Because airplanes must fly in the real world, the Air Force built a fake one.
- By Ed Regis
- Air & Space magazine, May 2006
U.S. Air Force
(Page 2 of 4)
During closed-loop operation, fans withdraw ambient air from inside the chamber, blow it over the cooling coils, and send the cold air up into ceiling ducts that distribute it through large circular diffusers. The same air is recycled through the system again and again, reducing the chamber’s temperature incrementally with each pass. (For high-temp tests, steam is run through the coils.)
The situation changes, however, when an aircraft’s jet engine is running, something that cannot happen in a closed system due to the process of combustion, which sucks in copious amounts of hangar air and forces it out through the engine exhaust system. All that air must be replaced at the precise rate at which it is being consumed. Plus, with the engines and exhaust ductwork radiating heat into the hangar, the incoming air must be cold enough to maintain the target temperature. Keeping up with all that heating and venting requires a whole new dimension in cooling and airflow management, which at McKinley is a masterpiece of engineering called the air makeup system.
“The air makeup system will take that outside air at 95 degrees, cool it down to –40 degrees, and dump it into the chamber at the exact same rate that the engines are using it,” says Velasco. The system has two primary components, the first of which is the air makeup building proper, which is a separate, free-standing, two-story structure housing an air-intake duct, one steam coil and two successive ranks of air conditioning coils, and an exhaust duct that pours a continuous volume of fresh-frozen air into the Main Chamber through a large square hole in the ceiling.
However, there is no way that even McKinley’s powerful compressors can produce the mass of cold air needed to balance a 747’s four engines running at cruise power for a solid hour. The solution: Chill a whole lot of coolant in advance and store it at low temperature until it’s needed. Accordingly, for two days prior to the Raytheon Hawker Horizon test, McKinley’s air makeup team has been cooling down the facility’s most potent refrigerant, R30, a.k.a. methylene chloride, to a temperature of –100 degrees, then shunting it into a 750,000-gallon cylindrical tank adjacent to the air makeup building. Big as a house, that tank of liquid frigidity is really the crux of the entire enterprise.
“This system can take outside air as hot as 105 degrees Fahrenheit and cool it down to minus 80 degrees Fahrenheit, at a rate of 1,000 pounds mass of air per second,” says Velasco. “So as long as your engines aren’t using more than a thousand pounds mass per second of air, we can maintain the conditions in there while those engines are operating.”
Lab personnel are kept informed of equipment functioning by the Facility Monitoring and Control System, a room that runs 24/7 to keep tabs on the facility’s hundreds of valves, miles of piping and electrical circuitry, and compressor pumps, steam boilers, heat exchangers, evaporators, condensers, nozzles, ducts, fans, backflow dampers, plenum doors, cooling towers, surge tanks, storage tanks, coil banks, pressure vessels, and large-scale fluid flows. A day’s use of the Main Chamber can cost between $10,000 to $25,000. If all you’re doing is a simple snow-load test of a tent, figure the lower end. “If you have something really complex,” says Velasco, “like a military Joint Strike Fighter, the F-35, the short-takeoff-and-landing version—it’s got a big lifting fan in the middle, it’s got variable exhaust in the back, a very complicated exhaust setup that we have to specially design and build—that gets up to $25,000 a day.”
Velasco, a thin, balding aeronautical engineer in his late 40s, came to Eglin in 1984 after spending three years doing climatic testing at Edwards Air Force Base in California’s Mojave Desert. One of the tests involved bringing a Lockheed F-16 to McKinley for the full treatment. The place impressed him so much that he transferred to Eglin and never left.
In the years since, Velasco has seen it all, but the tradition at McKinley is to maintain a strict code of omerta about test results. After all, any aircraft, military or civilian, is manufactured by a company whose board members would rather that the outside world not know that during one of the lab’s –40-degree tests, its precious airplane’s landing gear wouldn’t retract or, worse, extend.