I am looking for shortened but precise and understandable definitions for the 4 forces of flight for an essay I have to do (Lift, Weight, Thrust and Drag) or key parts I need to emphasise.

Any help or key tips are appreciated

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    $\begingroup$ Is this info not readily available from multiple sources on the internet already? $\endgroup$
    – abelenky
    Commented May 23, 2023 at 15:40
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    $\begingroup$ Hi, welcome to aviation stack exchange. This is a very broad question, and one which could (and does) fill books. It's not particularly well suited to a Question & Answer site. If you have specific things you're unclear about please feel free to post them as separate questions $\endgroup$
    – Jamiec
    Commented May 23, 2023 at 16:21
  • $\begingroup$ This NASA link will give you a very nice overview on the matter, especially this page. $\endgroup$
    – sophit
    Commented May 24, 2023 at 5:38
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    $\begingroup$ I rolled back the edit. Please do not deface your posts. $\endgroup$
    – Federico
    Commented May 25, 2023 at 7:05
  • $\begingroup$ Hello Nathan.moors2, please do not deface your own posts. If your original question has been answered, then please accept the answer that was most helpful by clicking the checkmark next to that answer. $\endgroup$
    – DeltaLima
    Commented May 25, 2023 at 7:16

2 Answers 2


Weight is the force due to gravity in the direction of the center of the earth (down).

Thrust is the force produced by the propulsion system. Usually generally pointing 'forward' (and fixed to the body), but sometimes otherwise.

Drag is the aerodynamic force parallel to velocity. It will always act to reduce the velocity.

There are infinite directions perpendicular to velocity. The aerodynamic force perpendicular to velocity is decomposed into two components -- lift and sideforce. Lift is the component of total aerodynamic force perpendicular to velocity and also in the symmetry plane of the aircraft.

Sideforce is perpendicular to velocity and also perpendicular to the symmetry plane of the aircraft.

In many simple analyses, the sideforce is zero and people forget about it. In these situations, people will say that lift is the force perpendicular to velocity.

  • $\begingroup$ What are you really trying to say in that middle paragraph introducing side force? $\endgroup$ Commented May 23, 2023 at 16:52
  • $\begingroup$ Simply that stating "Lift is perpendicular to velocity" is not specific enough when there is non-zero sideforce. In that case, lift is the part (of the aero force perpendicular to velocity) in the symmetry plane and sideforce is the part perpendicular to the symmetry plane. $\endgroup$ Commented May 23, 2023 at 18:23
  • $\begingroup$ But no discussion of the 4 basic forces the OP is asking about ever gets into side force. I don't disagree with the facts you are presenting, but it seems you are adding unasked for complexity to a model that is intended to be a very simple framework. $\endgroup$ Commented May 23, 2023 at 18:40
  • $\begingroup$ And of the infinite directions perpendicular to velocity, what purpose is served by breaking out lift and sideforce? Is not weight opposing lift more useful? Sideforce is only relevant and useful when discussing uncoordinated flight. $\endgroup$ Commented May 23, 2023 at 18:53
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    $\begingroup$ I always thought that the sideforce you describe is just considered part of lift. There's vertical lift and lateral lift... and, I suppose, longitudinal lift. $\endgroup$ Commented May 23, 2023 at 20:36

A picture is worth a thousand words:

enter image description here

In all seriousness though, these are each plain, (and plane) language terms that have commonly understood definitions. And if uncertain, they can be readily looked up from multiple sources. Their usefulness comes not from understanding the words themselves as much as integrating them into the visual model showing how they oppose and balance one another in flight. This forms the foundation for teaching more advanced concepts.

I would encourage you to either provide and cite a book definition, or explain them in your own words to come across naturally. It is doubtful that anybody here will be able to offer insight not already published in multiple places elsewhere.

NOTE: For those unfamiliar, the image posted above is in jest. The four forces of LIFT, WEIGHT, THRUST & DRAG mentioned in the question are typically applied to a generic airplane picture, and I would bet that a majority of non-aviators would probably be able to accurately figure out where each one goes... (if you are new here, give it a try!)


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