There are four forces that act on an
aircraft in flight: lift, weight, thrust, and drag.
having both a magnitude and a direction.
of the aircraft through the air depends on the relative magnitude
and direction of the various
of an airplane is determined by the size
and materials used in the airplane's construction and on the payload
and fuel that the airplane carries.
The weight is always directed towards the center of the earth.
by the size and type of propulsion system used
on the airplane and on the throttle setting selected by the pilot.
Thrust is normally directed forward along the center-line of the aircraft.
are aerodynamic forces that depend on the
shape and size of the aircraft, air conditions, and the flight
Lift is directed perpendicular to the flight path and drag is
directed along the flight path.
Just as the lift to drag ratio is an
efficiency parameter for total aircraft aerodynamics, the thrust
to weight ratio is an efficiency factor for total aircraft
propulsion. From Newton's
second law of motion
for constant mass, force F is equal to mass m times
F = m * a
If we consider
a horizontal acceleration and neglect the drag
the net external force is the thrust F.
From the Newtonian weight equation:
W = m * g
where W is the weight and g is the gravitational constant
equal to 32.2 ft/sec^s in English units and 9.8 m/sec^s in metric units.
Solving for the mass:
m = W / g
and substituting in the force equation:
F = W * a / g
F / W = a / g
F/W is the thrust to weight
and it is directly proportional
to the acceleration of the aircraft.
An aircraft with a high thrust to weight ratio has high acceleration.
For most flight conditions,
an aircraft with a high thrust to weight ratio will also have
a high value of excess thrust. High excess
thrust results in a high rate of climb. If
the thrust to weight ratio is greater than one and the drag is small,
the aircraft can accelerate straight up like a rocket. Similarly,
rockets must develop thrust greater than the weight of the rocket
in order to
NOTE: We must be very careful when using data concerning
the thrust to weight ratio. Because airframes and engines are
produced by different manufacturers and the same engine can go into
different airframes, the thrust to weight ratio of the engine alone
is often described in the literature. High
indication of the thrust efficiency of the engine. But when
determining aircraft performance, the important factor is the
thrust to weight of the aircraft, not just the engine alone. Another
problem occurs because the thrust of an engine decreases with
altitude while the weight remains constant. Thrust to weight ratios
for engines are often quoted at sea level static conditions, which
give the maximum value that the engine will produce.
Forces on an Airplane:
EngineSim - Engine Simulator:
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