Sphere vs Blimp

After many years of successful flying with spherical balloons, we switched to blimp shaped aerostats. When flying a Lighter Than Air (LTA), the overall weight is critical. Therefore, the weight of the envelope needs to be minimized by reducing the thickness of the material. 115cu ft of lift gas provides the required lift for our payloads and is below the threshold of FAA regulations for tethered balloons ( see“Regulations” above )

On paper, the sphere has the best weight-to-volume ratio. So, a 6 feet diameter balloon is the lightest LTA, and should work really well. But it has a high drag coefficient. 

At 2 knots, the Drag is at its lowest: 0.08. But for realistic world conditions (8 to 20 knots) the drag goes from 0.2 up to 0.7. This is very high! In addition, a lot of unsteady turbulence are created, making the flight unstable. To increase stability, some companies add a skirt, or kite-like surfaces. It helps a little with stability, but increases the drag coefficient and increases the overall weight. No free meal...

Yes, we flew many years with spherical balloons, but a sudden increase in wind would bring the LTA down which required constant attention from the operator(s).

For comparison, the following graph shows average the drag coefficient of a blimp -like LTA. [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091470.pdf]. The drag coefficient values are at least 3 to 10 times lower than a spherical balloon. In addition, the drag decreases when wind speeds increase. Turns out, you can get a free meal after all!!

So, the blimp shape is a clear winner. 

 Some additional benefits include: 

• Smaller diameter (4.5ft): it fits through doors for easy storage.

• Larger surface: custom advertisement using detachable banners.

• Rock steady stability: no external feature to create turbulence.