How 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 flights 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 or descending.

 

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
• The sum of all forward forces (not just thrust) equals

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 components.

 

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 in flight can be presented in their most essential without being involved in the technicalities of the In point of fact, considering only level flight, climbs and glides in a steady state, it is still true provided by the wing or rotor is the primary upward weight is the primary downward force.

 

To move, thrust must be exerted and be greater aircraft will continue to move and gain thrust and drag are equal. In order to maintain a 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

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. [Figure 4-2]

 

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 gases. [Figure 4-4]