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Beginner's Guide to Aerodynamics
Cruising Aircraft and Balanced Forces

Answers will vary. Suggested answers shown below.
  1. Define force.

     Force is a push or pull in a specific direction.

  2.  What are the four forces acting on an airplane?

    The four forces acting on an airplane are lift, weight, drag, and thrust.
  3. How do each of the four forces act on an airplane in flight? (Which direction does each force push or pull on the airplane in flight?)
    Lift pushes or pulls the airplane up. Weight pulls the airplane down towards earth. Drag pulls the airplane back to keep it from moving forward. And thrust pushes the airplane forward.

  4. Define balanced and unbalanced.
    Balanced means everything is equal. Unbalanced means not everything is equal. As in the case with forces, some are greater or less than others.

  5. In terms of the four forces acting on an airplane, what needs to happen so the forces are balanced? What do we call it when all four forces are balanced on an airplane?
    For the four forces to be balanced, thrust must be equal to drag and lift must equal weight. When this happens we say the airplane is cruising.

  6. You are traveling on an airplane and the pilot announces on the intercom that you have reached your cruising altitude of 32,000 feet. Explain what the pilot is telling you. (Include what is happening with the four forces on the airplane).
    The pilot is telling us that the four forces on the airplane are now balanced, the thrust equals the drag and the lift equals the weight. The pilot balanced the four forces at an altitude of 32,000 feet and the plane will continue to fly at that altitude at a constant speed until the forces become unbalanced.
  7. During a flight when we are cruising at a specific altitude, the four forces are balanced. At which points during a flight are the forces unbalanced?
    The four forces are unbalanced during takeoff, climbing, descending, maneuvering, landing, or basically anytime the airplane is not cruising.
  8. Define Newton's First Law of Motion.
    An object at rest will stay at rest and an object in motion will stay in motion at a constant velocity until acted upon by an outside force.

  9. In your own words, explain how Newton's First Law of Motion explains the motion of a cruising airplane.
    Newton's law says an object in motion will stay in motion unless acted upon by an outside force. A cruising airplane has all four forces balanced, with no external forces acting on it. As a result it will remain in its motion at a constant velocity until an external force acts upon it.

  10. What are some ways the forces on an airplane can become unbalanced? (Name at least three).
    Forces can become unbalanced when the airplane's speed slows down or gets faster, when the wing angle of attack is changed, when the weight of the plane changes significantly, or when there is a change in air pressure or density.

Related Pages:
Lesson Index
Aerodynamics Index


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Thu, Jun 12 04:46:42 PM EDT 2014

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