Page 192 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 179

Airplane Performance 179

10 percent increase in takeoff speed. But the real killer is that the takeoff
distance increases with the weight squared. This is a simple
consequence of Newton’s second law, which states that the acceleration
is the force divided by the mass. So, if the weight increases, for constant
thrust, the acceleration decreases and it will take a longer distance to
reach takeoff speed. Remember that the takeoff speed has increased to
exacerbate the problem. So, for example, a 20 percent increase in weight
increases the takeoff distance by about 44 percent for a high-powered
airplane. But an increase in weight of 20 percent will increase the takeoff
distance of low-powered general-aviation airplanes by about 60 percent
because of the lower acceleration.
The takeoff distance is also impacted by the wind. A headwind that
is 15 percent of the takeoff speed will shorten the takeoff distance by
about 30 percent while a tailwind of the same speed will lengthen the
takeoff distance by 33 percent. For a small airplane with a takeoff
speed of 70 mi/h (112 km/h) this is only a 10 mi/h (16 km/h) wind.
This is why airplanes always take off into the wind. Aircraft carriers
turn into the wind to launch and recover aircraft.

During the early years of aviation,WWI and earlier, airfields
were large, square, or circular fields.The airplanes could not
tolerate crosswinds as they do today so they would point into
the wind for takeoff.When runways were developed, this
meant that airplanes could no longer take off in any direction,
depending on the local wind. However, careful airport design
will place the main runway into the prevailing winds, and
sometimes a secondary runway is built perpendicular to it.
With a secondary runway it guarantees that the airplane will
at most see a 45-degree crosswind.

Altitude also contributes to takeoff performance. Recall that it is the
indicated airspeed that dictates airplane performance. So the takeoff
ground speed increases with altitude, though not the indicated
airspeed. Here the difference between a jet and a piston-powered
airplane is apparent. The thrust, and thus the acceleration, of a jet
engine is less affected by altitude. At an altitude of 6000 ft (1800 m)