What is the principle behind flight of airplanes? Need a simple answer for grade 7 student. Thank you
For Seventh Grade?
Because air consists of tiny little particles called atoms, or molecules, and when an airplane moves through the air, it collides with these particles and they bounce off the airplane and push on it, just like if you hit, or push on something with your hand.
Airplanes fly mainly because their wings push air downwards. As the air is being pushed down, an upwards force pushes the wing up.
The downwards movement of the air is achieved by two methods, both of which are used on airplanes:
- The leading edge of the wing is higher than the trailing edge, making the wing act as a wedge. The angle between the line from leading edge (front edge of the wing) to the trailing edge (rear edge of the wing) and the airflow coming towards the wing is called the angle of attack.
- The wings profile is such, that it forces the upper airflow go faster than the airflow under the wing. When these airflows meet at the trailing edge, the faster airflow coming from the top tilts the airflow down.
A portion of the lift comes from the faster airflow on top of the wing: as the airflow accelerates, the pressure of the air goes down, causing suction on the top of the wing.
To achieve lift, the airplane needs to go forward, so it either needs an engine to push or pull it, or it needs to glide downwards.
To get more lift, airplanes either raise the angle of attack (raising the leading edge even more higher than the trailing edge), or they go faster. To get less lift, the do the opposite. Adjusting lift will, of course, make the plane go up or down.
If the angle of attack gets too high, the air can no longer follow the curve of the wings upper surface, and the wing will lose its lift. This is called a stall. A wing stalls if the aircraft is going too slow, therefore all aircraft have a minimum safe speed.
Obviously, there is also a maximum speed for airplanes, usually limited by the airplanes structure which can no longer stand the stresses the fast moving air causes.
As Jpe61 wrote, the important thing is that the air gets pushed downwards. Contrary to what's often said, it doesn't really have anything to do with the Bernoulli effect.
Flow on the underside of the wing is quite easy to understand: air collides with the surface, which it can't penetrate, so it is forced to change its path from moving horizontally, to moving diagonally down along the wing.
At the point where the path bends, a downwards force needs to act on the air molecules. By Newton's third law, the opposite force must act on the wing, and that's what pushes the plane up.
...Well, that's actually not sufficient. Consider a wing shape like the following, where you also have the downwards path-deflection, but still don't get any net lift:
You can see there's no lift from the fact that the air flow after the wing is again horizontal, i.e. no vertical momentum has been imparted on the air.
Note in particular that two commonly mentioned factors are not sufficient to generate lift: 1. front edge of the wing higher than aft edge 2. upper surface longer cord than lower surface.
So the crucial thing is that you manage to keep the air streaming diagonally downwards after passing the wing. That's where the importance of the upper surface comes in: it must form a smooth enough curve so the air will follow it, streaming tangentially. In this case, the streams above- and below the wing will both be moving downwards at the trailing edge, so they can just merge again and continue diagonally.
But why would the air even bother curving down above the wing? After all, there's nothing in the way there that would prevent it from moving on in a straight line.
The reason is ambient air pressure. If the molecules did move in a straight line, there would be a whole region of vacuum above the wing. In the boundary between this vacuum and the above air stream, there would be a huge density- and pressure gradient, and that would push air molecules down into the vacuum region. This is a very violent process; it happens when space capsules re-enter the atmosphere at hypersonic speeds, but not in airplane wings.
Instead, with a proper wing profile at appropriate speed and AOA, you get the smooth flow over the wing, which involves no vacuum. It does however involve a lowered pressure right over the curved part of the wing. It is the pressure gradient between this low-pressure region and the higher-pressure ambient air above that maintains the curved flow. And the low-pressure region adds lift too, in addition to the excess-pressure lift from the lower surface.
It also causes a suction effect: higher-pressure air from in front of the wing is accelerated as it gets sucked into the low-pressure region. So the air flow is faster there – Bernoulli's effect. This is, however, not really relevant to the principle how lift works.
At too high AOA, or too low airspeed, or insufficient wing camber, something different happens though: the flow stalls, and instead of moving along the upper surface you get uncontrolled turbulence. This creates strong pressure fluctuations, which also disrupt the flow from under the wing behind the trailing edge. Ultimately the whole flow after the wing is then turbulent and without the downwards momentum, that's why a stalled wing produces no lift or drastically reduced lift.
In short, airplanes fly because the shape of their wings and the attitude they move through the air at generate lift.
While there still remains some debate as to exactly how wings do this, science has resolved the reason to a basic principle cause resulting from two different phenomena.
Wings generate lift by altering the momentum of air passing around them. They do this by the curvature, or camber, of their upper and lower surfaces, which accomplish two things. First, airflow which strikes the lower surface of the wing is deflected downward, resulting in a change of momentum and a resultant lift force through the conservation of momentum and Newton’s Third Law of Motion. Think about a child sticking their hand out of a car window with the palm down and fingers pointed toward the front of the car. As the car drives down the road at speed, the child experiences a force on their hand trying to push it upwards and backwards this is a result of air striking the child’s hand on the palm and being deflected downward, just as air striking the bottom of a wing does the same thing.
The second action is that air passing over the top of a wing is accelerated, which causes a drop in pressure over the top surface of the wing due to Bernoulli’s Principle. This low pressure area draws down high pressure air surrounding it altering its momentum and resulting in a lifting force. These two lifting forces constitute the overall force of lift, which counters the force of gravity. As a consequence the airplane can now fly.
The amount of lifting force that we can generate is directly proportional to the square of the airspeed, the density of the air, and the angle of attack, which is the angle between the chord line and the direction of the airflow to the wing.
According to Bernoulli's principle: Fluids at higher speeds produce low pressure at that point, which effects the surrounding high pressure to exert force on low pressure area.
The airplane wings are designed so that air speed above the wing is higher than below, causing a pressure difference (low pressure above and high pressure below the wings), Hence pushing the airplane upwards.