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Technicalities: Speaking of Jets

**Garrison's proposal for a four-seat
single-engine jet. It was judged less cool
than an X-tailed design with an engine
on a pylon above the rear fuselage.
**
Gemini Sparkle

Key Takeaways:

  • Jet airplanes cannot exceed their own exhaust velocity because their thrust comes from accelerating ingested air, a principle distinct from rockets which carry their own working fluid.
  • This limitation is not severe in practice, as the high temperature of jet exhaust results in a much higher speed of sound (and thus exhaust velocity) than ambient air, providing ample thrust for supersonic flight.
  • Austin Meyer, creator of the X-Plane flight simulator, is pursuing a project to design and eventually offer a four-seat jet as a kit for homebuilders.
  • Meyer's project faces significant challenges, including the difficulty of acquiring suitable modern jet engines for non-established manufacturers, real-world fuel consumption complexities, and intricate design issues concerning engine placement, aircraft balance, and stability for a single-engine personal jet.
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Certain universal questions pop up over and over, like “Why is there something, when there could be nothing?” or “Can a jet fly faster than its own exhaust velocity?” Let’s look at the second one; I’ll get back to the first in a future column.

Reciprocating engines inhale about 15 pounds of air for every pound of fuel they burn. This is the so-called “stoichiometric ratio” — the ratio at which every oxygen molecule in the air gets combined with a hydrocarbon molecule in the fuel, leaving no unconsumed oxygen or fuel at the end. Chemically combining the oxygen and fuel releases energy in the form of heat — which makes the engine go — as well as, in the ideal case, just carbon dioxide and water.

Peter Garrison

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, ""Technicalities"" and ""Aftermath,"" with FLYING readers.

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