Design News USA
04.04.05 Volume 60 no. 05
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Thinking Big, An Aerospace Engineer Takes on Hollywood
Charles J. Murray -- Design News
Shortly after producers of "The Aviator" began gearing up to make their epic film biography of Howard Hughes, aircraft engineer and USC grad Joe Bok received his first Hollywood contract. But when Bok, who normally builds flying drones for the military, pored over it, he noticed an unusual stipulation: It called for not one, but two copies of every aircraft model to be used in the film.
"I asked, 'Why two?'" recalls Bok (also correctly spelled "Bock"). "And they said, 'Because when you crash the first one, we want the second one on the runway with the propeller spinning.'" Thus began Bok's own high-pressure epic, which started in earnest when Bok, the head of the 35-employee shop Aero Telemetry Corp., brazenly told the big-money Hollywood producers that he didn't need to build duplicates because his models wouldn't crash. For Bok, however, the decision to forgo the obligatory Hollywood crash duplicate was a high-wire act based more on engineering intelligence than guts. Contrary to typical movie studio logic, Bok believed that radio-controlled model airplanes wouldn't crash if designed according to classical aerodynamic theory and endowed with sufficient size and weight. That's why he and a staff of six engineers decided to build models considered gargantuan in the world of radio-controlled aircraft, the biggest being a 30 ft wide, 648 lbs version of Hughes' infamous XF-11 reconnaissance plane. By most measures, even Bok's "small" models were big: His version of Hughes H-1B racer sported an 18-ft wingspan and weighed in at 450 lbs, while his scale-model of the fabled "Spruce Goose" measured 26 ft from wingtip to wingtip, and weighed 375 lbs.
"I've never seen an electric-powered model the size of his Spruce Goose," notes radio-controlled aircraft aficionado Don Hofeldt, who helped with the Goose's design. Indeed, Aero Telemetry's models were mammoth for Hollywood, which is typically more inclined to employ brief shots of tiny models. But Bok believed that size was a necessity, especially in light of Hollywood's track record.
"I've been out here for 20 years and I know almost everyone who has ever tried to fly an airplane for a movie," says Bok, who has resided in the area since playing football for, and earning an aerospace engineering at, the University of Southern California during the 1980s. "And all the models have crashed, and they've all looked hokey."
But Bok, by aiming to avoid that distinction, increased the pressure on himself and his staff. Hollywood executives, aware of Bok's no-net approach, all but threatened him with penalties for failure.
"They made it pretty clear that if we didn't show up on shooting day, or if we were late, or if the models didn't work, they would have $4 million worth of people and equipment standing idle," Bok recalls. "In terms of pressure, it was worse than any military customer we ever had."
Bok (at right) discusses pre-flight details for the H1 Racer with pilot Jason Somes
Hollywood Pressure Cooker
Size notwithstanding, Aero Telemetry's tasks would have been manageable were they not salted with complexity and with nearly impossible time constraints. The project, which started with the construction of a scale model of Hughes ill-fated XF-11 reconnaissance plane, suddenly took on greater proportions after "The Aviator's" producers lost their full-scale model of the H-1 racer following a crash that destroyed the plane and took the owner's life. Still in shock over the sudden turn of events, the producers approached Bok in August 2003 and asked if he would be willing to build an H-1 scale model in tandem with the XF-11 that his staff was already constructing.
"I told the producers, 'We've got to build it large enough so that it looks and flies like a real airplane,'" Bok recalls. After the two parties reached an agreement, Bok and his staff temporarily set the XF-11 aside and began work on the H-1 racer, which would become the Best Picture nominee's most important scale model. Hours after the agreement was reached, Aero Telemetry's staff began making 3D CAD models of the H-1, and then quickly hogged out of a foam block of the fuselage, adding wood and carbon-fiber box spars for the wings. Bok hired "sculptors" to carve the model's features into the foam fuselage and then approached expert surf board makers to lay up the carbon fiber and resins for the composite wings. Molds for the wings were designed using a software program called Rhinoceros from Robert McNeel & Associates and airfoils were completed with CompuFoil software from SoarSoft Software. "We had a whole army of sculptors working on it," Bok says. "Once they got it all per-fect, they began putting fiberglass over it."
In all, Aero Telemetry used six engineers, including one design engineer, one hydraulics engineer, one software engineer, and two electrical engineers. They also employed two machinists on lathes for the engines, two on Bridgeport mills, and one on a computer numerical control (CNC) mill.
Aero Telemetry engineers hold the XF-11 model while its engines warm up
prior to the first flight on Catalina Island.
Bok's staff, however, points to the retractable landing gears (used on both the XF-11 and H-1 racer) as the biggest design challenge. Because the models existed in a netherworld that lay somewhere between hobbyist aircraft and Cessna-sized planes, no commercial landing gear fit the bill. "There was simply nothing out there that we could use," notes Butch Fleck, an Aero Telemetry mechanical engineer who also had experience designing landing gears for Boeing 747s and DC-9s. "We needed something generic and simple, but strong enough to stand up to the impact of landing, and that just didn't exist." Out of necessity, the staff solved the problem by building landing gears from scratch. The H-1 racer used two 20-lb retractable wing gears that included a 2,000-psi, 24V hydraulic pump from Parker Hannifin's Oildyne Division, powered by a pair of 12V lead-acid batteries wired in series. The pump provided pressure to a 0.5-inch-diameter linear hydraulic actuator, specially machined by Aero Telemetry's staff to actuate the landing gear's wheels. The company's engineers say that the gear design, which required four weeks of 18-hr days for two engineers and four machinists to complete, was critical. If it didn't work, the planes would have crash-landed and set filming back by months. "We had to support a hard landing and a heavy aircraft, " Fleck says. "If it failed, it would have shut down the whole show."
"Tweener" Poses Power Train Challenges
Being a "tweener" project that existed between the world of commercial aircraft and hobbyist planes meant that the company's engineers also needed to develop their own power trains, including engines, electric motors, and gearboxes.
CAD drawings by Aero Telemetry show the retractable landing gear recessed in the wing and the hydraulic actuator assembly for the H-1 Racer main gear.
The H-1 racer model, for example, used a 360-cc two-stroke gasoline engine with twin cylinders re-bored to produce more power. Aero Telemetry's engineers also designed a special exhaust system and gearbox to squeeze more power from it. Bok's biggest departure from conventional radio controlled modeling, however, was his use of electric motors to lift the 26 ft wide, 375 lbs Hercules or "Spruce Goose." Working with Hofeldt, a hobbyist who buys gas models and converts them to electric, the company developed a system to power the model's eight 16-inch propellers.
In the end, Aero Telemetry employed eight small brushless dc motors, each powered by a tiny battery pack containing 20 nickel-metal hydride cells, thus providing enough thrust to lift the wooden model from the water in Long Beach Harbor, where its maiden flight was filmed. "There were 300 people in costume on barges there in the harbor, watching the model take off," Hofeldt recalls. "It was an amazing sight."
Eight tiny battery packs, each containing 20 nickel-metal hydride cells, provided enough power to lift the Spruce Goose model from the water in Long Beach Harbor on its maiden flight.
Equally important, however, was the company's use of telemetry to control the planes. From the outset, "The Aviator's" producers called for the radio-controlled models to fly in areas populated by actors on the ground. Serious operational errors, especially with models weighing between 400 and 700 lbs, could have been fatal for onlookers.
"It would have been like a cruise missile coming in," Bok states.
To control the models, Bok chose military-grade transmitters and receivers, augmented by custom-designed RF sections, operating at typical military frequencies (1.4 GHz). The higher frequency was used, he says, because such frequencies are more tolerant to electromagnetic interference.
Using the in-house-designed controls, Bok says the planes exceeded all expectations. The XF-11 made its first flight on Nov. 21, 2003, followed two days later by the flight of the lumbering Spruce Goose in Long Beach Harbor.
"We didn't crash a model and didn't have so much as a glitch," he says. "We never lost control of the planes for even a split second."
Moreover, Aero Telemetry's H-1 racer made its critical flights in the California Desert on Nov. 4, 2003, a scant three months after the company reached agreement with the movie's producers. Scenes of Bok's flying model ended up playing a critical role in the movie, along with computerized models (so-called "CG" models), and static models (for ground shots). Ultimately, Bok's H-1B model followed in the historical tracks of the record-setting original by flying 160 mph, reportedly the fastest half-scale model (manned or unmanned) ever built.
"The radio-controlled model was a huge component of the success for its sequence in the movie," notes Aviator executive producer, Chris Brigham. Shots of the XF-11 and Hercules models were also used in the film, along with computerized models.
Bok expects the impact made by his company's models to have a lasting effect on Hollywood, which in the past five years has made a hard turn toward computer graphics. He says his company is now negotiating with directors Steven Spielberg and Clint Eastwood to do aircraft models for the in-the-works film "Flags of Our Fathers."
"This is groundbreaking; Hollywood has never used models this big," Bok concludes. "It's going to change the way these kinds of special effects are done, for years going forward."
Aircraft Design: Why Bigger is Better
When engineer Joe Bok built models for "The Aviator," he broke Hollywood tradition by constructing planes with wingspans as large as 30 ft and masses approaching 650lbs.
Why? Because he listened to Osbourne Reynolds. Reynolds, whose pioneering fluid mechanics work in the 1880's is immortalized in the form of the unitless coefficient known as the "Reynolds number," defined the relationship between viscous forces and momentum forces. His number, still widely used today, grows larger when momentum forces rise.
For airplane wings of a given chord length, L, the Reynolds number is expresses as follows:
Re = (p / µ) V x L
Where p = density, µ = viscosity, V = velocity, put on one line.
In general, bigger aircraft have bigger Reynolds numbers. Thirty feet wide models like those built by Bok can have Reynolds numbers of a million or more, where smaller radio controlled models may have Reynolds numbers in the 100,000 range. A very small (and aerodynamically unstable) flying object, such as a butterfly, may have a Reynolds number of just 7,000.
All of this matters, Bok says, because momentum and viscosity affect the boundary layer of air on a wing's surface. And the boundary layer, in turn, is the singlemose important difference between the performance characteristics of full sized aircraft and models. For full sized airplane wings traveling at high speed, mass inertia is the more dominent factor. In contrast, for model airplanes traveling at lower speeds, viscous forces are more significant. That's why tiny models with 1ft and 2ft wing spans are more prone to crashing, Bok says.
"The bottom line on Reynolds number and airfoil design is that it directly affects the wing's stalling characteristics," Bok says. "A relatively low Reynolds number will result in an early stall."
Bok designs on the H1 racer and XF-11 spy plane models therefore maximized Reynolds number as a way of minimizing the chances of an inadvertent stall due to high bank angles at full power. Aerodynamics experts say that such principles manifest themselves in ways that can be grasped intuitively. Smaller models, they say, are more likely to be buffeted by gusts of winds. Unlike huge commercial planes, which steer like barges, smaller aircraft react quickly to control surface movements.
"What it comes down to is that aerodynamics don't scale well," Bok adds. "You never want to copy a big plane's airfoil onto a smaller plane. If the small plane goes too slow, or gets up too high in a turn, it could tip over and stall."
Article reprinted with permission of Design News
