Physics of Flight: Lift
When a beginner decides to embark into the world of rc airplanes their typical steps are:
- Find a trainer aircraft
- Choose electric or glow
- Get flying
And while these steps are not entirely incorrect in getting started it does create a large gap in your understanding why an airplane even behaves the way it does. Why should you care about the physics of flight? Have you ever heard someone say something like, “I don’t know what happened. It just stalled in the air and I lost control.”
Understanding the physics of flight is just as important as understanding and learning how to drive a car. If you choose to ignore any and all understanding of how a car functions and why (i.e. Acceleration & Breaking / Turn Signals / Rules of the Road) you will most certainly end your driving experience in a crash. Flying an aircraft is no different. Unfortunately it is common practice in the hobby of radio control aircraft to bypass or even ignore all the physics of flight to “just get flying”.
Control Chat isn’t about to take you down the road of a long classroom session with tests. Instead we want to pass along knowledge that can potentially prevent you from making fatal mistakes when learning to fly and in the event of something unfortunate, give you the tools to help understand why the unfortunate event happened.
Let’s start off with one very important physic of flight: Lift.
Lift is exactly what it sounds like. When you hear the word lift, usually you’ll think of an object being picked up and off of a surface. Be it a weight in the gym (lifting weights) or the European term for an elevator (a lift). Both imply an object leaving the ground where it once stood still. But how do you lift an airplane off the ground? No body is physically lifting the airplane, so how is it done?
Meet the Particles
The atmosphere around us is made of many elements. For our discussion, visualize the air we will be flying in as being packed with small objects called particles. These particles create the physical presence of our air.
Now let’s zoom in on a section of our air and place a wing in it.
You can see our particles are not moving and neither is our wing. Therefore nothing is really happening short of lazy particles floating around doing a whole lot of nothing. Not very exciting. So let’s turn it up a notch and make our wing start moving forward through the air with the help of an engine.
With the wing in motion take note of its shape. The bottom of the wing is flat while the top of the wing has a curve. When we move relatively slow through the air the particles zip by unaffected and also not effecting the wing. However, if we speed up the rate at which the wing is passing through our air, the particles start acting very differently and begin to interact with our wing and its surface.
Think of the particles zipping by our fast moving wing like a traffic jam. If a 4 lane highway is closed down to 1 lane, all the traffic will bottle neck to one point. Now what you have are the same 4 lanes of traffic trying to get through a single lane. When our wing moves forward through the air, our particles begin acting much the same way as traffic and begin to bottle neck.
It’s the curved shape of our wing that causes this bottle-neck. Now the same amount of air, which had 4 lanes, must now use 1 lane to pass. And this decrease in space for all the air to pass through causes a dramatic increase in speed of the air flow.
Because of this faster air flow there aren’t as many particles able to hit the wing (They don’t have time because they need to get moving over the wing and to the other side!) and an area of Low Pressure (not many particles hitting the wing) is created over the top of the wing.
Adversely, since there is not a curve on the bottom of the wing and the particles don’t have to squeeze into a smaller area to pass by, more particles can hit the wing surface. And this causes an area of High Pressure (lots of particles hitting the wing) below the wing.
It’s these differences in pressure, high vs. low, that together cause lift. These forces independently do not cause lift, let’s be clear about that. The high pressure alone does not ‘push’ the wing up, just as the low pressure above the wing doesn’t ‘suck’ the wing into the air. They work as a team.
What does all this mean?
In a nutshell, the reason your airplane flies is because you are pulling it through the air at a fast enough rate to cause the phenomenon of lift to occur on your wing. But keep in mind, speed does not always mean you’ll have lift. You can actually be moving quite fast in the air and suddenly drop out of the sky because you completely lost your lift. This loss of lift is called a stall.