Forces Acting on the Aircraft
Thrust, drag, lift, and weight are forces that act upon all
aircraft in flight. Understanding how these forces work and
knowing how to control them with the use of power and
flight controls are essential to flight. This chapter discusses
the aerodynamics of flight—how design, weight, load factors,
and gravity affect an aircraft during flight maneuvers.
The four forces acting on an aircraft in straight-and-level,
unaccelerated flight are thrust, drag, lift, and weight. They
are defined as follows:
• Thrust—the forward force produced by the powerplant/
propeller or rotor. It opposes or overcomes the force
of drag. As a general rule, it acts parallel to the
longitudinal axis. However, this is not always the case,
as explained later.
• Drag—a rearward, retarding force caused by
disruption of airflow by the wing, rotor, fuselage, and
other protruding objects. Drag opposes thrust, and acts
rearward parallel to the relative wind.
• Weight—the combined load of the aircraft itself, the
crew, the fuel, and the cargo or baggage. Weight pulls
the aircraft downward because of the force of gravity.
It opposes lift, and acts vertically downward through
the aircraft’s center of gravity (CG).
• Lift—opposes the downward force of weight, is
produced by the dynamic effect of the air acting on
the airfoil, and acts perpendicular to the flightpath
through the center of lift.
In steady flight, the sum of these opposing forces is always
zero. There can be no unbalanced forces in steady, straight
flight based upon Newton’s Third Law, which states that for
every action or force there is an equal, but opposite, reaction
or force. This is true whether flying level or when climbing
It does not mean the four forces are equal. It means the
opposing forces are equal to, and thereby cancel, the effects
of each other. In Figure 4-1 the force vectors of thrust,
drag, lift, and weight appear to be equal in value. The usual
explanation states (without stipulating that thrust and drag
do not equal weight and lift) that thrust equals drag and lift
equals weight. Although basically true, this statement can
be misleading. It should be understood that in straight, level,
unaccelerated flight, it is true that the opposing lift/weight
forces are equal. They are also greater than the opposing
forces of thrust/drag that are equal only to each other.
Therefore, in steady flight:
• The sum of all upward forces (not just lift) equals the
sum of all downward forces (not just weight).
• The sum of all forward forces (not just thrust) equals
the sum of all backward forces (not just drag).
This refinement of the old “thrust equals drag; lift equals
weight” formula explains that a portion of thrust is directed
upward in climbs and acts as if it were lift while a portion
of weight is directed backward and acts as if it were drag.
In glides, a portion of the weight vector is directed forward,
and, therefore, acts as thrust. In other words, any time
the flightpath of the aircraft is not horizontal, lift, weight,
thrust, and drag vectors must each be broken down into two
Discussions of the preceding concepts are frequently omitted
in aeronautical texts/handbooks/manuals. The reason is not
that they are inconsequential, but because the main ideas
with respect to the aerodynamic forces acting upon an
airplane in flight can be presented in their most essential
elements without being involved in the technicalities of the
aerodynamicist. In point of fact, considering only level flight,
and normal climbs and glides in a steady state, it is still true
that lift provided by the wing or rotor is the primary upward
force, and weight is the primary downward force.
By using the aerodynamic forces of thrust, drag, lift, and
weight, pilots can fly a controlled, safe flight. A more detailed
discussion of these forces follows.
For an aircraft to move, thrust must be exerted and be greater
than drag. The aircraft will continue to move and gain
speed until thrust and drag are equal. In order to maintain a
constant airspeed, thrust and drag must remain equal, just as
lift and weight must be equal to maintain a constant altitude.
If in level flight, the engine power is reduced, the thrust is
lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate until
its airspeed is insufficient to support it in the air.
Likewise, if the engine power is increased, thrust becomes
greater than drag and the airspeed increases. As long as
the thrust continues to be greater than the drag, the aircraft
continues to accelerate. When drag equals thrust, the aircraft
flies at a constant airspeed.
Straight-and-level flight may be sustained at a wide range
of speeds. The pilot coordinates angle of attack (AOA)—the
acute angle between the chord line of the airfoil and the
direction of the relative wind—and thrust in all speed regimes
if the aircraft is to be held in level flight. Roughly, these
regimes can be grouped in three categories: low-speed flight,
cruising flight, and high-speed flight.
When the airspeed is low, the AOA must be relatively high
if the balance between lift and weight is to be maintained.
[Figure 4-3] If thrust decreases and airspeed decreases, lift
becomes less than weight and the aircraft starts to descend.
To maintain level flight, the pilot can increase the AOA
an amount which will generate a lift force again equal to
the weight of the aircraft. While the aircraft will be flying
more slowly, it will still maintain level flight if the pilot has
properly coordinated thrust and AOA.
Straight-and-level flight in the slow-speed regime provides
some interesting conditions relative to the equilibrium of forces because with the aircraft in a nose-high attitude, there is a
vertical component of thrust that helps support it. For one thing,
wing loading tends to be less than would be expected. Most
pilots are aware that an airplane will stall, other conditions
being equal, at a slower speed with the power on than with the
power off. (Induced airflow over the wings from the propeller
also contributes to this.) However, if analysis is restricted to
the four forces as they are usually defined during slow-speed
flight the thrust is equal to drag, and lift is equal to weight.
During straight-and-level flight when thrust is increased and
the airspeed increases, the AOA must be decreased. That is,
if changes have been coordinated, the aircraft will remain in
level flight, but at a higher speed when the proper relationship
between thrust and AOA is established.
If the AOA were not coordinated (decreased) with an
increase of thrust, the aircraft would climb. But decreasing
the AOA modifies the lift, keeping it equal to the weight,
and the aircraft remains in level flight. Level flight at even
slightly negative AOA is possible at very high speed. It is
evident then, that level flight can be performed with any
AOA between stalling angle and the relatively small negative
angles found at high speed.
Some aircraft have the ability to change the direction of the
thrust rather than changing the AOA. This is accomplished
either by pivoting the engines or by vectoring the exhaust