Wings and propellers of all sorts and how they work


Wings are used not just on planes, but in a wide variety of roles to lift, move and keep things grounded. But how do they work and are they all the same? Jake Port explains.


The distinctive swept back wing shape of the Boeing 787 Dreamliner.
MARK WILLIAMSON

Aircraft use wings to direct air in such a way as to create lift, an aerodynamic phenomenon that is equally simple as it is critically important to racing, the military, commercial travel and the marine industry.

A wing can be described as an aerofoil, or airfoil, with a curved surface on top, a flatter underside.

One of the biggest misconceptions is that the curved surface causes the air above the wing to travel a further distance than the air below, causing it to accelerate and so generate a negative pressure, producing lift.

This is not the case.

If you put a wing with equal curvature in a wind tunnel you will see that if it is slightly angled the air above is still travelling much faster than below, reaching the end of the wing first. This is because as the air reaches the wing tip, it accelerates due to compression, racing over the top of the wing faster than the air underneath and so generating lift.

It’s just like when you let air out of a balloon. The tighter the hole from which the air is trying to escape, the higher the pitch of the sound due to air moving faster as it squeezes through. The wing tip causes the same squeezing of air as it is forced up and over.

You can see this in work by Holger Babinsky at the University of Cambridge in the video below.



Now let’s look at the applications and variations of different kinds of "wings".

Hydrofoils

If you want to make a boat go fast you have two options: you give it more power or you make it cut through the water more efficiently and cleanly. More power requires more fuel and bigger, heavier engines and is generally avoided in favour of more efficient design.

One of the most efficient ways to move a boat is to physically lift it out of the water.

This is the idea behind hydrofoils, named after the lifting surface that is fixed or adjusted underneath its hull – an aerofoil for the water. Just as the wings of a plane have control surfaces that adjust the flow of air, a hydrofoil can either change the angle of its wings to generate a lift force that lifts the boat out of the water.

A hydrofoil ferry lifted out of the water on its 'wings'.
Atlantide Phototravel

This massively reduces the power required to maintain the same speed as would be required on the water and so allows hydrofoils to travel much faster and more efficiently than their regular counterparts.

Hydrofoils have been used on commercial passenger boats, recreational and sports powerboats as well as military vessels.

Spoilers

One way of knowing that a car is meant to go fast is that it has some sort of aerodynamic package. This could include a big rear spoiler, splitters or other aerodynamic attachments. These do exactly the same job as the wings of a plane, but instead of generating lift they cause an opposite force called downforce. This pushes the wheels of the car into the road, giving more grip and control, important for when you’re trying to go fast.

Formula 1 cars are at the cutting edge of using downforce to get around a corner as fast as possible.
Dan Istitene

The more steeply angled the surface the more downforce is generated. But as Newton’s Third Law states: every action has an equal and opposite reaction. The more downforce is produced; the more drag is also generated. Fast cars must carefully balance how much downforce they generate with how much drag is produced. Too much will slow the car down.

This is where active aerodynamics comes in, which essentially uses mechanical movement of the surfaces, again just like the surfaces of a plane's wing or a hydrofoil, to generate force when needed but to then streamline when the force is not required.

Submarines

A Russian submarine just prior to launch showing its massive rear control surfaces and propeller that help it ‘fly’ through the water.
Ruslan Shamukov/Getty Images

Submarines are in many ways the planes of the sea. Because they travel beneath the water, it gives the impression that they are "flying" through the water. This, in fact, is a fairly accurate way of describing how they control their depth and direction in combination with ballast tanks. If you were to combine the downforce of a spoiler with the lift of a hydrofoil you’ve essentially described the wings used by a submarine, known as planes.

Attached on a pivoting arm, these wings direct the flow of the water to steer the submarine up and down, with a rudder controlling horizontal direction.

Propellers

Now that we have discussed how wings are used to control, let’s look at how they are used to move things. Water jets, jet engines, helicopter blades and propellers are all ways in which the wing has been used to produce thrust, propelling man and machine.

The best way to explain how all of these work is to think of a screw, the original name of the propeller. When you turn a screw it moves into whatever you’re screwing into. This is because the force of turning is converted into a forward force that drives the screw in.

The A400M by Airbus has some of the biggest and most powerful propellers ever mounted to a plane, giving it plenty of thrust to fly.
WOLFGANG KUMM / Getty Images

If the screw has thread that is highly angled, you get a lot more push for every turn, but it’s much harder to turn. A very low angle on the thread will make it easy to turn, but will also mean that you’ll have to turn the screw many times to drive it in.

When you’re talking about propellers, this is known as pitch, and describes the degree of angle on the propellers blades.

Air or water is nowhere near as dense as wood and so you need to spin a lot faster to generate the same level of forward force. This is why propellers on a plane turn at thousands of revolutions per minute (RPM). Because water is much denser than air, a boat or submarine propeller turns much slower.

Contrib jakeport.jpg?ixlib=rails 2.1
Jake Port contributes to the Cosmos explainer series.
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