Peter Garrison:” Why Can Aeroplanes Fly Upside-Down?

In 1945, the National Advisory Committee for Aeronautics, precursor of NASA and source of much of our knowledge about practical aerodynamics, published a hefty compendium of aerofoil data, NACA Technical Report 824, which later appeared in book form under the title Theory of Wing Sections.

The names of the authors, Abbott and von Doenhoff, have become household words wherever aerofoils are regularly discussed at the dinner table.

TR 824 presented, in the form of graphs, the characteristics of a great many aerofoils as measured in wind tunnel tests. Aeroplane designers and – perhaps even more numerous – would-be designers would pore over these graphs, alert to subtle differences that might make one aerofoil more suitable for a particular purpose than another.

To these aficionados of things round in front and pointed in back, aerofoils had distinct personalities: svelte and seductive 64-210, matronly 4418, dutiful and workmanlike 23012, exotic and mysterious 747A315.

But if one could free oneself from infatuation with one profile or family of profiles – “profile” and “aerofoil” are synonymous, by the way – and look with a cold eye on all these characters as a group, what was striking was that, as with people, their similarities were far greater than their differences.

If you superimposed all the graphs of lift coefficient against angle of attack upon one another, shifting them right and left as necessary to cancel the effects of camber, you found that they all formed a single slanting line that fanned out at its top and bottom ends into a spray resembling a spent firework. Remarkably, the line was practically straight; an aerofoil gained the same amount of lift when its angle of attack increased from five to six degrees as it did when it increased from one to two. 

The fraying at the ends represented the different ways different profiles stalled. Some aerofoils achieved greater maximum lift than others, so that the straight line extended farther before buckling and reversing its direction. Some, on stalling, lost lift suddenly and drastically: The straight line ended in a sharp peak and then plummeted. Others hung on stubbornly as angle of attack increased, rounding over gently and giving up lift a little at a time; these were the ones you wanted for STOL aeroplanes.

But if you looked only at the range of angles of attack actually used between climb and top speed – say, from plus eight degrees down to minus one – the lifting properties of all aerofoils were practically the same. Stranger still, you could flip the lines upside down and they still looked the same. In other words, they would behave the same way upside-down as they did right-side-up.

How could that be, if the whole magic of aerofoils depended, as so many books and articles said, on their special shape, and, in particular, on the upper surface having more of a hump than the lower?

The simple answer is that it doesn’t. We all know that wings without aerofoils – the wings of simple balsa wood gliders, for example, or those of folded-up sheets of paper – fly fine.

So what is a wing, anyway?

Basically, it’s a flat plate in a cute outfit. The rounded leading edge delays flow separation and stalling at large angles of attack. Camber, which gives many aerofoils their humped look, is just another way of helping air stick to the surface without separation. Camber also happens to give an aerofoil a top and a bottom: A cambered aerofoil will stall at a lower angle of attack inverted than right side up. For example, the NACA 65-415 laminar-flow aerofoil of the Piper Cherokee series stalls right side up at an angle of attack of 16 degrees and a lift coefficient of 1.35. Upside-down, the numbers are 13 degrees and 1.05. Roughly the same is true of the 23000-series profiles used on a great many Cessna and Beech aeroplanes.

(Lift coefficient is the ratio between the upward force generated by a wing and the impact pressure of moving air. For example, at 100 mph the impact pressure or “dynamic pressure” is about 25 pounds per square foot. A wing with a lift coefficient of 1.2 would generate 1.2 times 25, or 30 pounds of lift per square foot at 100 mph.)

It follows that, camber notwithstanding, it is not a problem for any wing to fly upside-down. Stalling behaviour and drag may change a bit, and so aeroplanes designed for aerobatics use symmetrical aerofoils to eliminate any preference.

When I first flew upside-down for any length of time it was in a Pitts Special. Much is made of the fact that turning controls are reversed in inverted flight, but what mainly surprised me, apart from the feeling that no matter how tightly I cinched my seat belt I still felt that I was falling out of my seat, was the amount of forward stick needed to hold the nose up and maintain level flight. To be sure, the aeroplane was, at least initially, trimmed for upright flight; but that ought to affect only the stick force needed to fly inverted, not the stick position.

Reflecting later on the strange sensation of holding the stick far out in front of me, I realized that it was due to the aeroplane having some built-in nose-up trim, in the form of decalage – a slightly negative incidence of the stabiliser relative to the wing. In upright flight, negative stabiliser incidence lifts the nose; in inverted flight, it makes the aeroplane want to dive, and has to be counteracted by extra “up” – that is, stick forward – elevator.

It was not long after people found out how to fly that they found they could fly upside down. The principal activity of pilots prior to World War One was making a spectacle of themselves, and despite the rickety appearance of early aeroplanes, pilots performed many daring stunts in them.

The first pilot to invert an aeroplane was Alphonse Pégoud, who did the trick in 1913 in a Blériot monoplane. This was the same type of aeroplane as Blériot himself had used for the 1909 crossing of the English Channel that made him famous, and it was singularly ill suited for inverted flight, its wing having far more camber than necessary even for upright flight. The Blériot’s wing was so severely cambered that I’m sure it would have been incapable of sustained inverted flight; but it could “loop the loop” because in those manoeuvres lift was always in a positive direction relative to the airframe.

Sustained inverted flight was not far behind, and it reached its reductio ad absurdum only a year later when a German daredevil named Gustav Tweer (pronounced Tvehr), whose specialty was diving straight at the ground and pulling out at the last possible moment, modified a Grade (pronounced GRAH-deh) monoplane by adding a mirror-image of its own landing gear and amused crowds by not only flying, but also landing, upside-down.

Hans Grade was Germany’s Blériot, and it is apparent from pictures of his aeroplanes that he realized that a wing did not need all the camber Blériot gave his; in fact, the wing aerofoil of Tweer’s invertible Grade appears to have been symmetrical.

The problem of inverted flight is really not an aerodynamic one at all. Aeroplanes – all aeroplanes – are capable of flying upside-down. It’s engines that are the problem. To keep them running, they need invertible fuel and lubrication systems. These typically involving a weighted hose that drops to whatever happens to be the bottom of the fuel or oil tank at the time. It is, come to think of it, one of the hitherto under-appreciated virtues of electric aircraft that you can fly them upside-down without any modification. Just remember to make your seat belt very, very tight.