For scientists, power is the rate at which
is performed by a
acting along a distance.
As an example shown on the slide, the Wright
is acted upon by the
force (F) from time (t) equals zero to some later time (t > 0)
and travels some distance (s). The work (W) done on the aircraft during this
time is F times s.
W = F * s
The average power (P) expended by the engine to perform
this work is equal to the work divided by the time.
P = W / t
P = F * s / t
The unit of power in the metric system is the watt, which is equal to
one joule per second.
In the English system the unit of power is the horsepower (hp)
which is equal to 550 foot-pounds per second.
In our simple example, the force is a constant value
aligned with the
of the aircraft and the velocity (V) is constant.
The power then becomes the product of the force and the velocity:
P = F * V
Let's look closely at our example. If our aircraft developed 100 pounds
of thrust and we flew 110 feet, it would require 11,000 foot-pounds of
work. If we fly the distance in 10 seconds, we need to exert 1100 foot-pounds
per second, or 2 horsepower. A perfectly efficient 2 horsepower engine would
deliver enough power to generate 100 pounds of thrust at a velocity of
11 feet/sec. Suppose we increase the speed to 22 feet/sec. We would now
cover the 110 feet distance in 5 seconds. The work would still be
11,000 foot-pounds (100 pounds of thrust for 110 feet). But the power
required is now 2200 foot-pounds per second, or 4 horsepower. Increased
horsepower lets you fly faster with the same amount of thrust, or it
lets you generate more thrust at the same velocity.
The Wright brothers used their knowledge of work and
power to determine
the minimum requirements for their
1903 engine design.
They were able to make rather accurate estimates of the drag of their
based on the flight tests of the
1901 wind tunnel results.
Knowing the drag and the desired flight velocity they computed the
power requirements for the engine in order to develop
to overcome the drag of the aircraft.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page