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Old 22nd Apr 2004, 11:09   #1 (permalink)
Join Date: Mar 2001
Location: Ireland
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When does a jet engine generate max thrust?

This seems like such a simple question I have been asked...

Under what conditions does a jet engine generate maximum thrust?

Thanks to anybody that can take up this question..
genius747 is offline  
Old 22nd Apr 2004, 12:29   #2 (permalink)
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Most gas turbines are flat rated to some extent. That is the rated thrust can be achieved over a range of altitudes and air temperatures without exceeding any engine limits.

If you are only limited by temperature (TET) and rpm limits, the highest thrust would be achieved at the highest air density i.e. low altitude and temperature. Other limits may come into play though, such as combustor pressure.
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Old 22nd Apr 2004, 12:32   #3 (permalink)

Why do it if it's not fun?
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Surely the answer would be "when the thrust levers are fully forward"?

Or a more sensible answer.... in addition to Mark's reply, is it not also true that jets develop most thrust at high true airspeeds? (Not a jet pilot... at least not yet... but that's the way I understand it in theory.)

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Old 22nd Apr 2004, 12:35   #4 (permalink)
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In fact, the highest thrust, as measured on the Pylon, would be when static.

Momentum drag in the intake is greater than the effect of increased jet velocities as speed increases.
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Old 22nd Apr 2004, 12:35   #5 (permalink)
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This site explains it quite nicely... also shows what mark means by flat rated.
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Old 22nd Apr 2004, 13:32   #6 (permalink)
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cheers gang...

Mark1 what do you mean max thrust is developed when static?
genius747 is offline  
Old 22nd Apr 2004, 13:57   #7 (permalink)
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Static = No forward speed.

Most engine makers quote the rating as sea-level static (SLS) thrust,as this gives them the biggest number to bandy about.
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Old 22nd Apr 2004, 16:50   #8 (permalink)
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ON FADEC engines, it is when they are turned off. FADEC will limit N1 and N2 for given flight phases. But as stated, the full thrust rating would only be available at static sea-level. Most FADEC controlled engines will set a limit for the thrust for flight phases based on pressure , temperature, AND velocity:

From a FADEC manual for the CFM56-7

Each N1 is calculated according to the following flight
conditions :
- Temperature : the thrust delivered depends on
outside air temperature (OAT). By design, the
engine provides a constant thrust up to a predetermined
OAT value, known as “corner point”,
after which the thrust decreases proportionally to
maintain a constant EGT value.
- Pressure : with an increase in altitude, thrust will
decrease when operating at a constant RPM due
to the reduction in air density, which reduces the
mass flow and fuel flow requirements.
- Mach : when mach number increases, the velocity of
the mass of air entering the engine changes,
decreasing thrust. To determine the fan speed, the
EEC calculates M0 from the static pressure, the
total pressure and the TAT values.
- Bleed : ECS bleed and anti-ice bleed are taken into
account in order to maintain the same EGT level
with and without bleeds.
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