A Parametric Study of a High Altitude Airship According to MAAT Multibody Advanced Airship for Transport

Transcription

1 A Parametric Study of a High Altitude Airship According to the Multibody Advanced Airship for Transport

2 Introduction 2 This work aims to analyze the role of the different design parameters associated with a conventional airship design concept to the development of a feeder according to the MAAT project. A conventional non-rigid airship, on which the hull profile is maintained by the pressure of the lifting gas contained inside the envelope, was the investigated concept. A crucial aspect of this conventional concept is that the air ballonets inside the envelope must have enough relative volume to allow the lifting gas expansion, necessary to reach the high cruiser altitude.

3 3 Fineness Ratio Influence The drag is proportional to required energy to complete a given path and to the required power to reach a given airspeed. For a fixed gross lift, and thus, lifting gas volume, the fineness ratio is the key parameter that influences the airship envelope drag.

4 4 Envelope Skin Materials

5 5 Fineness Ratio Influence The envelope skin weight is affected by envelope surface area and hoop pressure that, in turn, are affected by (l/d) ratio. An increase in the fineness ratio leads to an increase in envelope surface area and hoop pressure, thus, an increase in envelope skin weight.

6 6 Fineness Ratio Influence An increase in envelope skin weight together with drag leads to a reduction in a available useful load with an increasing in fineness ratio.

7 7 Length Influence For each skin material, a critical size is reached, where the useful load reaches a maximum.

8 8 Length Influence Nevertheless, if the fineness ratio if to be changed within the reasonable interval mentioned previously, that critical size also changes for the same material.

9 9 Altitude Influence For the conventional non rigid airship, the maximum altitude plays a crucial role in form of the required air ballonets relative volume. It is seen that, for the conventional non-rigid airship feeder concept, the 15 km altitude required by MAAT project is not reached with the prescribed conditions.

10 10 Altitude Influence Drag reduction due to lower air density in altitude is not enough to balance the increase in drag due to increased size of the airship.

11 11 Speed Influence An exponential increase in drag with the square of forward speed is obviously observed due to dynamic pressure, as shown in Figure 8. The relevance is that a 25% speed increase, e.g. from 20m/s to 25m/s produces a 56% drag increase!

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13 Conclusions A methodology to carry out parametric studies for conceptual design evolution of a high altitude airship was successfully implemented; It proved useful to identify key parameters in the design; It was found that the design fineness ratio can be kept within 4 to 8.5 interval from an aerodynamic point of view but the lower fineness ratio produce a smaller and lighter design; The airship s envelope material plays a crucial role in the weight and size (and thus: cost) of the system and the ultra-high-molecular-weight polyethylene (Dyneema ) seems to be the most suitable; Most of the materials could not reach a satisfactory design for 15 km design altitude and 5 Ton useful load; In all cases, a critical length was found beyond which, no matter the size increase, the useful load would not continue to grow. Notably this is contrary to the popular belief that the useful load of an airship grows exponentially with size; It was found that MAAT cruiser altitude of 15km may be to detrimental for the system performance. An altitude of 10 km should be the upper limit or a new concept that does no suffer the observed sharp drag increase above 12 km 13