The 4 forces

To a pilot,physics is the most interesting science. Our engines may run on a chemical reaction, but the resulting power is a force that launches us into the air. And among the various physical forces our airplane experiences in flight, there are four that really matter. The four forces are law, but they aren’t absolute. Thrust, drag, lift, and weight act independently, but a change in one results in a change in the others. Knowing how each works can help the pilot anticipate how the airplane will react.

Thrust.

The forward momentum of an airplane is its essence. Airplanes are built to move. Whether you call it thrust or power, the engine gives an airplane its push forward. We control thrust through the throttle. In an airplane with a fixed-pitch propeller, that increase in throttle means more fuel going to the cylinders. This results in more power to the crankshaft, which turns the propeller faster. And that means more speed.

Most important about thrust is that it gives us safety. As the amount of available thrust overcomes drag, we are able to accelerate, which can be turned into a climb. When thrust and drag are equal, we are flying at a constant airspeed. And when drag overcomes thrust, we can only slow down or go down.

Airplanes that are known for being fast are that way because they either have a massive engine or they reduce drag through aerodynamic efficiency, such as a Mooney with its laminar flow wing.

Of the four forces, thrust is one of the most direct and easiest to understand. We can control it, but only

Drag.

Physics gave us thrust, and physics hath taken away speed. Drag is aviation’s downer, the buzzkill of our power party. There are multiple types of drag, each changing depending on the speed. Unfortunately the two main types work opposite each other, so there is no escaping it.

Parasite drag includes pressure or form drag and skin or interference drag. Of the two major types of drag, parasite drag is more relatable. Imagine the wall of air hitting your car as you drive down the road and you have the basic idea. The mass of the car is pressure drag and the friction across the surface is skin drag. Everything that sticks out into the wind causes drag, from antennas to windows. The same goes for the airplane. The struts, skin, tires, rivets and everything else are constantly working against us. As speed increases, so too does this force. Basically we’re trying to ram the object even faster through the air and the air is fighting back even harder.

The other basic type of drag—induced—is unique to the wing. Induced drag is the byproduct of lift. As the air swirls off the sides and back of the wing it creates a series of vortices that create drag. Logic says the higher the angle of attack, the greater the vortices, and thus the greater the drag. And that’s exactly what happens. That means induced drag is highest at lower airspeeds when a higher angle of attack is necessary to maintain lift.

The pilot has some amount of control over drag. Parasite drag can be changed by slowing down or speeding up, and induced drag can be increased or decreased by changing the angle of attack or by changing the total aircraft weight. Drag also starts to explain why some airplanes are designed the way they are. Swept-wing jets and laminar flow wings have less parasite drag and are more efficient at high speeds.

There is also a changeable drag device built in to almost all modern airplanes—flaps. They are great when we want to either slow down or go down, such as when landing. But they are a hindrance when we want more speed, such as during a go-around.

Lift

This is what it’s all about. Lift pulls us into the sky. Learn how to manipulate this mysterious force with the wing and almost everything else falls into place.

There is a bit of debate about exactly what creates lift. The traditional explanation is Bernoulli’s principle, which that says a faster moving fluid decreases in pressure. In our case that means air must travel faster over the upper curve of the wing, causing the pressure to decrease and the wing to lift up. Some people say that’s Newton’s laws explain lift. Basically, the wing pounds on the air, the air pounds the wing back. Engineers generally agree upon a third explanation—circulation. The easiest way to think of this is with a pitcher’s curve ball. It’s the spinning ball that lifts it in toward the plate.

In a sense, the why of lift is irrelevant. Lift works, and that’s all a pilot really needs to know. What’s more important is when it works, when it doesn’t, how it can be changed, and why.

The pilot can affect lift through speed and angle of attack (the angle at which the wing strikes the relative wind). An aircraft designer can affect it through the shape, length, and width of the wing. Sometimes he also gives us flaps, which effectively change the shape of the wing, providing lift and drag.

Speeding up and increasing the angle of attack both provide more lift. And if things were that simple, flying would be easy. But it’s not and there are several caveats. First, at some predetermined point for each wing, the airflow will begin to separate from the upper surface of the wing, causing an extreme increase in drag and a corresponding loss of lift. We know it as a stall. Note that speed has little to do with a stall. There are published values for when we can expect an airplane to stall, but that’s for a very specific condition. You can go very slow and still be flying or very fast and stall. All that matters is the AOA.

If you’re flying at a constant speed and pull up, you will generate more lift and climb—up to a point. Do this with no power change and you’ll notice airspeed start to decrease. That’s because excess thrust is what sustains a climb. Think of a car going up a steep hill. The pavement is the lift in our wings. Keep the gas pedal constant and you’ll start to slow down, but if you add more power you can maintain speed.

There’s an expression in aviation that “speed is life.” The implication is that as long as you have some amount of forward momentum, the wing will keep flying, which is generally true. More speed equals more lift, and that means more options. This relationship is easy to see as you transition through the various speeds. To maintain altitude during slow flight the wing must be at a relatively high angle of attack. As you recover from slow flight, you add speed. To maintain altitude you must lower the angle of attack. If you keep speeding up until cruise speed you’ll notice that the angle of attack looks almost flat.

All this really matters close to the ground. We keep our speed at a safe and steady value on final approach because slowing down gives us two options—increase the angle of attack to add back the lift, which adds drag and risks a stall, or increase speed by going down, which flies us toward the ground.

Weight

For anything to fly it must overcome gravity, the one constant we have absolutely no control over. Mass multiplied by the acceleration caused by gravity creates the force that pulls us toward Mother Earth.

Weight is like a cancer to airplane designers. Every piece of equipment, every carpet, and every eighth-inch of aluminum adds weight, robbing performance. A lighter airplane can fly faster and burn less fuel because it requires less lift to fly, less thrust to climb, and so on. That’s why some parts on an airplane seem so flimsy. Every pound saved is extra speed or better efficiency.

The weight can even be so high that we can’t get off the ground, which is why we calculate beforehand the total weight of the airplane, fuel, people, bags, and everything else to be carried aloft. We also calculate a single theoretical point upon which all the weight acts called the center of gravity. Being within a certain range ensures the airplane is fully controllable and that it will be stable. This is of course the balance side of the weight and balance equation.

Loading an airplane toward the front of the center of gravity envelope results in a very stable airplane, but also a slower one that burns more fuel. A detailed guide from airplane manufacturer ATR describes it this way. "A more forward center of gravity requires a nose up pitching moment obtained through reduced tail plane lift, which is complensated for by more wing lift." And more lift obviously means more drag. ATR says that loading its airplane with a center of gravity 8 percent more forward can lead to .6 percent more fuel burn. Keep it light and keep it aft and you may find yourself going faster and burning less fuel.

Using this to your advantage by throwing your family in the baggage compartment probably isn’t a good idea, however.