Four forces and control surfaces


There are four forces that act on an airplane in flight: life, thrust, drag, and weight/gravity.

I’ve gone over lift, so today I’ll run through the other three and then talk about control surfaces.

Drag is what slows the airplane down. Drag is caused by the airframe hitting the air (wind resistance), by cooling ducts, by things sticking out from the airframe (antennas, for instance), and drag that’s caused by lift (when you are flying with an angle of attack that is not 0, some of the lift force generated goes to the rear, causing drag).

Thrust either pushes or pulls an aircraft through the air. This is done through propellers or jets. Thrust overcomes drag on airplanes. If you are playing with a paper airplane, you throwing it is the thrust.

Weight, or gravity, pulls the airplane down. This is made up of the airplane and whatever you’ve put in or on it.

Silly joke – an Alaskan bush pilot took a tourist on a hunting trip. They bagged a moose and strapped it onto the plane. They took off and the plane struggled and strained and finally ran into a mountain, about 3/4 of the way up. The pilot and passenger landed safely, amazingly enough, and the passenger was not pleased. He became even less pleased when he noticed that the pilot was laughing. “What are you laughing about? We’re stuck in the middle of nowhere and your plane is on top of my moose!” The pilot replied, “No! this is great! Last time I only got halfway up!”

Just a little too much weight.

During take off, thrust must be greater than drag and lift must be greater than weight. If those requirements are not met, the airplane doesn’t take off.

Control surfaces are fun. They’re especially fun once you get into training that deals with how to handle control surface failures and you learn that opening a door can help you control a plane in flight, but that’s probably another post.

There are three directions an airplane can move.

When you are looking at the front of the airplane, roll makes the wings go up and down but the nose stays pointed directly at you. Ailerons control roll, and the stick or yoke controls the ailerons. Ailerons are located on the trailing edge of the wing and are hinged so they go up and down in opposition – when one is up, the other is down.

When you are looking at an airplane from the side, pitch moves the nose up and down. The elevator, located on the trailing edge of the tail. Pushing the yoke or stick forward moves the elevator down and pushes the nose down as well.

When you are looking at an airplane from the top, yaw moves the nose and tail from side to side. The pivot point is basically the cockpit, and the nose and tail circle around that on the horizontal axis. Yaw is controlled by the rudder pedals. If you are making a turn in an airplane, you have to control yaw or the tail slides out. If you have passengers, they are more likely to turn green from yaw than from pretty much anything else. It feels weird, like your rear end is trying to escape from your spine and move off to the side. That’s an overstatement, of course, but it’s the reason “flying by the seat of your pants” starts to make sense when you are in the pilot’s seat. You can feel yaw.

The flaps are the final control surface. I discussed them some in the section on lift. They are most commonly used for takeoff and landing.

That about does it for the very basic parts of control surfaces. I think perhaps I’ll get into weather next time.

One thought on “Four forces and control surfaces

Comments are closed.