Fundamental Principles Of Aircraft Flight

An aircraft  contains dozens of different technologies. Everything from the engine to the wings is its own special universe of design and engineering .We take for it granted that we can fly from one side of the world to the other in a matter of hours, but a century ago people can hardly imagine some things that weight several hundreds tons can float through  the air at 2-3 times speed of sounds .

so is this how they fly : 9f6aee9ed517084e9b2774620615d340

Alright , not like that , more like this :

forces_of_flight

To understand how an aircraft fly, we should have a closer look at some common misconception bout flying

Physics students when asked how aircraft fly often quote  Bernoulli’s principle, which says that if air speeds up the pressure is lowered .And because   aircraft wings often curved on top and flat n the bottom  hence when aircraft moving through the air , air goes faster over the top of the wing than at the bottom , creating a region of low pressure, and thus lift to counter gravity

centurion-solar-wing (1)

cub_airplanes_lesson02_fig2

While this explanation of how wings work is widely repeated, it is wrong . Think about it for a second , if such explanation was correct aircraft wouldn’t be able to  fly upside down, and paper planes wouldn’t be able to fly.

Another problem with the common explanation is that , it assume  the air shooting over the wing will always to stay in step with the air going underneath it, thus   has to travel a bigger distance in the same time, that cannot be further from the truth.

So, how does a wing generate lift? To begin to understand lift we must  review Newton’s first and third laws.  Newton’s first law states a body at rest will remain at rest, or a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is a force acting on it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction.As Newton’s laws suggests, the wing must change something of the air to get lift. Changes in the air’s momentum will result in forces on the wing. To generate lift a wing must divert air down; lots of air.As a curved airfoil wing flies through the sky, it deflects air and alters the air pressure above and below it. That’s intuitively obvious. Think how it feels when you slowly walk through a swimming pool and feel the force of the water pushing against your body: your body is diverting the flow of water as it pushes through it, and an airfoil wing does the same thing (much more dramatically—because that’s what it’s designed to do). As a plane flies forward, the curved upper part of the wing lowers the air pressure directly above it, so it moves upward.Why does this happen? As air flows over the curved upper surface, its natural inclination is to move in a straight line, but the curve of the wing pulls it around and back down. For this reason, the air is effectively stretched out into a bigger volume—the same number of air molecules forced to occupy more space—and this is what lowers its pressure. For exactly the opposite reason, the pressure of the air under the wing increases: the advancing wing squashes the air molecules in front of it into a smaller space. The difference in air pressure between the upper and lower surfaces causes a big difference in air speed (not the other way around, as in the traditional answer given by physics students ).

how-airfoil-wing-makes-lift

 

Most of you will find it easy to understand how the lower surface  of the wing push air downward but the common question is  “how does the upper surface of the  wing pull the air down?” When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass

There are many types of wing: conventional, symmetric, conventional in inverted flight, the early biplane wings that looked like warped boards, and even the proverbial “barn door”. In all cases, the wing is forcing the air down, or more accurately pulling air down from above. What each of these wings have in common is an angle of attack with respect to the oncoming air.

aoa5

It is this angle of attack that is the primary parameter in determining lift. The inverted wing can be explained by its angle of attack, despite the apparent contradiction with the popular explanation involving the Bernoulli principle. A pilot adjusts the angle of attack to adjust the lift for the speed and load. The   shape and thickness of the wing gives the pilot  the speed to adjust.

 

sav9a

Typically, the lift increase as angle of attack go up , but it  begins to decrease at an angle of attack of about 15 degrees. The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing. This separation of the airflow from the top of the wing is a stall..

 

To sum up ,

The lift of a wing is equal to the rate of change in momentum of the air it is diverting down. Momentum is the product of mass and velocity. The lift of a wing is proportional to the amount of air diverted down per second times the downward velocity of that air.

So more speed , more angle of attack ( upto about 15 degrees ) mean more lift

lifteq

To be continued …..

Advertisements

5 thoughts on “Fundamental Principles Of Aircraft Flight

  1. Thank God someone actually explains aerodynamic lift with some accuracy. Your articles hopefully will finally get people rejecting old school myths about aircraft .
    Also your comments about LO and radar detection are some of the best I’ve seen.

    Like

  2. Aircraft 101 is very helpful many thanks for producing this i have started to learn some very interesting for a change. Very well written i will digesting this data for ages. Many Thanks

    Liked by 1 person

  3. Super informative. Very interesting to read and extremely well written. Still trying to understand most of the information guessing i’m going to looking at this info for a very long time. Thanks so much for producing this! really helpful material.

    Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s