Reverse Engineering a Blimp Fan Blade

Besides the fact that it flies, an airship—better known as a blimp—has about as much in common with other aircraft as a whale has with fellow sea creatures.

Among a blimp’s unique design elements are custom fan blades used to cool its engine. The fan blades were especially problematic for American Blimp Corp., maker of the world’s most popular airship, after its initial suppliers went out of business.

The solution came from Advanced Design Concepts Inc. (ADC), which used Geomagic software to create injection molds based on scans of the original fans. The new process improves quality, cuts manufacturing time, and reduces cost by nearly 10 fold.

Strange beasts in the air

The mechanics of an airship make it nearly as intriguing as its peculiar form. Once inflated, the airship becomes “alive.” An air bladder inside the envelope—called a ballonet—is pressurized by the blast from the engine propellers or by electric fans incorporated into the airbox. The pressure inside the ballonet acts on the helium inside the envelope to maintain the shape. The airbox has various valves that control and regulate the pressure semi-automatically, and manual controls allow the pilot to override the airbox if necessary. 

As an airship rises, the helium expands and at some point the ballonet—which encompasses a very small percentage of the envelope volume—becomes empty. This condition is referred to as “pressure height.”  Pressure height limits the airship’s maximum altitude, as helium must be vented for the airship to go higher.

Helium valves can be used to release helium manually or automatically, but care must be taken not to release too much helium—which can cause the ballonet to become too full at lower altitudes. When that happens, a replaceable panel in the ballonet is ripped out, allowing air into the helium chamber. The airship can then descend, but the contaminated helium will have to be replaced before flying again.

“Airships are strange beasts and are totally different from ‘normal’ aircraft,” says Lance Nordby, project engineer at American Blimp. “They have a number of systems on board that have no counterpart in the airplane world.”

Up the proverbial creek

The difference between airships and other aircraft is exemplified by the American Blimp fan blades, which are modeled after those in the Moulton B. Taylor Aerocar of the 1950s. The car/airplane incorporated a fan that American Blimp thought suitable for cooling airship engines. Fans are particularly important for airships, because relatively low cruising speeds make the engines difficult to cool.

For nearly a decade, American Blimp purchased the blades from Mouton B. Taylor’s aircraft manufacturing business. Unfortunately, when Taylor passed away in 1995, his company and American Blimp’s engine blades went with him. With no access to the original molds, American Blimp had a third-party vendor use a CNC machine to manufacture the blades out of acetyl from a scan of an original blade. Eventually that source went out of business as well.

“The vendor had ownership of the scan file,” Nordby says. “Suddenly they went out of business with no notice and we had no way to buy the scan file if we had wanted to. We were up the proverbial creek.”

Recreating the exact shape

The shape of the blade—now incorporated into all of the company’s airship designs—had to be exact to provide the appropriate cooling characteristics and to fit on existing hardware. That made designing a new blade from scratch difficult or even impossible.

Given the reduction of cost for short-run injection molding, American Blimp had already been contemplating having molds made for manufacturing the blades. ADC was a company that could do the whole job—scanning, mold-making, and production.

American Blimp sent an original fan blade to ADC, and it was scanned with a Perceptron laser scanner mounted on an 8-foot Romer arm. The blade was fixtured using one of its existing holes, so that the scanner could see all of the part’s surfaces.

A Perceptron laser scanner captured the original fan blade, generating scans that totaled 2.2 million points.

“Perceptron allows you to orientate the head in many different positions to capture every necessary angle,” says Greg Groth, senior designer at ADC. The scanner can capture more than 23,000 points per second with 50-micron accuracy. “No 2-D data had to be recorded, because the high resolution of the scanner allowed us to capture everything we needed to reverse-engineer the part.”

Once the data was collected, a point cloud with around 2.2 million points was brought into Geomagic Studio software. Groth did a uniform sampling of the individual points, automatically converted each scan into polygons, and merged them into one model. To maintain the edge on the blade, the merged points were sampled using a curvature-based setting in the software, then converted to polygons again. Groth smoothed out the polygons to remove any imperfections captured from the original part, rebuilt the parting line on the blade, filled in existing holes, and created a reference to be exported for future use.

In Geomagic Studio, scans were merged and automatically converted into a polygon model. The model was smoothed out to remove any imperfections from the original part, a parting line on the blade was rebuilt, and holes were filled.

Once the polygon model was finalized, Geomagic Studio was used to create a NURBS surface model that could be imported into Pro/ENGINEER software.

Once the polygon model was finalized, the software was used to automatically generate a NURBS surface model. The model was imported into Parametric Technology’s Pro/ENGINEER software, where the mechanical features were added and an injection mold tool was built. IGES files of the tool components were exported to Surfware’s Surfcam system to manufacture the injection mold with a CNC machine.

IGES files of the tool components were exported to Surfcam software to manufacture the injection mold with a CNC machine.

In the final step, the aluminum mold was mounted on a JSW injection-molding machine, and injected with acetal to make the blade. ADC used Geomagic Qualify computer-aided inspection software to automatically align and compare measurement data from cross-sections of the physical part with the digital model, verifying that the molded blade matched the original design.

Since the final model shared the same coordinate system of the point cloud, there was no need to register the data sets. This allowed Groth to automatically generate a color plot in Geomagic Qualify. The final tolerances for the mold were +/- 0.005 in.

Geomagic Qualify was used to automatically align and compare measurement data from cross-sections of the physical part with the digital model, generating a color plot that showed final tolerances of +/- .005 in.

Better, faster, cheaper

Using the injection-mold process, the cost for American Blimp to make each fan blade is slightly more than $5, compared to about $50 when the parts were manufactured directly on a computer numerical controlled machine. The process allows the blades to be molded from glass-filled acetal, which has better fatigue and UV resistance, as well as high stiffness, low warpage, and better resistance to deformities.

“The parts have a much better surface finish and are more consistent,” Nordby says. “The smoother surface and greater stiffness also improve the blade's efficiency.”

The new process is so successful that American Blimp is looking for other possible applications, that are almost certain to be unique to this one-of-a-kind aircraft.

The new process allows blades to be molded from glass-filled acetal, which has better fatigue and UV resistance, as well as high stiffness, low warpage, and low creep.

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