Thrust Variation with Speed and Altitude

The following pages provide examples of the kind of information provided by engine manufacturers. Data on s.f.c. and thrust as a function of Mach number, altitude, throttle setting and power extraction are generally provided in the form of plots and now as software based cycle decks.

Unlike propeller-powered aircraft for which the power output is approximately constant with changes in speed, turbojets produce a more constant thrust with speed. Modern turbofans are somewhat in-between constant thrust jets and constant power propeller systems. Significant reductions in net thrust are associated with increasing speed and altitude.


Typical trend of thrust vs. speed for turbojets and turbofans with varying bypass ratio at seal level.

A particular engine's thrust performance usually cannot be inferred well from generic cycle decks and it is common now to begin an aircraft design study with a number of computer decks from the different engine manufacturers. This is because many possible constraints on engine pressures, temperatures, and RPM's may be critical at different operating points. Many engines are flat-rated, meaning that they might actually be able to produce mush more thrust at low altitudes and speeds, but they are limited (often in software) to lower thrust levels to extend engine life and reduce maximum loads. Thus some supersonic engines show very little reduction in thrust from sea-level static conditions to Mach 1 at 30,000 ft.

Actual engine performance differs from the basic engine data in a number of ways. The air bled from the compressor for air conditioning, the power extracted for hydraulic pumps and alternators, and inlet and exhaust duct losses reduce engine thrust. The exact amount depends, of course, on the requirements of the accessories, the engine size, and the inlet and duct design, but reasonable estimates for conventional inlets are:

1) Thrust is reduced by 3.5% below engine specification levels
2) Specific fuel consumption is increased by 2.0%

During the take-off the air conditioning bleed is often shut-off automatically to avoid the thrust loss. The remaining thrust loss is about 1%. If a long or curved (S-bend) inlet is involved as in center engine installations, an additional thrust loss of 3% and a specific fuel consumption increase of 1-1/2% may be assumed. This additional loss applies only to the affected engine.