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Proficiency: No equations requiredProficiency: No equations required

The secret of flight doesn’t need to be

As a newly minted pilot, I read every aviation magazine I could find cover to cover. Tucked in the back of one almost 20 years ago, I found an advertisement regarding a collection of videos by aerodynamicist Alexander Lippisch that had been gathering dust in university archives since the 1950s. The ad promised that, if enough interested parties pitched in $150 each, the University of Iowa would digitize The Secret of Flight and send each contributor a set. I’d been fascinated by the science of flight from the start, so I sent off a check and wondered what I would get in return.
P&E November 2019
This series of smoke tunnel clips shows the extent to which the flow across the top of the cambered airfoil is accelerated. We also see that the higher velocity flow over the top corresponds to narrower streamlines. Note that two air molecules that occupy the same timeline (top) and pass on opposite sides of the airfoil will never meet again as the one moving over the top has a much greater velocity. This demonstration debunks the equal transit theory proposed in many aviation manuals.

As the angle of attack of the wing increases, the point at which the flow separates from the airfoil surface moves toward the leading edge. When the separation bubble, or the region of reversed flow that occupies the region between the airfoil and the flow, envelopes most of the airfoil, the wing stalls. Interestingly, even at low angles of attack, there is a small bit of separated flow near the trailing edge of the airfoil. It is minimized with—and the reason that wings have—a pointed trailing edge.It was the same anticipation I experienced as a child when I had amassed enough breakfast cereal box tops to send in and claim my prize. Invariably, though, my hopes were dashed when a flimsy piece of plastic or some other throwaway arrived in the mailbox. Despite this poor track record, I still deemed the video collection worth the risk. At worst, I would see the way aerodynamics was explained in the olden days, and I’d learn a bit of history. When the envelope from University of Iowa arrived, I ran to put the disc in the DVD player and found the contents of this time capsule far better than I ever could have imagined.

Lippisch was a German aerodynamicist whose important contributions included work on the delta wing, tailless aircraft, and high-performance gliders. In 1946, he immigrated to the United States as part of Operation Paperclip, the same program that brought more than 1,600 German scientists, including Wernher von Braun, to propel American flight research. In 1950 Lippisch began a 14-year tenure with the Collins Radio Corporation, during which time he published scientific papers on his smoke tunnel experiments and aeronautical innovations. In 1955, he created The Secret of Flight film series with the University of Iowa.

The series consists of 13 half-hour episodes in which Lippisch uses simple physical models and smoke tunnel videos that allow the viewer to witness the way wings produce lift, how they stall, and the nature of induced drag, to name just a few. The best part is, despite being at the top of his field, Lippisch targets a viewer with no technical background and introduces few scientific terms and equations. During the first episode alone, I realized I had been duped regarding the story of lift by the manuals I had studied as a student pilot.

I knew about the principle that eighteenth-century mathematician and physicist Daniel Bernoulli observed about fluid flow—when the velocity of the flow increases, the pressure it exerts decreases. It follows from Newton’s Second Law of Motion, which says the rate of change of momentum of a body is proportional to the force applied and in the direction of the applied force. But these facts don’t provide an intuitive understanding of lift, so some instructors and authors try to offer one.

The “equal transit theory” is a popular one that goes like this: Consider two air particles that start at the front of a cambered airfoil with one of them traveling across the top and the other below. Since the top surface of the airfoil is longer, the particle that travels across it must move faster in order to meet up with the particle that travels below. It’s a tidy theory, and perhaps it’s somehow romantic to think these long-lost particles will reconvene following their solo journeys. The trouble is, it’s just plain wrong, and the negative effects of teaching this flimsy theory last longer than the disappointment of a useless toy. Student pilots become flight instructors who propagate this fallacy through the aviation community at an alarming rate.Last year, several aviation authors offered mea culpas for presenting the equal transit theory. But one author defended its use by claiming that understanding what really causes lift necessitates learning big confusing words. Not true at all, as Lippisch deftly demonstrates.

P&E November 2019
Induced drag is an inevitable byproduct of lift and can be visualized through wingtip vortices. In this sequence of frames, we see from above a wing in the smoke tunnel with negative, zero and positive angles of attack. A wing with a symmetric airfoil at a zero angle of attack produces no lift and, therefore, no induced drag (left). With a positive angle of attack, the higher-pressure air below the wing seeks the lower pressure environment on top as is evidenced by the streamlines curling around the wing tips (right). WIth a negative angle of attack, the higher-pressure air atop the wing curls to the lower side around the wing tips (center)

The Secret of Flight allows the viewer to actually witness the production of lift. Lippisch uses a wind tunnel in which a vertical strip with evenly spaced holes emits colored particles (or smoke) into the flow. The smoke along the streamlines reveals the shape of the flow around the airfoil. Many of the videos feature pulsated smoke to show the time history of the flow as well. The timelines prove that the flow across the top of the wing is much greater than the flow along the bottom. Those two air particles of the equal transit theory will never meet again! Note also that faster flow corresponds to streamlines that are more closely spaced. Finally, Lippisch uses a gas manometer to show the pressure is lowest along those regions of the airfoil at which the velocity is highest. Therefore, narrow streamlines correspond to faster flow which, in turn, results in lower pressure. With low pressure above the wing and high pressure below it, lift is the net force in the upward direction. That’s it!

All this represents just a few minutes of The Secret of Flight video series. Other highlights include the nature of a wing stall (above) and a visualization of induced drag via wingtip vortices (p. 86). In Episode 6, Lippisch starts with a simple propeller, increases its efficiency by enclosing it with a shroud, and then ultimately motivates the modern jet engine. In Episode 12, he explains why the smooth wings on a bird and the rough wings on an insect each minimize drag. These are just a few of the topics guaranteed to fascinate student pilots and aerodynamicists alike.

The Secret of Flight is indeed a time capsule produced when color films were rare. I chuckled when Lippisch predicted that “rocket engines will someday take man to space.” But while the age of the series is evident in quaint ways, the laws of aerodynamics are timeless; it’s our attempts to model and understand them that evolve over time. These smoke tunnel videos and simple demonstrations are as valuable as instructional tools today as ever. I have watched the series several times and I glean new facts with each viewing.

In recent years, The Secret of Flight became unavailable for purchase after the supply of DVDs was exhausted. But in August of this year, the University of Iowa bestowed upon the aviation community an incredible gift by uploading the series in its entirety on YouTube. No money, anxious waiting times, nor impending disappointment is necessary. Just sit back, relax, and enjoy as the secret of flight is revealed.

Catherine Cavagnaro is an aerobatics instructor and professor of mathematics at Sewanee: The University of the South.

SEcret of Flight

Pulsated flow reveals interesting features of the flow. Streamlines are parallel to the flow in the tunnel.  Particles along a timeline (for example, the left edge of the blue colored flow) enter the tunnel at the same time. As the flow across the top is accelerated to a higher velocity compared with the flow along the bottom, the timelines break into two. Note also that the streamlines above are narrower. This sequence of frames shows the wing tip of a cambered wing in the smoke tunnel.  The angle of attack is (from top to bottom), positive, a slightly negative angle of attack that results in approximately zero lift and a negative angle of attack. The lowest frame shows that even a positively-cambered wing can create a down force since its lower pressure side is the bottom.  In a conventional aircraft, the horizontal tail-elevator combination can provide lift in either direction.  Aerobatic aircraft in inverted flight shows that the wing is capable of lifting toward the lower side of the airplane. To facilitate inverted flight, many acrobatic category aircraft feature a wing using a symmetric airfoil. A gas manometer shows pressure values at various points along the airfoil surface. Taller bars indicate regions of lower pressure.

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