There are four forces working on an aircraft traveling at a constant speed. They are lift, thrust, drag, and weight. In straight and level flight, thrust directly opposes drag, and weight directly opposes lift.

Thrust

Thrust pushes the plane through the sky and is produced by the aircraft's engine.


Weight

Weight is how a mass is accelerated under the influence of gravity. On earth, the acceleration of gravity is 32.2 ft/s2, or 9.8 m/s2. For most aircraft flying close to the earth, the acceleration of gravity is a constant, until you start moving away from the earth. For space shuttles, rockets, and ICBMs, the influence of gravity becomes variable and the assumption of constant gravity, and thus weight is not valid. But for our purposes, it is constant.

The weight of an aircraft will change appreciably over a long time period due to the burning of the aircraft’s fuel, since fuel can take up an appreciable amount of the total aircraft’s weight. But, this change of mass over time changes slowly, so it is OK to assume that the weight is constant at any point in time (unless you are carrying a big bomb load, and suddenly drop your ordinance).

Lift

The aerodynamicist’s primary job is to design an aircraft that generates the maximum amount of lift, the least amount of drag, and uses minimum weight to do it. In addition, the aerodynamicist must design the lifting surfaces that also meet the performance requirements of the aircraft over the entire range of operation.

Lift on an aircraft is due primarily due to the contributions of the lifting surfaces on the aircraft. The primary surface for generating lift is the aircraft wing. At a given speed, the curved upper surface of the wing requires the air to have to travel farther than the lower surface of the wing. If the streamlines are to meet at the trailing edge of the wing, then the air must have to accelerate faster on the top surface than on the bottom.

In essence, the air on the top surface has given up some of its static pressure energy and converts this static pressure energy into kinetic energy. Therefore, there is lower pressure on the upper part of the wing compared to the lower part of the wing. The sum total of static pressure plus the pressure due to movement of the air is called total pressure, and is constant at any point in the flow. This phenomenon is called the Bernoulli effect, and the net result is a lifting force.

Common depiction of airflow over a wing. This wing has no lift.

True airflow over a wing with lift, showing upwash and downwash.

Drag

Drag forces are forces that are generally opposite to the direction of flight. There are generally three types of drag that manifest themselves during flight. Minimizing drag results in the increased range and lower engine power requirements for an aircraft. They are listed and described below:

Skin Friction Drag

Skin friction drag is a force that is in the same direction as the airflow. If you can visualize the airflow over a wing, there has to be a point on the wing surface where the flow is stopped since the air is moving at zero velocity with respect to the moving wing or vice-versa in the situation of a wind tunnel test.

To bring the flow to rest at the surface, there has to be some force that causes this. This force is called a viscous shearing force and is what causes the skin friction drag. This is why airliner companies and militaries stress the importance of washing their aircraft routinely. In addition, there are very strict engineering specifications regarding the surface roughness profiles of high performance aircraft. The smoother the surface is, the less skin drag there is.

Pressure or Profile Drag

When you stick your hand out the window of a moving car you feel a force that pushes your hand back when you turn your palm into the wind. On the other hand (no pun intended), have you noticed that the force pushing your hand is significantly less when you turn your palm down and 90° to the wind? You have experienced first hand (pun?) drag due to pressure or due to surface profile.

Pressure drag is exactly what the name implies; there is a pressure unbalance between the front/leading surface, and the trailing surface. This is why aircraft are essentially streamlined, to minimize the profile or pressure drag.

Drag Due to Lift or Induced Drag

Flight sims today have made some incredible advances in the area of graphics. Wingtip vortices are even modeled very realistically. Did you know that wingtip vortices cause induced or lift drag? The explanation for induced drag is that since a wing is of finite length on a real aircraft, the high-pressure air on the bottom of the wing will migrate towards the upper part of the wing where there is lower pressure. The air on the wingtips will then roll off and turn counter-clockwise on the left wing and turn clockwise on the right wing when viewed from the front of the aircraft.

The vortices cause a downward motion on the wing and reduce the effective angle of attack. This effect is called downwash. Since there is a reduction of the effective angle of attack, the lift vector itself is tilted forward by this downwash angle. The lift vector will then have some component in the drag direction. This component is called induced drag. Hence, lift is not free as it causes drag itself.

In fact, the amount of induced drag rises as the square of the amount of lift generated, so induced drag is large for aircraft on takeoff/climbout and landing and can account for as much as 25% of the total amount of drag on the aircraft. This induced drag can be reduced by increasing the aspect ratio of the wing (span2/wing area), or decreasing the amount of lift generated (not practical).

However, there is some optimum (and expensive) wing profile that is designed to purposely minimize the amount of induced drag generated by a wing. One famous example is the British Supermarine Spitfire. It has an elliptical wing planform, which is desirable from an induced drag standpoint. This is not to say that the Spitfire has no induced drag, but it has the optimum wing planform shape to minimize it.