Development Steps of HAPS Aircraft: Flight Test of Sub-scale Model Aircraft



SoftBank has been developing Sunglider, a large HAPS aircraft, in preparation for the launch of mobile service provision from a stratospheric communications platform (High Altitude Platform System, HAPS). The flight test for the sub-scale model of this next-generation Sunglider aircraft was successfully held at the Willcox Playa dry lake bed in Arizona, USA. This is a preparatory step for developing a next-generation Sunglider prototype, which will be an improved version of the aircraft which went under a stratospheric flight demonstration in 2020, in preparation for commercial use. In this article, the purpose and background of this experiment will be explained.

Press Release: SoftBank Corp. Successfully Completes Flight Test of Sub-scale Model of Next-generation HAPS UAS

1. Why build a sub-scale model aircraft first?

In the development of aircraft, flight tests and wind tunnel tests are often conducted using sub-scale model aircrafts that are smaller in scale than the actual aircraft. The models used here are different from models used by hobbyists, since they are built by devising similarities in physical characteristics, such as shape, weight distribution, and degree of deformation. Since similarity models provide a wide range of data for aircraft design, the production of a sub-scale model aircraft is an important confirmation step and milestone in the development of an aircraft.

2. Relationship between sub-scale model aircraft and large Aircraft

A sub-scale model aircraft is subject to the same physical laws of gravity, inertia, aero-viscous forces, etc., as the actual aircraft, so the properties of the actual enlarged model can be determined from the scale model. By determining the scale and considering the important physical laws at work in the model, we can test the large aircraft model before its actual development.

However, halving the size of the aircraft does not halve the gravity experienced on Earth. Air density, material density and strength does not reflect at the same scale. Also, it would take too much effort to fly the model into the stratosphere. These various limitations are taken into account when considering the performance of the actual aircraft.

Control Method confirmed during our experiment with Sub-scale Model Aircraft

3. Comparison with computer simulation

Now that computer simulations have advanced to a high degree, it is possible to use computer-modeled aircraft with computers and estimate their static strength calculation along with theirs dynamic response during flight. On the other hand there is a risk that mistakes in assumptions made with a computer model may not be noticed until the actual aircraft is completed, especially when building a new aircraft model, such as a HAPS aircraft. In flight tests using a sub-scale model aircraft, the aircraft is exposed to the actual laws of physics, so design errors can be noticed quickly and are obvious for improvement. Furthermore, by applying the results of flight tests with a sub-scale model aircraft itself to computer simulations, the results can be used to evaluate the validity of the calculations, rather than relying on the results of a larger aircraft.

4. Information obtained from sub-scale model aircrafts

Data from sub-scale model aircraft can be used to obtain information on aircraft behavior, control issues, and other information that is important to understand before building a large aircraft, without relying solely on computer simulations. Sub-scale model aircraft can be easily modified multiple times to find the optimal configuration. For example, to study winglets to reduce pressure changes at the wingtips, a quick comparison can be done by analyzing the effects of winglet additions, removals, and modifications from different flight tests. Other unexpected discoveries, difficulties, and possibilities for larger aircraft can be discovered by experimenting with sub-scale model aircraft.

Sub-scale model aircraft can be applied for tests that are difficult to conduct with actual aircraft, or to test the extreme limitations of an aircraft. Testing with an actual aircraft may lead to a serious air accident, so the data obtained in advance from a test with a sub-scale model aircraft is valuable.

5. Difficulties and limitations of sub-scale model aircraft experiments

Building, adjusting, and analyzing the results of a sub-scale model aircraft requires sufficient experience in developing aircraft. One important issue is at what fraction of the size to make the aircraft. In the case of wind tunnel testing, the size of the model can be reduced because the air velocity can be adjusted. However, when testing outdoors in a real environment, a 2- to 3-meter model is too small to study the performance of the Sunglider, which is actually 78 meters long. If the model is too small, the air flow and deformation around the aircraft would be different from that of the actual aircraft, and the control methods to be verified would not be understood. Therefore, the sub-scale model aircraft was made larger, taking these factors into consideration.
Recently, in the development of a domestically produced business jet, a sub-scale model aircraft was used to visualize airflow and succeeded in developing an aircraft with efficiency.

6. Flight testing on dry lake bed

A dry lake bed was used for this flight test. As the name implies, the dry, flat lake bed extends for several kilometers. It is a suitable land environment for takeoffs and landings of sub-scale model airplanes, which are easily affected by wind speed conditions and have the advantage of being able to land anywhere if something unexpected happens during overflight. It is so ideal for testing airplanes that many experimental aircraft airfields are built on dry lake beds. However, because of the tendency for water to accumulate during rainy seasons, it can be difficult to conduct tests in years of bad weather.

In our test, when all the conditions were finally met and the flight succeeded for the first time, our members rejoiced intensely on the chilly dry lake bed. The experiment was held as planned, although our joy over the successful flight was that of a child flying his or her first model aircraft. We repeated the flight multiple times to ensure we had valid flight test results.

7. Future developments

With the successful flight of the sub-scale model aircraft, SoftBank believes that it has taken a definite step forward in the technical realization of the next-generation Sunglider, which will have even higher performance capabilities. SoftBank will continue to promote development with the aim of achieving steady results, including the design of an aircraft concept, prototype production of a full-size actual aircraft, development of subsystems, and studies toward obtaining FAA type certification.

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