Effect of Gravity on a Stalled Jet
Date: 09/05/97 at 13:11:34 From: Travis George Subject: Impact of gravity on jet's fwd momentum If a 747 jet is flying at 30,000 feet and the engines stop working, as gravity pulls the plane closer to the ground, will the wings provided forward momentum, in essence allowing the plane to glide to a reasonably safe landing? Or will the plane fall straight down?
Date: 09/15/97 at 11:31:10 From: Doctor Pipe Subject: Re: Impact of gravity on jet's fwd momentum Travis, First off, the plane will not fall straight down! So if you e-mailed this from 30,000 feet, just relax. It's been many years since my pilot training, but here is a brief discussion of how a plane flies: A plane's engine, be it propeller or jet, provides the force, called thrust, that accelerates the plane forward; this enables the plane to roll along the runway. As the plane moves down the runway, air moves over the wings and the movement of the air over the wings creates the force, called lift, that lifts the plane off the ground and into the air. The speed at which an airplane's wings must move through the air to provide sufficient lift for flight is specific for each aircraft and is determined by both aircraft and weather conditions at the time of takeoff. Some of the aircraft conditions are the shape and size of the wing and the weight of the aircraft at takeoff; some of the weather conditions are the temperature, humidity, and barometric pressure. Cold dry air allows for takeoff at a slower speed than in hot humid air. As long as the plane can maintain a speed faster than this minimum speed for flight, called the stall speed, the airplane will fly. When we talk about an airplane "stalling" we mean that the flow of air over the wings (that produces lift) has been disrupted; the wings are not producing lift and the plane is no longer flying, but has stalled. An aircraft that has stalled IS FALLING! When learning to fly, pilots ascend to a safe altitude and then deliberately stall the plane in order to familiarize themselves with the characteristics of the aircraft immediately before a stall and to practice the procedure for recovering from a stall. I have done this in a single engine propeller plane called the Grumman Tiger. I'm not sure if 747 pilots do the same thing with a 747 - that would be quite an experience! Getting back to your question, if a 747 loses power at 30,000 feet then it has lost its thrust but not its lift. The loss of thrust means that the plane has lost the force that moves the plane through the air enabling the wings to produce lift. The momentum imparted to the plane when the engines were on is what causes the plane to continue traveling forward (relative to the ground). The pilot will allow the plane to descend and give up altitude in order to maintain airspeed. Here, gravity (rather than thrust) is the force accelerating the aircraft. In the scenario of your question, the pilot's job, along with the copilot, would be to locate an airfield close enough and with a long enough runway to land his 747. I'm not sure how far a 747 can fly without power from 30,000 feet, but if there is an airport within that distance the pilot can land the plane. It would be a landing similar to those made by Space Shuttle pilots (the Shuttle falls out of earth orbit and flies without power to a landing in Florida or California) - it can be done, but you only get one chance to do it right. (Pilots have many witty sayings like that. Another one is: Any landing you can walk away from is a good landing.) If you are interested in learning more about aircraft and how they fly, visit your local library. It is a subject that has fascinated me since I was six years old. -Doctor Pipe, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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