Aircraft move forward because their engines push air backward. This is a direct example of Newton's Third Law: when the aircraft exerts a backward force on air, the air exerts an equal and opposite forward force on the aircraft. That forward force is called thrust, and it is essential for takeoff, climbing, cruising, and overcoming drag.
The subtitle idea, Pushing Air Back to Move Forward, captures the central physics in one sentence.
A jet engine creates thrust by pulling in air, compressing it, mixing it with fuel, burning the mixture, and ejecting hot gas backward at high speed. A propeller creates thrust by spinning airfoil-shaped blades that accelerate a large mass of air backward, usually at a lower speed than a jet. In both cases, thrust depends on how much air is moved each second and how much its velocity changes.
A useful model is F = m_dot delta-v, where m_dot is the mass flow rate of air and delta-v is the change in air speed.
Understanding Aviation: Thrust and Newton's Third Law
The important quantity behind thrust is momentum. Momentum describes how hard it is to stop a moving mass. Each second, an engine gives some air or exhaust gas extra backward momentum.
The aircraft receives a forward momentum change of the same size. This is why the speed of the exhaust matters, but it is not the whole story. The amount of air matters too.
A fan engine at the front of an airliner moves a very large stream of air. Much of its useful thrust comes from the fan rather than from the hot core exhaust. This design works well at airline speeds because it produces substantial thrust without throwing all the air backward extremely fast.
Energy use gives an important reason for this difference. Giving a small mass of air a huge speed requires more energy than giving a large mass a modest speed when both choices provide similar thrust. The extra energy ends up as fast moving air in the wake.
That leftover motion is not useful for moving the aircraft. Propellers and high bypass turbofan engines are therefore efficient when an aircraft flies well below the speed of sound. Their large disks or fans spread the force across a wide area of air.
Fighter jets accept faster exhaust because they need compact engines and strong thrust at high speed. Rockets are different because they carry their own reaction mass and can work where there is almost no air.
Thrust does not by itself tell whether an aircraft speeds up. The forces on the whole aircraft must be compared. Drag acts backward and grows strongly as airspeed rises.
Rolling resistance matters while wheels are on the runway. Weight acts downward, while lift acts upward. During a takeoff roll, thrust must exceed drag and rolling resistance for the aircraft to accelerate.
In steady cruise, thrust is usually adjusted until it equals drag. The aircraft then keeps a roughly constant speed. During a climb, engines may provide more thrust than is needed for level flight, allowing some of the aircraft's energy to increase its height.
Students often picture a propeller as a screw pulling itself through the air. A better model treats each blade as a rotating wing. Its curved shape and angle guide air backward.
The blade feels a force with a forward part that pulls the aircraft. Blade angle must be chosen carefully. If the angle is too small, the blade does little work.
If it is too large, airflow can separate from the blade surface, much like a stalled wing. Many aircraft use variable pitch propellers so the blades can keep an effective angle during takeoff, climb, and cruise.
Real engines have limits that simple thrust calculations leave out. Air entering an engine already moves relative to the aircraft, so the engine must first take in that moving air before accelerating it rearward. At high aircraft speed, this reduces the net gain from the exhaust.
Air density changes with altitude and weather, changing how much mass reaches a propeller or engine each second. Jet engines lose available thrust high in the atmosphere, although lower air density reduces drag. When studying aircraft performance, pay attention to the reference frame, the direction of each force, and the difference between thrust produced by an engine and the net force that actually accelerates the aircraft.
Key Facts
- Newton's Third Law: for every action force, there is an equal and opposite reaction force.
- Thrust is the forward force on an aircraft caused by accelerating air backward.
- Basic thrust model: F = m_dot delta-v.
- Mass flow rate is m_dot = mass of air moved per second, measured in kg/s.
- A jet engine usually gives a smaller mass of air a larger change in velocity.
- A propeller usually gives a larger mass of air a smaller change in velocity.
Vocabulary
- Thrust
- Thrust is the force that pushes an aircraft forward through the air.
- Newton's Third Law
- Newton's Third Law states that forces occur in equal and opposite pairs between interacting objects.
- Mass flow rate
- Mass flow rate is the amount of mass that passes through a system each second.
- Delta-v
- Delta-v is the change in velocity of an object or fluid, such as air moving through an engine.
- Propeller
- A propeller is a rotating set of airfoil-shaped blades that accelerates air backward to produce thrust.
Common Mistakes to Avoid
- Thinking the engine pushes against still air like a solid wall. Air is a fluid, and thrust comes from changing the momentum of moving air.
- Forgetting the direction of the reaction force. If the aircraft pushes air backward, the air pushes the aircraft forward.
- Using only air speed and ignoring mass flow rate. A small amount of very fast air and a large amount of slower air can both produce thrust depending on m_dot delta-v.
- Assuming propellers and jets use different laws of physics. Both create thrust by accelerating air backward and both obey Newton's Third Law.
Practice Questions
- 1 A small jet engine moves 20 kg of air each second and increases the air speed by 300 m/s. Using F = m_dot delta-v, what thrust does it produce?
- 2 A propeller accelerates 80 kg of air each second by 25 m/s. Calculate the thrust, then compare it to a jet that moves 10 kg of air each second by 200 m/s.
- 3 Two aircraft produce the same thrust. One uses a propeller that moves a large mass of air slowly, and the other uses a jet that moves a smaller mass of air very fast. Explain how both can satisfy F = m_dot delta-v and Newton's Third Law.