• Question: Why do some planes have different airfoils?

    Asked by anon-74191 on 30 Apr 2020.
    • Photo: Katie Sparks

      Katie Sparks answered on 30 Apr 2020:

      There are so many different things that go into designing an aeroplane – the airfoil is just one of those.
      In all engineering there is no perfect or right answer – instead, you look to balance lots of different things and find something that works the best. “Works the best” means something that doesn’t cost too much, is possible to build, uses materials that are well known, can carry the right around of weight (and how that weight is spread out), as well as how it uses fuel.
      One way you could try out balancing lots of different ideas is to make a paper aeroplane – but one that can spend the longest time in the air and travel the furthest and goes the highest. Try out lots of different designs, some changes will help make it stay in the air longer, others something else. Which one meets the challenge the best?

    • Photo: Paul Pitcher

      Paul Pitcher answered on 30 Apr 2020:

      The airfoil or aerofoil is the shape of the wing if you were to cut through it. It looks like a slender tear drop. This shape is determined by the speed of the air flowing over, the expected extreme angle of the air flowing over, altitude, and several other factors. For example, the airfoil shape for a passenger aircraft might be tuned to fly at their usual speed @550mph at their usual altitude 30,000ft to give minimum fuel burn per km flown. This tends to make it a bit fatter of an airfoil shape. Whereas a fighter jet needs to go fast, high, be capable of extreme angles of attach (like flying vertically up the way) so often will be sharp, and thin.

    • Photo: Tom Gunter

      Tom Gunter answered on 30 Apr 2020:

      The shape of the airfoil and the shape of the wing itself depends on the what the aircraft is designed to do. Aircraft designed to fly at very high speeds must have very low air resistance (called drag) and therefore have thin airfoils. The high speed of the aircraft means that although the wing is thin, it can still generate the lift needed. These aircraft also have wings that are swept back, a design feature that also helps to reduce drag at very high speeds.
      Aircraft designed to carry heavy loads must generate high lift at lower speeds, and therefore have thicker airfoils. All airfoils force air to travel faster over the top of the wing that the bottom, causing a pressure difference that leads to lift. The thicker the airfoil, the greater the lift (and also greater drag).
      Aircraft use slats and flaps that extend to change the shape of the airfoil so that more lift can be generated at lower speeds, i.e. during take-off and landing. You may have seen flaps on a commerical airliner.
      Concorde had a very clever wing design. Its swept wing allowed concorde to travel at supersonic speeds but the wing also generated vortices (swirling currents of air) to generate higher lift when landing at low speed.
      Some military planes such as the Tornado and Tomcat have wings that can sweep forwards or backwards depending on the speed they are flying at.

    • Photo: Peter Yates

      Peter Yates answered on 30 Apr 2020:

      The effective aerofoil section on real planes can be varied by the pilot using a number of different add-ons. These are normally slats – which are extra plates that move out of the front of the wing and flaps which are at the back of the wing.
      These are needed to keep the plane flying slow enough to land and take off efficiently. Because the aerofoil section to fly economically at high speed is very different from the section needed to land and take-off.
      These being deployed gives the whirring noises you hear when coming into land (along with the pumps whirring to put the wheels down).

    • Photo: Owen Jeffreys

      Owen Jeffreys answered on 30 Apr 2020: last edited 2 May 2020 7:30 pm

      Hi Alfie,
      The shape of the airfoil controls several things, but the most important is called “lift”. This is an upward force which is created when the airfoil moves forward through the air. So, as the plane goes forward, lift is generated and therefore the plane lifts off the ground.

      Now that you know the relationship between these forces, you can figure out that the shape of the airfoil and the speed of the plane travelling forward affects the amount of lift. So if you change the shape of the airfoil, it will alter:
      1. The amount of lift generated
      2. The speed at which the plane needs to go forward to generate that lift

      These are both very important things to control. For example, you might want the plane to take off really quickly (if you have a very short runway) or you may want your plane to go up really quickly, but you don’t care how fast it goes.

      These factors will also affect other forces and properties of the plane (such as drag and agility).

      Here’s a little homework:
      Now you know all that, think about different types of planes you see and I bet you can start guessing what shape airfoil they might have. Have a go with these:
      * A super fast military jet with 1 pilot
      * A very big jumbo jet with 300+ passengers
      * A little aerobatic plane for doing air shows

      I could talk for hours about this, I fly model planes for a hobby, but I hope this answered your question. If it’s been helpful, don’t forget to smash that like button 🙂

    • Photo: Graeme Ralph

      Graeme Ralph answered on 30 Apr 2020:

      Great question!! Basically different aerofoils enable the plane to perform better at certain tasks, if you need a plane to fly fast (like a jet) you need a swept profile but long range aircraft that carry high loads need a different profile to work best for this task. Overall its a balance of the function of the plane, the aerofoil and a range of other design factors.

    • Photo: Conrad Manning

      Conrad Manning answered on 1 May 2020:

      Much the same way different boats have different shapes, the aerofoil has a different shape depending on how it will be used.

      http://airfoiltools.com/

      This website shows you the vast array of different shapes and their equivalent lift and drag coefficients (basically the lift and drag without knowing the shape or speed that the aerofoil is moving). These different lift vs drag profiles then get sorted through so that the foil performs best in the area where it will get most used (we regularly look at speeds leading up to that use zone so that we can see if it’s still useful).

    • Photo: Martin McKie

      Martin McKie answered on 21 May 2020:

      Hi There, Alfie,

      Please see the graphic below which describes the basic shapes and why they are used.

      https://www.pinterest.ru/pin/601160250230231769/

      Best Regards

      Martin

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