Specific thrust

Specific thrust

Specific Thrust is a term used in Gas Turbine Engineering to show the relative bulk of a jet engine (e.g. turbojet, turbofan, etc) and is defined as the ratio: net thrust/total intake airflow.

Low specific thrust engines

A modern civil turbofan has a low specific thrust (~30lbf/lb/s) to keep the jet noise at an acceptable level, and to achieve low fuel consumption, as a low specific thrust helps to improve specific fuel consumption (SFC). This low specific thrust is usually achieved with a high bypass ratio. Additionally a low specific thrust implies that the engine is relatively large in diameter, for the net thrust it produces. Consequently such aircraft engines are normally located externally, in a separate nacelle or pod, attached to the wing, or the rear fuselage.

High specific thrust engines

On the other hand, a modern military turbofan has a fairly high specific thrust (45-110 lbf/lb/s), which keeps the cross-sectional area of the engine low, so that (normally) the engine can be easily accommodated within the fuselage, thus minimizing aircraft drag. A high specific thrust implies a high jet noise level, which is not an important consideration for most military applications.

Maximum airspeed

Specific thrust has significant bearing on thrust lapse rate: the low jet velocity associated with a low specific thrust engine implies that there are large reductions in net thrust with increasing flight velocity, which can only be partially offset by throttle changes at rated conditions (e.g. Maximum Recommended Climb Rating).

upersonic aircraft

Supersonic aircraft require a high specific thrust, since that gives a high exhaust speed- exhaust speed must exceed airspeed to maintain forward flight.

Afterburners

Dry specific thrust has impact upon the performance of afterburning turbofans.

A low(dry) specific thrust engine has a low tailpipe temperature, which means that the temperature rise across the afterburner can be high, giving a good thrust boost in afterburning. Nevertheless, the afterburning specific thrust is still relatively low. The total fuel flow (main combustor plus afterburner) is fixed by the temperature rise from air intake to nozzle and, for a given airflow, changes little with dry specific thrust. Consequently, the low afterburning thrust implies a high afterburning SFC. However, the dry SFC is low.

The situation is completely reversed for a high (dry) specific thrust.

Consequently engine designers must select a level of dry specific thrust that is suitable for the engine application. A compromise may be required.

References

ee also

*Specific impulse/exhaust velocity
*jet engine


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • specific thrust — The ratio of total thrust produced by an engine and the mass of the engine. It is a measure of the efficiency of the engine. It is 1/specific weight of engine …   Aviation dictionary

  • specific thrust — noun the thrust per unit weight of a vehicle or engine …   Wiktionary

  • Specific impulse — (usually abbreviated I sp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount …   Wikipedia

  • Thrust bearing — A thrust bearing is a particular type of rotary bearing. Like other rotary bearings they permit rotation between parts, but they are designed to support a high axial load while doing this.Thrust bearings come in several varieties. * Ball thrust… …   Wikipedia

  • Specific fuel consumption (thrust) — Specific fuel consumption, often shortened to SFC, or TSFC is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. It allows the efficiency of different sized engines to be directly… …   Wikipedia

  • Specific orbital energy — In the gravitational two body problem, the specific orbital energy (or vis viva energy) of two orbiting bodies is the constant sum of their mutual potential energy ( ) and their total kinetic energy ( ), divided by the reduced mass. According to… …   Wikipedia

  • specific impulse — noun : the thrust produced per unit rate of consumption of the propellant usually specified in pounds of thrust per pound of propellant used per second and forming a measure of the efficiency of performance of a rocket engine * * * Rocketry. 1. a …   Useful english dictionary

  • specific impulse — noun Date: 1947 the thrust produced per unit rate of consumption of the propellant that is usually expressed in pounds of thrust per pound of propellant used per second and that is a measure of the efficiency of a rocket engine …   New Collegiate Dictionary

  • thrust-specific fuel consumption — A measure of the fuel efficiency of a turbojet or turbofan engine. It is the mass of fuel burned per hour for each unit of thrust produced …   Aviation dictionary

  • specific impulse — Rocketry. 1. a measure, usually in seconds, of the efficiency with which a rocket engine utilizes its propellants, equal to the number of pounds of thrust produced per pound of propellant burned per second. 2. the specific impulse that a given… …   Universalium

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”