People who are apprehensive about flying often ask, “What if the engine quits? Won’t we fall out of the sky?” A pilot will usually answer, “Of course not. We just glide down.” Sounds simple; but what’s with “gliding”, anyway?
The first thing to know about airplanes (and helicopters) is that they have wings. Wings generate lift. There are four forces acting on an aircraft: Lift, gravity, thrust and drag. The opposing forces (lift and gravity, or “weight”, and thrust and drag) must be balanced for flight. (There’ll be more on that later.)
How do wings make “lift”? The answer they teach in the classroom is Bernoulli’s Principle. Bernoulli was a 17th-century Swiss mathematician (d. 1705) who found that by restricting the flow of a fluid (and air acts as a fluid), the pressure at the constricting point increases as the speed of the fluid through the point increases. Why does the fluid increase its speed? If you’re trying to push something through a restriction (a venturi in a tube, for example), the “something” (air or water, for example) behind it pushes harder to get through. The greater pressure is trying to put the same amount of fluid through a smaller place; therefore, the fluid must go through faster. Think back to your elementary school algebra. Both sides of the equations must be equal. So in the fluids example, you get more speed, but you lose pressure in the area where the speed is greater.
Wings are like half of a venturi; the upper part is curved. This means that the air flowing over the top must travel faster than the air flowing under the bottom because it has a farther distance to travel. (There’s that ol’ algebra again!) With lower pressure on top of the wing, the wing is “sucked” upward, carrying the rest of the airframe with it. (There is a debate going on which is more responsible for flight: The Bernoulli Effect, or downwash from a surface that is not parallel to the airflow. I’ll avoid that here. For this post, just remember that wings need air flowing over them to produce lift; which they do, regardless of how they do it.)
So now you have lift. When you have enough of it, it can overcome gravity, or weight, and the aircraft climbs. But how do you get the airflow to product the lift? In a powered aircraft, you use the engine. In a propeller-driven aircraft the propeller is like a miniature wing; except instead of lifting upwards, it “lifts” forewards. When you have enough “foreward lift”, you overcome drag and move foreward. Eventually, the foreward speed is great enough to move the aircraft through the air fast enough for the wings to generate lift. And you fly.
To recap: Thrust overcomes drag, and lift overcomes weight.
There are control surfaces on an airframe. One of those is the “elevator”, which is on the back of the fixed horizontal stabilizer; or the “stabilator”, which is a horizontal surface at the rear of the typical airplane that pitches up and down as a unit. Pitch. That’s the movement of the airframe about the axis that goes through one wingtip and leaves the other. “Up and down”. There’s a saying: “Pull the wheel back, the houses get smaller. Push the wheel foreward, the houses get bigger.” That’s true enough, but not quite accurate. An airplane must balance lift and weight and thrust and drag to remain in level flight. Remember I said that both sides of an equation have to be equal? Change one thing, and something else has to change. If you increase lift, then you are overcoming weight. But you need to get the force from somewhere to climb. So you need more power to maintain the same speed. If you don’t increase power, you can climb but you give up speed. Give up speed, and the wing will stop generating enough lift to overcome gravity. Keep losing speed and the wing stops flying. This is called a “stall”.
Think of it this way: The engine thrust actually controls alititude, and the elvator controls speed.
So you’re flying along and you hear a loud BANG! The engine quits. But you still have speed! And the elevator controls speed! You lower the nose enough to maintain your best glide speed. This is called “trading altitude for airspeed”. That is, you use one of the counteracting forces – gravity – as power. It pulls you toward the ground. You control the speed with pitch control using the elevator. By using gravity, you still have the air flowing over your wings and that generates lift so that you don’t “fall out of the sky”.
I hope you have a place picked out to land, because what goes up must come down! Oh, good; you’ve selected a nice flat field. That’ll do. But now you have a problem. You’re still coming down, but the rate is too great for a comfortable landing. And you don’t really want to touch down at 70 knots on those little wheels. OMYGOD! WE’RE GOING TO CRASH!!! Relax. Just as you traded altitude for airspeed, you can also trade airspeed for altitude (or, rather, a slower sink rate and slower speed). As you approach your landing field and are getting rather close to the ground, “flare” the aircraft by pulling back on the yoke or stick (“up elevator”). By increasing your pitch you are generating more lift at the expense of speed. Your rate of closure with the ground decreases and your airspeed decreases. As your main wheels touch the ground, keep pulling back on the elevators. Keeping the nose up will slow you down and keep the nose wheel from digging in.
So there you have it: a very simple explanation of why airplanes don’t fall out of the air if the engine quits. Or to sum up the preceeding in a sentence: You won’t fall out of the sky because the wings still develop lift, and you can control the “flying equation” so that you can fly all the way to a landing.
(Now to sit back and watch the 20 or 30 other Flying Dopers pick apart my explanation, add details both trivial and important, and relate stories of their own forced landings – which don’t really happen very often, by the way.)
And when the engine in your car stops, you don’t go from 70 mph to zero in an instant. Same with an airplane. You don’t just suddenly stop flying.