Regarding Engine Thrust Ratings (757)

[Guest Iz]

Hi Guys,I seem to get the question "Why does the 37,000 lb (eg. PW4037)engine version perform so much worse than the 40,000 lb (eg. RR 535E4) version?" a lot.A mistake that's quickly made is to think that the PW4037 has roughly 8% less thrust so it should climb about 8% slower. This is not the case.If it were the case, it would mean that an aircraft would climb at 100% with 40,000 lb thrust and an aircraft with ZERO lb thrust would fly straight and level (0% climb). Imagine a 757 without engines flying 310 knots at FL330 for an extended period. :) Basic aerodynamics. Any situation where there is no acceleration in any direction, requires an equilibrium of forces in every direction.An acceleration (positive) regarding aircraft can be seen as an acceleration in speed or increase in altitude. In the first case, thrust is higher than drag, in the second, lift is higher than gravity. Lift however, is directly linked to thrust.So in level flight, holding a certain airspeed, a certain amount of thrust is required to equal drag for that speed. Less thrust would cause the aircraft to decelerate.Therefore, acceleration (either in altitude or speed) is only achieved with EXCESS thrust. When an aircraft is at its absolute ceiling (highest it will go), it requires full thrust to maintain airspeed. This means it cannot accelerate or climb any further. There is no excess thrust available.Now say you need 25,000 lb of thrust per engine just to maintain a certain airspeed at a certain altitude (the figure is just meant as an example, actual thrust in lb is fluctuates greatly depending on airspeed, air density etc). This means that in theory you have 12,000 lb of excess thrust available in the PW4037 and 15,000 lb in the RR 535E4. So here's your difference: 25% more excess thrust in the Rolls Royce engine.This is just a fictive sample with fictive numbers and depending on density altitude and airspeed, the difference can be greater or smaller than 25%. It does explain why a 757 with 37,000 lb engines performs significantly less than a 40,000 (or even 43,000lb!) version.Now go out and enjoy your flying :)Iz