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Old 11th April 2005 | 20:03
  #31 (permalink)  
Alex Whittingham
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Joined: May 1999
Posts: 1,846
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From: Bristol, England
Old Smokey,

I've a couple of problems with your diagram.

The first is that the thrust available curve shows a reduction in thrust with forward speed then an increase. This is the classic thrust line from aerodynamics textbooks produced in the 1960s which, of course, reflects a 1960s engine, typically an Avon. Modern, high bypass ratio, jets tend never to recover the thrust by ram effect.

The second problem is that you have shown the thrust available curve just touching the thrust required curve in such a way as to suggest that the point of contact is significant. If the way you have illustrated it was correct the thrust available would always be less than the thrust required and you'd never get off the deck. In fact, thrust available is (usually) greater than thrust required and what is important is not where the graphs touch when they are artificially brought together but the vertical gap between them, which indicates angle of climb. If you displace your jet thrust available curve upwards to where it belongs, well above the thrust required, the greatest difference between thrust available and thrust required occurs AT VMD not at the artificial point of contact you illustrate.
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