Retrotechtacular: Forces Acting On An Airfoil

floating film title We’ve probably all experimented with a very clear demonstration of the basic principles of lift: if you’re riding in a car and you put your flattened hand out the window at different angles, your hand will rise and fall like an airplane’s wing, or airfoil. This week’s Retrotechtacular explains exactly how flight is possible through the principles of lift and drag. It’s an Army training documentary from 1941 titled “Aerodynamics: Forces Acting on an Air Foil“.

What is an airfoil? Contextually speaking, it’s the shape of an airplane’s wing. In the face of pressure differences acting upon their surfaces, airfoils produce a useful aerodynamic reaction, such as the lift that makes flight possible. As the film explains, the ideas of lift and drag are measured against the yardstick of relative wind. The force of this wind on the airfoil changes according to the acute angle formed between the airfoil and the direction of the air flow acting upon it. As you may already know, lift is measured at right angles to the relative wind, and drag occurs parallel to it. Lift is opposed by the weight of the foil, and drag by tension.

wind tunnel testing

Airfoils come in several types of thicknesses and curvatures, and the film shows how a chord is derived from each shape. These chords are used to measure and describe the angle of attack in relation to the relative wind.

The forces that act upon an airfoil are measured in wind tunnels which provide straight and predictable airflow. A model airplane is supported by wires that lead to scales. These scales measure drag as well as front and rear lift.

In experimenting with angles of attack, lift and drag increase toward what is known as the stalling angle. After this point, lift decreases abruptly, and drag takes over. Lift and drag are proportional to the area of the wing, the relative wind velocity squared, and the air density. When a plane is in the air, drag is a retarding force that equals the thrust of the craft, or the propelling force.

monometer tubesAirfoil models are also unit tested in wind tunnels. They are built with small tubes running along many points of the foil that sit just under the surface. The tubes leave the model at a single point and are connected to a bank of manometer tubes. These tubes compare the pressures acting on the airfoil model to the reference point of atmospheric pressure. The different liquid levels in the manometer tubes give clear proof of the pressure values along the airfoil. These levels are photographed and mapped to a pressure curve. Now, a diagram can be made to show the positive and negative pressures relative to the angle of attack.

In closing, we are shown the effects of a dive on lift as an aircraft approaches and reaches terminal velocity, and that lift is attained again by pulling slowly out of the dive. Remember that the next time you fly your hand-plane out the window.

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

20 thoughts on “Retrotechtacular: Forces Acting On An Airfoil

  1. Excellent. A question which has always puzzled me is, how can an aircraft fly upside down if the airfoil shaped wings produce lift in the downward direction! This doco might provide an answer.

    1. angle of attack – most inverted flights I’ve witnessed had the tail end visibly lower than the nose end, so the wings act more like a deflector than a classic airfoil. Helicopter blades are more flat than airfoil shaped, and that is how they generate more or less lift, by changing the angle of attack – or the angle they slice into the wind.

      This article (googled inverted airfoil aerodynamics) covers it mostly, but he mostly dismisses the classic physics of airfoil design, which i’m not sure is true, but the angle of attack part is.
      in further googleing, it appears the classic view of an airfoil is actually less true than i previously know, the one that says there’s a force pushing the wing up because of lower pressure and such – there is an effect like that, but it is less than most believe. the truth is an airfoil shape inherently creates a downwash, and the equal and opposite force is the airfoil goes up. so angle of attack is most of that force, the airfoil just exaggerates it:

      Most acrobatic planes actualy have symmetric airfoil wings, so any air pressure force lift is cancelled out and it is all up to angle of attack.

      Even wikipedia has it – first two sentences of the third paragraph: “The lift on an airfoil is primarily the result of its angle of attack and shape. When oriented at a suitable angle, the airfoil deflects the oncoming air, resulting in a force on the airfoil in the direction opposite to the deflection.” -

      1. Ah, good, I was hoping this might pop up in this thread. The very basics of the aerofoil they teach you at school are wrong, but there doesn’t seem to be a solid consensus of opinion as to why. Is it simply deflecting air downward, is it downwash, or vortices, or what? HAD thread is as good a place as any to have a science-fight!

        1. Downwash is caused by the air being deflected downwards. Vortices cause parasitic drag (hence winglets) and become more important at transsonic speeds. For low speed airfoils they are less relevant to the overall lift effects on an airfoil.

          The primary cause of lift is deflection of the airstream. Even a flat plane (a barndoor for instance) could be used as a wing. The problem is it’ll have the stall characteristics of a brick and any plane built that way will become quickly uncontrollable. Because of the shape the airflow at the boundry layer would be turbulent, leading to increased drag and less efficiency.

          A second effect is that most airfoils, will create a pressure difference between the top and bottom surfaces. This pressure difference then contributes to lift. This pressure difference has a number of reasons, but is not due to the old “the air must flow faster over the wing because it has more distance to cover than the air flowing around the bottom”. Air over the top of the wing DOES speed up but not enough flow around the wing at the same average speed as the airflow around the bottom. Air being deflected by the bottom surface of the wing has a slightly increased pressure.
          This pressure difference is what is shown with the bourdon/pressure tubes at around the 17 minute mark. Because of the airflow effects over a 3 dimensional, finite span wing, the air over the top of the wing tends to flow outwards towards the tip of the wing, where it then peels off and sucks in the higher pressure air from the bottom. Creating the well know tip vortex (And most of the aformentioned parasitic drag) This is why winglets work. They redirect the spanwise airflow at the tip, causing cleaner vortex separation and better airflow over the ailerons.

          I’m a glider pilot and mechanical engineer with only some basic aerodynamics in me. If someone thinks I’m wrong or has aditions I’d love to hear it

    2. Just change angle of attack to compensate for negative lift, though you get a higher stall speed with inverted flight. With sufficient engine power, just about anything can fly.

      1. Oops. Chris beat me with his post while I was composing mine. Yes, angle of attack is the key… However, a good airfoil design can allow lower speed flight with less risk of a stall.

          1. More in expansion of what cHRIS is saying than anything. When you say fast jets I’m gonna assume over mach 0.8 as thats where stuff gets interesting.

            Basically at mach 0.8 we’re talking transonic aircraft (commonly big jets), so at some point over the wing the flow will become supersonic. The flatness the flatness of the aerofoil section helps delay the resulting shockwave ie. The flow doesn’t become supersonic until we’re further to the rear of the foil. This’ll reduce the drag due to the shock wave, great for long range transport.

            Supersonic foils are more like diamonds, this has more to do with keeping the shock attached and preventing a leading bow shock. Again more efficient. If I wasn’t on my phone I’d go into more detail but definitely an interesting topic.

    3. It’s all about angle of attack. While a non-symmetrical airfoil is most efficient at producing lift when upright, it can still produce lift when upside down. Part of the problem is the way the aerodynamics concept is presented in grade school text books where they explain that the air traveling over the top has to go faster blah blah blah. A more accurate and probably more intuitive would be to think of an airfoil as something that pumps air downward (newton’s laws and whatnot). It can easily do the same thing when the airplane is upside down, just not as efficiently.

  2. If lift was purely due to the Bernoulli principle, the wings of most modern aircraft would need to be about as thick as they are wide. Instead, deflection due to the Coandă effect is the major factor. This is also why planes can fly super-sonic.

    In regard to how they fly upside-down. The top of the wing is still deflecting massive amounts of air down… it’s just doing so inefficiently. Most aircraft can’t do this for long. Jet fighters have adequate power to deal with it. Stunt planes simply have symmetrical wing profiles so they fly equally well upside-down as right-side-up.

    1. Modern aerobatic aircraft “stunt planes” also have huge amounts of power. Planes like the Extra line often use the same engine as a 6 seat single but in a small, light single seater.

    1. An aircraft is just a formation of parts more or less flying in the same direction. It’s the pilots job to keep this collection of parts going the right way. The mechanics job is to keep the formation as tight as possible.

      A helicopter is a collection of rotating parts going round and round and reciprocating parts going up and down – all of them trying to become random in motion.

      It only takes two things to fly, airspeed and money.

      Definition of ‘pilot’: The first one to arrive at the scene of an aircraft accident.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.