The physics of flight

Some independent research from one of our RGS Physics pupils:

How is it possible that a 568Tonne pressurised metal tube can become airborne and fly half way across the world?

Basically, a plane stays in the air due to a balance of forces. The main forces acting on an aircraft are:

  • Thrust, weight, lift and drag.

To keep the plane flying straight and level, all of the four forces must be equal. (Drag=Thrust), (Lift=Weight). If these forces become unbalanced then the aircraft will change its flight profile and will either change its pitch and altitude or speed.

But how does a plane get into the air in the first place?

That is explained by the Bernoulli Principle. The Bernoulli Principle states that an increase in flow results in a decrease in pressure. This means the faster (the air) moves around the wing, the lower pressure it will have.

An aerofoil (or wing) is shaped in such a way that the air particles travel both underneath and above the wing. The curved top of the aerofoil means that the air has to move faster over the top of the wing to reach the same point as the air going beneath the wing. This is an increase of flow for the air particles travelling above the wing. Going back to Bernoulli’s Principle, we know that an increase in flow gives us a lower pressure. So we have a lower pressure on the top of the wing surface and a higher pressure below the wing. This means, that when the pressure difference is greater than the weight /gravity the plane will move up into the air. In the airline industry this is called ‘unsticking’.

So how do we know how much force is needed to overcome the weight/gravity? This is explained by Newton’s Second Law; F=ma. A body of mass (m) subject to a force (F) undergoes acceleration (a).


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