Theory of Flight: Aerodynamics

slide001Clouds and balloons float. They are lighter than air. But airplanes are heavier. How does that work?

slide002This is a page from Wolfgang Langewiesche’s book, Stick and Rudder. It was written in 1944, the year I was born. It is still one of the best books on learning to fly.  On this page he shows with pictures that the wing is pushing air down.  It follows from Newton’s Third Law of Motion that the airplane is pushed up.

slide003Recently I came across this photo – yes, it’s real – that shows what’s going on in even more detail. The Cessna Citation is flying slow and clean (no flaps), just above a layer of cloud. You can see not only that it is pushing air down, but that air is escaping around the wingtips and creating horizontal tornadoes. We’ll talk more about that later. But this is one of those images that is important to keep in mind as you learn about flying.

slide004Humankind has been dreaming about flying for millennia. Philosophers (that’s what scientists were called back in the day) have been pondering the mysteries of mass and movement. Then they teach and write and sometimes their insights survive to be developed further by later generations. My favorite example is Newton following Galileo: Newton was born in 1642,  the year Galileo died.

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But first – Leonardo da Vinci! In addition to all his other accomplishments, Leonardo dreamed of flying and studied bats. These sketches survive. Notice how the pilot flaps the wings with (her?) legs through linkages.

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slide026This is the aircraft’s Drag Curve.  It is VERY IMPORTANT that a pilot know this curve well. Make it a goal to be able to draw it from memory before you get to exercises 10 and 11 in your flight course.

You can see that an aircraft’s Total Drag is the sum of two curves:

  1. The Parasite Drag (also called Form Drag) which is just what you think it would be – like sticking your hand out of the window in a car.  Also – as you would predict – the faster you go, the greater the drag.
  2. The Induced Drag, which is the price you pay for making a wing produce lift. Much of this drag goes into making those horizontal tornadoes that form at the wingtips (or flap tips) – otherwise known as wing tip vortices, or wake turbulence. Induced drag is greater at slow speeds (higher angles of attack). It is helpful to consider that a wing is designed to be most efficient at cruise speed.

Why exercises 10 and 11? Because these are:

  • speeds for range and endurance, and
  • slow flight

Slow flight is defined as flight at speeds left of the minimum drag speed.  Notice that if you fly slower than that, the drag increases.

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slide096This last subject is a tough one, but it is of the utmost importance to your job as a pilot. The recent loss of control accidents (Colgan, Air France, Asiana) have demonstrated just how important an understanding of this subject is.

At a certain Angle of Attack (AOA) a wing essentially stops producing lift. It stops working the way it is supposed to. Held in a stall, the airplane will fall to the ground. Since the pilot’s job is survival, that’s not good. The pilot, then, has to understand what aerodynamic stall is, understand when and why it might happen, and know how to recover from a stall.

Notice that I did not say the a stall will happen at a certain speed. I said it will happen at a certain Angle of Attack. True – there is a relationship between speed and Angle of Attack.

 

slide097This A320 is slowing down. It has glided down at 300 knots from FL370 with the engines at idle. Now, at 10,000 feet, it has to slow to 250 knots (the speed limit below 10,000 feet).

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