Page 137 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 124

124 CHAPTER FIVE

Fig. 5.4. Photos of propeller slices. (Photo courtesy of Albert Dyer.)

Since the propeller works by pushing a certain amount of
The world indoor FF (Free Flight)
air mass through each revolution and accelerates this mass of
record as of January 1, 2000, was
air to a higher velocity, we can begin to understand the
60 minutes and 1 second. On
tradeoffs in propeller design. A large-area propeller will be
June 1, 1997, this was achieved
more efficient than a smaller one because it can push more air.
by Steve Brown’s Time Traveler.
To obtain the same thrust, the smaller propeller will accelerate
Free Flight is a competition
less air but to a higher velocity, making it less efficient.
using rubber-band-powered
Rubber-band-powered model airplane enthusiasts know that a
airplanes. This same airplane flew
large, slow-turning propeller will result in the longest flights.
for an unofficial 63 minutes and
Propeller size and rotation speeds are dictated by many
54 seconds on another flight.
factors. First, a large, slow-turning propeller may not be
practical because of ground clearance. But of equal