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Time Compression Technologies September 2001
RE Firm Helps Recreate Wright Brothers' Airplane
A pair of self-taught engineers working in a bicycle shop opened the era of aviation when their plane first took to the air nearly a century ago. Before that time, no one in aviation seemed to do anything right, but since that time, no one has done anything fundamentally different.
By John Connolly
The Wright Brothers accomplished this feat employing innovative design techniques that they themselves pioneered. Helping to recreate the propellers that once helped lift Orville Wright off the sands of Kitty Hawk, one reverse engineering company is using its own brand of technology to follow in the brothers' footsteps.
The story begins two years ago when a Virginia-based company that rebuilds old airplanes approached Direct Dimensions, Inc. (Baltimore, MD) - a rapid solutions company - to say that they were rebuilding the Wright Brothers' plane for the 100th anniversary of the flight in 2003 and could they help them measure and reverse CAD model the original propellers into a full-scale, complex, 3-D surface model.
Before the work could begin, however, company design engineers had to look at the intricate blueprints, wind tunnel tests and performance calculations that the brothers had logged on the original propellers. Early in the century, very few really understood that the propeller was nothing more than a wing that rotates around its axis and provided lift. The Wrights read shipbuilding manuals and used the empirical principles within to develop a theory of propulsion. They reasoned that the basic model of the propeller was a moveable wing - testing various propeller shapes and sizes in their wind tunnel until ones were perfected - a pair of eight-foot propellers carved out of laminated spruce that turned in opposite directions to offset the twisting effect on the planes structure. Tested at 350 rpm, the reproduction 1903 propeller produced 64.2 pounds of thrust while the original propeller's static thrust measurement was 67.1 pounds at 350 rpm. Wind tunnel tests performed at the NASA Langley Aerospace Center in Virginia found that the propellers of today performed only marginally better than those first developed by the brothers almost a century before.
"The brothers took copious notes on all of their design and performance calculations," says Michael Raphael, owner and engineer with Direct Dimensions. "This was an incredible feat of engineering because, as we've been told, the Wright Brothers perfected the propeller to where it was 80 percent efficient in power. Compare that to today's propellers, which only produce 85 percent efficiency - a difference of only five percent. By 1911 they had it right! Engineers today are trying to figure out how they did it."
Another group involved in supervising the computer imaging and evaluation of the original propellers, The Wright Experience (Warrenton, VA), said that it was very pleased but not surprised that both thrust coefficient plots showed very good to excellent agreement between the measurements taken by the brothers and the measurements taken during the current test series.
"The static thrust measurements produced by the propeller reproductions are in excellent agreement with the thrust measurements recorded by the brothers," says Ken Hyde, an engineer with The Wright Experience aircraft production team, which has been designing, building and testing full-scale reproductions of the Wright Brothers' aircraft and its engines. "It made life on this end a little easier."
Employing the Wright Techniques
Two reverse engineering technologies were employed for the measurement of the complex propellers. The main system was the FARO Arm portable CMM from FARO Technologies (Lake Mary, FL) - a technology service provider. This system is the backbone of Direct Dimensions' 3-D measurement capability. Engineers employed the FARO Arm because it reads accurate data that they reverse engineered into an accurate surface model. They also employed laser scanning on the original 1903 propeller to capture the fine wood details, cracks and splits on the props.
The FARO Arm was used for all of the propellers that were measured. The technique captured the basic geometric characteristics of the shape of the propellers - including the mounting face planes and the center shaft bolt hole. The company's engineers used the same features to establish the primary X, Y, Z coordinate system for the data and model. They scanned parallel section cuts through a complex contoured shape. This results in 2-D slices at even increments along the length of the propeller being measured. Similar cross-sections were in fact taken in several directions to identify the constantly changing contours. Direct Dimensions used this data in the surfacing process similar to the lofting techniques from early shipbuilding.
Overcoming the Obstacles
Direct Dimensions' engineers measured the props on-site with the FARO Arm portable CMM and converted the raw 3-D scan data into accurate, fully-surfaced CAD models of the original props. One obstacle encountered by the company was intrusiveness. DDI measured eight different propellers on-site at various locations - mostly museums. The curators, while cooperative, were concerned about the length of time that would be required to do the work, the support and facilities that would be needed, and the safety of the propellers. DDI designers were limited on handling - much less touching - the propellers. They created a basic measurement procedure during the first propeller measurement, tested it for completeness, and followed the same plan for each propeller they modeled. By doing this, DDI designers ensured the museum curators that they would be quick, safe and successful at obtaining the proper data the first time. While other technologies required a lot of technical baggage to do their work, Direct Dimensions' FARO Arm had just one requirement: a steady table and a power source. The equipment setup time was about 15 minutes, which enabled DDI to obtain the required data in about four hours per propeller.
Direct Dimensions' designers found that the one thing that other engineers should be aware of is the use of design intent, which is a huge part of the reverse engineering process. Some software works on the basics of simply wrapping a surface around a densely scanned cloud of points. This can be a valid approach for very rapid modeling of very clean dense data, but for this type of RE project, DDI engineers were required to incorporate information from multiple sources and did not want to recreate the prop models exactly "as is" with the twists, deformations, cracks and dents. This aspect of RE is common to DDI in that the company is often asked to reverse engineer real parts.
"Real parts are not exactly flat or not exactly round, or exactly symmetric, or have exact square corners. Real parts have imperfections, however slight. We think that reverse engineering requires just that - engineering," says Raphael. "We think of the task as a problem to be solved or engineered. We develop and incorporate information from various sources to solve the problem of how the part was intended to be by the original designer. We can reverse engineer the model to be better than the actual part, but at the same time balance these changes and assumptions against the accuracy of the measured data."
For more information contact Michael Raphael of Direct Dimensions (Baltimore, MD) at (410) 455-5599 or Ken Hyde of The Wright Experience (Warrenton, VA) at (540) 347-1909.ght. We think that reverse engineering requires just that - engineering," says Raphael. "We think of the task as a problem to be solved or engineered. We develop and incorporate information from various sources to solve the problem of how the part was intended to be by the original designer. We can reverse engineer the model to be better than the actual part, but at the same time balance these changes and assumptions against the accuracy of the measured data."
For more information contact Michael Raphael of Direct Dimensions (Baltimore, MD) at (410) 455-5599 or Ken Hyde of The Wright Experience (Warrenton, VA) at (540) 347-1909.