The Science of Flight - Intro

OPENING QUESTION:  We can generally describe 4 forces acting on an airplane in flight.

  1. List them
  2. Now check your work against *this* NASA page
  3. Imagine that you are on a flight from Seattle to New York. You're about half way to New York and your plane is flying straight and level at a constant speed. Sketch the forces acting on that plane at that time (Hint: Bigger arrows mean bigger forces, smaller arrows mean smaller forces)

LEARNING GOAL FOR TODAY: I will be able to show that weight depends on mass and gravity during today's class.

I will be able to sketch how lift pulls an airplane upward during today's class.

WORDS O' THE DAY:

  • Force: ("A push or a pull")
  • Thrust ("forward pushing force")
  • Air Friction ("resists forward motion")
  • Lift ("Lower pressure, upward pull")
  • Weight ("downward pull")

WORK O' THE DAY:

Please consider the following: How are gravity and weight related?

With that in mind, sketch the forces acting on a person about my size (185 lbs or so).

Now sketch me on the moon (gravity = 1/6 Earth)

Now sketch me on Mars (gravity = 1/3 Earth)

What do you notice about my weight?

But I'm still the same person, how can that change?

We have known since Newton's time that the weight of an object is *directly* related to the mass of the object:

W = mg

If you remember back to the dim dark days of Middle School, you may recollect that looks very much like Newton's 2nd Law:

F = ma

That's because it is just another (but more specific) way of writing Newton's 2nd Law

Imagine a fully loaded Boeing 747. At takeoff it is absolutely massive at 400,000 kg. When we multiply that by the acceleration due to gravity (about 10 m/s2) we come up with an incredible amount of weight trying to pull that 747 back to earth.

If there is not enough air moving over the wings of that 747 it can go into a stall (as we've discussed).

There is a rather famous instance of a 747 cargo plane doing just that a number of years ago in an air base in Afghanistan.

The cargo shifted on take off and slid to the back of the plane, changing the center of mass on the plane which pulled the nose up. As the nose went up, not enough air was moving over the wings (we talked about this with the 737-Max) and the plane stalled and crashed tail first.

    

This video can be kind of rough to watch so if you are feeling a bit out of sorts I'd go ahead and skip it.

═══════════════════════════

Now let's get back to Earth... we all know that it takes more effort to lift a more massive object upwards against the force of gravity.

It is the same thing for airplanes in flight. The weight (mass x gravity) of the aircraft is constantly pulling the airplane towards the Earth

Now let's take a gander at the weight of air molecules... why do we care?

 

Recall that the air above us and all around us is essentially billions x billions x billions of individual air molecules:

Each of those bubbles has a mass.

Gravity works on each of those bubbles so each of those bubbles has *weight*

Stacked up on top of us we say that those bubbles cause *air pressure* of about 14.7 pounds on each square inch of us!

═══════════════════════════

Why do we care about air pressure?

The weight (mass x gravity) of all that air pushes down on everything on or flying above the surface of the Earth.

At first it seems like that is yet more of a problem working against an aircraft in flight (and it is, to a degree).

However, there is a very interesting phenomenon that occurs when a moving fluid strikes an object, or when an object moves through a fluid:

It's kind of odd to suggest that air is a fluid. Although it isn't *actually* a fluid we can model moving air just like it was a very non-dense liquid and the math works out very well.

We won't get into the math, but the interesting phenomenon of *LIFT* is critical to our understanding of flight.

Recall last week's conversation of *lift*:

Please take a moment to sketch an airplane wing.... what about the shape of that wing causes air to move faster over the wing and slower under the wing.

Why is that critical for generating lift?

 

 

Does your sketch look like this:

Notice that the LOW pressure above the wing is so much lower than the high pressure area pushing up on the wing that the plane is actually lifted upwards.

The shape of the wing helps control how air moves over and under the wing.

You don't need to remember the specific details in the photo below, but I thought you might find it interesting to note the different aircraft wing designs ("camber" refers to the curved surface of the wing):

═══════════════════════════

Let's recap:

    • What is weight and how is it related to mass?

    • What is air pressure and how is that related to the mass of air molecules?

    • How is lift generated by an airplane wing?