## March 06, 2009

### principles of flight

Principles of flight
This is a simplisitic summary of how I understand paper airplane aerodynamics

(Fig1)
How does plane fly?
Plane flys because when it flys lift is created on its wings.

How is the lift created?
This lift is created because when the air pressure is smaller above the wing and greater underneath the wing.

Why is airpressure different above and under the wing?
This happens because when wings travel through air, the air travels faster above the wing and slower underneath the wing.

Why air travels faster above wing?

(Fig 2a)
The ways airfoil is designed (curvature on the top) means the distance to travel from the front end to the rear end is greater on the top surface than on the bottom surface (more distance to travel at the top, less distance underneath). When to bottom air reaches the rear end of the airfoil, the top end is still 3/4 of the way (see fig 2a), this creates a vacuum of air (vortex) at the rear end that pulls the top air to make it go faster (like how a vacuum cleaner pulls air) to meet the bottom air at the rear. (Note as of 16:40 06/03 a mistake in the description about the 'distance difference as a cause of lift generation' is recongised and will be corrected in a future entry)

How is air pressure difference related to lift?
Consider this; a motionless object is simply in a state where all forces applied to is equal in all directions. Pressure difference creates unequal balance in force. As the pressure above the wing lessens, the downward 'push' by gravity and weight of atomphsphere is overcomed by the upward 'push' of a denser under-wing air (like a hydrogen balloon, air inside ballon is 'lighter' (less pressure) than the 'heavier' air outside the ballon (higher pressure), therefore the plane rises up.

### 2 comments by 1 or more people

1. Paper aeroplanes don’t have “thrust” and they are actually gliders.

Really planes don’t create lift like you describe either. Read:
http://en.wikipedia.org/wiki/Aerodynamic_lift#Equal_transit-time
http://en.wikipedia.org/wiki/Angle_of_attack

06 Mar 2009, 10:49

2. Thanks Nathan for your input and your provided links. I spent sometime researching this in more depth and I agree that my description is not entirely correct but it isn’t completely wrong either. At least the part where lift is generated from different air pressure on ‘top’ and ‘bottom’ of wing is true by the scientific community (well, according to your wiki link anyway). Second, the same applies to the different airflow velocity that created this difference in air pressure. The point of controversy I guess you are referring to is the ‘Equal transit time’ theory, or the idea that ‘above wing’ air flows faster to meet the ‘under wing’ airflow because because of the different distance the air has to travel. That I agree with you. The distance difference has nothing to do with this. I will correct this information in my next blog entry.

06 Mar 2009, 16:39

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