Thrust
Thrust


Thrust is a reaction force described quantitatively
by Newton’s second and third laws. When a system expels or accelerates mass in one direction,
the accelerated mass will cause a force of equal magnitude but opposite direction on
that system. The force applied on a surface in a direction perpendicular or normal to
the surface is called thrust. Force, and thus thrust, is measured in the International System
of Units as the newton, and represents the amount needed to accelerate 1 kilogram of
mass at the rate of 1 metre per second squared. In mechanical engineering, force orthogonal
to the main load is referred to as thrust. Examples A fixed-wing aircraft generates forward thrust
when air is pushed in the direction opposite to flight. This can be done in several ways
including by the spinning blades of a propeller, or a rotating fan pushing air out from the
back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is
proportional to the mass of the airstream multiplied by the difference in velocity of
the airstream. Reverse thrust can be generated to aid braking after landing by reversing
the pitch of variable pitch propeller blades, or using a thrust reverser on a jet engine.
Rotary wing aircraft and thrust vectoring V/STOL aircraft use engine thrust to support
the weight of the aircraft, and vector sum of this thrust fore and aft to control forward
speed. Birds normally achieve thrust during flight
by flapping their wings. A motorboat generates thrust when the propellers
are turned to accelerate water backwards. The resulting thrust pushes the boat in the
opposite direction to the sum of the momentum change in the water flowing through the propeller.
A rocket is propelled forward by a thrust force equal in magnitude, but opposite in
direction, to the time-rate of momentum change of the exhaust gas accelerated from the combustion
chamber through the rocket engine nozzle. This is the exhaust velocity with respect
to the rocket, times the time-rate at which the mass is expelled, or in mathematical terms: where T is the thrust generated, is the rate
of change of mass with respect to time, and v is the speed of the exhaust gases measured
relative to the rocket. For vertical launch of a rocket the initial
thrust at liftoff must be more than the weight. Each of the three Space Shuttle Main Engines
could produce a thrust of 1.8 MN, and each of the Space Shuttle’s two Solid Rocket Boosters
14.7 MN, together 29.4 MN. Compare with the mass at lift-off of 2,040,000 kg, hence a
weight of 20 MN. By contrast, the simplified Aid for EVA Rescue
has 24 thrusters of 3.56 N each. In the air-breathing category, the AMT-USA
AT-180 jet engine developed for radio-controlled aircraft produce 90 N of thrust. The GE90-115B
engine fitted on the Boeing 777-300ER, recognized by the Guinness Book of World Records as the
“World’s Most Powerful Commercial Jet Engine,” has a thrust of 569 kN.
Thrust to power The power needed to generate thrust and the
force of the thrust can be related in a non-linear way. In general, . The proportionality constant
varies, and can be solved for a uniform flow: Note that these calculations are only valid
for when the incoming air is accelerated from a standstill – for example when hovering.
The inverse of the proportionality constant, the “efficiency” of an otherwise-perfect thruster,
is proportional to the area of the cross section of the propelled volume of fluid and the density
of the fluid. This helps to explain why moving through water is easier and why aircraft have
much larger propellers than watercraft do. Thrust to propulsive power
A very common question is how to contrast the thrust rating of a jet engine with the
power rating of a piston engine. Such comparison is difficult, as these quantities are not
equivalent. A piston engine does not move the aircraft by itself, so piston engines
are usually rated by how much power they deliver to the propeller. Except for changes in temperature
and air pressure, this quantity depends basically on the throttle setting.
A jet engine has no propeller, so the propulsive power of a jet engine is determined from its
thrust as follows. Power is the force it takes to move something over some distance divided
by the time it takes to move that distance: In case of a rocket or a jet aircraft, the
force is exactly the thrust produced by the engine. If the rocket or aircraft is moving
at about a constant speed, then distance divided by time is just speed, so power is thrust
times speed: This formula looks very surprising, but it
is correct: the propulsive power of a jet engine increases with its speed. If the speed
is zero, then the propulsive power is zero. If a jet aircraft is at full throttle but
is tied to a very strong tree with a very strong chain, then the jet engine produces
no propulsive power. It certainly transfers a lot of power around, but all that is wasted.
Compare that to a piston engine. The combination piston engine–propeller also has a propulsive
power with exactly the same formula, and it will also be zero at zero speed –- but that
is for the engine–propeller set. The engine alone will continue to produce its rated power
at a constant rate, whether the aircraft is moving or not.
Now, imagine the strong chain is broken, and the jet and the piston aircraft start to move.
At low speeds: The piston engine will have constant 100%
power, and the propeller’s thrust will vary with speed
The jet engine will have constant 100% thrust, and the engine’s power will vary with speed This shows why one cannot compare the rated
power of a piston engine with the propulsive power of a jet engine – these are different
quantities. There isn’t any useful power measurement in a jet engine that compares directly to
a piston engine rated power. However, instead of comparing engine performance, the gross
aircraft performance as complete systems can be compared using first principle definitions
of power, force and work with the requisite considerations of constantly changing effects
like drag and the mass in both systems. There is of course an implicit relationship between
thrust and their engines. Thrust specific fuel consumption is a useful measure for comparing
engines. See also
Aerodynamic force Astern propulsion
Gimballed thrust, the most common thrust system in modern rockets
Stream thrust averaging Thrust-to-weight ratio
Thrust vectoring Tractive effort
References

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