Avgeek Alert: How Airplanes Fly – the ‘Venturi Effect’


Do you ever still wonder how airplanes stay in the air and fly? Well, it’s been well known and studied for many decades now, applying various physical principles. And one of the main ones is the so-called “Venturi effect” – and get this: it wasn’t discovered in the 20th century but rather 225 years ago. Intrigued?


What Exactly Is the ‘Venturi Effect’?

Named after the Italian physicist Giovanni Battista Venturi, who demonstrated the principle in 1797, the Venturi effect is essentially is the reduction in fluid pressure when it flows through a constricted section of a pipe or tube. To explain it in depth we’d have to get into complex mathematical formulas and arduous definitions related to physics and aerodynamics, but here we’ll try to summarise it in simpler terms. Imagine a circular duct, for example, with the same diameter except at one point (let’s say in the middle), where  it narrows (picture a drinking straw you pinch with your fingers in the centre, without closing it completely). If we pass a fluid through the conduit, it increases its speed as it approaches the constriction, and its static pressure decreases. As you go all the way through that narrowing and flow again through a wider area, the velocity decreases and the static pressure increases. The Venturi effect is based on something called Bernoulli’s principle, which holds that the speed and pressure of an ideal fluid through a closed circuit will result in a constant. That is, if the speed increases, the pressure decreases (and vice versa) to maintain the same value. Another thing that happens is that, when going through the narrow part of the duct, the temperature drops significantly. This is very interesting, as well as being and the easiest test to do at home (as we will see later).


Fine, but What Does all this Have to Do with Airplanes?

First let’s keep in mind the idea of that narrowing duct through which air passes at a greater speed. Now let’s think about what an airplane wing profile looks like. The upper part of the wing is more curved than the lower part. If we apply Bernoulli’s principle – and by extension the Venturi effect -we must think of the extrados as the narrowing curve of a duct. The air, which is essentially a fluid, runs through the upper part (the most curved) of the wing as if it were the narrowing of the duct. That is, its pressure decreases and its speed increases. On the contrary, that same air slows down and increases its pressure on the lower part of the wing, which is flatter. In this way the air arrives at the same point at the same time (the trailing edge of the wing), even though it has to travel along more surface at the top.

That is the reason why airplane wings are designed like they are. In short, the Venturi effect causes them to generate lift – one of the forces that operate on the aircraft and which we briefly discussed in our post dedicated about how fuel is stored in the wings. To put it succinctly, lift is the force opposite the weight, which pulls the plane up (while the weight pulls it down). The other forces are push and resistance.


Other Ways the ‘Venturi Effect’ Comes into Play

Engines in General
Engine carburetors draw fuel using “Venturi tubes” passing through a narrowing in the duct.

The Venturi effect is used to inject fertiliser into water for irrigating crops.

Believe it or not. Though these days those little saliva-suction devices work like little engines, in the past they used Venturi tubes.