Concept Evaluation

At its current state, the concept may not be as competitive compared to current vertical take-off (VTOL) air taxi concepts. Most air taxi concepts utilize electric flight as part of their main feature while still being capable of performing VTOL. Some competitors include the Kitty Hawk Heavyside which was already proven for electric flight and demonstrated100 mile (161km) flight at 180mph (156.5 kts) [1]. While it has less range, it is much more economical and more affordable to operate compared to the proposed 'Niska' concept. However, it is possible to replace the aircraft's propulsion system with a fully electric system with a caveat. The aircraft can only be converted to an electric aircraft with the basis that the aircraft would only be utilized in a conventional flight. For fully electric VTOL operation, the aircraft's range would be reduced greatly to only within city limits of around 60km radius. In a conventional operation, however, the aircraft can meet the proposed range of 300km using the entirety of its battery charge. This would require modifications to the aircraft structure to remove the integrated VTOL design aspect such as the tiltrotor system and extra reinforcements for the tilting wing system.

The proposed concept was estimated to have a 300km (162 nmi) range with an additional 30 km diversion range and reserve fuel enough for 45 minutes of loitering. With these specifications, it meets the IFR requirement provided by the FAA and CAR [2]. The aircraft can perform two attempts on lading on top of the previously mentioned specifications. Additionally, the aircraft would also be equipped with a Garmin G1000 flight system on top of the traditional analog six-pack instrument cluster for redundancy. The six-pack instrumentation would indicate the airspeed, altitude, vertical speed, altitude indicator, heading, and turn coordinator. Furthermore, to meet the Canadian IFR regulations, engine conditions would also be monitored for fuel, pressure, and temperature as well as the addition of anti-icing equipment for the pitot tube and wing leading edge [3]. 

The aircraft's main feature, its amphibious capability, while useful in various regions with large water bodies, was found to be impractical for the purpose of urban mobility. The added requirement for amphibious requirement added weight and complexity for the aircraft especially considering the reinforcement and watertight compartment requirements [4]. The aircraft's water operation must also be considered carefully as it requires the aircraft to be stable on the water. This would certainly require the addition of pontoons or fuselage extension considering the high wing design.  This is especially important considering the size and the aircraft's weight of 8767 lbm. The size and design of the aircraft along with its weight proved to be a challenge to design safe floatation parameters for. The concept aircraft has a similar wing loading to a Grumman G73 Mallard at 31.44 lb/ft2 for Niska compared to 31.53 lb/ft2 for the Mallard [5]. While they have similar wing loading, the Mallard is a considerably larger aircraft with heavier weight (almost double) but more volume to displace water. The aircraft's passenger capacity of 20 people shows how large the aircraft is. When comparing Niska to a smaller amphibious aircraft at the same weight category such as the G21 Goose, Niska is considerably heavy for its category. The wing loading of a G21 is lower at 30 lb/ft2 which is 5lb/ft2 lower than the concept aircraft [6]. The G21 aircraft design follows a more traditional taildragger flying boat design which displaces a large volume of water. This is more advantageous compared to Niska's design which was focused in weight saving and lightweight structure design where the structure extremely tapers as it approaches the tail. The concept aircraft profile can be compared to the G44 Widgeon which is similar to the G21 as seen in Figure 1.

Figure 1, Side comparison between the Widgeon and Niska[7].

As seen in the figure, the main fuselage of Niska that would have to support the aircraft on water has a similar size as the Widgeon. However, the Widgeon has an MTOW of 5500lb which is considerably smaller than that is on Niska [8].  For these reasons, pontoon or fuselage extension may be required for the aircraft to properly float. This modification can greatly affect the aircraft's cruise performance as the addition of surface area would increase the parasitic drag. 

The aircraft design style falls in line with more common flying boats design and can be somewhat attractive to customers. The specifications for the aircraft are also reasonable for within and short hops between cities with a range of 300km at the speed of 150kts. The aircraft's maximum take-off weight is considerably high at 8767lb (3977kg) but it can also have a reasonable useful weight. However, maintenance of the aircraft can prove to be cumbersome as the two PT6 engines can be expensive to overhaul [9]. The engines by themselves could cost over one million U.S dollars to acquire and can cost upward of 500,000 USD to overhaul [9]. The overhaul cost does not include expensive replacement of the life-limited parts that the aircraft would need every 3600 hours [9]. Similarly, composite technology, while very useful in weight-saving, has a high cost in manufacturing. Despite extensive tests, the idea was abandoned presumably due to its complexity and technology at the time. The proposed engine for the aircraft, the pair of PT6A 67F, may also have been overpowered at the total power of 3400hp. A smaller and more economical version of the PT6 engine could have been chosen considering that the aircraft would require 4lb/hp of power loading for VTOL which translates to roughly 2200hp. Initially, the engine was chosen to allow the aircraft to hover and perform VTOL without the shelf of ground effect. 

As a designer of this aircraft, I would consider the manufacturing and purchase of this aircraft if funding was not a constraint. However, I would also consider this aircraft as a niche and novelty design due to its shortcoming. This aircraft proposes a rare concept of an amphibious aircraft capable of performing VTOL and cruise in a fixed-wing configuration. This idea is not new considering that the U.S Navy had done experiments utilizing a modified Grumman G21 Goose in a tiltwing configuration named the Kaman K-16B [10]. Realistically, smaller and less powerful variants of the PT6 may have been a good candidate to lower the cost and possibly the weight of the aircraft. For these reasons, a considerable reiteration of the design is required to make the aircraft more appealing. At its current state, the aircraft can overperform the requirements significantly at the expense of high cost, and complex design. 

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References

[1]        “Heaviside – Kitty Hawk.” https://kittyhawk.aero/heaviside/.

[2]        “CAR 602 Canadian Aviation Regulations.” https://laws-lois.justice.gc.ca/eng/regulations/sor-96-433/page-91.html.

[3]       CAR 605.18  Justice Laws Canada, Canadian Aviation Regulations. Canada, 2021.

[4]        U.S Department of Transportation Federal Aviation Administration, “Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations Handbook,” Civil Aviation: Standards and Liabilities, 2004, doi: 10.4324/9781003123187-21.

[5]        “Airliners.net.” https://www.airliners.net/aircraft-data/grumman-g-73-mallard/232 .

[6]        J. W. R. Taylor, F. R. S. Hisf, and A. F. R. S. Ae, Jane’s All The World’s Aircraft 1974-75. Jane’s Information Group;, 1975.

[7]        “Grumman Widgeon, March 1967 American Modeler - Airplanes and Rockets.” https://www.airplanesandrockets.com/airplanes/grumman-widgeon-march-1967-american-modeler.htm .

[8]     W. R. Taylor, Jane’s all the world’s aircraft, 1984-85. London: Jane’s Publishing Company, 1984.

[9]  “PT6A Engine: What’s it Worth in Today’s Market? | AvBuyer.” https://www.avbuyer.com/articles/engines-biz-av/pt6a-engine-what-s-it-worth-in-today-s-market-112271.

[10] “New England Air Museum.” https://www.neam.org/shell.php?page=aircraft_collection_detail&name=kamank16b.