Imagine a submarine floating through the sky, and you’ll essentially have an airship; a.k.a., a blimp. Strange but true: beyond a physical likeness, the design and mechanical features of a blimp are closer to a submarine than an airplane.
Such a melding of concepts introduces design elements unlike those in any other aircraft – something American Blimp Corp. realized first hand when the manufacturer of its custom fan blades went defunct. The company turned to engineering design firm Advanced Design Concepts (ADC) to create injection molds from the actual part. The new process improved quality, cut manufacturing time, and reduced costs by nearly 90 percent.
ADC offers rapid prototyping, reverse engineering, and first-run manufacturing services. The company uses Geomagic Studio and Geomagic Qualify software to automatically convert point clouds from a scanned physical part into accurate digital models, and to graphically compare those models to the part for quality inspection. The models are then used for downstream CAD/CAM, engineering analysis, and machining. For American Blimp, ADC used the digital models to reconstruct the blade, create an injection mold for manufacturing, and inspect the mold for accuracy.
Up the proverbial creek
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 to spur a rise in altitude, but care must be taken to not release too much helium – which can cause the ballonet to conversely 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.”
One distinctive design aspect of American Blimp airships are the fan blades, modeled after the Moulton B. Taylor Aerocar of the 1950s. The car/airplane incorporated a cooling fan that American Blimp thought suitable for cooling the airship engines. This is particularly important, because the relatively low airspeeds of airships make their engines difficult to cool.
For nearly a decade, American Blimp purchased the blades from Mouton B. Taylor’s aircraft manufacturing business. Unfortunately, Taylor passed away in 1995, his business fizzled, and the supply of these blades eventually ran out. 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 dried up 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.”
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 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, which scanned it with a Perceptron laser scanner mounted on an 8-foot Romer arm. The blade was fixtured using one of its existing holes, so the scanner could see all of the part’s surfaces.
“Perceptron allows you to orient 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 2D data had to be recorded, because the high resolution of the scanner allowed us to capture everything we needed to reverse-engineer the part.”
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
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 Geomagic Studio, then polygonized again. Groth smoothed out the polygons to remove any imperfections captured on the original part, rebuilt the parting line on the blade, filled in existing holes, and created a reference to be exported for future use.
“The parting line was the most important area to define when generating the surface,” Groth says.
Once the polygon model was finalized, Geomagic Studio 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.
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 +/- .005.
Using the new injection-mold process, it costs American Blimp less than $6 to make each fan blade, compared to about $50 when the parts were manufactured directly on a CNC machine. The new 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 low creep – particularly in applications above room temperature.
“The parts have a much better surface finish and are more consistent,” Nordby says. “The smoother surface and greater stiffness should also improve the blade’s efficiency.”
The company is currently considering the same process for the blocks of its engines, which tend to have odd free-form shapes. After Geomagic software digitally reconstructs an accurate model of the engine, the company can use SolidWorks software to create interfacing parts such as sheet aluminum cooling baffles.
“Creating work in 3D forces greater discipline upon the designer to get things right from the beginning,” Nordby says. “Instead of having to fix work that’s already been done, we can move on to new projects that benefit the company.”