Lift/Velocity
Relationship Problems Open the slide called Velocity Effects (with text) and read the explanation on how lift varies with the square of the velocity. At the bottom of the Mass and Velocity Effects slide is a problem that supplies weight and velocity data. Let's work through that problem and discover the effect a large change in weight has upon velocity. The problem gives us a 15,000 pound airplane with 5,000 pounds of fuel traveling at 250 mph. The lift must equal a total weight of 20,000 pounds, which is equal to the velocity squared multiplied by some constant (assuming constant air density) . Therefore, we have L = k V2. Solving for k, k = L/ V2 , and substituting in our data, k = 20,000 pounds/ (250 mph)2 = 0.32 pounds-hours2/mile2. Now we wish to know the velocity the pilot should maintain to hold the same altitude after he has burned half of his fuel. Our weight (and necessarily lift) becomes 17,500 pounds, which is then equal to 0.32 V2. Solving for V, V = (17,500/0.32)1/2 = 234 mph. You may have been wondering about the photo of the "Spirit of St. Louis" above. We will now use our lift/velocity relationship information on an imaginary flight in the Spirit of St. Louis where we again maintain altitude. Data needed to continue is found at "Ryan, Spirit of St. Louis." (http://www.letsfindout.com/subjects/aviation/rpistlou.html)
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Curator: Tom.Benson@grc.nasa.gov Responsible Official: Kathy.Zona@grc.nasa.gov
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