If you've ever watched a plane taking off or coming in to land, the first thing that strikes us is the noise of the engines. One might think that engines are the key to make a plane fly, but that's not the case. Things can fly quite happily without engines, as gliders (planes with no engines), paper planes, and indeed gliding birds who readily show us the ease of flight without any engine embedded in their bodies. So what is the basic principle of flight?
Principle of Flight
If you ever tried flying paper airplanes in your childhood, you can refresh your memory and thus realize that unknowingly you actually implemented some of the basic physics laws practically that concluded to the flight motion of your paper plane.
When we blow faster-moving air above the surface of paper plane or launch it with a force and an upward inclination to some angle, we unintentionally create a lower air pressure region above the paper. Now the air pressure below the paper is higher and this pressure difference is what that creates lift. Lift does exactly what it sounds like; it lifts objects off the ground when everything is just right.
How can you create the pressure difference for birds and planes?
We blew air over the top of the sheet of paper to create the lower pressure. You cannot do that for birds and planes. For them, wings play this role. Wings of both birds and planes have a curved upper surface and a flatter lower surface. When air crushes over the curved upper wing surface, it has to travel more than the air that passes underneath, so it has to go faster (to cover more distance in the same time). Here a beautiful law comes into play called the Bernoulli's law, it states that "fast-moving air is at lower pressure than slow-moving air", so the pressure above the wing is lower than the pressure below it, and this creates the lift that moves a bird or a plane upward.
So we can conclude from here that for a flight motion, the upward motion is provided by the wings. But what about the forward thrust?
Getting the air to move over and under the wing also requires the wing to be moving with the air. This is called thrust. In case of birds, thrust is created when birds flap their wings using their strong breast muscles. In a way, birds use a swimming motion to get the lift needed to fly. Whereas this is not the case when it comes to airplanes. Planes use another method for thrust. They use engines. Plane wings have a similar shape as bird wings, but instead of flapping their wings, they use engines to thrust them into the air and create the lift needed to fly. So for both birds and planes, thrust is the other part of creating lift and the ability to fly. So the shape of the wing and the ability to move it through the air are the two things needed for bird and plane flight.
But wait. What about Rockets? They have no wings but they too fly upwards!! So what generates lift in a rocket?
How is the flight of an Airplane different from that of a Rocket?
Both an airplane/jet engine and a rocket engine function by burning fuel and ejecting this fuel mass opposite to the direction of desired acceleration to gain momentum.
The main difference between the two types of engine is that a rocket carries its own supply of oxygen for combustion (burning fuel). A jet engine takes oxygen from the atmosphere for combustion, and so cannot operate in the vacuum of space.
An airplane sucks in air from the front of the engine. This air is burned with the fuel within the engine. The resulting large mass of gas is ejected towards the rear at high velocity, which propels the airplane forward. The uplift is achieved by the wings as discussed above.
A rocket, in contrast, carries both fuel (which may be solid or liquid) and oxygen. Therefore it does not suck in air from the front. All it does is burn the fuel with the oxygen, and eject it at very high velocities backward. This momentum is used to both lift and propel the rocket. Thus there are no wings required for uplift.
Most airplanes can not possibly climb vertically, while rockets are built for that purpose only. An airplane is pretty much useless outside the atmosphere, but a rocket will work just fine since it carries its own fuel and oxygen.