Supersonic flight over the continental U.S. is prohibited because of the potential damage from sonic booms, i.e., pressure waves from the jet that slam into the ground.
That’s why NASA engineers at Edwards Air Force Base in California have been exploring noise-reduction techniques using a modified F-15 fighter. The so-called Lancets (lift-and-nozzlechange- effect-on-tail-shock) program measures the benefits of redistributing aircraft lift to reduce shock-wave pressure, thereby lessening the sonic boom.
But measuring pressures on the surface of a supersonic plane from a chase plane 200 ft away is risky and expensive. So NASA decided to see if CFD software could simulate flight conditions accurately enough to predict how minor adjustments would reduce noise levels. To do this, they needed a digital stand-in for the F-15.
Sectional CAD drawings of the plane were available, but assembling them into a representation of the real airframe would not yield a model accurate enough to generate the needed data. Also, the test airplane had been hard landed, twisting the airframe enough that the dimensions differed fractionally from the original drawings. What NASA needed was a dimensionally accurate 3D photo of the F-15.
The space agency turned to testing engineers at Direct Dimensions Inc. (directdimensions. com), Owings Mills, Md., for a computer model accurate to ±0.25 in. and a resolution of 0.125 in. A 3D image-capturing scanner from Faro Technologies of Lake Mary, Fla. (faro.com), digitizes objects up to 250 ft away, which meant scanning the 64ftlong F-15 would not present a problem.
The scanner captures so many points in a given scene (120,000 pps) that the resulting files resemble 3D gray-scale photographs. In less than a minute, the scanner can take an 8-megapixel image of the surrounding area, with resolutions of 0.009° vertical and 0.00076° horizontal. The point-cloud images can be viewed from any angle, or enlarged to show excellent detail.
The scanner sees everything within its field of vision. So the team cleaned up the background, removing anything that would have had to be digitally erased. To a laser, engine intakes and exhausts appear as large black holes. The landing gear and panels that cover gear compartments were other unknowns.
To represent supersonic-flight conditions, the team had to find a way to image the parked plane, then digitally retract the gear. The canopy presented another challenge. To keep the laser from passing right through it, they treated it with a light-reflecting material.
As for the curvature of the plane’s surface, there is a natural fall-off of reflected data. So the team made additional scans over these areas to be more nearly perpendicular to points around bends. And most of the scans took place in the evening to minimize washout of the laser by sunlight.
More than 50 scans of the airplane, over and under, fore and aft, port and starboard, yielded a total point-cloud harvest of about 50 million points. Removing overlapping data and stitching scans into a 3D point cloud of the complete aircraft reduced the file size to about 1 million points.
The clean image went to PolyWorks (Innovmetric Inc., Quebec City, Canada), which extracted geometric shapes of the airframe, essentially reverse engineering the airframe from the point cloud. The team established the centerline and contours, put in cross sections, located axes of rotation all to build a wire frame suitable for CAD. Images of the engines were eliminated; NASA would add them later. Direct Dimensions supplied six versions of two models, including formats in IGES, STEP, and one containing Nurbs surfaces.
Up the coast from Edwards, at NASA’s Ames Research Center, the CFD team assembled a virtual wind tunnel. The learning curve to adapt the F-15 models to NASA’s CFD program (Cart 3D) was short, in part because of the model’s realism.
The engineers don’t expect to soften the F-15’s sonic boom to the point it is legal to fly over the U.S. at Mach 1, but CFD tools may help with future aircraft development.