For the average person, what constitutes a hot day is subjective. The same with a cold day.
For aerospace engineers, though, it’s not subjective—and there’s a new SAE standard to prove it. The S-15, Gas Turbine Performance Simulation Nomenclature and Interfaces Committee, recently published ARP210, “Definition of Commonly Used Day Types (Atmospheric Ambient Temperature Characteristics Versus Pressure Altitude).”
The standard recognizes that an aircraft performs differently, depending on meteorological factors such as temperature and other atmospheric conditions. The gas-turbine industry has long used nonstandard atmospheric temperature profiles in the design and testing of aircraft and their engines. There really are “hot” days and “cold” days. “And aircraft really do not perform the same on hot days as they do on cold days,” said committee member Thomas Roadinger, Project Manager at Pratt & Whitney.
Going back to 1953, the military defined specifications for what was a hot day, a cold day, a tropical day, etc. The aircraft industry used these specifications as a good, convenient way to describe performance at varying conditions, according to Roadinger. The problem is that the military canceled the specifications. So the common definitions of hot day, cold day, etc., disappeared.
In ARP210 can be found all the nonstandard atmospheric temperature profiles that used to be in the military specifications. But the S-15 committee went a little further than the original specifications, Roadinger said. In addition to the complete temperature profiles, the committee has a recommended set of profiles that are smaller and have been scrutinized for consistency.
The atmospheric temperature profiles are presented in terms of ambient temperature as a function of pressure altitude. And just to make sure everybody is on the same page, Roadinger noted, ARP210 includes an appendix that gives the details of exactly what altitude means, including physical altitude, geometric altitude, geopotential altitude, GPS altitude, and pressure altitude.
"Honeywell and other gas turbine engine manufacturers conduct ground and flight tests to assure they meet customer requirements,” said S-15 Committee Chairman Mark Steele, Manager of Advanced Engines, APU, Commercial Propulsion Performance, Honeywell. “The customers and gas-turbine manufacturers depend on atmospheric standards to avoid confusion and disagreements. Honeywell has used the definitions that are now incorporated in ARP210 for many years, and they have saved untold hours of disagreements and retesting."
Honeywell uses a Boeing 720—one of the few remaining in operation—for its in-flight testing of turbofan engines. It has a cell in Phoenix for ground testing. Between the two, Honeywell is able to cover 99% of all engine rating tests, according to Steele.
Test engines are mounted on the front fuselage of the 720 and are small enough in comparison with the four engines powering the airplane that a turbofan under test can be run through its entire power range while the main engines maintain the aircraft’s altitude and speed, said Steele.
“The test engine can even be shut down, and the aircraft can still maintain the desired flight condition,” he said. “This allows the gathering of ‘windmilling’ data, and also engine starting data can be obtained for a multitude of flight conditions. With the 720 we can set just about any desired altitude and Mach number within the engine's flight envelope. The one thing we can't control is the ambient temperature, which is where ARP210 comes into play. We adjust the data from the tested temperature to the appropriate temperature from ARP210.”
Most gas-turbine engine testing is done in ambient cells, according to Steele, and the data is corrected to the desired inlet temperature. “Again, this is where ARP210 can come into play.”