Date: 13 Jul 97 19:52:40 From: Wolfgang Decker <email@example.com> Organization: Flashnet Communications, http://www.flash.net References: 1
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187 wrote: > > Some questions:- > > 2) What is the best EPR number to be attained? Is it the smaller the > number the better or the greater the better? Since EPR=turbine > discharge pressure: compressor inlet pressure. And why? Basically thrust is generated by accelerating mass into the opposite direction you are going. This is, as Ken Ishiguro wrote, like standing on a skateboard and throwing a basketball. Mathematically this looks like F = (m/t) * (v2 - v1) where F is the thrust, (m/t) is the mass of air per time (t) unit, also called mass flow, v2 is the outlet speed and v1 the inlet speed, which is lowest when standing on the ground. The outlet speed of course is determined by the pressure in the nozzle. Now one can see that increasing the thrust is possible either by increasing the mass flow or the speed gradient. Latter was done in the very first time of jet engines, where no bypass was used. Looking at the efficiency coefficient (unfortunately I don't remember the derivation and the formula exactly, but it basically has the form e = v1 / (v2-v1) ), the efficieny of a jet egine is higher, if you maximize the mass flow by minimizing the speed gradient. This is why modern engines have this very high bypass ratio of larger than 6:1. Now, the question which epr is best can't be answered that easy. It is first determined by the engine type, and secondly by the current flight condition, which includes air density (therefor altitude), inlet air temperature and speed of aircraft. I hope, this gives some additional aspects Wolfgang Decker firstname.lastname@example.org I prefer to fly without engines!