Thrust and Newton’s Third Law in Aviation

A good pilot must understand how Newton’s Third Law applies to thrust and how an aircraft flies. It is essential to any student pilot to have a strong grasp on this basic understanding of physics. Doing so will strongly contribute to safe and efficient flying. The first two laws are also important to know and remember; however, the Third Law is uniquely at work every second an aircraft is in flight.

The Third Law states that “For every action, there is an equal and opposite reaction.”  It was developed by Sir Issac Newton in the 17^th century. The four forces of flight are always acting on an aircraft: thrust (forward), drag (rearward), lift (up), and weight (down).  Managing those forces and their equal and opposite reactions to each other is how a pilot makes an aircraft break free of gravity, then maintain control.

 

What is Thrust?

Thrust is the unseen “push” or force that moves an object through the air or on the ground. In the case of pushing a ball across the room, the acting thrust is the hand that shoved it. Where aviation is concerned, the thrust that enables the airplane to overcome gravity and take off is mechanical. The thrust comes from the engine, whether it’s a jet, propeller, or turboprop. What scientists learned about mechanical thrust in the development of balloons and airplanes was quickly applied to the rockets which pushed spacecrafts out of Earth’s atmosphere.

The energy of the thrust of the engine is derived from the fuel powering the machine. It creates a chemical reaction that propels the airplane into the opposite direction of the chemical reaction generated by this controlled combustion. In order to maximize the power of the propulsion system, engineers and airplane designers perform a thermodynamic analysis to predict thrust amongst a variety of variables. The weight, type, and propulsion type of an aircraft means that thrust can vary widely from type to type.

 

Thrust and Newton’s Third Law

Now that we have a working understanding of thrust, we can focus on how it fits in with Newton’s Third Law. The Third Law is most dramatically applied with this force of flight.  Thrust is most often thought of as simply acting opposite to drag. However, this is a simplistic understanding of how the Third Law works in the aviation world.

There is much more to understand about thrust and its relation to flight, thanks to Newton’s understanding and explanation.  An aircraft gains altitude entirely because of thrust; either the thrust is pointed upward, as is the case with helicopters, or an airplane tilting its nose up, or in level flight, excess thrust overcomes drag, increasing airflow over the wings, generating more lift, which increases altitude.  The Third Law eventually “catches up” with the airplane, because the byproduct of lift is drag, so drag increases to match the extra lift, and the aircraft returns to equilibrium without a further increase in thrust.

 

Balancing the Four Forces of Flight

When an aircraft is in straight, level, un-accelerated flight, called cruise phase, all four forces are balanced and acting with equal strength and direction.  To maneuver the aircraft, the pilot must intentionally disrupt the balance between the forces, and then quickly restore the balance or risk losing control.  For example, to make the aircraft turn, the pilot banks the wings or tilts the rotor, which points to the side some of the lift normally pointing up.  That is called creating a horizontal component of lift, and that moves the aircraft to the left or right of its original course.

However, the horizontal component of lift doesn’t come from thin air; it must be subtracted from the vertical component usually generated by the wings or rotor.  That means in a bank, there is less vertical lift to counteract gravity, or weight, trying to pull the aircraft lower.  Uncorrected, the aircraft will descend. This means that the pilot, using Newton’s Third Law to maintain altitude, must increase the angle of attack to generate more total lift, so that the vertical component will have the equal and opposite reaction to gravity.

Still on the topic of turning, when an airplane banks its wings, the wing on the outside of the turn travels faster through the air, generating more lift.  One will recall that a byproduct of lift generation is drag, so the outer wing will tend to be “pulled back” by that extra drag.  The pilot uses Newton’s Third Law again and applies rudder in the direction of the turn, overcoming the excess drag, and the airplane will stay in the turn. Flight, then, is a constant maintenance and equilibrium of all these forces.

Taken another step further, the act of deflecting the rudder also generates sideways lift to turn the airplane, which creates more drag.  Newton’s Third Law demands that the additional drag be overcome, or the airplane will slow down.  That is why, to maintain speed in a turn, the pilot must increase the thrust generated by the engine or engines.

 

Lift, Thrust, and Motion: Bernoulli vs Newton

The understanding of the role of Newton’s Third Law in thrust and lift is generally embraced by aeronautical scientists, engineers, and physicists, but some scientists and pilots disagree about how the Third Law fits in with Daniel Bernoulli’s principle of lift, which basically states that pressure in airflow drops as the air flows over the surface of the wing.

Newton lived before Bernoulli, and Bernoulli studied Newton’s work while developing his theory. Even the most celebrated scientists in the field favor varying theories about how lift, the Third Law, and Bernoulli’s theories work together. Student pilots can find a variety of explanations, depending up on which text the study or even the theory their flight school favors. Some of this information can at, times, seem to conflict. What is important to understand is the underpinnings of the theories and how the forces of flight act upon an airframe, not to mention the role of the propulsion system.