Published in the December
1997 issue of Continental,
Continental Airlines' in-flight magazine
Climb Performance
By Dave Esser
One of the most significant aspects of the overall performance
of jet transport aircraft is their ability to climb. Because
aircraft performance is enhanced at high altitudes, a
quick climb is important to maximize efficiency. Safe
obstacle clearance is obviously an important consideration
in climb, but noise abatement is also a critical factor.
Jet aircraft must climb steeply so as not to cause undue
noise to those who live near busy airports.
Let's examine the various components of climb performance
related to jet transport aircraft. An aircraft can climb
only if it can produce excess thrust. In a climb, a component
of weight acts along the flight path. The steeper the
climb angle, the greater this component. In a steady state
climb, the forward acting thrust must equal the rearward
acting forces of drag and the weight component. The greater
the thrust, the steeper the climb. Jet fighters capable
of producing more thrust than they weigh can climb at
a 90-degree angle -- straight up. To maximize excess thrust
a jet aircraft will climb at a speed where drag is minimum.
This is called the best angle of climb speed, abbreviated
Vx.
Another way to measure a climb is by the rate rather
than the angle of ascent. The ability of an aircraft to
climb in terms of feet per minute is a function of excess
power. You may recall from physics that power is the rate
at which work is done and that work is a force applied
through a distance. To lift a 600,000 pound jet to an
altitude of 10,000 feet takes six billion foot pounds
of work (600,000 pounds x 10,000 feet). To complete this
climb in 10 minutes (or 600 seconds) would require 10,000,000
foot pounds per second of power (6,000,000,000 foot pounds/600
seconds). Thanks to British physicist James Watt, who
determined that a horse is capable of producing roughly
550 foot pounds of power per second, we can express this
figure in terms of horsepower. In our example, the amount
of power required would be a little more than 18,000 horsepower.
An aircraft's maximum rate of climb occurs at an airspeed
where there is maximum excess power. The best rate of
climb speed, abbreviated as Vy, is at a faster speed than
Vx. For each flight the pilot must determine whether it
is more important to climb at the steepest angle to clear
obstacles, or at the fastest rate.
By federal aviation regulations a four-engine jet transport
must be able to climb at a three percent gradient on takeoff,
or about two degrees. This may not sound like much until
you realize that regulations require this capability even
if one of the jet engines is not working. It is comforting
to know just how well a jet transport aircraft can climb.
After all, without the climb, a jet would just be a really
fast bus with wings.
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