From: email@example.com () Organization: AT&T GBCS/Bell Labs Date: 17 Aug 95 04:58:49 References: 1
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In article <firstname.lastname@example.org>, Mark Wiklund <Mark_Wiklund@hmco.com> wrote: >Having watched jet departures from the open air decks at >Logan airport since a boy, and more recently having done >much travel, I have always noticced that when a jet >revs up for take off, it seems to go to half-power >for a second or two before proceeding to take-off >power - as if it was a two step process. Is this half-step >process with all engines, or spooling up engines >individually, or simply a misperception on my part? Note that in a carburetted piston engine, fuel lags behind air. You open the throttle, letting in more air, which picks up more fuel. In a turbine engine, air lags behind throttle. You add more fuel and RPM come up giving more air. Carb'd engines starve a bit when you punch it at low RPM. Turbines go rich and risk dousing the flame with liquid fuel. What you observed varies by jet engine, and I don't have all of the specifics handy. But in general, it's difficult to make a turbine engine that will accept sudden throttle transitions. In particular, they don't like going from low to high throttle settings too fast. Flameout is the big worry. The Me262 was known for this. Piston trained pilots wanted to slam the throttle all the way forward when it came time for air combat, and this would flame out an engine. The F14 Tomcat was given new engines which were free from restrictions in throttle movement, a welcome change. For example, you could go directly into afterburner without waiting for RPM to climb. I would expect that some engine management systems might decouple the throttle inputs from being directly tied to fuel output. This would allow the engine controller to add more fuel as RPM rose. This allows the pilot to set the throttles as a "request" and then they can concentrate on something other than slowly advancing the throttle according to RPM. --- Neil Kirby DoD# 0783 email@example.com AT&T Bell Labs Columbus OH USA (614) 860-5304 President Internet BMW Riders The BMW R1100RSL - Because the Britten V 1000 is not street legal.