Fluid Mechanics

Fluid Mechanics Explained

fluid-mechanics1. Aerospace Engineers and physicists alike classify all liquids and gases as “Fluids”

2. Even though air, water and syrup may seem like very different substances, they all conform to the same set of mathematical relationships

  • Often, basic aerodynamic tests are performed underwater
  • Another way of looking at this is that fish essentially fly through water and birds swim through the air
  • Empty sky is not really empty at all – the atmosphere is a massive layer of fluid 1


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3. All flight comes down to a matter of controlling 4 separate forces that act on the object trying to fly: (1) Thrust (2) Lift (3) Drag (4) Weight

  • Thrust is the force produced by the engines of the aircraft that move it forward
  • Drag is the force produced by the resistance of air to the forward motion of the airplane
  • Weight is the force created by the pull of gravity toward the center of the earth
  • Lift is the force created as air pushes up against the airfoil (wing) and for that matter, the rest of the plane as the aircraft moves forward 2

4. The net impact of these forces:

  • When the force of Thrust produced by the engine(s) is greater than the force of Drag, the airplane moves forward.
  • When the forward motion is enough to produce a force of Lift that is greater than the Weight, the airplane moves upward. 3


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5. Every object on earth has Weight (which equals its mass multiplied by the force of gravity)…weight + mass x gravity. This is the force which draws the aircraft toward earth.

6. Weight’s opposing force is Lift, which holds the aircraft in the air. This is accomplished through the use of a wing, also known as an airfoil.

7. As is the case with the Drag, Lift can exist only in the presence of a moving fluid

  • Lift can work regardless of whether the object is stationary and the fluid is moving (as with a kite on a windy day), or the fluid is stationary and the object is moving (as with a flying jet on a windless day)
  • The key is the relative difference in speeds between the object and the fluid

8. As for the actual mechanics of lift, the force occurs when a moving fluid is deflected by an airfoil.

  • The wing splits the fluid in two directions: up and over the wing and down along the underside of the wing

9. The wing shape and tilt enables air moving over it to travel faster than the air moving underneath

  • When moving air runs into the aircraft and particularly the wing (and confronts its front and the sudden increase in wing angle), its path narrows and the flow speeds up as all the molecules rush though
  • Once past the obstacle, the path widens and the flow slows down again
  • The same principle is at work when water in a river is choked going through a narrow pass – the speed of the water necessarily speeds up until the ravine widens again

10.  As air speeds up going over the top of the wing, the associated pressure drops. As a result, the faster-moving air moving over the wing exerts less pressure on it than the slower air moving underneath the wing.

  • The result is an upward force of Lift or push that is exerted against the aircraft
  • In Fluid Mechanics, this is known as Bernoulli’s principle 4