a scale version at 12 meters called "SkyKitten", built by the Advanced Technologies Group Ltd, flew in 2000. The U.S. Defense Advanced Research Projects Agency (DARPA) initiated the Walrus Hybrid Ultra Large Aircraft program in 2005, a technology development initiative focused on ultra heavy air lift technology explorations. The program was terminated in 2007.
In 2006, the Lockheed Martin P-791 underwent manned flight tests. It was an unsuccessful candidate for the military Long Endurance Multi-intelligence Vehicle program even though it was the only successful Hybrid Airship to have ever flown until 7th August 2016.
The Hybrid Air Vehicles HAV 304 was built for the US Army Long Endurance Multi-intelligence Vehicle (LEMV) program. It flew unsuccessfully for 90 minutes in August 2012. The long duration of the first flight was primarily due to problems with center of gravity control and failure of one of the forward engines during the first few minutes of the test flight. The flight ended with the LEMV striking the ground in a nose high attitude which resulted in significant damage to the tail of the airship. Following cancellation of the LEMV project, Hybrid Air Vehicles re-purchased the HAV 304 vehicle and brought it back to the UK. It has been refurbished and renamed the Airlander 10. On August 17th, 2016 the Airlander 10 had it's first successful test flight outside the Cardington Hangars at RAF Cardington. Chief Test Pilot Dave Burns said in a statement "It was privilege to fly the Airlander for the first time and it flew wonderfully. I’m really excited about getting it airborne. It flew like a dream."  Over 200 more flight hours are needed for full certification, which HAV intends to complete over the summer.
Other current projects
An Australian-based company is working on a project to develop an air crane called the SkyLifter, a "vertical pick-up and delivery aircraft" being capable of lifting up to 150 tons.
A Canadian start-up, Solar Ship Inc, is developing solar powered hybrid airships that can run on solar power alone. The idea is to create a viable platform that can travel anywhere in the world delivering cold medical supplies and other necessitates to locations in Africa and Northern Canada without needing any kind of fuel or infrastructure. The hope is that technology developments in solar cells and the large surface area provided by the hybrid airship are enough to make a practical solar powered aircraft. Some key features of the Solarship are that it can fly on aerodynamic lift alone without any lifting gas,[not in citation given] and the solar cells along with the large volume of the envelope allow the hybrid airship to be reconfigured into a mobile shelter that can recharge batteries and other equipment.
The Hunt GravityPlane (not to be confused with the ground-based gravity plane) is a proposed gravity-powered glider by Hunt Aviation in the USA. It also has aerofoil wings, improving its lift-drag ratio and making it more efficient. The GravityPlane requires a large size in order to obtain a large enough volume-to-weight ratio to support this wing structure, and no example has yet been built. Unlike a powered glider, the GravityPlane does not consume power during the climbing phase of flight. It does however consume power at the points where it changes its buoyancy between positive and negative values. Hunt claim that this can nevertheless improve the energy efficiency of the craft, similar to the improved energy efficiency of underwater gliders over conventional methods of propulsion. Hunt suggest that the low power consumption should allow the craft to harvest sufficient energy to stay aloft indefinitely. The conventional approach to this requirement is the use of solar panels in a solar-powered aircraft. Hunt has proposed two alternative approaches. One is to use a wind turbine and harvest energy from the airflow generated by the gliding motion, the other is a thermal cycle to extract energy from the differences in air temperature at different altitudes.
|ATG SkyKitten||United Kingdom||2000||Experimental||Prototype||scale demonstrator for the proposed SkyCat.|
|Hybrid Air Vehicles HAV-3||United Kingdom||2008||Experimental||Prototype||Technology demonstrator.|
|Hybrid Air Vehicles HAV 304 Airlander 10||United Kingdom||2012||Multi-role||Prototype||Built in conjunction with Northrop Grumman for the US Army LEMV programme. Subsequently rebuilt as the Airlander 10.|
|Lockheed Martin P-791||USA||2006||Experimental||Prototype|
|Nimbus EosXi||Italy||2006||UAV||Small (7 m) delta-wing hybrid|
|Walrus HULA||USA||2010||Transport||Project||DARPA project, cancelled 2010.|
|AeroLift CycloCrane||USA||1984||Flying crane||Prototype|
|Piasecki PA-97 Helistat||USA||1986||Flying crane||Prototype|
|SkyHook JHL-40||USA||Flying crane||Project||Joint project with Boeing|
|Andrews Aereon||USA||Experimental||Prototype||Propulsion by alternately dropping ballast and venting gas.|
|Andrews Aereon 2||USA||Experimental||Prototype||Propulsion by alternately dropping ballast and venting gas.|
|Physicist Robert D. Hunt's GravityPlane||USA||Project||Various means of energy harvesting proposed. Ballast Control by phase change of the working fluid lifting gas i.e. water vapor to water; and, compression to increase density of lifting gas and lift loss as currently proposed by Aeroscraft to which Hunt advised.|
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For every 1,000 lbs. lift carried by the planes, approximately 60 lbs. resistance must be overcome by the thrust of the propellers. On the other hand, a 5,000,000 cu.ft. airship flying at 60 mph experiences only about 20 lbs. resistance per 1,000 lbs lift, and the relative resistance decreases with increasing size and diminishing speed. It is apparent, therefore, that the increase in lift obtained by the use of planes on an airship would require a disproportionate increase in engine power and fuel consumption.
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