Why do wind turbines have three narrow blades, whereas my fan at home has five wide blades?
—J. Lester, Stroudsburg, Pa.

Dale E. Berg, a member of the technical staff in the wind energy technology department at Sandia National Laboratories, replies:

The differences between wind turbine and ceiling fan blades arise from the contrasting design criteria: the wind turbine is intended to capture high-velocity wind to generate electricity efficiently; the ceiling fan needs to move air at low velocity with inexpensive components.

To keep drivetrain costs low, a wind turbine must capture the energy in fast-moving air and rotate at relatively high speed—within limits, so as to avoid excessive noise generation. (Slow rotation would increase the torque and require heavier and more expensive drivetrain components.) Such high-efficiency energy conversion dictates the use of lift-type turbine blades, similar to airplane wings, of twisted and tapered airfoil shapes. The blade design creates a pressure difference in wind—high pressure on one side and low pressure on the other—that causes the blades to turn. A combination of structural and economic considerations drives the use of three slender blades on most wind turbines—using one or two blades means more complex structural dynamics, and more blades means greater expense for the blades and the blade attachments to the turbine.

The ceiling fan, on the other hand, is built to keep the occupants of a room comfortable by moving air gently. Its engineers work to minimize noise while the fan rotates at low speed (for safety reasons) and to keep the construction costs, and therefore the purchase price, low. Energy efficiency is not a primary concern, because operation is inexpensive—a typical ceiling fan running 24 hours a day consumes about 60 kilowatt-hours a month, for an average electricity cost of about six dollars. For this reason, most ceiling fans incorporate blades that are comparatively inefficient drag devices; rotating the pitched blades pushes air vertically out of the way. Wide, flat blades are inexpensive to build and work well as drag devices. More blades are better, up to a point, and the usual layout of four or five blades is the result of balancing trade-offs between efficiency and expense.

A 2001 article in Mechanical Engineering chronicled the quest of a man named Danny Parker to create a more efficient ceiling fan. Parker's initial blade prototype looked a lot like a wind turbine blade, but the end result (because of manufacturing, safety and operating concerns) was a hybrid between a standard ceiling fan blade and a wind turbine blade.

What happens to the donor's DNA in a blood transfusion?
—W. McFarland, Winter Springs, Fla.

Michelle N. Gong, an assistant professor at the Mount Sinai School of Medicine, explains:

Studies have shown that donor DNA in blood transfusion recipients persists for a number of days, sometimes longer, but its presence is unlikely to alter genetic tests significantly. Red blood cells, the primary component in transfusions, have no nucleus and no DNA. Transfused blood does, however, host a significant amount of DNA-containing white blood cells, or leukocytes—around a billion cells per unit (roughly one pint) of blood. Even blood components that have been filtered to remove donor white cells can have millions of leukocytes per unit.

Investigators have detected donor DNA after transfusion with a process called polymerase chain reaction (PCR) that amplifies minuscule amounts of genetic material for detection and identification of specific genes. Studies using PCR to amplify male genes in female recipients of transfusions from male donors have demonstrated that donor DNA endures in recipients for up to seven days. And a study of female trauma patients receiving large transfusions showed the presence of donor leukocytes for up to a year and a half.

All these results, however, were found using very sensitive techniques whereby donor DNA was selectively amplified over the more plentiful recipient DNA. In studies where genes common to both donors and recipients were amplified, the results reflected the dominance of the transfusion recipient's own DNA, showing the donor's DNA to be a relatively inconsequential interloper.

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