Bjorn’s Corner: Aircraft drag reduction, Part 4

By Bjorn Fehrm

October 27, 2017, ©. Leeham Co: In this and the following Corners, we will review the competition for first manned flight between Samuel Langley, high ranking scientist of Allegheny Observatory in Pittsburgh, and the Wright Brothers, bicycle manufacturers in Dayton, (OH).

We described in the last Corner how Langley researched airfoil characteristics with a giant whirling arm, built beside the Observatory. The Wright Brothers went another route; they built a wind tunnel in their workshop.

Figure 1. The wind tunnel built by the Wright Brothers. Source: Wikipedia.

The aerodynamic information that puzzled the Wright Brothers

Wilbur and Orville Wright had a small bicycle manufacturing shop in Dayton. The brothers were intelligent, disciplined and inventive. They got interested in manned flight from reading about Lilienthal’s glider activities in Berlin.

In 1899, Wilbur wrote to the Smithsonian Institution and requested a list of all relevant literature on flight. The secretary of the Smithsonian sent Wilbur references to available reports and books. Among them was “Progress in Flying Machines” by Octave Chanute (published 1894). It summarized the aeronautical knowledge of the time and included excerpts from Lilienthal’s articles that he published in German aeronautical journals.

Chanute was in contact with the Wrights over the years and kept them informed about new information, like the publication of Langley’s Experiments in Aerodynamics 1902. Wilbur Wright read the reports and books and compared it to their own experiences with their gliders. A lot didn’t fit.

The Wrights decided to measure all parameters themselves, as they no longer trusted what was in the reports. And with good reason. A critical parameter, used by many at the time, was the coefficient for air pressure working on a flat plate perpendicular to the air stream. Smeaton had measured the coefficient with a whirling arm in 1759. He publicized this to have a coefficient of 0.005. The correct value (with the units he used) was 0.0029, a fault of 72%!

Wind tunnel measurements

The Wrights designed a wind tunnel (Figure 1) to do their own aerodynamic measurements. It wasn’t the first wind tunnel. The first was built in the UK in 1871, beside the Greenwich observatory, by Francis Whenham. He measured the forces of angled flat plates placed in a 3m by 0.5*0.5m tunnel, where a fan drove the air to 20m/s. As the tunnel was not baffled to get a non-turbulent stream and the measurement was made with coil spring balances, the data was not very reliable.

Wenham made some important discoveries, however. He found the center of lift was concentrated to 1/4 of the cord from the leading edge of the plate. Therefore, he suggested it was a waste to make the cord long; it was better to make a wide and narrow plate, i.e., give the plate a high aspect ratio.

Langley made the same discovery and wrote about it in his 1902 book, Experiments in Aerodynamics. Chanute sent the Wrights this book but by then they had decided not to trust others, better to do the fundamental research themselves. They can’t be blamed for not taking Langley’s findings to heart.

While Langley was right about the superiority of higher aspect ratio plates in the book, a lot of other stuff was suspect. Langley advocated the use of flat plates, despite his visit to Lilienthal in Berlin in 1895 and what he learned about the higher lift with curved profiles (Langley didn’t think highly of Lilienthal). He advocated the use of flat plates, as he was convinced lift came from air colliding with the bottom side, pushing the wing up.

The flow over the top was non-important, despite his test with cambered airfoils showing better lift to drag ratio. He also pushed his “inverse” drag law (see last Corner) as universal and a big discovery.

The Wright brothers by now were convinced. The way out of the contradicting information received through Chanute was to do their own basic research. The wind tunnel was similar to Whenham’s (they had access to his work and later wind tunnel tests by Horatio Phillips).

The tunnel made of wood was 1.8m (6ft) long, 0.46m (18in) wide and 0.46m (18in) high. Its fan was driven by the workshops 1hp gasoline engine via belt drive, it could get the air up to 13m/s (30mph). To get a non-circulating air at the windowed measurement section at the back, it was baffled internally.

In this tunnel, the Wrights placed their research objects on a measurement balance of very smart design. We will talk about why in the next Corner.

17 Comments on “Bjorn’s Corner: Aircraft drag reduction, Part 4

  1. Even in todays world with powerful modeling, CFD and theory, when someone has a radical idea, the cheapest, quickest way is to make a scale model, put it in a tunnel and see what happens at different speeds/angles.

    Most tunnel tests today are however to confirm CFD modeling.

  2. “He found the center of lift was concentrated to 1/4 of the cord of the plate.” Is that 1/4 the distance of the cord from the leading edge or trailing edge?

      • Thanks. I take it a cambered airfoil is more balanced with the center of lift more towards the center of the cord. I’ve always wondered how much force was on the flaps. How much the center of lift moves aft on the 727 between no flaps and maximum flaps?

          • I’m still trying to figure out how a wing works. Looking at figure 3.9 on page 96, shows the profile of a wing on an A320, a 757, and a 777 with a line from the leading edge to trailing edge. If the wing is at that zero degree angle of attack, will the force be up, lift, or will the force be down? Because I have a hunch, that Langley was mostly right, lift is created by air hitting the underside and being forced down. Every description of lift I have been taught is that it is from a shorter distance below the wing, and a longer more curved path on top of the wing. If this was the main component of lift, wings would fly at a zero degree angle of attack, right?

          • You get lift as a reactive force from moving (air) mass down relative the plane via the wing. 🙂
            Pressure is how you explain transfer of force.

            If you use the accelerated air -> lower pressure on top ~= lift explanation:
            On an inclined flat plate the upper airflow will separate and turn turbulent. Thus no accelerated airflow on top. Lift goes away.

            you can do another explanation:
            For all airflow situations you can draw a demarcation line separating ordered from unordered flow. Will that virtual profile create lift or not?

    • “He found the center of lift was concentrated to 1/4 of the cord of the plate.”

      This is only true for symmetric airfoils and presumably flat plates since they are fairly symmetric. When an airfoil has positive camber the center of lift is aft of the quarter chord point. In this case the lift of the airfoil can still be expressed as a combination of a lift force and a pitching moment at the 1/4 chord.

      Now, the 1/4 chord point is significant because it has been shown (both theoretically and through measurement) to be the point along the airfoil chord where the pitching moment of the wing section is constant with angle of attack (strictly only true for thin airfoils at speeds much less than sonic). Because of this the 1/4 chord point is called the aerodynamic center (ac) of the airfoil section. For symmetric airfoils the pitching moment at the ac is zero while for positively cambered airfoils the pitching moment is non-zero in the counterclockwise direction (center of lift is aft of the ac).

      Keep in mind that the above is valid for wing sections, not entire wings. If a wing has a rectangular planform (or tapered such that the ac of each section lies on a straight line perpendicular to flight direction) then these results for wing sections apply directly. However, if the wing is swept or has a planform where the section ac’s do not line up on a straight line, then the section properties need to be averaged along the span. One then talks about the mean aerodynamic center (mac) of the wing.

    • By the way, I’m not trying to diminish the aeronautical contribution of Whenham in any way. I just think the statement of his 1/4 chord discovery should’ve been qualified as being only for flat plates or symmetrical airfoils.

  3. Also,. Models are not magic. They are only as good as the data.

    If its new concept or approach and there is no data, then a model is not going to portray the results well, or even at all.

    I also seem to recall that the faster you go (maybe of Mach) then things change.

    My memory is a fighter that had a flat top and a curved bottom.

    And I never discounted the pressure at the bottom, its both in combination.

    Aeronautics aircraft fly upside down fairly well, not optimum of course but a bit more complex.

    I always thought that it is faith that keeps airplane in the air myself!

  4. Since the early pioneers figured that a cambered plate is better than a flat plate, what about the trailing edge? Why not have a reverse camber on the trailing edge so the air was returned at zero degree angle for a more laminar exit? The shape looking more like the graph of the function y=-x^3 instead of y=-x^2.

    • The up front thick profile is copied from birds.
      Bird wings have a single wing spar. It is preferable that most lift is created near that spar. evolutionary “design” processes lead to the observable in nature profiles.

      • Forgot: displacing flow ( at the front ) produces less disturbance than reuniting flow ( having it follow the upper surface further back.)

  5. Dear Bjorn,

    I’m surprised to find your initial statement fails to reference Gustav Weisskopf as an aviation pioneer. What are your thoughts on his alleged flights in 1901-02 in Bridgeport, Connecticut? Especially considering the aerodynamic deficiencies of Wright Flyer I.

    • The article series is not about who made the first manned flight, it’s about aerodynamic research and the different drag types that exit and their source. A lot of people have researched the subject on how flew first (I have not) and they claim the Wright brothers were the first sustained and controlled manned flight.

      • Understood. I had figured as much. I got hung up on the wording in the first paragraph. Thanks for the great writing/research/insight as always.


  6. Emirate is taking 40 787-10.

    Most interesting.

    Still struggling with Airbus over the A380 order.

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