Date: 12 Oct 98 00:02:15 From: Jim Crowder <email@example.com> Organization: The Boeing Company References: 1 2
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This process is recognized in the experimental aerodynamics business and is call a sunlight shadowgraph. It can be a very convenient way to document the location and extent of the wing shock wave system. All commercial jet transports operate at a high enough Mach number (.75 to .85) so that the the flow over the front part of the wing accelerates to a local Mach number of 1.2 to 1.4. This kind of flow usually can only decelerate through a shock wave. The shock wave can be pictured as a surface standing perpendicular to the wing and running approximately spanwise. Often the shock wave over the inboard part of the wing will split into two shock waves going off in different directions. When rays from the sun are arranged to be parallel to the shock wave surface a strong shadow is cast onto the wing which is easily visible. It is relatively rare for an airliner to be coincidently traveling at the optimum time, place and direction, so most of time sunlight shadowgraph viewing is much less distinct. When an airplane is flying in smooth air the shadowgraph should be essentially stationary. In rough air it will jiggle back and forth as the wing loading and the local Mach number fluctuates. I remember one flight where I observed the shadowgraph slowing moving forward and back a distance of about 3 feet over a 10 minute period. When I asked the crew about this, they were surprised that I could tell, but they were trimming the airspeed to hold a constant altitude as they flew through smooth updrafts and downdrafts caused by wavy jet stream winds. I have published a description of this in an AIAA paper, number 90-1273, titled Flow Visualization in Flight Testing.