reply to post by ULTIMA1
ULTIMA, looked at your 'jet engine sim'....you do realize, don't you, that the effects of the thrust diminish by the square of the distance?
What I mean is, to the outsider, you see a tremendous drop-off of the thrust vector...
What's important, though, is how the engine affects the vehicle it is attached to. The energy imparted by the engine, propels the vehicle, at the
moment by moment process, if you want to think of it that way.
In an airplane, there are other forces at work as well...go to a Flight School, and you'd learn about the 'four' forces....thrust, which we're
discussing, and 'drag', commonly thought to be anti-thrust, but we'll keep it simple. There is always gravity (weight) of course, and
finally...lift.
Lift is produced by forward motion through the air. Gravity can produce a forward motion through air, since we're talling about a wing producing
such lift. Ever notice how a glider can stay aloft for hours at a time?
Other way, if you're not in a glider, to produce lift and not have to rely on thermals, is to have an engine. Thus, you have a powerful component of
thrust, another source of energy besides gravity and thermal uplifts, and now you can design airplanes that are effecient in their own way...and can
actually take you somewhere, rather than just be a toy (no offense to any glider pilots out there, I know it is a sport...an art...)
Back to point....the engine provides thrust, and the result of the energy imparted to keeping the airframe aloft is devoted to, well, keeping the
airframe aloft!
If you want to examine the exhaust gas speed from an engine, whil the airplane is on the ground, then what you are looking at is....the exhaust gas
speeds as energy is being exerted to move a mass (the airplane) from a standstill, to another speed.
If an airplane is taxiing, there is no need to advance the throttles, once motion is started. In fact, in some cases, just idle power provides too
much 'thrust', once the airplane is in motion. We may have to 'ride' the brakes....but that's a poor idea, since it induces brake wear, and just
adds heat, and we need those brakes in case of a rejected take-off.
There was a time, years ago, when some pilots would put a few engines into reverse, during taxi, to keep taxi speed down, without brakes....but, then
there's greater potential for FOD....finally, and this is the solution today...taxi with one or more (if more that two engines) shut down. Saves
fuel, prevents the fast taxi, and subsequent brake prob! Bingo!
At my airline, we were authorized for the single engine taxi, on the B737, but not the B757 nor B767. That may have changed in the last few years, I
don't know. Reason is, there has to be a procedure, and a checklist, written for such conditions. AND, since we flew both the B757 and B767, and at
times, a B767 would be so heavy that it would NEED both engines to initiate taxi, then there was no procedure to cover all contingencies, hence...all
two-engine taxis was the rule.
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[edit on 4/7/0808 by weedwhacker]
