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• Instrument for Determining Satellite Rotational Losses and Windage
Losses of Airfoil Shapes
When a satellite rotates in space, the gravitational
attraction of the earth causes the structure to flex. This results in a small hysteresis loss,
which causes a gradual decrease in the rotation rate over a long period of time. This loss is
dramatically increased if relative motion occurs between parts in the assembly. Such motion may
occur from slip at bolted joints or sloshing of fluids present in the body. We have developed a
simulator that rotates an object such as a satellite on a frictionless gas bearing and measures
the decay in spin rate resulting from the mechanical losses described above.
Losses corresponding to retarding torques as small as 0.001 lb-inch can be
measured. The test machine also determines the moment of inertia of the payload and corrects for
static and dynamic unbalance of the rotating assembly. Since windage losses would obscure the
data, a helium chamber is built into the machine. The chamber used with this gas bearing system
uses a novel fabric membrane concept that allows the air to be thoroughly expelled before the
helium is introduced.
This machine is an extremely sensitive device for determining the windage losses
for various airfoil shapes. A baseline windage loss is first established using a smooth cylinder
spinning in a helium atmosphere. Various airfoils can then be attached to the cylinder and the
decay rate measured in both air and helium at different speeds. The machine reliably measures
the drag due to projections as small as the head of a 1/4-inch socket head cap screw located at
a radius of 4 inches rotating at speeds below two revolutions per second.
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