Fundamental Forces of Flight
In aeronautics, there are four fundamental forces that act upon the aircraft during inner-atmospheric flight. The four forces are: lift, weight, thrust and drag. The picture below show a graphical representation of the forces.
*NOTE: these four forces only apply within a planet’s atmosphere, there is de facto no lift, weight or drag in space due to the absent of a working fluid and the insufficient amount of gravity. Not to say there’s no gravity in empty space, but it is just not enough to make a rocket go down like how it would near a planet.
We will start with learning about weight as a force, since that is what we deal with everyday of our life, given that you are not an astronaut. Weight is the force that keep us on the ground, it is the thing that keep us from flying for centuries. Recalling from high school Physics, the first thing we learn is Newton’s Law:
With Earth’s gravitation acceleration being at the surface, we often simplify the weight or gravitational force equation to . It is important to know that in the Metric system, mass is measured in kilograms (kg) and weight force is in Newtons (N). On the other hand, the Imperial system, mass in measured in slugs and weight force is measure in pound force (lbf). There’s this thing in the Imperial system that is called pound mass (lbm), that pretty much just add to the mess and confuses everyone. I will be mostly using Metric units here since I absolutely hate the English system, but just keep that in mind. The rest of the world use the Metric system while only the United States stick with the Imperial English system. Read this article for a better clarification on the outdated system that I really think they should get rid of for good. NASA even crashed a Mars orbiter back in 1999 because of this unit mess.
Okay, back to aerospace. Basically, weight is caused by gravitation attraction force between the object and Earth. is a simple estimation for the force, treating gravity as constant. In reality, it is important to know that gravitation acceleration is not constant, it changes with respect to the distance between the two objects. In our case, the gravitational acceleration changes as the aircraft changes altitude. It is given by , where G is the universal gravitational constant, G = 6.67300 × 10-11 m3 kg-1 s-2. M is the mass of the planet (Mass of Earth = 5.9722E24 kg). r is the distance from the center of the planet to the object, pretty much add the altitude to Earth’s radius, 6371 km or 6.371E6 m. Remember that when doing calculation, always use kg for mass and m for distance.
Mean while weight is the force that keep us always “falling” to the center of the planet, lift is the counter force of that. Lift is the force that allow flight to happens. Lift is the force that pull a plane up toward the sky. When lift is stronger than weight, the plane ascends. The other way around, then the plane descends. If lift is equals to weight in the air, then, the plane maintain level flight.
An airplane’s lifting force is usually generated by the wing. The airfoil cross-section create the upward force by setting an uneven pressure above and below itself when air passes through it. When the pressure is lower above the wing and higher below, the lifting force is created. You can feel this force first handed by sticking your hand outside the window when driving on a highway, just arrange your fingers in an arc like shape.
As mentioned above, lift is created by having air rushes through the airfoil. In order to create the effect of air rushing through the airfoil, we need to move the airplane. The force that move the airplane forward is called Thrust. Thrust is usually created by the engine. Generally, the more thrust there is, the more air rushes through the airfoil thus creating more lift. If a plane reduces thrust and travel too slow, not enough lift is generated by the wing and the plane will descends.
For every force, there’s usually a counter force. The counter force of thrust is drag. Drag is created by air rubbing against the airplane’s surface. As drag increases, the thrust required to maintain a certain speed also increases, which required more fuel. A streamline design will reduce drag, thus letting the plane to move travel with less thrust, thus more efficient. Remember that drag is a reaction force, it is not possible for drag to move you backward. Though it is more than capable to slow you down. Again, you can feel this force yourself by sticking your hand out the window on a highway (simulating a wind tunnel pretty much). With your palm facing down, you might feel some lifting force or downward pulling force. With your palm facing the direction you are traveling, you will feel a force pushing it back, that is drag.