Understanding the Relationship Between Available Thrust and Required Thrust in Aviation

In aviation, understanding how thrust works is crucial for safe and efficient flight. Pilots and engineers rely on the concepts of available thrust and required thrust to determine whether an aircraft can maintain steady flight, climb, or accelerate. Graphing these two forces against variables like airspeed provides valuable insights into aircraft performance. This post explains the relationship between available thrust and required thrust, how to interpret their graphs, and why this knowledge matters for pilots and aviation enthusiasts.

Graph showing available thrust and required thrust curves for an aircraft at different speeds

What Is Available Thrust?

Available thrust refers to the force produced by an aircraft’s engines at any given moment. It depends on several factors:

• Engine type (jet, turboprop, piston)

• Engine power setting (throttle position)

• Air density (affected by altitude and temperature)

• Airspeed

  
  

For example, a jet engine produces maximum thrust at low speeds and sea level conditions. As speed increases, the thrust available usually decreases due to changes in airflow and engine efficiency. Pilots can control available thrust by adjusting the throttle, but environmental factors limit the maximum thrust achievable.

What Is Required Thrust?

Required thrust is the amount of thrust an aircraft needs to overcome drag and maintain steady flight at a particular speed and altitude. Drag increases with speed, so the required thrust changes accordingly. It depends on:

• Aircraft weight

• Drag characteristics (parasite drag and induced drag)

• Airspeed

• Altitude

  
  

At low speeds, induced drag (caused by lift) is high, so required thrust is higher. At higher speeds, parasite drag (caused by air resistance) dominates, increasing required thrust again. The required thrust curve typically has a U-shape when graphed against airspeed.

How Graphing Thrust Helps Understand Aircraft Performance

Plotting available thrust and required thrust on the same graph against airspeed reveals critical performance points:

• Equilibrium points where available thrust equals required thrust indicate steady flight speeds.

• Speeds below equilibrium mean available thrust exceeds required thrust, allowing acceleration or climb.

• Speeds above equilibrium mean required thrust exceeds available thrust, causing deceleration or descent.

  
  

This graph helps pilots understand safe operating speeds, maximum climb rates, and stall speeds.

Example: Interpreting a Thrust Graph

Imagine a graph where the horizontal axis is airspeed and the vertical axis is thrust force.

• The available thrust curve starts high at low speeds and slopes downward as speed increases.

• The required thrust curve forms a U-shape, high at low and high speeds, lowest at some intermediate speed.

  
  

Where these curves cross marks the maximum and minimum steady flight speeds. Between these points, the aircraft can maintain level flight. Outside this range, the aircraft either cannot maintain speed or must change altitude.

Practical Applications for Pilots

Understanding these thrust curves helps pilots make informed decisions:

• Takeoff and climb: Pilots ensure available thrust exceeds required thrust to gain altitude safely.

• Cruise: Pilots select speeds where thrust required is minimal to save fuel.

• Approach and landing: Pilots monitor thrust to maintain control at lower speeds without stalling.

  
  

For example, if a pilot notices the aircraft speed dropping near the minimum equilibrium point, they know to increase throttle to avoid stall.

Factors Affecting Thrust Curves

Several factors shift these curves:

• Weight changes: Heavier aircraft increase required thrust.

• Altitude: Thinner air reduces available thrust and changes drag.

• Engine condition: Engine wear or damage reduces available thrust.

• Configuration: Flaps and landing gear increase drag, raising required thrust.

  
  

Pilots must consider these when planning flights and adjusting power settings.

Summary