Science Sunday 12-2-10

How Airplanes Fly

A good place to start discussing how airplanes fly is to look at the four forces acting on the plane while in flight:

four forces

Thrust comes from the planes jets or propellers, and is a third law effect: as the air is pushed to the rear, it pushes back, causing the plane to go forward.

Weight of course is the Earth’s attraction of the plane to it, and is universally downward.

Drag is just air friction.

Lift is what causes the airplane to go up and stay up.  It is caused by the flow of air over the plane’s wings.

Lift happens because of the cross-sectional shape of the wings (called the airfoil) and the wing’s angle of attack:


The airfoil, the shape of which can change on takeoff and landing, creates a difference in pressure between the top and bottom of the plane’s wings.  This difference in pressure is caused by two factors: Bernoulli’s Principle and Newton’s Third Law.

According to Bernoulli, air travelling faster over a surface exhibits less pressure on that surface than air travelling slower over that surface.

Air travels faster over the top of the wing than the bottom of the wing, so there is a pressure difference between the bottom of the wing (higher pressure) and the top of the wing (lower pressure).  That pressure difference is up, and the plane is lifted up, once the lift force exceeds the plane’s weight.

Lift can also be explained as Third Law action-reaction pairs between the air flowing under and over the wing and the wing itself.   Because there is much confusion about whether the Bernoulli explanation or the Third Law explanation is dominantly correct in explaining lift, let’s look at a video on that very same subject:

When a plane is taking off, the angle of attack is changed to create the lift needed to lift the plane off the ground.  This is accomplished by the wing’s flaps and slats:

flaps and slats

Flaps (at the back of the wing) and slats (at the front of the wing) increase the wing’s surface area, which increases lift.

[The spoiler shown in the illustration above is located on the top and back of the wing, and is used to decrease lift on landing, or to aid in slowing down the plane once it has landed.]

Lift is also enhanced by the Coanda Effect, which states that a fluid will tend to hug a convex surface.  On the top of the wing (with flaps and slats extended, or not), the air flow over the wing is redirected downward due to the Coanda Effect.  By Newtons Third Law, the air pushed downward by the wing reacts by pushing back upward on the wing, causing lift.  (It also induces some additional drag.)

[Of course air flowing under the wing is also directed downward, because of the angle of attack.  So Newton’s Third Law will result in additional lift here as well.]

Notice the separation between successive flaps in the illustration above.  Air flowing below the flap is redirected to above the flap, at which point the Coanda Effect causes it to hug the top of the flap so as to redirect that air downward.  The overall lift enhancement due to the Coanda Effect is shown here:

Lift Coanda

[Here we see that the slats also enhance the Coanda Effect by redirecting air flow to above the wing.]

In researching the subject of the Coanda Effect, I ran across a video that illustrates it nicely:

Note that flaps and slats are normally retracted during steady flight.

Of course, once airborne, it is necessary to control the airplane as to its direction and its altitude (collectively called navigation).  This is done using three devices: the ailerons, the rudder and the elevator.

The ailerons are at the wing tips and work at cross purposes: when one aileron goes up, the other goes down.  The aileron that is up causes its wing to go down (because it has less lift).  The aileron that is down causes its wing to go up (because it has more lift).

When ailerons are used, the plane rolls around its front to back axis of rotation, and this causes a change in the plane’s overall direction and altitude, while maintaining its pointing direction.

The rudder is on the vertical stabilizer at the end of the plane.  When it is used, the plane will yaw around its top to bottom axis of rotation, and this causes a change in the plane’s pointing direction but not its altitude.   The rudder is sometimes used to inhibit a yawing motion that occurs when the ailerons are used to change direction and altitude.

The elevator is on the horizontal stabilizer at the end of the plane.  When it is used, the plane’s pitch will change, meaning it will rotate around the horizontal axis going through the plane’s wings.  This changes the plane’s altitude (up if the elevator is up, and down if the elevator is down) without changing the plane’s direction.

Here is a nice illustration of the three axes of rotation of a plane in flight:

roll pitch yaw

Here also is a nice illustration of the three in-flight control devices just described, and their purposes:


Notice that a plane has to maintain a minimum speed in flight to keep the lift force equal to the plane’s weight.  When a plane is landing, the flaps and slats are opened to increase wing area and lift to compensate for the slower plane speed.

A plane also needs to avoid too great an angle of attack, because then the plane will stall due to insufficient lift.

Here is video that summarizes everything regarding takeoff, landing, and in-flight navigation:

One final comment worth noting: recently, some commercial jets were grounded in Phoenix because it was too hot to fly.  It turns out that lift force is affected by air density.  If the air is too hot, the air density will be too low for safe flight.

You can read about this density issue here.