For scientists, power is the rate at which
work
is performed by a
force
acting along a distance.^M
As an example shown on the slide, the Wright
1903 aircraft
is acted upon by the
thrust
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
displacement
of the aircraft and the velocity V is constant.
The power then becomes the product of the force and the velocity:
P = F * V
The Wright aircraft developed 130 pounds
of thrust and required 32 miles per hour airspeed to fly.
The motor was then developing about 11 horsepower:
[130 pounds times 32 mph (converted to
feet per second 88 fps = 60 mph) divided by 550 foot-pounds per second
per horsepower = 11.09 hp]
For comparison, a modern lawn mower engine is rated at 6 horsepower.
The Wright brothers used their knowledge of work and
power to determine
the minimum requirements for their engine.
They were able to make rather accurate estimates of the drag of their
aircraft based on wind tunnel tests and flight tests of their earlier
gliders.
Knowing the
drag
and the desired flight velocity, they computed the
power requirements for the engine needed to develop enough
thrust
to overcome the drag.
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